Patent Publication Number: US-2021171322-A1

Title: Mobile crane

Description:
TECHNICAL FIELD 
     The present invention relates to a mobile crane. 
     BACKGROUND ART 
     There have been conventionally known mobile cranes each including a lower travelling body of a self-running type, an upper slewing body slewably mounted on the lower travelling body, and a tiltable attachment including a boom tiltably attached to the upper slewing body. Such a mobile crane performs a hoisting work of hoisting a hanged load in a state (a raised state) where the boom is raised from the upper slewing body. Further, in an assembly work of assembling the mobile crane, the boom is attached to the upper slewing body in a state (a lowered state) where the boom is lowered with a posture substantially parallel to the ground. For the hoisting work, the posture of the boom is then changed from the lowered state to the raised state by way of a raising operation of gradually increasing an inclination angle of the boom to the ground. Conversely, for a disassembly work of disassembling the mobile crane, the posture of the boom in the raised state is rechanged to the lowered state by way of a lowering operation of gradually decreasing the inclination angle of the boom to the ground. 
     In the aforementioned crane, a gravity center position of the tiltable attachment including the boom shifts in accordance with a change in the inclination angle of the boom to the ground. Accordingly, a moment correlating with the weight and the gravity center position of the tiltable attachment changes. The mobile crane includes a moment limiter to prevent the change in the moment from causing the mobile crane to turn over. Moreover, the moment limiter gives an alarm to suspend the operation of the mobile crane or the like when a turning-over moment of the mobile crane reaches a predetermined threshold in accordance with the change in the inclination angle of the boom in the hoisting work, resulting in ensuring the safety. 
     Meanwhile, the assembly work and the disassembly work involving the extensive raising and lowering operations between the raised state and the lowered state as described above differ from the hoisting work in the following respects. Specifically, the moment limiter is a device basically directed to the stability during the hoisting work. Therefore, the moment limiter sets a hoisting performance within a work range presumed to be necessary for the hoisting work. In contrast, the assembly work and the disassembly work may be performed in a situation with a range deviating from the work range of the hoisting work, e.g., may be performed in the situation of the aforementioned lowered state where the inclination angle of the boom to the ground is small, in addition to the work range of the hoisting work. In this regard, the moment limiter sets no hoisting performance for the range deviating from the work range of the hoisting work. Under the circumstances, the angle of the boom is decreased for the assembly work and the disassembly work while the moment limiter is stopped or a limit set by the moment limiter is released without stopping the moment limiter. Hence, an operator of the mobile crane is required to have experiences and knowledge for determining whether the inclination angle of the boom assures safety in the assembly work and the disassembly work. Various technologies have been proposed to increase the safety in the assembly work and the disassembly work. 
     Patent Literature 1 discloses an operation of raising a boom having a posture extending from an upper slewing body in one of leftward and rightward directions. Patent Literature 1 discloses that a distance from a turning-over fulcrum increases by a side jack attached to a lateral part of a side frame that faces the boom when the boom having the posture is raised (paragraph [0015] of Patent Literature 1). 
     Patent Literature 2 discloses an operation assisting device for a crane. A certain combination of a boom length and a jib length of a front attachment (a tiltable attachment) attains a stability during an operation of lowering the front attachment in the crane including the operation assisting device under the condition that a relative angle of a boom to a jib is defined as a first target angle. In this case, the operation of lowering the front attachment is continued until a distal end of the jib comes into contact with the ground in the state where the relative angle of the jib to the boom is maintained at the first target angle. The technology disclosed in Patent Literature 2 requires an operator to input in advance, to the operation assisting device, various information including information concerning the boom and the jib, and a target value of the relative angle of the boom to the jib. 
     However, the operator of the crane is still required to have the experiences and knowledge for determining whether the inclination angle of the boom assures safety in the assembly work and the disassembly work even with the increased distance from the turning-over fulcrum owing to the side jack of Patent Literature 1. That is to say, success or failure in safe operations of raising and lowering the boom depends on the operator&#39;s experiences and knowledge. 
     Additionally, different mobile cranes have different specifications, e.g., a crane including a boom, a jib, and a strut like the crane disclosed in Patent Literature 2, a crane including a boom without a jib, a crane including a lattice mast, and other cranes with various specifications. A tiltable attachment is suitably selected, and a boom length and a jib length are adjusted to meet a required performance and a type of work for each of the mobile cranes. With the technology disclosed in Patent Literature 2, it is necessary to input information concerning a boom and a jib for all the specifications, and a target value suitably for each of the specifications. In this regard, however, the operator is overburden with the grasp and the input of the information and the target values for all the specifications, which may lead to a typographical error made by the operator. 
     This problem may occur in other works as well as the assembly work and the disassembly work. Such other works include, for example, a work for an overload test related to the mobile crane. The overload test is a test of confirming a hoisting work of hoisting a predetermined hanged load to apply a load exceeding a rated load to the mobile crane while the moment limiter is stopped or a limit of the moment limiter is released without stopping the moment limiter. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Publication No. 2016-221993 
     Patent Literature 2: Japanese Unexamined Patent Publication No. 2014-162607 
     SUMMARY OF INVENTION 
     The present invention has been accomplished to solve the aforementioned problem, and an object of the present invention is to provide a mobile crane which can detect information necessary to safely raise and lower a tiltable attachment without an overburdened input operation by an operator. 
     Provided is a mobile crane, including: a lower traveling body which includes a pair of crawlers each extending in forward and rearward directions and spaced apart from each other in leftward and rightward directions; an upper slewing body supported on the lower traveling body slewably about a slewing axis; a tiltable attachment including a boom tiltably supported on the upper slewing body; and a physical quantity detector. The lower traveling body has a reaction force receiving part for receiving a reaction force from the ground at a position away from the slewing axis in a boom direction in a state where the pair of crawlers is in contact with the ground, the boom direction coinciding with a horizontal component of a direction in which the boom extends from the upper slewing body. The physical quantity detector is configured to detect a physical quantity which changes in accordance with a change in the reaction force received from the ground by the reaction force receiving part. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sideview of a mobile crane according to embodiments including a tiltable attachment having a posture in a raised state in a hoisting work. 
         FIG. 2  is a block diagram showing an operative configuration of the mobile crane shown in  FIG. 1 . 
         FIG. 3  is a plan view of a lower traveling body of the mobile crane according to a first embodiment. 
         FIG. 4  is a sideview of the lower traveling body of the mobile crane according to the first embodiment. 
         FIG. 5  is a sideview of a crawler frame of the lower traveling body of the mobile crane according to the first embodiment. 
         FIG. 6  is a sideview of a front end of the crawler frame seen in a direction of the arrow VI in  FIG. 3 . 
         FIG. 7  is a schematic view showing a stress distribution in a target cross section of the front end of the crawler frame for which a strain is measured. 
         FIG. 8  is a schematic sideview of the mobile crane having a specific posture that the tiltable attachment is in a lowered state in an assembly work or a disassembly work of the mobile crane. 
         FIG. 9  is a schematic sideview of the mobile crane having a specific posture that the tiltable attachment is being raised or lowered in the assembly work or the disassembly work. 
         FIG. 10  is a schematic sideview of the mobile crane having a specific posture that the tiltable attachment is being raised or lowered in the assembly work or the disassembly work. 
         FIG. 11  is a schematic sideview of the mobile crane having a specific posture with a moment balanced position close to a turning-over fulcrum in the assembly work or the disassembly work. 
         FIG. 12  is a schematic sideview of the mobile crane having a specific posture that the tiltable attachment is being raised or lowered in the assembly work or the disassembly work. 
         FIG. 13  is a schematic sideview of the mobile crane having a specific posture that the tiltable attachment is being raised or lowered in the assembly work or the disassembly work. 
         FIG. 14  is a sideview of a front end of a crawler frame seen in the direction of the arrow VI in  FIG. 3  in a first modification of the first embodiment. 
         FIG. 15  is a sideview of a front end of a crawler frame seen in the direction of the arrow VI in  FIG. 3  in a second modification of the first embodiment. 
         FIG. 16  is a sideview of a front end of a crawler frame seen in the direction of the arrow VI in  FIG. 3  in a third modification of the first embodiment. 
         FIG. 17  is a perspective view schematically showing a fourth modification of the first embodiment. 
         FIG. 18  is a flowchart showing exemplary arithmetic processing by a controller in the mobile crane. 
         FIG. 19  shows specific examples of stability information notified on a display part of a notification device in the mobile crane. 
         FIG. 20  is a flowchart showing another exemplary arithmetic processing by the controller. 
         FIG. 21  is the flowchart showing another exemplary arithmetic processing by the controller. 
         FIG. 22  is a plan view of a lower traveling body of a mobile crane according to second and third embodiments. 
         FIG. 23  is a sideview of a support member (a receiving member) to be attached to a crawler frame of the mobile crane in  FIG. 22 . 
         FIG. 24  is a sideview of the support member having been attached to the crawler frame in  FIG. 22 . 
         FIG. 25A  is an exemplary cross-sectional view of a beam of the support member taken along the line XXV-XXV in  FIG. 24 . 
         FIG. 25B  is another exemplary cross-sectional view of the beam of the support member taken along the line XXV-XXV in  FIG. 24 . 
         FIG. 26  is a schematic sideview of the mobile crane in  FIG. 1  according to the second and the third embodiments having a specific posture that a tiltable attachment is in a lowered state in an assembly work or a disassembly work of the mobile crane. 
         FIG. 27  is a schematic sideview of the mobile crane in  FIG. 1  according to the second and the third embodiments having a specific posture that the tiltable attachment is being raised or lowered in the assembly work or the disassembly work. 
         FIG. 28  is a schematic sideview of the mobile crane in  FIG. 1  according to the second and the third embodiments having a specific posture that the tiltable attachment is being raised or lowered in the assembly work or the disassembly work. 
         FIG. 29  is a schematic sideview of the mobile crane in  FIG. 1  according to the second and the third embodiments having a specific posture with a moment balanced position close to a turning-over fulcrum in the assembly work or the disassembly work. 
         FIG. 30  is a schematic sideview of the mobile crane in  FIG. 1  according to the second and the third embodiments having a specific posture that the tiltable attachment is being raised or lowered in the assembly work or the disassembly work. 
         FIG. 31  is a schematic sideview of the mobile crane in  FIG. 1  according to the second and the third embodiments having a specific posture that the tiltable attachment is being raised or lowered in the assembly work and the disassembly work. 
         FIG. 32  is a sideview of a lower traveling body of a mobile crane according to a first modification of the embodiments. 
         FIG. 33A  is an exemplary cross-sectional view of a beam of a support member taken along the line XXXIII-XXXIII in  FIG. 32 . 
         FIG. 33B  is another exemplary cross-sectional view of the beam of the support member taken along the line XXXIII-XXXIII in  FIG. 32 . 
         FIG. 34  is a sideview of a lower traveling body of a mobile crane according to a second modification of the second embodiment. 
         FIG. 35  is a cross-sectional view of a beam of a support member taken along the line XXXV-XXXV in  FIG. 34 . 
         FIG. 36  is a plan view of a lower traveling body of a mobile crane according to a third modification of the second embodiment. 
         FIG. 37  is a plan view of a lower traveling body of a mobile crane according to a fourth modification of the second embodiment. 
         FIG. 38  is a perspective view schematically showing a fifth modification of the second embodiment. 
         FIG. 39  shows an exemplary hydraulic circuit of the mobile crane according to the third embodiment. 
         FIG. 40  is a plan view of a lower traveling body of a mobile crane according to fourth and fifth embodiments, and shows a state where a trans-lifter is engaged with an engaging portion of a frame. 
         FIG. 41  is a sideview of the lower traveling body of the mobile crane in  FIG. 40 , and shows a state where the trans-lifter is engaged with the engaging portion of the frame. 
         FIG. 42  is a partially broken sideview of the engaging portion of the frame of the mobile crane in  FIG. 40  and the trans-lifter engaged with the engaging portion. 
         FIG. 43  is a plan view of the lower traveling body of the mobile crane in  FIG. 40 , and shows a state where a support member (a receiving member) is engaged with the engaging portion of the frame. 
         FIG. 44  is a sideview of the lower traveling body of the mobile crane in  FIG. 40 , and shows a state where the support member (the receiving member) is engaged with the engaging portion of the frame. 
         FIG. 45  is a partially broken sideview of the engaging portion of the frame of the mobile crane in  FIG. 40  and the support member (the receiving member) engaged with the engaging portion. 
         FIG. 46  is a cross-sectional view taken along the line XXXXVI-XXXXVI in  FIG. 45 . 
         FIG. 47  is a perspective view schematically showing a modification of the fourth embodiment. 
         FIG. 48  is a plan view of a lower traveling body of a mobile crane according to sixth and seventh embodiments. 
         FIG. 49  is a sideview of the lower traveling body of the mobile crane in  FIG. 48 . 
         FIG. 50  is a cross-sectional view of a beam of a support member (a receiving member), taken along the line XXXXX-XXXXX in  FIG. 48 , in a crawler of the lower traveling body in  FIG. 48 . 
         FIG. 51  is a sideview of a front portion of the lower traveling body of the mobile crane in  FIG. 48 . 
         FIG. 52  is a plan view of the front portion of the lower traveling body of the mobile crane in  FIG. 48 . 
         FIG. 53  is a plan view of a lower traveling body of a mobile crane according to a first modification of the sixth and the seventh embodiments. 
         FIG. 54  is a perspective view of a crawler frame and a support member (a receiving member) attached to the crawler frame in the mobile crane according to the first modification of the sixth and the seventh embodiments, and shows a state where a part of a beam of the support member is disengaged from an engaging portion. 
         FIG. 55  is a perspective view of the crawler frame and the support member (the receiving member) in  FIG. 48 , and shows a state where a part of the beam of the support member is engaged with the engaging portion. 
         FIG. 56  is a sideview of the crawler frame and the support member (the receiving member) in  FIG. 48 , and shows a state where a part of the beam of the support member is disengaged from the engaging portion. 
         FIG. 57  is a sideview of the crawler frame and the support member (the receiving member) in  FIG. 48 , and shows a state where a part of the beam of the support member is engaged with the engaging portion. 
         FIG. 58  is a perspective view schematically showing a second modification of the sixth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a mobile crane according to each of the embodiments will be described. 
     [Mobile Crane] 
       FIG. 1  is a sideview of a mobile crane  10  according to each of the embodiments having a specific posture that a tiltable attachment is in a raised state in a hoisting work of hoisting a hanged load on a work site.  FIG. 2  is a block diagram showing an operative configuration of the mobile crane  10  in  FIG. 1 . 
     The directions denoted by “UPWARD”, “DOWNWARD”, “FORWARD”, “REARWARD”, “RIGHTWARD”, and “LEFTWARD” in the drawings are defined with respect to a lower traveling body  11  of the mobile crane. The forward and rearward directions in the drawings represent directions in which the lower traveling body  11  moves forward and rearward. Specifically, in the embodiments, a direction from a longitudinal center of a crawler frame  1  to a wheel  4   a  (see  FIG. 3 ) to be described later is defined as a forward direction, and a direction from the longitudinal center of the crawler frame  1  to a wheel  4   c  (see  FIG. 3 ) to be described later is defined as a rearward direction. Leftward and rightward directions are determined in relation to the forward and rearward directions. However, the way of defining the forward and rearward directions is just an example, and thus should not be limited thereto. For instance, the direction from the longitudinal center of the crawler frame  1  to the wheel  4   a  may be defined as a rearward direction, and the direction from the longitudinal center of the crawler frame  1  to the wheel  4   c  may be defined as a forward direction. 
     As shown in  FIG. 1 , the crane  10  includes the lower traveling body  11  of a self-travelling type, an upper slewing body  12  mounted on the lower traveling body  11  slewably about a slewing axis C (see  FIGS. 3, 4 and 10 ), a tiltable attachment, a mast  20 , a counterweight  13  carried on a rear part of the upper slewing body, at least one physical quantity detector  90  (see  FIG. 2 ), a controller  100  (see  FIG. 2 ), and a notification device  110  (see  FIG. 2 ). In the embodiments, the tiltable attachment includes a boom  14 , a jib  17 , an upper strut  22 , and a lower strut  21 . 
     The boom  14  is tiltably and detachably attached to the upper slewing body  12 . The boom  14  shown in  FIG. 1  has a boom main body  15  of a lattice-type, a proximal end  14 A, and a distal end  14 B. 
     The boom main body  15  has a proximal end member  15 A, one or more (two in the illustrated example) intermediate members  15 B,  15 C, and a distal end member  15 D. The proximal end member  15 A is coupled to a front part of the upper slewing body  12  swingably in raising and lowering directions. The intermediate members  15 B,  15 C are detachably connected with a distal end of the proximal end member  15 A in this order. The distal end member  15 D is detachably connected with a distal end of the intermediate member  15 C. The intermediate members  15 B,  15 C are excludable. 
     The jib  17  is rotatably and detachably attached to a distal end of the boom  14 . The jib  17  has a lattice configuration in the illustrated example. The jib  17  has a proximal end  17 A rotatably coupled to the distal end  14 B of the boom  14 . A rotational axis of the jib  17  is parallel to a rotational axis of the boom main body  15  with respect to the upper slewing body  12 . As shown in  FIG. 1 , the jib  17  has a distal end  17 B provided with a roller  17 R which can support the jib  17  and rotate on the ground while the distal end  17 B is in contact with the jib  17 . 
     The upper strut  22  and the lower strut  21  enable the jib  17  to rotate. The upper strut  22  is rotatably attached to the distal end  14 B of the boom  14 . The lower strut  21  is rotatably attached to the distal end  14 B of the boom  14  in the rear of or below the upper strut  22 . The upper strut  22  and the lower strut  21  are attachable to and detachable from the distal end  14 B of the boom  14 . 
     A pair of left and right backstops  23  is disposed above the upper slewing body  12 . The left and right backstops  23  respectively come into contact with the left and right opposite side ends of the proximal end member  15 A of the boom  14  when the boom  14  is raised and reaches the posture shown in  FIG. 1 . The contact restricts the boom  14  from being blown rearward with a strong wind or the like. 
     The lower strut  21  is held in a posture of protruding from the distal end  14 B of the boom  14  in a boom raising direction (a leftward direction in  FIG. 1 ). A way of holding the posture includes arranging a pair of left and right backstops  25  and a pair of left and right strut guide lines  26  between the lower strut  21  and the boom  14 . Each of the backstops  25  connects the distal end member  15 D and a middle portion of the lower strut  21  with each other for supporting the lower strut  21  from below. The guide line  26  extends under a tension to connect the distal end  21 B of the lower strut  21  and the proximal end member  15 A with each other, and restricts the position of the lower strut  21  by using its tension force. 
     The upper strut  22  is rotatably coupled to the jib  17  to cooperate with the jib  17 . Specifically, a pair of left and right jib guide lines  28  extends under a tension to connect a distal end  22 B of the upper strut  22  and the distal end  17 B of the jib  17  with each other. Accordingly, the jib  17  rotates in cooperation with the rotation of the upper strut  22 . 
     The mast  20  has a proximal end  20 A and a rotatable end  20 B. The proximal end  20 A of the mast  20  is swingably coupled to the upper slewing body  12 . The mast  20  has a rotational axis extending in parallel to a rotational axis of the boom  14  and just in the rear of the rotational axis of the boom  14 . In other words, the mast  20  is swingable in a direction corresponding to the raising direction of the boom  14 . In contrast, the rotatable end  20 B of the mast  20  is connected with the distal end  14 B of the boom  14  via a pair of left and right boom guide lines  24 . The connection allows the mast  20  and the boom  14  to swing in cooperation with each other. 
     The crane  10  is mounted with various winches, specifically, a boom raising and lowering winch  30  for raising and lowering the boom  14 , a jib raising and lowering winch  32  for causing the jib  17  to swing in the raising and lowering directions, a main winch  34  and an auxiliary winch  36  for lifting and lowering the hanged load. 
     The boom raising and lowering winch  30  executes winding and unwinding of a boom raising and lowering rope  38 . The boom raising and lowering rope  38  is arranged so that the mast  20  swings in accordance with the winding and the unwinding. Specifically, the rotatable end  20 B of the mast  20  and a rear end of the upper slewing body  12  are provided with their respective sheave blocks  40 ,  42  each having a plurality of sheaves arrayed in a width direction. The boom raising and lowering rope  38  drawn out from the boom raising and lowering winch  30  is supported on the sheave blocks  40 ,  42  and extends therebetween. Consequently, the winding and unwinding of the boom raising and lowering rope  38  by the boom raising and lowering winch  30  brings a change in a distance between the sheave blocks  40 ,  42 , thereby allowing the mast  20  and the boom  14  to swing in cooperation with each other in the raising and lowering directions. 
     The jib raising and lowering winch  32  executes winding and unwinding of a jib raising and lowering rope  44 . The jib raising and lowering rope  44  is arranged so that the upper strut  22  rotates in accordance with the winding and unwinding. Specifically, the lower strut  21  has a longitudinally intermediate portion provided with a guide sheave  46 . Further, the distal end  21 B of the lower strut  21  and the distal end  22 B of the upper strut  22  are provided with their respective spreaders  47 ,  48  (sheave blocks) each having a plurality of sheaves arrayed in a width direction. The jib raising and lowering rope  44  drawn out from the jib raising and lowering winch  32  is supported on the guide sheave  46  and extends between the spreaders  47 , 48 . Consequently, the winding and unwinding of the jib raising and lowering rope  44  by the jib raising and lowering winch  32  brings a change in the distance between the spreaders  47 ,  48 , thereby allowing the upper strut  22  and the jib  17  to rotate in cooperation with each other in the raising and lowering directions. 
     The main winch  34  executes the lifting and lowering of the hanged load by using a main rope  50 . For the main lifting and lowering, main guide sheaves  52 ,  53 ,  54  are rotatably provided in the vicinities of a proximal end  21 A of the lower strut  21 , a proximal end  22 A of the upper strut  22 , and the distal end  17 B of the jib  17 , respectively. Furthermore, a jib point sheave  56  is located adjacently to the main guide sheave  54  (at the distal end  17 B of the jib  17 ). The main rope  50  drawn out from the main winch  34  is sequentially supported on the main guide sheaves  52 ,  53 ,  54 , and extends between the jib point sheave  56  and a hook sheave  58  provided at a main hook  57  for the hanged load. Consequently, winding and unwinding of the main rope  50  by the main winch  34  brings a change in the distance between the sheaves  56 ,  58 , thereby achieving lifting and lowering of the main hook  57 . 
     Similarly, the auxiliary winch  36  executes lifting and lowering of the hanged load by using an auxiliary rope  60 . For the auxiliary lifting and lowering, auxiliary guide sheaves  62 ,  63 ,  64  are provided rotatably and coaxially with the corresponding main guide sheaves  52 ,  53 ,  54 . The roller  17 R (serves as an assistive sheave) is rotatably located adjacently to the auxiliary guide sheave  64  (at the distal end of the jib  17 ). The auxiliary rope  60  is supported on the assistive sheave. Specifically, the auxiliary rope  60  drawn out from the auxiliary winch  36  is sequentially supported on the auxiliary guide sheaves  62 ,  63 ,  64 , and eventually hangs down from the assistive sheave. Consequently, winding and unwinding of the auxiliary rope  60  by the auxiliary winch  36  leads to lifting and lowering of an unillustrated auxiliary hook for the hanged load attached to a leading end of the auxiliary rope  60 . 
     The physical quantity detector  90  detects information necessary to safely raise and lower the boom  14  in a specific work of the crane  10 . The physical quantity detector  90  is configured to detect a physical quantity which changes in accordance with a change in a reaction force received from the ground by a reaction force receiving part to be described later in the lower traveling body  11 . The physical quantity includes, for example, a strain occurring in the crawler frame  1  to be described later, and at least one of a pressure in a head chamber and a pressure in a rod chamber of a hydraulic cylinder to be described later, but should not be limited thereto. A signal representing the physical quantity detected by the physical quantity detector  90  is input to the controller  100 . 
     The specific work means a work accompanied by an occurrence of a large moment in a direction of causing the crane  10  to turn over. The specific work includes, for example, the assembly work and the disassembly work. The specific work further includes, for example, a work for the overload test. Hereinafter, each of the assembly work and the disassemble work will be described as the specific work. 
     The notification device  110  shown in  FIG. 2  is a device for notifying an operator of stability information concerning a stability of the crane  10  based on the physical quantity detected by the physical quantity detector  90 . For instance, the stability information includes information concerning a front and rear balance of the crane  10 . 
     The notification device  110  includes, for example, at least one of a sound emitter for emitting a sound, a light emitter for emitting a light beam, and a display part for displaying a character, a geometric shape, or the like. The notification device  110  is disposed in a place easily recognizable for the operator, specifically, in a cab  12 A on the upper slewing body  12 . 
     The sound emitter has a function of emitting a sound hearable by the operator through the operator&#39;s auditory. For example, the sound emitter has an alarming buzzer, a speaker or the like. The light emitter has a function of emitting a light beam visible by the operator through the operator&#39;s sight. For example, the light emitter has a display lump, a revolving lump, a signal lump or the like, which is unillustrated. The display part has a function of displaying a character, a geometric shape or the like understandable by the operator through the operator&#39;s sight. For example, the display part has an unillustrated display element. 
     The controller  100  is composed of a central processing unit (CPU), a ROM which stores various control programs, a RAM for use as a working area of the CPU, and the like. As shown in  FIG. 2 , the controller  100  includes a calculation section  101 , a stability determination section  102 , a notification control section  103 , and an operation control section  104 . 
     The calculation section  101  has a parameter calculation part and a ratio calculation part. The parameter calculation part of the calculation section  101  calculates, based on the physical quantity detected by the physical quantity detector  90 , a moment caused by the weight of the tiltable attachment (a gravity acting on the tiltable attachment) in the crane  10  to cause the crane  10  to turn over. The ratio calculation part of the calculation section  101  calculates a ratio between a first parameter in connection with a first moment Mf to be described later and a second parameter in connection with a second moment Mb to be described later. In the embodiments, the first parameter serves as the first moment Mf, and the second parameter serves as the second moment Mb. However, the first parameter may be other parameter which changes in accordance with a change in the first moment Mf, and hence is not necessarily limited to the first moment Mf. Similarly, the second parameter may be other parameter which changes in accordance with a change in the second moment Mb, and hence is not necessarily limited to the second moment Mb. 
     The stability determination section  102  determines the stability based on the physical quantity detected by the physical quantity detector. 
     The notification control section  103  outputs a notification instruction of notifying in the notification device  110  the operator of the stability information concerning the stability determined by the stability determination section  102 . The notification device  110  notifies the operator of the stability information in response to the notification instruction output from the notification control section  103 . 
     The operation control section  104  outputs an operation instruction directing an operation required to avoid a decrease in the stability of the crane  10  based on the stability determined by the stability determination section  102 . Each of the boom raising and lowering winch  30  and the jib raising and lowering winch  32  is controlled to operate in accordance with the operation instruction output from the operation control section  104 . Specifically, each of the winches  30 ,  32  suspends or decelerates the operation of winding or unwinding of the corresponding rope, for example. 
     More specifically, the crane  10  includes a hydraulic circuit having a boom winch control valve and a jib winch control valve each operating in response to the operation instruction output from the operation control section  104 . The valves accordingly control a flow rate and a flow direction of hydraulic fluid supplied from a hydraulic pump to the boom raising and lowering winch  30  and the jib raising and lowering winch  32 . As a result, each of the winches  30 ,  32  suspends or decelerates the operation of winding or unwinding the corresponding rope in accordance with the operation instruction. 
     The above-described configuration is common in first to seventh embodiments to be described below. Hereinafter, the mobile crane  10  according to each of the first to the seventh embodiments will be sequentially described. 
     First Embodiment 
       FIG. 3  is a plan view of a lower traveling body  11  of a crane  10  according to the first embodiment.  FIG. 4  is a sideview of the lower traveling body  11 .  FIG. 5  is a sideview of a crawler frame  1  of the lower traveling body  11  of the crane  10  in  FIG. 1 .  FIG. 6  is a sideview of a front end of the crawler frame  1  of a right crawler  3  seen in the direction of the arrow VI in  FIG. 3 . 
     As shown in  FIGS. 3 and 4 , the lower traveling body  11  is of a crawler type. The lower traveling body  11  includes a pair of crawlers  3 ,  3 , a slewing bearing  2   a  which bears the upper slewing body  12  thereon, and a center frame  2  connecting the pair of crawlers  3 ,  3  with each other and supporting the slewing bearing  2   a . The pair of crawlers  3 ,  3  includes a first crawler  3  and a second crawler  3 . 
     The center frame  2  has a car body  2   d  for retaining the slewing bearing  2   a  under the slewing bearing  2   a , a front axle  2   b  extending in the leftward and rightward directions at the front of the car body  2   d , and a rear axle  2   c  extending in the leftward and rightward directions at the rear of the car body  2   d . The first crawler  3  is attached to one end (a right end) of the front axle  2   b  and one end (a right end) of the rear axle  2   c , and the second crawler  3  is attached to another end (a left end) of the front axle  2   b  and another end (a left end) of the rear axle  2   c.    
     The first crawler  3  and the second crawler  3  have the same configuration except that their structural elements are symmetrically arranged in the leftward and rightward directions. The crawlers  3  extend in the forward and rearward directions, and are spaced apart from each other in the leftward and rightward directions. Each of the crawlers  3  includes the crawler frame  1 , a pair of wheels  4   a ,  4   c  (a first wheel  4   a  and a second wheel  4   c ), a drive mechanism  4   b , a crawling member  7 , a plurality of upper rollers  5 , and a plurality of lower rollers  6 . The crawler frame  1  of the first crawler  3 , the crawler frame  1  of the second crawler  3 , and the center frame  2  constitute a frame unit. 
     The drive mechanism  4   b  has a hydraulic motor (a traveling motor) and a travelling speed reducer, which are unillustrated. The crawling member  7  is formed of multiple shoes connected with one another. The crawling member  7  laid over the pair of wheels  4   a ,  4   c  is endlessly supported (in a loop) by the pair of wheels  4   a ,  4   c  and cyclically movable. In this embodiment, the first wheel  4   a  serves as a drive tumbler  4   a , and the second wheel  4   c  serves as an idler  4   c.    
     As shown in  FIG. 5 , the crawler frame  1  has a shape extending in the forward and rearward directions. The crawler frame  1  includes a frame main body  1 A and a tumbler bracket  1 B (a bracket). The tumbler bracket  1 B constitutes an end (the front end) of the crawler frame  1 . The frame main body  1 A has a shape extending in the forward and rearward directions, and has a proximal end  1 A 1  that is a rear end thereof and a distal end  1 A 2  that is a front end thereof. The tumbler bracket  1 B has a proximal end  1 B 1  (a rear end) attached to the distal end  1 A 2  of the frame main body  1 A and a distal end  1 B 2  that is a front end thereof, and extends from the proximal end  1 B 1  to the distal end  1 B 2  in the forward and rearward directions. The proximal end  1 B 1  of the tumbler bracket  1 B is joined to the distal end  1 A 2  of the frame main body  1 A by a joining way such as welding. The tumbler bracket  1 B bears the drive tumbler  4   a  and the drive mechanism  4   b.    
     As shown in  FIGS. 3, 5, and 6 , the tumbler bracket  1 B has a bracket main body P 1  and an outer periphery P 2 . The bracket main body P 1  is a plate-shaped part substantially perpendicularly intersecting a rotational axis CB of the drive tumbler  4   a  and facing the drive mechanism  4   b  in the leftward and rightward directions. The outer periphery P 2  is a plate-shaped part surrounding the bracket main body P 1  in substantially parallel to the rotational axis CB. The outer periphery P 2  covers a part of or all the periphery of the drive mechanism  4   b.    
     The drive tumbler  4   a  is rotatably supported on the tumbler bracket  1 B constituting the front end of the crawler frame  1 . The drive tumbler  4   a  is a wheel to rotate under a rotational force transmitted from the traveling motor to the traveling speed reducer, thereby driving the crawling member  7 . The drive tumbler  4   a  serves as a reaction force receiving part. 
     The reaction force receiving part is a part of the lower traveling body  11 , and receives a reaction force from the ground at a position away from the slewing axis C in a boom direction in a state where the pair of crawlers  3 ,  3  is in contact with the ground. The boom direction coincides with a horizontal component of a direction in which the boom  14  extends from the upper slewing body  12 . In the first embodiment, the boom direction corresponds to a first direction D 1  (the forward direction) denoted in  FIGS. 4 and 5 . The tumbler bracket  1 B constituting the front end of the crawler frame  1  is away from the slewing axis C in the boom direction D 1 . 
     The idler  4   c  is rotatably supported on a proximal end of the crawler frame  1  (the proximal end  1 A 1  of the frame main body  1 A). The idler  4   c  is a wheel for guiding the crawling member  7  at the opposite position to the drive tumbler  4   a  in the forward and rearward directions. 
     The plurality of upper rollers  5  is rotatably supported on an upper portion of the crawler frame  1 . The upper rollers  5  are arranged at intervals between the drive tumbler  4   a  and the idler  4   c  in the forward and rearward directions for guiding the crawling member  7 . 
     The plurality of lower rollers  6  is rotatably supported on a lower portion of the crawler frame  1 . The lower rollers  6  are arranged at intervals between the drive tumbler  4   a  and the idler  4   c  in the forward and rearward directions for guiding the crawling member  7 . Hereinafter, a lower roller closest to the drive tumbler  4   a  (the first wheel  4   a ) among the lower rollers  6  is called a first lower roller  6 A. 
     [Physical Quantity Detector] 
     The physical quantity detector  90  is configured to detect, as the physical quantity, a strain that is caused in each of the pair of crawler frames  1 ,  1  by the reaction force which the drive tumbler  4   a  (the reaction force receiving part) receives from the ground via the crawling member  7  in the assembly work and the disassembly work including operations such as the raising operation, the lowing operation and the like. Specifically, the physical quantity detector  90  serves as a strain detector that can detect a strain which changes in accordance with a change in a moment in a direction in which the weight of the tiltable attachment causes the crane  10  to turn over. The raising operation includes increasing an inclination angle of the boom  14  to the ground, and the lowing operation includes decreasing the inclination angle. 
     In the first embodiment, the crane  10  includes a plurality of physical quantity detectors  90 . The plurality of physical quantity detectors  90  includes a first physical quantity detector  90  for detecting a strain occurring in the crawler frame  1  (a first crawler frame  1 ) of the first crawler  3 , and a second physical quantity detector  90  for detecting a strain occurring in the crawler frame  1  (a second crawler frame  1 ) of the second crawler  3 . The first physical quantity detector  90  and the second physical quantity detector  90  have the same configuration, and each of the detectors is provided at the same position in the corresponding crawler frame  1 . Therefore, only one of the physical quantity detectors  90  is mainly focused below. 
     As shown in  FIGS. 3 and 4 , a specific portion of the crawler frame  1  where the strain is to be detected by the physical quantity detector  90  in the forward and rearward directions is at a position (a detection position) away in the boom direction D 1  from a rotational axis of the first lower roller  6 A. The detection position is preferably located in a region R between the rotational axis CA of the first lower roller  6 A and the rotational axis CB of the drive tumbler  4   a . In the detailed example shown in  FIGS. 5 and 6 , the physical quantity detector  90  is configured to detect a strain occurring in the tumbler bracket  1 B of the crawler frame  1  that supports the drive tumbler  4   a.    
     The physical quantity detector  90  includes at least one device for detecting the strain in the crawler frame  1 . For instance, a strain gauge, such as a metal strain gauge and a semiconductor strain gauge, is adaptable to the device. The strain gauge is attached to the crawler frame  1  by, for example, applying the same on a surface of the crawler frame  1 . However, the device of the physical quantity detector  90  should not be limited to the strain gauge, and may be other device which can detect the strain in the crawler frame  1 . Such other device may be, for example, a loadcell like a pin-typed loadcell. 
     The metal strain gauge has a configuration in which, for example, a metal resistor (a metal foil) is arranged on a thin insulator in a zigzag manner for detecting a change in an electric resistance accompanied by a deformation of the resistor. The change in the measured electric resistance is converted into a strain quantity of the crawler frame  1 . The semiconductor strain gauge utilizes, for example, a piezo resistance effect that an electric resistance ratio of a semiconductor changes depending on a stress. 
     As shown in  FIGS. 5 and 6 , the physical quantity detector  90  in the first embodiment is provided at the proximal end  1 B 1  of the tumbler bracket  1 B of the crawler frame  1 . The physical quantity detector  90  includes a plurality of strain gauges (a first strain gauge  90 A and a second strain gauge  90 B in the illustrated example). 
     As shown in  FIG. 6 , the first strain gauge  90 A is provided in an upper portion of the front end of the crawler frame  1 , and the second strain gauge  90 B is provided in a lower portion of the front end of the crawler frame  1 . Specifically, the first strain gauge  90 A is provided in an upper portion of the proximal end  1 B 1  of the tumbler bracket  1 B, and the second strain gauge  90 B is provided in a lower portion of the proximal end  1 B 1  of the tumbler bracket  1 B. 
     Region T enclosed by a long-dashed double-dotted line in  FIG. 6  contains the distal end  1 A 2  of the frame main body  1 A and the proximal end  1 B 1  of the tumbler bracket  1 B, the distal end  1 A 2  and the proximal end  1 B being connected with each other. The region T thus includes the connection portion between the frame main body  1 A and the tumbler bracket  1 B, and adjacent portions that are adjacent to the connection portion. The region T has an I-shaped cross section defined by a plate-shaped web section S extending in the upward and downward directions, a plate-shaped upper flange section S 2  connected with an upper end of the web section S 1  and extending in the forward and rearward directions, and a plate-shaped lower flange section S 3  connected with a lower end of the web section S and extending in the forward and rearward directions. 
     The web section S 1  is constituted by at least one of a part of the distal end  1 A 2  of the frame main body  1 A and apart of the proximal end  1 B 1  of the tumbler bracket  1 B. The upper flange section S 2  is constituted by a part of the distal end  1 A 2  of the frame main body  1 A and a part of the proximal end  1 B 1  of the tumbler bracket  1 B. The lower flange section S 3  is constituted by a part of the distal end  1 A 2  of the frame main body  1 A and a part of the proximal end  1 B 1  of the tumbler bracket  1 B. 
     In the first embodiment, the first strain gauge  90 A is provided at the proximal end  1 B 1  of the tumbler bracket  1 B defining the upper flange section S 2  (in the upper portion of the outer periphery P 2  of the tumbler bracket  1 B described above). The second strain gauge  90 B is provided at the proximal end  1 B 1  of the tumbler bracket  1 B defining the lower flange section S 3  (in the lower portion of the outer periphery P 2  of the tumbler bracket  1 B described above). 
     The physical quantity detector  90  detects a strain occurring in the crawler frame  1  in the assembly work and the disassembly work of the crane  10  including the raising and lowering operations. A signal representing the strain detected by the physical quantity detector  90  is input to the controller  100  shown in  FIG. 2 . For instance, the calculation section  101  calculates, based on the physical quantity, a moment in a direction of causing the crane  10  to turn over in the boom direction D 1 . The stability determination section  102  determines the stability based on the physical quantity detected by the physical quantity detector  90 , specifically, based on the moment calculated by the calculation section  101 . The notification control section  103  controls the notification device  110  to notify the operator of the stability information concerning the stability (the information concerning the front and rear balance of the crane  10 ) by using a sound, a light beam, a character, a geometric shape or the like. The operation control section  104  outputs an operation instruction directing an operation required to avoid a decrease in the stability of the crane  10  based on the stability determined by the stability determination section  102 . 
     [Assembly Work and Disassembly Work] 
     Next, the assembly work and the disassembly work of the crane  10  will be described. It should be noted here that a basic sequence of each of the assembly work and the disassembly work described below is common in the first to the seventh embodiments. 
     Each of  FIGS. 8 to 13  is a schematic sideview of the mobile crane in  FIG. 1  having a specific posture in the assembly work or the disassembly work thereof.  FIG. 8  shows the tiltable attachment in a lowered state. Each of  FIGS. 9 and 10  shows the tiltable attachment being raised or lowered with a large relative angle of the jib  17  to the boom  14 .  FIG. 11  shows a state where a moment balanced position is close to a turning-over fulcrum. Each of  FIGS. 12 and 13  shows the tiltable attachment being raised or lowered with a small relative angle of the jib  17  to the boom  14 .  FIGS. 8 to 13  illustrate only the structural elements necessary to explain the moment that the crane  10  receives, and hence some structural elements are omitted. 
     As shown in  FIGS. 10 and 13 , in the crane  10 , the tiltable attachment including the boom  14  and the jib  17  extends in the boom direction D 1  (the first direction D 1 ), and the counterweight  13  is arranged in the opposite direction D 2  to the boom direction D 1  with respect to the slewing axis C. Hereinafter, a moment acting on the crane  10  will be described mainly with respect to the slewing axis C of the upper slewing body  12 . 
     It can be said that a moment (hereinafter, referred to as a moment Mt) of causing the crane  10  to turn over in the boom direction D 1  is determined in accordance with a first moment Mf and a second moment Mb. The first moment Mf is mainly caused by the gravity acting on the tiltable attachment. In other words, the first moment Mf is caused by the weight and the posture of the tiltable attachment. The second moment Mb is mainly caused by the gravity acting on the counterweight. The second moment Mb is caused by the weight of the counterweight  13  and a part of the weight of the upper slewing body  12 . The second moment Mb is further caused to oppose to the first moment Mf and prevent the crane  10  from turning over. In summary, the moment Mt is obtained by subtracting the second moment Mb from the first moment Mf (Mt=Mf−Mb). 
     As shown in  FIGS. 10 and 13 , under the condition of no change in a loading weight of the counterweight  13  and a distance from the slewing axis C thereto, a gravity center position Gb thereof in relation to the second moment Mb are substantially unchanged. Therefore, the second moment Mb remains substantially constant. Specifically, when the weight of which the weight of the counterweight  13  accounts for a large proportion (in a portion away from the slewing axis C in the direction D 2 ) is defined as “Wb”, the second moment Mb is expressed by a product of the weight Wb and a distance Lb from the slewing axis C to the gravity center position Gb (Mb=Wb×Lb). In this respect, the second moment Mb may be calculated and stored in a storage of the controller  100  in advance. Furthermore, the second moment Mb may be calculated by the parameter calculation part of the calculation section  101  based on the weight Wb and the distance Lb in the assembly work and the disassembly work. 
     In contrast, a distance L (e.g., a distance L 1  shown in  FIG. 10 , and a distance L 2  shown in  FIG. 13 ) from the slewing axis C to a gravity center position Gf of the tiltable attachment including the boom  14  and the jib  17  changes depending on the posture of the tiltable attachment. Accordingly, the first moment Mf changes depending on the posture of the tiltable attachment. The gravity center position Gf is mainly determined in accordance with an inclination angle of the boom  14  to the ground and a relative angle of the jib  17  to the boom  14 . Specifically, when the weight of the tiltable attachment is defined as “Wat”, the first moment Mf is expressed by a product of the weight Wat and the distance L (e.g., the distance L 1 , the distance L 2 ) from the slewing axis C to the gravity center position Gf (Mf=Wat×L). The first moment Mf is calculated by the parameter calculation part of the calculation section  101  based on the weight Wat and the distance L in the assembly work and the disassembly work. 
     For instance, as shown in  FIG. 13 , the first moment Mf and the second moment Mb around the center on the slewing axis C become substantially equal to each other in a state where the boom  14  is so sufficiently raised from the upper slewing body  12  that the inclination angle of the boom  14  to the ground is relatively large and the relative angle of the jib  17  to the boom  14  is relatively small (at a relative angle θ 2 ). The moment Mt reaches approximately zero in this substantially equal situation. Eventually, the crawling member  7  undertakes the weight of the crane  10  almost equally over the entirety thereof in the forward and rearward directions. 
     In comparison with the substantially equal situation, as shown in  FIG. 10 , the gravity center position Gf shifts in the boom direction D 1  in a state where the boom  14  is so sufficiently lowered with respect to the upper slewing body  12  that the inclination angle of the boom  14  to the ground is relatively small and the relative angle of the jib  17  to the boom  14  is relatively large (at a relative angel θ 1 ). Accordingly, the first moment Mf occurring around the center on the slewing axis C increases. In such a biased situation where the moment increases in the boom direction D 1 , the moment Mt takes a larger positive value than in the substantially equal situation (Mt=Mf−Mb&gt;0). 
     Here, the center of the moment for calculating the moment is shifted in the forward and rearward directions from the center on the slewing axis C, and a forward moment and a rearward moment around the shifted center are calculated. At this time, a certain shifted center where the magnitude of the forward moment becomes the same as the magnitude of the rearward moment is defined as a “moment balanced position”. Furthermore, as shown in  FIGS. 10, 11 , and  13 , a position S of the crawler frame  1  corresponding to the rotational axis CB of the drive tumbler  4   a  in the forward and rearward directions is called a turning-over fulcrum S. 
     In the substantially equal situation described above (e.g., the situation shown in  FIG. 13 ), a moment balanced position P 1  substantially coincides with the slewing axis C. In contrast, in the biased situation (e.g., the situation shown in  FIG. 10 ), a moment balanced position P 2  is away from the slewing axis C in the boom direction D 1 . It is not that the crane  10  is caused to turn over as soon as the moment Mt takes a positive value and the moment balanced position shifts from the slewing axis C to the position P 2  in the boom direction D 1  as shown in  FIG. 10 . In other words, the biased situation shows that the crawler frame  1  of the lower traveling body  11  opposes to the moment Mt of causing the crane  10  to turn over in the boom direction D 1 . 
     In the biased situation shown in  FIG. 10 , the moment Mt reaches zero (Mt=0) at the moment balanced position P 2 . In the biased situation, a bending moment mainly acts on a portion of the crawler frame  1  extending from the moment balanced position P 2  to the turning-over fulcrum S. From these perspectives, not the entirety of the crawler frame  1  but the front end (in the portion extending from the moment balanced position P 2  to the turning-over fulcrum S) of the crawler frame  1  opposes to the moment Mt in the biased situation. In this way, a flexural rigidity at the front end of the crawler frame  1  results in preventing the crane  10  from turning-over, thereby maintaining the posture thereof. 
     For instance, as shown in  FIG. 11 , a moment balanced position P 3  mostly coincides with the turning-over fulcrum S in a situation immediately before the crane  10  is caused to turn over after the gravity center position Gf of the tiltable attachment shifts furtherer away from the position shown in  FIG. 13  in the first direction D 1 . In this situation immediately before the turning-over, the front end of each of the pair of crawler frames  1  undertakes almost all the moment Mt. A circular arrow Mr in  FIG. 11  indicates that the front end of the crawler frame  1  opposes to the moment Mt. 
     In this configuration, in a case where the physical quantity detector  90  can detect a strain occurring in the front end of the crawler frame  1  appropriately for various situations including the substantially equal situation shown in  FIG. 10 , the biased situation shown in  FIG. 13 , and the situation immediately before the turning-over shown in  FIG. 11 , and the parameter calculation part of the calculation section  101  can calculate, based on the detected strain, a moment that the front end of the crawler frame  1  receives, the stability determination section  102  can determine various possible states of the crane  10  based on the calculated moment. 
     In the first embodiment, the physical quantity detector  90  provided at the front end of the crawler frame  1 , i.e., in a portion located in the region R described above, can effectively detect a strain occurring in the front end of the crawler frame  1  in the biased situation. As a result, it is possible to obtain a criterion for determining a state of the front and rear balance of the crane  10 . 
     In the first embodiment, the information concerning the strain occurring in the front end of the crawler frame  1  and detected by the physical quantity detector  90  is obtainable in the above-described manner. Thus, information concerning a state of the crane  10  necessary to safely raise and lower the tiltable attachment in the assembly work and the disassembly work of the crane  10  is detectable without an overburdened work by the operator. The crane  10  then utilizes the detected information to safely execute the raising and lowering operations. Details will be described below. 
     As shown in  FIG. 8 , the boom  14  and the jib  17  are mounted on the upper slewing body  12  in a state (a lowered state) where each of the boom  14  and the jib  17  is lowered with a posture substantially parallel to the ground GR in the assembly work of assembling the crane  10 . For the hoisting work, the posture of the boom  14  in the lowered state is changed to the raised state (shown in  FIG. 1 ) by the raising operation of gradually increasing the inclination angle of the boom  14  to the ground GR. 
     First, the inclination angle of the boom  14  is gradually increased in a state where the roller  17 R provided at the distal end  171  of the jib  17  is in contact with the ground OR to change the posture of the tiltable attachment from the lowered state to the raised state described above. During this operation, the relative angle of the jib  17  to the boom  14  gradually decreases. 
     For instance, after the relative angle has reached the angle θ 1  shown in  FIG. 9 , further increasing the inclination angle of the boom  14  while keeping the relative angle at the angle θ 1  as shown in  FIG. 10  causes the roller  17 R at the distal end  17 B of the jib  17  to leave the ground GR. Accordingly, the moment Mf acts on the crane  10 . 
     At this time, the notification control section  103  of the controller  100  controls the notification device  110  to notify the operator of the information concerning the front and rear balance of the crane  10  via the notification device  110  based on a detection signal output from the physical quantity detector  90 . Owing to the notification, the operator can acquire the stability information concerning the front and rear balance of the crane  10  via the notification device  110 . For instance, upon recognition of an unstable state of the crane  10  shown in  FIG. 10 , the operator decreases the inclination angle of the boom  14  again to bring, for example, the roller  17 R at the distal end  17 B of the jib  17  into contact with the ground as shown in  FIG. 9 . Thereafter, the operator gradually increases the inclination angle of the boom  14  in the state where the roller  17 R at the distal end  17 B of the jib  17  is in contact with the ground GR. The relative angle of the jib  17  to the boom  14  gradually decreases during this operation (e.g., the state shown in  FIG. 12 ). 
     For example, after the relative angle has reached the angle θ 2  shown in  FIG. 12 , further increasing the inclination angle of the boom  14  while keeping the relative angle at the angle θ 2  as shown in  FIG. 13  causes the roller  17 R at the distal end  17 B of the jib  17  to leave the ground GR. Accordingly, the moment Mf acts on the crane  10 . At this time, the moment balanced position P 1  is close to the slewing axis C. Therefore, it is possible to safely raise the tiltable attachment of the crane  10 . 
     The disassembly work of disassembling the crane  10  can be safely performed in a reverse sequence of the assembly work. 
     [Way of Calculating Turning-Over Moment] 
     Hereinafter, a way of calculating a moment acting on the front end of the crawler frame  1  will be described in detail. 
     As shown in  FIG. 6 , in the first embodiment, the distal end  1 A 2  of the frame main body  1 A and the proximal end  1 B of the tumbler bracket B. i.e., the connection portion between the frame main body  1 A and the tumbler bracket  1 B, and the adjacent portions that are adjacent to the connection portion in the forward and rearward directions have the I-shaped (or H-shaped) cross section defined by the web section S 1 , the upper flange section S 2 , and the lower flange section S 3 . 
     Upon occurrence of a bending deformation in each of the pair of crawler frames  1  having received a bending load in the upward and downward directions during the raising and lowering operations for the tiltable attachment, an upper portion of the front end of the crawler frame  1  from a neutral plane (a neutral axis) thereof is compressed and contracted, and the lower portion from the neutral plane is pulled and stretched. 
     Further, a strain occurring in each of the upper portion and the lower portion of the crawler frame  1  increases in proportion to a distance from the neutral plane. Each of the upper and lower flange sections S 2 , S 3  is at a relatively large distance from the neutral plane in the I-shaped (or I-shaped) cross-section, and hence a relatively large strain (a relatively large bending stress) occurs in each of the upper flange section S 2  and the lower flange section S 3 . In this case, the physical quantity detector  90  provided in each of the upper flange section S 2  and the lower flange section S 3  in the above-described manner in the first embodiment can detect the relatively large strain. 
     Here, a strain occurring in the upper flange section S 2  is defined as “ε1”, and a strain occurring in the lower flange section S 3  is defined as “ε2”. Besides, a distance from the neutral plane of the front end of the crawler frame  1  to the first strain gauge  90 A is defined as “r1”, and a distance from the neutral plane to the second strain gauge  90 B is defined as “r2”. Further, a moment of inertia of area in a cross section where the strain is measured is defined as “I”, and a Young&#39;s modulus is defined as “E”. Moreover, an upward stress of stresses caused only by the bending moment at the front end of the crawler frame  1  is defined as “σmt”, and a downward stress thereof is defined as “σmc”. 
     The neutral plane (the neutral axis) of the front end of the crawler frame  1  in connection with the bending deformation is not limited to the center of the front end in the upward and downward directions. Thus, the upward stress σmt and the downward stress σmc can differ from each other. In this case, the ratio between the upward stress σmt and the downward stress σmc (σmt:σmc) corresponds to the ratio between the distances (r1:r2) from the neutral plane (σmt:σmc=r1:r2). 
     Additionally, the crawling member  7  is wound around the crawler frame  1  without being slackened. In this state, the crawler frame  1  receives a compressive force an (an axial force) in the forward and rearward directions. Taking these premises into consideration, the way of calculating the moment will be described below. 
       FIG. 7  is a schematic view showing a stress distribution in a target cross section of the front end (the tumbler bracket  1 B in the first embodiment) of the crawler frame  1  for which a strain is measured. As shown in  FIG. 7 , the stress distribution is obtainable by a sum of the stresses (σmt, σmc) attributed to the bending moment and the aforementioned compressive force on. 
     Accordingly, a bending stress σmt required to obtain the moment M is calculated by the following Formula (1): 
         E×ε 1− E×ε 2=(σ mt+σn )−(−σ mc+σn )=σ mt+σmc=σmt (1+ r 2/ r 1)  (1).
 
     The following Formula (2) is obtainable as a result of Formula (1): 
       σ mt=E (ε1−ε2)/(1+ r 2/ r 1)  (2).
 
     From these Formulas, the moment M applied to the front end of the crawler frame  1  is calculated by the following Formula (3): 
         M=E×σmt×I/r 1  (3).
 
     The crane  10  includes the pair of crawler frames  1 . The moments obtainable from the strains respectively in the front ends of the left and right crawler frames  1  are defined as “ML”, “MR”. The turning-over moment Mt is calculated by the following Formula (4) where: 
         Mt=MR+ML   (4).
 
     The calculation section  101  (specifically, the parameter calculation part of the calculation section  101 ) of the controller  100  calculates the turning-over moment Mt in the manner described above based on a signal (a detection signal) representing the physical quantity input from the physical quantity detector  90 . In this way, the turning-over moment Mt of causing the crane  10  to turn over is obtained. The frequency of detection by the physical quantity detector  90  and the frequency of calculation by the calculation section  101  are not particularly limited. For instance, the detection by the physical quantity detector  90  and the calculation by the calculation section  101  may be executed per predetermined time period, or continuously (always) executed. 
     Meanwhile, the moment M is calculated by the following Formula (5) in no consideration of the compressive force an (the axial force): 
         M=E×I (|ε1/ r 1|+|ε2/ r 2|)/2  (5).
 
     [Operations] 
     Next, the arithmetic processing by the controller  100  in the crane  10  will be described.  FIG. 18  is a flowchart of exemplary arithmetic processing by the controller  100 . As shown in  FIG. 18 , the controller  100  determines whether or not an operation mode is set at an independent mode (step S 1 ). In the raising operation for the tiltable attachment in the specific work such as the assembly work, the independent mode permits the controller  100  to determine the stability of the crane  10  and to automatically execute an avoidance operation required to avoid a decrease in the stability when the stability is determined to be low. The independent mode is selectable by the operator in the cab  12 A of the upper slewing body  12 . 
     When the independent mode is selected (YS in step S 1 ), the controller  100  determines whether or not a manipulation lever is manipulated by the operator to activate the boom raising and lowering winch  30  (step S 2 ). When it is determined that the manipulation lever is manipulated (YES in step S 2 ), the operation control section  104  of the controller  100  controls the boom raising and lowering winch  30  to wind the boom raising and lowering rope  38  (step S 3 ). 
     Subsequently, the controller  100  acquires a physical quantity (a strain in the first embodiment) detected by the physical quantity detector  90  (step S 4 ). 
     Next, the calculation section  101  (specifically, the parameter calculation part) of the controller  100  calculates the turning-over moment Mt by using, for example, the calculation way described above (step S 5 ). 
     Then, the stability determination section  102  of the controller  100  determines the stability of the crane  10  (step S 6 ). Specifically, the stability determination section  102  determines whether or not the turning-over moment Mt is larger than a predetermined threshold m (step S 6 ). The threshold m indicates a value for determining the stability of the crane  10 , and is set in advance for each type of the crane  10  with reference to a limit value of the turning-over moment Mt of causing the crane  10  to turn over. 
     When the stability determination section  102  determines that the turning-over moment Mt is larger than the threshold m (YES in step S 6 ), the notification control section  103  outputs a notification instruction of notifying in the notification device  110  the operator of the stability information concerning the stability determined by the stability determination section  102  (step S 7 ). 
     As described above, the notification device  110  includes, for example, at least one of a sound emitter for emitting a sound, a light emitter for emitting a light beam, and a display part for displaying a character, a geometric shape, or the like. The stability information is notified to the operator by way of at least one of the sound emitter, the light emitter, and the display part in response to the notification instruction. 
       FIG. 19  shows specific examples of the stability information notified on the display part of the notification device  110 . In  FIG. 19A , each of the directions of the first moment Mf and the second moment Mb is denoted by an arrow. The magnitude of each of the first moment Mf and the second moment Mb is expressed with, for example, a thickness of the arrow, a length of the arrow, and a numerical value given therefor. For instance, each arrow may be displayed with an image of the crane  10  as shown in  FIGS. 10 and 13 . In  FIG. 1913 , the magnitude of each of the first moment Mf and the second moment Mb is shown in the form of a graph such as a bar graph and a pie chart. In  FIG. 19C , the magnitude of each of the first moment Mf and the second moment Mb is shown with a numeric value given therefor. 
     When the stability determination section  102  determines that the turning-over moment Mt is larger than the threshold m, the notification control section  103  may control the notification device  110  to display the arrows and numeric values shown in  FIGS. 19A to 19C  while flashing the same, or control the notification device  110  to change a period of the flashing depending on the stability. Further, when the stability determination section  102  determines that the turning-over moment Mt is larger than the threshold m, the notification control section  103  may control the notification device  110  to notify the stability information by using a voice in addition to the displaying ways shown in  FIGS. 19A to 19C . The stability information using the voice may include massages, for example, “The value approaches the turning-over limit.”, “There is a risk of turning-over.”, or the like. 
     Subsequently, the operation control section  104  outputs an operation instruction directing an operation required to avoid a decrease in the stability of the crane  10  based on the stability determined by the stability determination section  102  (step S 8 ). Specifically, the boom raising and lowering winch  30  suspends, for example, the operation of winding the rope  38  in accordance with the operation instruction. In a case where the jib raising and lowering winch  32  is operating, the jib raising and lowering winch  32  suspends, for example, the operation of winding (or unwinding) the rope  44  in accordance with the operation instruction. 
     Each of  FIGS. 20 and 21  is a flowchart showing another exemplary arithmetic processing by the controller  100 . Steps S 11  to S 15  in  FIG. 20  are the same as steps S 1  to S 5  in  FIG. 18 , and thus the descriptions therefor are omitted. Hereinafter, the differences from the arithmetic processing shown in  FIG. 18  will be described. 
     As shown in  FIGS. 20 and 21 , the stability determination section  102  of the controller  100  determines the stability of the crane  10  (steps S 16 , S 17 , S 20 ). Specifically, the ratio calculation part of the calculation section  101  calculates a ratio (Mf/Mb) between the first moment Mf and the second moment Mb, and the stability determination section  102  compares the ratio (Mf/Mb) with each of thresholds α, β, γ. The thresholds α, β, γ are values for determining the stability of the crane  10 , and are set in advance so as to satisfy, for example, the relation of 0&lt;α&lt;β&lt;γ&lt;1.0. 
     When the ratio (Mf/Mb) is equal to or lower than the threshold α (NO in step S 16 ), the stability is not low. Thus, the operation control section  104  controls the boom raising and lowering winch  30  to continue the operation of winding the boom raising and lowering rope  38  (step S 13 ). 
     Conversely, when the ratio (Mf/Mb) is higher than the threshold α (YES in step  16 ), the stability determination section  102  determines whether the ratio (Mf/Mb) is higher than the threshold α and lower than the threshold β (step S 17 ). When the ratio (Mf/Mb) is higher than the threshold α and lower than the threshold β (YES in step S 17 ), the notification control section  103  outputs a notification instruction of notifying in the notification device  110  the operator of the stability information concerning the stability determined by the stability determination section  102  (step S 18 ). In this case, the stability information includes information of warning the operator of a low stability of the claim  10 . 
     Next, the operation control section  104  outputs an operation instruction directing an operation required to avoid a decrease in the stability of the crane  10  based on the stability determined by the stability determination section  102  (step S 19 ). In this case, the operation instruction includes, for example, an instruction of reducing the rotational speed of the boom raising and lowering winch to A %. Then, the operation control section  104  controls the boom raising and lowering winch  30  to continue the operation of winding the boom raising and lowering rope  38  at the reduced rotational speed (step S 13 ). 
     When the ratio (Mf/Mb) is equal to or higher than the threshold P (NO in step S 17 ), the stability determination section  102  determines whether the ratio (Mf/Mb) is higher than the threshold β and lower than the threshold γ (step S 20 ). When the ratio (Mf/Mb) is higher than the threshold β and lower than the threshold γ (YES in step S 20 ), the notification control section  103  outputs a notification instruction of notifying in the notification device  110  the operator of the stability information concerning the stability determined by the stability determination section  102  (step S 21 ). In this case, the stability information may include information of warning the operator of the low stability of the crane  10  in a more persuading manner than in step S 18 . 
     Subsequently, the operation control section  104  outputs an operation instruction directing an operation required to avoid a decrease in the stability of the crane  10  based on the stability determined by the stability determination section  102  (step S 22 ). In this case, the operation instruction includes, for example, an instruction of reducing the rotational speed of the boom raising and lowering winch  30  to B %. Then, the operation control section  104  controls the boom raising and lowering winch  30  to continue the operation of winding the boom raising and lowering rope  38  at the reduced rotational speed (step S 13 ). The values A, B indicate degrees of reduction in the rotational speed of the raising and lowering winch  30 , and are set in advance so as to satisfy the relation 100&gt;A&gt;B&gt;0. 
     When the ratio (Mf/Mb) is equal to or higher than the threshold γ (NO in step S 20 ), the notification control section  103  outputs a notification instruction of notifying in the notification device  110  the operator of the stability information concerning the stability determined by the stability determination section  102  (step S 23 ). In this case, the stability information includes information (warning information) of warning the operator of the low stability of the crane  10  in a still further persuading manner than in step S 21 . 
     Next, the operation control section  104  outputs an operation instruction directing an operation required to avoid a decrease in the stability of the crane  10  based on the stability determined by the stability determination section  102  (step S 24 ). Specifically, the boom raising and lowering winch  30  suspends, for example, the operation of winding the rope  38  in accordance with the operation instruction. In a case where the jib raising and lowering winch  32  is operating, the jib raising and lowering winch  32  suspends, for example, the operation of winding (or unwinding) the rope  44  in accordance with the operation instruction. 
     Modifications of First Embodiment 
     Each of  FIGS. 14 to 16  is a sideview of a front end of a crawler frame  1  seen in the direction of the arrow VI in  FIG. 3 ,  FIG. 14  showing a first modification of the embodiment,  FIG. 15  showing a second modification of the embodiment, and  FIG. 16  showing a third modification of the embodiment. 
     The first to the third modifications shown in  FIGS. 14 to 16  are equivalent to the embodiment shown in  FIG. 6  in that a specific portion of the crawler frame  1  where a strain is to be detected by a physical quantity detector  90  in the forward and rearward directions is located at a position (a detection position) in a region R between a rotational axis CB of a drive tumbler  4   a  and a rotational axis CA of a first lower roller  6 A. Further, the first to the third modifications are equivalent to the embodiment shown in  FIG. 6  in that a first strain gauge  90 A is provided in an upper portion of the crawler frame  1 , and a second strain gauge  90 B is provided in a lower portion of the crawler frame  1 . 
     In contrast, the first to the third modifications differ from the embodiment shown in  FIG. 6  in the location of the physical quantity detector  90  within the region R of the crawler frame  1 . Details will be described below. 
     In the first modification shown in  FIG. 14 , the first strain gauge  90 A and the second strain gauge  90 B are provided at a plate-shaped web section S 1  extending in the upward and downward directions. In the second modification shown in  FIG. 15 , the first strain gauge  90 A extends along the web section S 1  and an upper flange section S 2 , and the second strain gauge  90 B extends along the web section S 1  and a lower flange section S 3 . 
     In the third modification shown in  FIG. 16 , the first strain gauge  90 A and the second strain gauge  90 B are provided at a distal end  1 A 2  of a frame main body  1 A. Specifically, in the third modification, the first strain gauge  90 A is provided at the distal end  1 A 2  of the frame main body  1 A defining an upper flange section S 2 . The second strain gauge  90 B is provided at the distal end  1 A 2  of the frame main body  1 A defining a lower flange section S 3 . 
       FIG. 17  is a perspective view schematically showing a fourth modification of the first embodiment. In the fourth modification shown in  FIG. 17 , a crawler frame  1  further includes a measurement support base  200  (a deformation member) to which a strain gauge (a physical quantity detector) is attached. The measurement support base  200  is located at such a position as to sensitively detect a strain occurring in the crawler frame  1  in a state where a distal end  14 B of a boom  14  is away in a boom direction D 1  from a proximal end  14 A of the boom  14  in the forward and rearward directions. Specifically, the measurement support base  200  is disposed in a specific portion of the crawler frame  1  where a detection position in the forward and rearward directions is within the region R (sec  FIG. 4 ) between the rotational axis CB of the drive tumbler  4   a  and the rotational axis CA of the first lower roller  6 A. 
     For instance, the measurement support base  200  may be disposed in a specific portion of the crawler frame  1  where each of the strain gauge  90 A and the strain gauge  90 B is provided as shown in  FIGS. 6, 14, 15, and 16 . However, the specific portion for disposing the measurement support base  200  should not be limited thereto. In the detailed example shown in  FIG. 17 , the measurement support base  200  is in the same portion as the portion where each of the strain gauges  90 A and  90 B is provided in the second modification shown in  FIG. 15 . Details will be described below. 
     As shown in  FIG. 17 , the measurement support base  200  extends along the web section S 1  and the upper flange section S 2 . In other words, the measurement support base  200  is located at the corner between the web section S 1  and the upper flange section S 2 . 
     The measurement support base  200  includes a first surface  200 A, a second surface  200 B, and a retaining surface  200 C. The first surface  200 A faces the web section S 1  and is attached to the web section S 1 . The second surface  200 B faces the upper flange section S 2  and is attached to the upper flange section S 2 . The retaining surface  200 C connects an end edge of the first surface  200 A and an end edge of the second surface  200 B with each other, and retains the strain gauge  90 A. In the detailed example shown in  FIG. 17 , the retaining surface  200 C has a slope inclined upward as advancing forward and retaining the strain gauge  90 A thereon. In the detailed example shown in  FIG. 17 , the slope is formed of a curve surface (a concave) in an arc shape, but may be formed of a flat surface or a convex. Moreover, in the detailed example shown in  FIG. 17 , the measurement support base  200  has a substantially L-shape. However, the shape of the measurement support base  200  should not be limited to the substantially L-shape. 
     Another measurement support base  200  is arranged at another corner between the web section S 1  and a lower flange section S 3  in addition to the measurement support base at the corner between the web section S 1  and the upper flange section S 2  as described above. Here, a bending moment is applied to the crawler frame  1  and a bending deformation occurs in each of a tumbler bracket  1 B and a frame main body  1 A when the tiltable attachment is raised and lowered. As a result, a strain occurs on the retaining surface  200 C of the upper measurement support base  200  in a direction of being pulled and stretched. Similarly, a strain occurs on the retaining surface  200 C of the lower measurement base in a direction of being compressed and contracted. Under the circumstances, the strain gauge provided along each of the retaining surfaces  200 C can detect the corresponding strain necessary to calculate the bending moment. In the case where the retaining surface  200 C is a curve surface in an arc shape, it is possible to adjust the magnitude of the strain by changing the radius of curvature in the arc shape. 
     Hereinafter, the second to the seventh embodiments will be described. A mobile crane  10  according to each of the second to the seventh embodiments differs from the mobile crane according to the first embodiment in that a lower traveling body  11  includes at least one receiving member  80  (one support member  80 ). In the second to the seventh embodiments, the lower traveling body  11  has almost the same configuration as the configuration of the lower traveling body  11  in the first embodiment except the aforementioned difference. Accordingly, in the following descriptions of the second to the seventh embodiments, the same structural elements as those in the first embodiment are given with the same reference signs and numerals, and the descriptions therefor will be omitted. 
     Second Embodiment 
       FIG. 22  is a plan view of a lower traveling body  11  of a mobile crane  10  according to the second embodiment.  FIG. 23  is a sideview of a support member  80  to be attached to a crawler frame  1  of the crane  10  in  FIG. 22 .  FIG. 24  is a side view of the support member  80  having been attached to the crawler frame  1  in  FIG. 22 . 
     In the second embodiment shown in  FIG. 22 , the lower traveling body  11  has a plurality of support members  80  (specifically, a pair of support members  80 ). Each of the pair of support members  80  has a connection part connected with the crawler frame  1  in a frame unit, and a contact part being in contact with the ground. The connection part of the support member  80  is constituted by a proximal end  8 A of a beam  81  to be described later, and the contact part of the support member  80  is constituted by a float  85  that is a lower end of a leg  82  to be described later. 
     A boom direction in the second embodiment coincides with a horizontal component of a direction in which a boom  14  extends from an upper slewing body  12  in the assembly work and the disassembly work. In the second embodiment, the boom direction D 1  corresponds to a first direction D 1  (the rightward direction) shown in  FIG. 22 . As shown in  FIG. 22 , the float  85  (the contact part) serves as a part (a reaction force receiving part) for receiving a reaction force from the ground at a position away from a slewing axis C in the boom direction D 1 . The float  85  (the contact part) is away from the proximal end  8 A (the connection part) of the beam  81  in the boom direction D 1 . 
     Hereinafter, the second embodiment will be described in detail. As shown in  FIGS. 22 and 23 , the crawler frame  1  includes a frame main body  1 A and a tumbler bracket  1 B. The frame main body  1 A has a shape extending in the forward and rearward directions. The frame main body  1 A has a top plate  111  extending in a longitudinal direction of the frame main body  1 A, a bottom plate  112  spaced downward from the top plate Ill and extending in the longitudinal direction, and a pair of side plates  113 ,  114  each extending in the longitudinal direction. The one side plate  113  connects right ends of the top plate  111  and the bottom plate  112  with each other. The other side plate  114  connects left ends of the top plate  111  and the bottom plate  112  with each other. 
     As shown in  FIG. 23 , the frame main body  1 A has a closed cross section defined by the top plate Ill, the bottom plate  112 , and the pair of side plates  113 ,  114 . The closed cross section is specifically defined by the top plate  111  and the bottom plate  112  each extending in the leftward and rightward directions, and the pair of side plates  113 ,  114  each extending in the upward and downward directions. For instance, a flat plate  115  (a reinforcing plate) having a posture perpendicularly intersecting the longitudinal direction may be arranged in the inside of the closed cross section. In the embodiment, the flat plate  115  is provided in a specific portion of the frame main body  1 A to which the corresponding support member  80  is attached or in the vicinity thereof. The specific portion of the frame main body  1 A receives a bending moment caused in the support member  80  by a turning-over moment of the crane  10 . Therefore, the flat plate  115  provided in the specific portion of the frame main body  1 A or in the vicinity thereof can reinforce the specific portion. The rigidity of the crawler frame  1  is consequently enhanced. 
     [Support Member (Receiving Member)] 
     As shown in  FIG. 22 , in the embodiment, the pair of support members  80  is supported by the crawler frame  1  (a first crawler frame  1 ) of a first crawler  3  of a pair of crawlers  3 . The pair of support members  80  is arranged at a distance between the support members in the forward and rearward directions. The pair of support members  80  includes a first support member  80  and a second support member  80  located in the rear of the first support member  80 . The support members  80  have the same configuration except their different attachment positions to the first crawler frame  1 . 
     As shown in  FIG. 22 , the pair of support members  80  is preferably attached to the first crawler frame  1  so that at least a part of the proximal end  8 A of the first support member  80  and at least a part of the proximal end  8 A of the second support member  80  are located in a region between a position of the first crawler frame  1  corresponding to a front edge  2   e  of a front axle  2   b  and another position of the first crawler frame  1  corresponding to a rear edge  2   f  of a rear axle  2   c  in the forward and rearward directions. 
     The pair of support members  80  is more preferably attached to the first crawler frame  1  so that at least a part of the proximal end  8 A of the first support member  80  is located in a region of the first crawler frame  1  corresponding to the front axle  2   b  in the forward and rearward directions, and at least a part of the proximal end  8 A of the second support member  80  is located in another region of the first crawler frame  1  corresponding to the rear axle  2   c  in the forward and rearward directions. 
     A first distance from the attachment position of the proximal end  8 A of the first support member  80  to the first crawler frame  1  to the slewing axis C of the upper slewing body  12 , and a second distance from the attachment position of the proximal end  8 A of the second support member  80  to the first crawler frame  1  to the slewing axis C of the upper slewing body  12  are preferably equivalent to each other, and are more preferably the same. An excessively large difference between the first distance and the second distance is likely to cause a large difference between deflection and torsion occurring in the beam  81  of the first support member  80 , and deflection and torsion occurring in the beam  81  of the second support member  80 . 
     Each of the support members  80  includes the beam  81  and the leg  82 . The beam  81  extends outward from the first crawler frame  1  in the leftward and rightward directions. The beam  81  has the proximal end  8 A and a distal end  8 B. The proximal end  8 A of the beam  81  is attached to the first crawler frame  1 . The distal end  8 B of the beam  81  is away from the first crawler frame  1  in the boom direction D 1  (the rightward direction in  FIG. 3 ) corresponding to one of the leftward and rightward directions. 
     In the embodiment, the beam  81  linearly extends in a plan view shown in  FIG. 22 . The beam  81  extends in a direction parallel to the leftward and rightward directions in the plan view, but should not be limited thereto and may extend in a direction oblique to the leftward and rightward directions. Specifically, the configuration in the embodiment where the beam  81  extends outward from the crawler frame  1  in the leftward and rightward directions involves a case where the beam  81  extends in a direction oblique to the leftward and rightward directions as well as the case where the beam  81  extends in the direction parallel to the leftward and rightward directions, in the plan view of the beam  81 . For instance, the front beam  81  of the pair of beams  81  may extend diagonally forward, and the rear beam  81  thereof may extend diagonally rearward. 
     The leg  82  is supported on the distal end  8 B of the beam  81  and extends downward from the distal end  8 B so that the lower end  85  comes into contact with the ground. In the embodiment, the leg  82  includes a hydraulic cylinder. Specifically, the leg  82  includes a cylinder main body  83  supported on the distal end  8 B of the beam  81  and extending downward from the distal end  8 B, a rod  84  slidable along the cylinder main body  83  in the upward and downward directions, and the float  85  (see  FIG. 5 ) attached to a lower end  84 A of the rod  84  (see  FIG. 4 ). The float  85  constitutes the lower end  85  (the contact part) of the leg  82 . 
     The leg  82  of the support member  80  is away from the first crawler  3  in the boom direction D 1 . This position of the leg  82  away from the first crawler  3  in the boom direction D 1  means that a central axis CC of the leg  82  is at a position away from the first crawler  3  in the boom direction D 1 . In the embodiment, the central axis CC of the leg  82  serves as a central axis CC of the hydraulic cylinder (i.e., the central axis CC of the rod  84 ) extending in the upward and downward directions. 
       FIG. 25A  shows an exemplary cross-sectional view of the beam  81  of the support member  80  taken along the line XXV-XXV in  FIG. 24 .  FIG. 25B  shows another exemplary cross-sectional view of the beam  81  of the support member  80  taken along the line XXV-XXV in  FIG. 24 . 
     The beam  81  of the support member  80  may have an I-shaped cross section perpendicularly intersecting the longitudinal direction of the beam  81  as shown in  FIG. 25A , or a closed cross section perpendicularly intersecting the longitudinal direction of the beam  81  as shown in  FIG. 25B . 
     In the case of the cross section shown in  FIG. 25A , the beam  81  has a top plate  811  extending in the longitudinal direction of the beam  81 , a bottom plate  812  spaced downward from the top plate  811  and extending in the longitudinal direction, and a side plate  813  extending in the longitudinal direction. The side plate  813  connects the top plate  811  and the bottom plate  812  with each other. The I-shaped cross section shown in  FIG. 25A  is defined by the top plate  811  and the bottom plate  812  each extending in the leftward and rightward directions, and the side plate  813  extending in the upward and downward directions. 
     In the case of the cross section shown in  FIG. 25B , the beam  81  has a top plate  811  extending in the longitudinal direction of the beam  81 , a bottom plate  812  spaced downward from the top plate  811  and extending in the longitudinal direction, and a pair of side plates  813 ,  814  each extending in the longitudinal direction. The one side plate  813  connects rear ends of the top plate  811  and the bottom plate  812  with each other, and the other side plate  814  connects front ends of the top plate  811  and the bottom plate  812  with each other. The closed cross section shown in  FIG. 25B  is defined by the top plate  811  and the bottom plate  812  each extending in the leftward and rightward directions, and the pair of side plates  813 ,  814  each extending in the upward and downward directions. 
     Although the top plate  811  is inclined downward as advancing to the distal end  8 B of the beam  81  in the detailed examples shown in  FIGS. 25A, 25B , the arrangement should not be limited thereto. The top plate  811  may be horizontally arranged. 
     In the embodiment, each of the support members  80  is configured to be detachably attachable to the crawler frame  1 . Details will be described below. 
     As shown in  FIG. 23 , the proximal end  8 A of the beam  81  is formed with an engaged portion for attaching the beam  81  to the crawler frame  1 . The engaged portion is engageable with an engaging portion provided in the frame main body  1 A of the crawler frame  1 . In the embodiment, the engaged portion includes a pair of upper through holes  8 C, a pair of lower through holes  8 D, and a pair of pins. The upper through holes  8 C and the lower through holes  8 D are spaced apart from each other in the upward and downward directions. One of the pair of pins is inserted in the pair of upper through holes  8 C for fastening in advance. 
     In contrast, the engaging portion provided in the frame main body  1 A of the crawler frame  1  includes a pair of hooks  1 C and a pair of lower through holes  1 D located below the hooks  1 C. As shown in  FIGS. 23 and 24 , the pin extending in the upper through holes  8 C of the engaged portion is hooked by the hooks  1 C of the engaging portion. Further, the other of the pair of pins is inserted in the lower through holes  8 D of the engaged portion and the lower through holes  1 D of the engaging portion in a state where the through holes  8 D and the through holes  1 D face each other. Moreover, the lower end  84 A of the rod  84  of the leg  82  shown in  FIG. 23  fits in a recess on a top surface of the float  85  constituting the lower end  85  of the leg  82  shown in  FIG. 24 . Consequently, the support member  8  is attached to the crawler frame  1 . 
     Each of the support members  80  is detachable from the crawler frame  1  in a reverse sequence of the above-described attachment work. 
     In the embodiment, the support members  80  (side jacks) detached from the crawler frame  1  can serve as members (a front jack and a rear jack) of a trans-lifter provided at each of the front axle  2   b  and the rear axle  2   c  of the lower traveling body  11 . Details will be described below. 
     The trans-lifter includes a plurality of support members and is aimed at lifting a frame  2  from the ground for attaching the crawler  3  to the front axle  2   b  and the rear axle  2   c  of the frame  2 , and detaching the crawler  3  therefrom. The front axle  2   b  is provided with two engaging portions each having the same configuration as the engaging portion provided in the crawler frame  1 , and the rear axle  2   c  is also provided with two engaging portions each having the same configuration as the engaging portion provided in the crawler frame  1 . In the embodiment, at least a part of the plurality of (typically, four) support members of the trans-lifter serves as the pair of support members  80  provided in the crawler frame  1  shown in  FIG. 22 . However, the support members  80  (the side jacks) may not serve as the support members (the front jack and the rear jack) of the trans-lifter. 
     [Physical Quantity Detector] 
     A physical quantity detector  90  is configured to detect information necessary to safely raise and lower the boom  14  in the assembly work and the disassembly work of the crane  10 . Specifically, the physical quantity detector  90  detects a strain occurring in the beam  81  of the support member  80 . The physical quantity detector  90  is configured to detect a strain occurring in the beam  81  of the support member  80  and corresponding to a moment in a direction of causing the crane  10  to turn over in one of the leftward and rightward directions. 
     In the embodiment, the crane  10  includes a plurality of physical quantity detectors  90  as shown in  FIG. 22 . Specifically, the support members  80  are provided with their respective physical quantity detectors  90 . With this configuration, a strain occurring in the beam  81  of each of the support members  80  is detectable. In the embodiment, the two physical quantity detectors  90  have the same configuration, and each of the detectors is provided at the same position in the corresponding crawler frame  1  as shown in  FIG. 22 . Therefore, one of the physical quantity detectors  90  is mainly focused below. 
     As shown in  FIGS. 23 and 24 , the physical quantity detector  90  in the embodiment is closer to the proximal end  8 A than the distal end  8 B of the beam  81  in the beam  81 . However, the physical quantity detector  90  may be closer to the distal end  8 B than the proximal end  8 A of the beam  81  in the beam  81 , or may be at the longitudinal center of the beam  81 . 
     The physical quantity detector  90  is preferably arranged in a portion of the beam  81  where a strain is likely to occur. In the arrangement, a strain caused in the beam  81  by the moment is sensitively detectable. Such a portion where the strain is likely to occur may be, for example, a connection portion between the beam  81  and the crawler frame  1  or an adjacent portion that is adjacent to the connection portion, or a connection portion between the beam  81  and the leg  82  or an adjacent portion that is adjacent to the connection portion. 
     The physical quantity detector  90  includes at least one device for detecting the strain in the beam  81 . Adoptable for this device is the exemplary device described in the first embodiment. 
     As shown in  FIG. 24 , the physical quantity detector  90  in the embodiment includes a plurality of strain gauges (a first strain gauge  90 A and a second strain gauge  90 B in the illustrated example). The first strain gauge  90 A is provided in an upper portion of the beam  81 , and the second strain gauge  90 B is provided in a lower portion of the beam  81 . The strain gauge  90 A can detect a strain occurring in the upper portion of the beam  81 , and the strain gauge  90 B can detect a strain occurring in the lower portion of the beam  81 . 
     In the exemplary arrangements of the strain gauges shown in  FIGS. 25A and 25B , the first strain gauge  90 A is provided on the top plate  811 , and the second strain gauge  90 B is provided on the bottom plate  812 . This leads to an increased distance from a neutral plane of the beam  81  to each of the strain gauges. Thus, a strain occurring in the beam is sensitively detectable. The first strain gauge  90 A may be provided in an upper portion of the side plate  813 , and the second strain gauge  90 B may be provided in a lower portion of the side plate  813 . 
     As shown in  FIG. 25A , in the I-shaped cross section of the beam  81 , the first strain gauge  90 A is located, for example, around a boundary between the top plate  811  and the side plate  813 , and the second strain gauge  90 B is located, for example, around a boundary between the bottom plate  812  and the side plate  813 . However, their locations should not be limited thereto. Each of the strain gauges may be located at a position away from the boundary. 
     As shown in  FIG. 25B , in the closed cross section of the beam  81 , the first strain gauge  90 A is located at the width-center of the top plate  811  of the beam  81 , and the second strain gauge  90 B is located at the width-center of the bottom plate  812  of the beam  81 . However, their locations should not be limited thereto. Each of the strain gauges may be located at a position away from the width-center. 
     The physical quantity detector  90  detects a strain occurring in the beam  81  of the support member  80  in the raising operation and the lowering operation by the crane  10 . A detection signal output from the physical quantity detector  90  is input to the controller  100  shown in  FIG. 2 . Arithmetic processing to be executed by the controller  100  is the same as that executed in the first embodiment, and thus the description therefor is omitted. 
     [Assembly Work and Disassembly Work] 
     Next, the assembly work and the disassembly work of the crane  10  according to the second embodiment will be described. Each of  FIGS. 26 to 31  is a schematic side view of the crane  10  according to the second embodiment having a specific posture in the assembly work or the disassembly work thereof. As shown in  FIGS. 26 to 31 , the second embodiment differs from the first embodiment in that the boom direction corresponds to one of the leftward and rightward directions (the rightward direction in the detailed example), that the float  85  constituting the lower end of a beam  82  of a support member  80  serves as a reaction force receiving part, and the physical quantity detector  90  is provided in the support member  80 . In contrast, a basic sequence of each of the assembly work and the disassembly work in the second embodiment is the same as the sequence described with reference to  FIGS. 8 to 13 , and hence detailed description therefor is omitted. 
     Modifications of Second Embodiment 
     In the crane  10  according to the second embodiment, the physical quantity detector  90  (a strain detector) is sufficiently configured to detect a strain occurring in the beam  81  of the support member  80 , and hence the location of the physical quantity detector  90  should not be limited to those described above. 
     For example, as shown in  FIGS. 32, 33A, and 33B , the physical quantity detector  90  may be provided on an outer surface (an upper surface) of the top plate  811  and an outer surface (a lower surface) of the bottom plate  812  of the beam  81 . 
     Moreover, the physical quantity detector  90  may not be necessarily provided in the beam  81 , and may be provided, for example, in a specific portion of the leg  82  that is adjacent to the distal end  8 B of the beam  81 . 
     Furthermore, the physical quantity detector  90  may not be necessarily provided in the support member  80 , and may be provided, for example, in a specific portion of the frame main body  1 A of the crawler frame  1  that is adjacent to the proximal end  8 A of the beam  81  as shown in  FIGS. 34 and 35 . In the modifications shown in  FIGS. 34 and 35 , a first strain gauge  90 A is provided on a top plate  111  of the frame main body  1 A, and a second strain gauge  90 B is provided on a bottom plate  112  of the frame main body  1 A. The portions where the strain gauges  90 A,  90 B are provided are adjacent to an engaging portion including hooks  1 C and through holes  1 D as described above. 
     In the crane  10  according to the second embodiment, two or more support members are preferably provided to maintain the posture of the crane  10  stable when a moment in a direction of causing the crane  10  to turn over in one of the leftward and rightward directions occurs. The number of support members  80  should not be limited to those described in the embodiment. 
     The lower traveling body  11  of the crane  10  may include three support members  80  as shown in  FIG. 36 , may include four support members  80  as shown in  FIG. 37 , or may include five or more support members  80 . In any of the modifications, the support members  80  are arranged at intervals in the frame main body  1 A of the crawler frame  1  in the forward and rearward directions. 
       FIG. 38  is a perspective view schematically showing a fifth modification of the second embodiment. In the fifth modification shown in  FIG. 20 , the beam  81  further includes a measurement support base  200  (a deformation member) to which a strain gauge (a physical quantity detector  90 ) is attached. The measurement support base  200  is located at such a position as to detect a strain occurring in a beam  81  in a state where a distal end  14 B of a boom  14  is away leftward or rightward from the proximal end  14 A of the boom  14  in the leftward and rightward directions. The measurement support base  200  is preferably arranged in this manner, and thus the location of the measurement support base  200  is not particularly limited. 
     Moreover, as shown in  FIG. 38 , the beam  81  in the fifth modification includes a reinforcing plate  815  extending in a direction perpendicularly intersecting a longitudinal direction of the beam  81  and having a surface oriented in the longitudinal direction (the leftward direction or the rightward direction). The measurement support base  200  in the fifth modification shown in  FIG. 38  extends along the reinforcing plate  815  and atop plate  811 . In other words, the measurement support base  200  is located at the corner between the reinforcing plate  815  and the top plate  811 . 
     The measurement support base  200  includes a first surface  200 A, a second surface  200 B, and a retaining surface  200 C. The first surface  200 A faces the reinforcing plate  815  and attached to the reinforcing plate  815 . The second surface  200 B faces the top plate  811  and attached to the top plate  811 . The retaining surface  200 C connects an end edge of the first surface  200 A and an end edge of the second surface  200 B with each other, and retains the strain gauge  90 A. In the detailed example shown in  FIG. 38 , the retaining surface  200 C has a slope inclined upward as advancing rightward and retaining the strain gauge  90 A thereon. In the detailed example shown in  FIG. 38 , the slope is formed of a curve surface (a concave) in an arc shape, but may be formed of a flat surface or a convex. Moreover, in the detailed example shown in  FIG. 38 , the measurement support base  200  has a substantially L-shape. However, the shape of the measurement support base  200  should not be limited to the substantially L-shape. 
     Another measurement support base  200  is arranged at another corner between the reinforcing plate  815  and a bottom plate  812  in addition to the measurement support base at the corner between the reinforcing plate  815  and the top plate  811  as described above. Here, a bending moment is applied to the beam  81  and a bending deformation occurs in the beam  81  when the tiltable attachment is raised and lowered. As a result, a strain occurs on the retaining surface  200 C of the upper measurement support base  200  in a direction of being pulled and stretched. Similarly, a strain occurs on the retaining surface of the lower measurement base in a direction of being compressed and contracted. Under the circumstances, the strain gauge provided along each of the retaining surfaces  200 C can detect the corresponding strain necessary to calculate the bending moment. In the case where the retaining surface  200 C is a curve surface in an arc shape, it is possible to adjust the magnitude of the strain by changing the radius of curvature in the arc shape. 
     Third Embodiment 
     A crane  10  according to the third embodiment has a configuration equivalent to that according to the second embodiment described above with reference to  FIGS. 22 to 31 . The third embodiment differs from the second embodiment in that a physical quantity detector is constituted by a reaction force detector  93  in place of the strain detector  90  in the second embodiment. With this configuration in the third embodiment, the strain detector  90  shown in  FIGS. 22 to 25  may be excluded. 
     Hereinafter, the difference of the third embodiment from the second embodiment will be mainly described. 
     A lower traveling body  11  of the crane  10  according to the third embodiment shown in  FIG. 22  includes a pair of support members  80  (a pair of receiving members  80 ) in the same manner as the second embodiment. The pair of receiving members  80  includes a first support member  80  and a second support member  80  located in the rear of the first support member  80  in the same manner as the second embodiment. Each of the pair of support members  80  has a connection part connected with the crawler frame  1  in a frame unit, and a contact part being in contact with the ground. The connection part of the support member  80  is constituted by a proximal end  8 A of a beam  81  to be described later, and the contact part of the support member  80  is constituted by a float  85  that is a lower end of a leg  82  to be described later. 
     In the third embodiment, a boom direction D 1  corresponds to a first direction D 1  (the rightward direction) shown in  FIG. 22  in the same manner as the second embodiment. As shown in  FIG. 22 , the float  85  (the contact part) serves as a part (a reaction force receiving part) for receiving a reaction force from the ground at a position away from a slewing axis C in the boom direction D 1 . The float  85  (the contact part) is away from the proximal end  8 A (the connection part) of the beam  81  in the boom direction D 1 . 
     [Physical Quantity Detector] 
     In the third embodiment, the physical quantity detector  93  serves as the reaction force detector  93  ( FIG. 6 ) for detecting information necessary to safely raise and lower the boom  14  in the assembly work and the disassembly work of the crane  10 . Specifically, the physical quantity detector  93  detects a physical quantity which changes in accordance with a change in a reaction force received from the ground by the support member  80 . The physical quantity detector  93  is configured to detect a pressure corresponding to a moment in a direction of causing the crane  10  to turn over in one of the leftward and rightward direction. In the third embodiment, the support member  80  is arranged so that the float  85  (the contact part) that is the lower end of the leg  82  is at a position away from the proximal end  8 A (the connection part) of the beam  81  in the boom direction D 1  in the assembly work and the disassembly work in the same manner as the second embodiment. 
       FIG. 39  shows an exemplary hydraulic circuit in the crane  10 . As shown in  FIG. 39 , the physical quantity detector  93  in the embodiment includes a first pressure sensor  91  for detecting a physical quantity which changes in accordance with a change in a reaction force received from the ground by the first support member  80 , and a second pressure sensor  92  for detecting a physical quantity which changes in accordance with a reaction force received from the ground by the second support member  80 . Specifically, the first pressure sensor  91  detects, as the physical quantity, a pressure on a head side of the hydraulic cylinder  86  of the first support member  80 . The second pressure sensor  92  detects, as the physical quantity, a pressure on a head side of the hydraulic cylinder  86  of the second support member  80 . Each of the first pressure sensor  91  and the second pressure sensor  92  may be configured to detect a pressure in a head chamber of the corresponding hydraulic cylinder  86 , or may be configured to detect a pressure in a hydraulic pipe L connected with the head chamber to be described later. Hereinafter, the hydraulic circuit shown in  FIG. 39  will be described. 
     As shown in  FIG. 39 , the crane  10  includes a hydraulic pump  170 , a pair of control valves  171 ,  172 , an instruction device  174 , the pair of hydraulic cylinders  86 ,  86 , and the physical quantity detector  93 . 
     The hydraulic pump  170  discharges hydraulic fluid. The hydraulic pump  170  is driven by an unillustrated drive source (e.g., an engine). 
     Each of the pair of control valves  171 ,  172  is disposed between the hydraulic pump  170  and the corresponding hydraulic cylinder  86 . A pipe leading to a tank and provided with relief valves is connected with a pipe connecting the hydraulic pump  170  and the control valves  171 ,  172  with each other. Each of the pair of the control valves  171 ,  172  is shiftable between a supply position (an upper position or a lower position in  FIG. 39 ) for supplying the hydraulic fluid discharged from the hydraulic pump  170  to the corresponding hydraulic cylinder  86  through a hydraulic path and a suspension position (a middle position in  FIG. 39 ) for suspending the supply of the hydraulic fluid discharged from the hydraulic pump  170  to the hydraulic cylinder  86 . 
     The instruction device  174  instructs each of the control valves  171 ,  172  to shift between the supply position and the suspension position. The instruction device  174  may be configured to be operable by, for example, an operator, or may be configured to be operable in response to an instruction from a controller  100 . The instruction device  174  supplies an instruction current from a power source  178  to a solenoid of the corresponding control valve in response to an operation for setting the control valve at the supply position. In this way, the control valve is shifted to the supply position. 
     Specifically, upon shifting of the control valve to the upper position shown in  FIG. 39 , the hydraulic fluid discharged from the hydraulic pump  170  flows into the head chamber of the corresponding hydraulic cylinder  86  through the hydraulic pipe L 1 , and the hydraulic fluid in the rod chamber of the hydraulic cylinder  86  flows out to a hydraulic pipe L 2 . As a result, the leg  82  of the support member  80  extends. Conversely, upon shifting of the control valve to the lower position shown in  FIG. 39 , the hydraulic fluid discharged from the hydraulic pump  170  flows into the rod chamber of the corresponding hydraulic cylinder  86  through the hydraulic pipe L 2 , and the hydraulic fluid in the head chamber of the hydraulic cylinder  86  flows out to the hydraulic pipe L 1 . As a result, the leg  82  of the support member  80  retracts. 
     The hydraulic pipe L 1  connected with the head chamber of one hydraulic cylinder  86  is provided with a check valve  176 . Similarly, the hydraulic pipe L 1  connected with the head chamber of the other hydraulic cylinder  86  is provided with a check valve  77 . Each of the check valves  176 ,  177  suspends the hydraulic fluid in the head chamber from flowing out in a direction of outflowing from the head chamber while the support member  80  receives a reaction force of the load applied to the ground by the crane  10 . In this manner, the hydraulic cylinder  86  is prevented from retracting. In contrast, upon shifting of each of the control valves to the lower position shown in  FIG. 39 , each of the check valves  176 ,  177  permits the hydraulic fluid in the head chamber to flow out from the head chamber in a direction of outflowing from the head chamber under a pressure of the hydraulic fluid serving as a pilot pressure (a pilot source) in the hydraulic pipe L 2  connected with the rod chamber. Here, each of the pair of control valves  171 ,  172  at the center position (a neutral position) suspends the supply of the hydraulic fluid discharged from the hydraulic pump  170  to the head chamber of the hydraulic cylinder  86 . In contrast, each of the control valves at the center position (the neutral position) permits the hydraulic fluid in the hydraulic pipe L 2  connected with the rod chamber to flow into the tank so as to inhibit the corresponding check valve from being in an open state under the pressure of the hydraulic fluid in the hydraulic pipe L 2 . 
     A signal representing the pressure detected by the physical quantity detector  93  is input to the controller  100  shown in  FIG. 2 . 
     [Way of Calculating Counterforce] 
     The reaction force received from the ground by the float  85  (the reaction force receiving part) of the support member  80  is calculated by, for example, the following Formula (5): 
       reaction force  RF =pressure on head side× Ah —pressure on rod side×( Ah−Ar )   (5),
 
     where “Ah” denotes a cross-sectional area (a bore cross-sectional area) of the head chamber of the hydraulic cylinder  86 , “Ar” denotes a cross-sectional area of a cylinder rod of the hydraulic cylinder  86 . Accordingly, the expression (Ah−Ar) in the formula represents a substantial cross-sectional area of the rod chamber. 
     The rod chamber of the hydraulic cylinder  86  is connected with the tank via the hydraulic pipe L 2 , and hence the pressure on the rod side can be regarded as indicating substantially zero. In this case, the reaction force RF may be calculated by the following Formula (6): 
       reaction force  RF =pressure on head side× Ah   (6)
 
     As shown in  FIG. 22 , the lower traveling body  11  includes the pair of support members  80 . Thus, a supportive reaction force RFt that represents a sum of supportive reaction forces of supporting the crane  10  is calculated by the following formula (7): 
         RFt=RF 1+ RF 2  (7),
 
     where “RF1” denotes a reaction force received from the ground by the float  85  of the first support member  80 , and “RF2” denotes a reaction force received from the ground by the float  85  of the second support member  80 . 
     [Operations] 
     In the third embodiment, the controller  100  stores in advance a maximal value RFmax (a maximally permissible reaction force) of the reaction force RFt received from the ground by the floats  85  (the contact parts) of the pair of support members  80  on the premise that the pair of support members  80  in the crane  10  undertakes the weight (corresponding to the turning-over moment Mt) of the crane  10 . 
     The calculation section  101  shown in  FIG. 2  calculates the supportive reaction force RFt based on the pressures detected by the physical quantity detector  93  by using Formulas (5) and (7), or Formulas (6) and (7). 
     The stability determination section  102  compares the supportive reaction force RFt with the maximally permissible reaction force RFmax. The stability determination section  102  determines that the supportive reaction force RFt falls within a safe range and thus the crane  10  is in a stable state when the supportive reaction force RFt is smaller than the maximally permissible reaction force RFmax (supportive reaction force RFt&lt;maximally permissible reaction force RFmax). Conversely, the stability determination section  102  determines that the supportive reaction force RFt falls within a risky range and thus the crane  10  is in an unstable state when the supportive reaction force RFt is larger than the maximally permissible reaction force RFmax (supportive reaction force RFt&gt;maximally permissible reaction force RFmax). 
     When the stability determination section  102  determines that the crane  10  is in the unstable state, the notification control section  103  outputs a notification instruction of notifying in the notification device  110  the operator of the stability information concerning the stability determined by the stability determination section  102 . 
     The operation control section  104  outputs an operation instruction directing an operation required to avoid a decrease in the stability of the crane  10  based on the stability determined by the stability determination section  102 . Specifically, the boom raising and lowering winch  30  suspends or decelerates, for example, the operation of winding the rope  38  in accordance with the operation instruction. In a case where the jib raising and lowering winch  32  is operating, the jib raising and lowering winch  32  suspends or decelerates, for example, the operation of winding (or unwinding) the rope  44  in accordance with the operation instruction. 
     Fourth Embodiment 
       FIG. 40  is a plan view of a lower traveling body  11  of a mobile crane  10  according to the fourth embodiment, and shows a state where a trans-lifter is engaged with an engaging portion of a center frame  2 .  FIG. 41  is a sideview of the lower traveling body  11  of the crane  10  in  FIG. 40 , and shows a state where the trans-lifter is engaged with the engaging portion of the center frame  2 .  FIG. 42  is a partially broken sideview of the engaging portion of the center frame  2  of the crane  10  in  FIG. 40  and the trans-lifter engaged with the engaging portion. 
       FIG. 43  is a plan view of the lower traveling body  11  of the crane  10  in  FIG. 40 , and shows a state where a support member  80  (a receiving member  80 ) is engaged with the engaging portion  201   a  of the center frame  2 .  FIG. 44  is a sideview of the lower traveling body  11  of the crane  10  in  FIG. 40 , and shows a state where the support member  80  is engaged with the engaging portion of the center frame  2 .  FIG. 45  is a partially broken sideview of the engaging portion  201   a  of the center frame  2  of the crane  10  in  FIG. 40  and the support member  80  engaged with the engaging portion  201   a .  FIG. 46  is a cross-sectional view taken along the line XXXXVI-XXXXVI in  FIG. 45 . 
     In the fourth embodiment shown in  FIGS. 40 to 46 , the lower traveling body  11  includes a plurality of trans-lifters  70 , and a plurality of support members  80  (a plurality of receiving members  80 ). In the detailed example shown in  FIG. 43 , the plurality of support members  80  includes a pair of support members  80 . 
     As shown in  FIGS. 40 and 43 , each of the support members  80  is configured to be replaceable with a corresponding trans-lifter  70 . Specifically, the crane  10  according to the fourth embodiment is configured such that its use is changeable between a trans-lifter use that the trans-lifters  70  are attached to the center frame  2 , and a support member use that the support members  80  (the receiving members  80 ) are attached to the center frame  2 . 
     Each of the pair of support members  80  has a connection part connected with the center frame  2  in a frame unit, and a contact part being in contact with the ground. The connection part of the support member  80  is constituted by a proximal end  8 A of a beam  81 , and the contact part of the support member  80  is constituted by a float  85  that is a lower end  85  of a leg  82 . 
     A boom direction in the fourth embodiment coincides with a horizontal component of a direction in which a boom  14  extends from an upper slewing body  12  in the assembly work and the disassembly work. In the fourth embodiment, the boom direction D 1  corresponds to a first direction D 1  (the forward direction) shown in  FIG. 43 . As shown in  FIGS. 43 and 44 , the float  85  (the contact part) serves as a part (a reaction force receiving part) for receiving a reaction force from the ground at a position away from a slewing axis C in the boom direction D 1 . The float  85  (the contact part) is away from the proximal end  8 A (the connection part) of the beam  81  in the boom direction D 1 . 
     Hereinafter, the fourth embodiment will be described in detail. First, the trans-lifter use shown in  FIGS. 40 to 42  will be described. 
     As shown in  FIGS. 40 to 42 , in the fourth embodiment, the crane  10  includes four trans-lifters  70 . The trans-lifters  70  are aimed at lifting the center frame  2  from the ground when a pair of crawlers  3 ,  3  is attached to the center frame  2  and detached from the center frame  2 . Two trans-lifters  70  among the four trans-lifters  70  are attached to a front axle  2   b , and the remaining two trans-lifters  70  are attached to a rear axle  2   c.    
     Each of the trans-lifters  70  includes a beam  71  and a leg  72 . The beam  71  has a proximal end supported by the front axle  2   b  or the rear axle  2   c  of the frame  2 . The proximal end of the beam  71  is engaged with the engaging portions  201   a ,  202   a  provided at the front axle  2   b  or the rear axle  2   c  of the frame  2 . 
     In the embodiment, the engaging portions  201   a ,  202   a  are respectively constituted by through holes  201   a ,  202  formed in the front axle  2   b  and the rear axle  2   c . Specifically, each of the axles of the frame  2  includes a top plate  201  extending in the leftward and rightward directions, a bottom plate  202  spaced downward from the top plate  201  and extending in the leftward and rightward directions. The through hole  201   a  is formed in the top plate  201  of the axle, and the through hole  202   a  is formed in the bottom plate  202  thereof. The through hole  201   a  and the through hole  202   a  are spaced apart from each other in the leftward and rightward directions. 
     The proximal end of the beam  71  is also formed with through holes  711   a ,  712   a . A pin  203  is inserted in the through holes in a state where the holes face each other. Consequently, the beam  71  is attached to the frame  2 . 
     The beam  71  of the trans-lifter  70  attached to the front axle  2   b  extends in a boom direction D 1  that is the first direction D 1  corresponding to one of the forward and rearward directions, or in a direction oblique to the boom direction D 1 . The beam  71  of the trans-lifter  70  attached to the rear axle  2   c  extends in a second direction D 2  opposite to the boom direction D 1  or a direction oblique to the second direction D 2 . Since the trans-lifters  70  are used to lift the frame  2  from the ground for the attachment or detachment of the pair of crawlers  3 ,  3 , a distal end of the beam  71  is located between the frame  2  and a distal end of each of the crawlers  3  in the forward and rearward directions (on an inner side) as shown in  FIG. 3 . 
     The leg  72  includes a hydraulic cylinder, the hydraulic cylinder including a cylinder main body  73  supported on the distal end of the beam  71  by an attachment member  76  and extending downward from the distal end, and a rod  74  slidable along the cylinder main body  73  in the upward and downward directions. 
     [Support Member (Receiving Member)] 
     Next, the support member use will be described. 
     In the embodiment, the lower traveling body  11  of the crane  10  includes a first support member  80  and a second support member  80 . Each of the support members  80  includes the beam  81  and the leg  82 . The beam  81  has a proximal end  8 A (a connection part) supported by the center frame  2  between the pair of crawlers  3 ,  3 , and extends from the frame  2  in the boom direction D 1 . The beam  81  of the first support member  80  and the beam  81  of the second support member  80  are spaced apart from each other in the leftward and rightward directions. 
     The leg  82  is supported on the distal end  8 B of the beam  81  by an attachment member  186  and extends downward from the distal end  8 B. The float  85  that is the lower end  85  of the leg  82  is configured to come into contact with the ground GR at a position away from the pair of crawlers  3 ,  3  in the boom direction D 1 . The leg  82  is located at a position away from a rotational axis CB of a drive tumbler  4   a  in the boom direction D 1 . In the embodiment, the leg  82  includes a hydraulic cylinder. Specifically, the leg  82  includes a cylinder main body  83  supported on the distal end of the beam  81  and extending downward from the distal end, and a rod  84  slidable along the cylinder main body  83  in the upward and downward directions. 
     As shown in  FIG. 46 , the beam  81  of the support member  80  has a closed cross section perpendicularly intersecting a longitudinal direction of the beam  81 . Specifically, the beam  81  has a top plate  811  extending in the longitudinal direction of the beam  81 , a bottom plate  812  spaced downward from the top plate  811  and extending in the longitudinal direction, and a pair of side plates  813 ,  814  each extending in the longitudinal direction, as shown in  FIGS. 45 and 46 . The one side plate  813  connects right ends of the top plate  811  and the bottom plate  812  with each other, and the other side plate  814  connects right ends of the top plate  811  and the bottom plate  812  with each other. 
     The proximal end of the beam  81  is formed with through holes  811   a ,  812   a  serving as an engaged portion for attaching the beam  81  to the center frame  2 . Specifically, the through hole  811   a  is formed in the proximal end of the top plate  811 , and the through hole  812   a  is formed in the proximal end of the bottom plate  812 . In a state where each of the through holes  811   a ,  812   a  and the corresponding one of the through holes  201   a ,  202   a  respectively formed in the top plate  201  and the bottom plate  202  of the front axle  2   b  face each other, a pin  203  is inserted in the through holes. Consequently, the beam  71  is attached to the frame  2 . 
     As described above, in the embodiment, the support member  80  is detachably attachable to the center frame  2 . The beam  81  of the support member  80  is further configured to be engageable with the engaging portions  201   a ,  202   a  in place of the trans-lifter  70  having been disengaged from the engaging portions  201   a ,  202   a . In other words, the engaging portions of the frame  2  are available for both the attachment of the beam  71  of the trans-lifter  70  and the attachment of the beam  81  of the support member  80 . 
     [Physical Quantity Detector] 
     A physical quantity detector  90  is configured to detect information necessary to safely raise and lower the boom  14  in the assembly work and the disassembly work of the crane  10 . Specifically, the physical quantity detector  90  detects a strain occurring in the beam  81  of the support member  80  in the raising operation and the lowering operation. The physical quantity detector  90  is configured to detect a strain occurring in the beam  81  of the support member  80  and changing in accordance with a change in a moment in a direction of causing the crane  10  to turn over in the boom direction D 1 . 
     In the embodiment, the lower traveling body  11  of the crane  10  includes a first physical quantity detector  90  for detecting a strain occurring in the first support member  80 , and a second physical quantity detector  90  for detecting a strain occurring in the second support member  80 . The first physical quantity detector  90  and the second physical quantity detector  90  have the same configuration, and each of the detectors is provided at the same position in the corresponding support member  80 . Therefore, one of the physical quantity detectors  90  is mainly focused below. 
     In the embodiment, as shown in  FIGS. 43 to 45 , the physical quantity detector  90  is provided in the beam  81  of the corresponding support member  80 . The physical quantity detector  90  includes at least one device for detecting the strain in the beam  81  of the support member  80 . Adoptable for this device is the exemplary device described in the first embodiment. 
     Specifically, as shown in  FIGS. 43 and 45 , the physical quantity detector  90  in the embodiment is provided at the proximal end of the beam  81  of the corresponding support member  80 . More specifically, the physical quantity detector  90  is provided in a portion of the proximal end of the beam  81  that is adjacent to a front end of the axle  2   b  of the center frame  2  in the forward and rearward directions. Here, the proximal end of the beam  81  is closer to the engaging portion  201   a  of the center frame  2  than the longitudinal center of the beam  81 . The distal end of the beam  81  is closer to the leg  82  than the longitudinal center of the beam  81  in the beam  81 . 
     As shown in  FIG. 46 , the physical quantity detector  90  includes a plurality of strain gauges (four strain gauge  90 A,  90 B,  90 C,  90 D in the illustrated example). The strain gauges  90 A,  90 B (serving as a first device) are provided in an upper portion of the proximal end of the beam  81 . The strain gauges  90 C,  90 D (serving as a second device) are provided in a lower portion of the proximal end of the beam  81 . 
     More specifically, the strain gauge  90 A is located in a boundary portion between the top plate  811  and the one side plate  813  of the beam  81 . The strain gauge  90 B is located in a boundary portion between the top plate  811  and the other side plate  814  of the beam  81 . The strain gauge  90 C is located in a boundary portion between the bottom plate  812  and the one side plate  813  of the beam  81 . The strain gauge  90 D is located in a boundary portion between the bottom plate  812  and the other side plate  814  of the beam  81 . In the embodiment, each of the strain gauges is attached to an outer surface of the beam  81 , but may be attached to an inner surface of the beam  81 . 
     The physical quantity detector  90  detects a strain occurring in the beam  81  of the support member  80  in the raising operation and the lowering operation by the crane  10 . A detection signal representing the strain detected as the physical quantity from the physical quantity detector  90  is input to the controller  100  shown in  FIG. 2 . Arithmetic processing to be executed by the controller  100  is the same as that executed in the first embodiment, and thus the description therefor is omitted. 
     [Assembly Work and Disassembly Work] 
     Next, the assembly work and the disassembly work of the crane  10  according to the fourth embodiment will be described. As shown in  FIGS. 43 and 44 , the fourth embodiment is equivalent to the first embodiment in that the boom direction D 1  corresponds to one of the forward and rearward directions (the forward direction in the detailed example). Furthermore, the fourth embodiment is equivalent to the second embodiment in that the float  85  constituting the lower end of the beam  82  of the support member  80  serves as a reaction force receiving part, and that the physical quantity detector  90  is provided in the support member  80 . From these perspectives, a basic sequence of each of the assembly work and the disassembly work in the fourth embodiment is the same as the sequence described with reference to  FIGS. 8 to 13, and 26 to 31 , and hence detailed description therefor is omitted. 
     Modifications of Fourth Embodiment 
       FIG. 47  is a perspective view schematically showing a modification of the fourth embodiment. In the modification shown in  FIG. 47 , a beam  81  further includes a measurement support base  200  (a deformation member) to which a strain gauge (a strain detector) is attached. The measurement support base  200  is located at such a position as to detect a strain occurring in the beam  81  in a state where a distal end  14 B of a boom  14  is away in a boom direction D 1  from a proximal end  14 A of the boom  14  in the forward and rearward directions. The measurement support base  200  is preferably arranged in this manner, and thus the location of the measurement support base  200  is not particularly limited. The details of the configuration of the measurement support base  200  is the same as those in the fifth modification of the second embodiment shown in  FIG. 38 , and thus the detailed description therefor is omitted. 
     Fifth Embodiment 
     A crane  10  according to the fifth embodiment has a configuration equivalent to that according to the fourth embodiment described above with reference to  FIGS. 43 to 47 . The fifth embodiment differs from the fourth embodiment in that the physical quantity detector is constituted by a reaction force detector  93  (see  FIG. 6 ) in place of the strain detector  90  in the fourth embodiment. With this configuration in the fifth embodiment, the strain detector  90  shown in  FIGS. 43 to 47  may be excluded. 
     Hereinafter, the difference of the fifth embodiment from the fourth embodiment will be mainly described. 
     A lower traveling body  11  of the crane  10  according to the fifth embodiment shown in  FIG. 43  includes a pair of support members  80  (a pair of receiving members  80 ) in the same manner as the fourth embodiment. The pair of receiving members  80  includes a first support member  80  and a second support member  80  located at a distance from the first support member  80  in the leftward and rightward directions in the same manner as the fourth embodiment. Each of the pair of support members  80  has a connection part connected with a center frame  2  in a frame unit, and a contact part being in contact with the ground. The connection part of the support member  80  is constituted by a proximal end  8 A of a beam  81 , and the contact part of the support member  80  is constituted by a float  85  that is a lower end of a leg  82 . 
     In the fifth embodiment, a boom direction D 1  corresponds to a first direction D 1  (the forward direction) shown in  FIG. 43  in the same manner as the fourth embodiment. As shown in  FIG. 22 , the float  85  (the contact part) serves as a part (a reaction force receiving part) for receiving a reaction force from the ground at a position away from a slewing axis C in the boom direction D 1 . The float  85  (the contact part) is away from the proximal end  8 A (the connection part) of the beam  81  in the boom direction D 1 . 
     [Physical Quantity Detector] 
     In the fifth embodiment, the physical quantity detector  90  serves as a reaction force detector  93  (see  FIG. 6 ) for detecting information necessary to safely raise and lower the boom  14  in the assembly work and the disassembly work of the crane  10 . Specifically, the physical quantity detector  90  detects a physical quantity which changes in accordance with a change in a reaction force received from the ground by the support member  80 . The physical quantity detector  93  is configured to detect a pressure corresponding to a moment in a direction of causing the crane  10  to turn over in one of the forward and rearward directions. In the fifth embodiment, the support member  80  is arranged so that the float  85  (the contact part) that is the lower end of the leg  82  is away from the proximal end  8 A (the connection part) of the beam  81  in the boom direction D 1  in the assembly work and the disassembly work in the same manner as the fourth embodiment. 
     A hydraulic circuit included in the crane  10  according to the fifth embodiment is the same as that in the crane according to the third embodiment described with reference to  FIG. 39 . Accordingly, the physical quantity detector  93  in the fifth embodiment includes a first pressure sensor  91  for detecting a physical quantity which changes in accordance with a change in a reaction force received from the ground by the first support member  80 , and a second pressure sensor  92  for detecting a physical quantity which changes in accordance with a change in a reaction force received from the ground by the second support member  80 . In the fifth embodiment, a physical quantity detected by each of the first pressure sensor  91  and the second pressure sensor  92  is a pressure on a head side of a corresponding hydraulic cylinder  86  in the same manner as the third embodiment. 
     A signal representing the pressure detected by the physical quantity detector  93  is input to the controller  100  shown in  FIG. 2 . 
     In the fifth embodiment, a way of calculating the reaction force is the same as the way described in the third embodiment. 
     Besides, in the fifth embodiment, the controller  100  stores in advance a maximal value RFmax (a maximally permissible reaction force) of the reaction force RFt received from the ground by the floats  85  (the contact parts) of the pair of support members  80  in the same manner as the third embodiment. The calculation section  101  shown in  FIG. 2  calculates the supportive reaction force RFt based on the pressures detected by the physical quantity detector  93  using Formulas (5) and (7), or Formulas (6) and (7). 
     The stability determination section  102  compares the supportive reaction force RFt with the maximally permissible reaction force RFmax, and determines whether the crane  10  is in a stable state or an unstable state in the same manner as the third embodiment. 
     When the stability determination section  102  determines that the crane  10  is in the unstable state, the notification control section  103  outputs a notification instruction of notifying in the notification device  110  the operator of the stability information concerning the stability determined by the stability determination section  102 . 
     The operation control section  104  outputs an operation instruction directing an operation required to avoid a decrease in the stability of the crane  10  based on the stability determined by the stability determination section  102  in the same manner as the third embodiment. 
     Sixth Embodiment 
       FIG. 48  is a plan view of a lower traveling body  11  of a mobile crane  10  according to the sixth embodiment.  FIG. 49  is a sideview of the lower traveling body  11  of the crane  10  in  FIG. 48 .  FIG. 50  is across-sectional view of abeam  81  of a support member  80 , taken along the line XXXXX-XXXXX in  FIG. 48 , in a crawler  3  of the lower traveling body  11  in  FIG. 48 . 
     In the sixth embodiment shown in  FIGS. 48 to 50 , the lower traveling body  11  has a plurality of support members  80  (a plurality of receiving members  80 ). In the detailed example shown in  FIG. 48 , the plurality of support members  80  includes a first right support member  80  (a first right receiving member  80 ), a first left support member  80  (a first left receiving member  80 ), a second right support member  80  (a second right receiving member  80 ), and a second left support member  80  (a second left receiving member  80 ). The first right support member  80  and the first left support member  80  are attached to a first crawler frame  1  that is the crawler frame  1  of the first crawler  3 . The second right support member  80  and the second left support member  80  are attached to a second crawler frame  1  that is the crawler frame  1  of the second crawler  3 . 
     Each of the support members  80  has a connection part connected with the crawler frame  1  in a frame unit, and a contact part being in contact with the ground. The connection part of the support member  80  is constituted by a proximal end  8 A of the beam  81 , and the contact part of the support member  80  is constituted by a float  85  that is a lower end  85  of a leg  82 . 
     A boom direction in the sixth embodiment coincides with a horizontal component of a direction in which a boom  14  extends from an upper slewing body  12  in the assembly work and the disassembly work. In the sixth embodiment, the boom direction D 1  corresponds to a first direction D 1  (the forward direction) shown in  FIG. 48 . As shown in  FIG. 48 , the float  85  (the contact part) serves as a part (a reaction force receiving part) for receiving a reaction force from the ground at a position away from a slewing axis C in the boom direction D 1 . The float  85  (the contact part) is away from the proximal end  8 A (the connection part) of the beam  81  in the boom direction D 1 . 
     As shown in  FIG. 48 , the first right support member  80  is arranged so that the float  85  (the contact part) of the first right support member  80  is away rightward from the first crawler frame  1  and is away from the proximal end  8 A (the connection part) of the beam  81  of the first right support member  80  in the boom direction D 1 . 
     The first left support member  80  is arranged so that the float  85  (the contact part) of the first left support member  80  is away leftward from the first crawler frame  1 , and is away from the proximal end  8 A (the connection part) of the beam  81  of the first left support member  80  in the boom direction D 1 . 
     The second right support member  80  is arranged so that the float  85  (the contact part) of the second right support member  80  is away rightward from the second crawler frame  1 , and is away from the proximal end  8 A (the connection part) of the beam  81  of the second right support member  80  in the boom direction D 1 . 
     The second left support member  80  is arranged so that the float  85  (the contact part) of the second left support member  80  is away leftward from the second crawler frame  1 , and is away from the proximal end  8 A (the connection part) of the beam  81  of the second left support member  80  in the boom direction D 1 . 
     Hereinafter, the sixth embodiment will be described in detail. As shown in  FIGS. 48 and 49 , the crawler frame  1  includes a frame main body  1 A and a tumbler bracket  1 B. The frame main body  1 A has a shape extending in the forward and rearward directions. The frame main body  1 A includes a top plate  111  extending in a longitudinal direction of the frame main body  1 A, a bottom plate  112  spaced downward from the top plate  111  and extending in the longitudinal direction, and a side plate  113  connecting the top plate  11  and the bottom plate  112  with each other. 
     [Support Member (Receiving Member)] 
     As shown in  FIGS. 48 and 49 , each of the support members  80  includes the beam  81  and the leg  82 . The beam  81  includes the proximal end RA supported by the crawler frame  1 , and a distal end  8 B away in the boom direction D 1  from the proximal end  8 A in the forward and rearward directions. In the embodiment, the beam  81  linearly extends in a plan view shown in  FIG. 48 . 
     The proximal end  8 A of the beam  81  is attached to the crawler frame  1 , and the beam  81  extends in a direction oblique to the boom direction D 1 . Specifically, the proximal end  8 A of the beam  81  is attached to a portion of the frame main body  1 A of the crawler frame  1  away from a front axle  2   b  in the boom direction D 1 . 
     Specifically, the proximal end  8 A of the beam  81  of the right support member  80  is attached to a right portion of the frame main body  1 A of the crawler frame  1 , and the beam  81  extends from the right portion in a direction obliquely rightward to the boom direction D 1  (extends diagonally forward to the right). The proximal end  8 A of the beam  81  of the left support member  80  is attached to a left portion of the frame main body  1 A of the crawler frame  1 , and the beam  81  extends from the left portion in a direction obliquely leftward to the boom direction D 1  (extends diagonally forward to the left). 
     The leg  82  is supported on the distal end  8 B of the beam  81  and extends downward from the distal end  8 B so that the lower end  85  comes into contact with the ground. In the embodiment, the leg  82  includes a hydraulic cylinder. Specifically, the leg  82  includes a cylinder main body  83  supported on the distal end  8 B of the beam  81  and extending downward from the distal end  8 B, and a rod  84  slidable along the cylinder main body  83  in the upward and downward directions. 
     The leg  82  of the support member  80  is located at a position away from a rotational axis CB of a drive tumbler  4   a  in the first direction D 1 . Here, the arrangement that “the leg  82  is located at a position away from a rotational axis CB of a drive tumbler  4   a  (a first wheel) in the first direction D 1 ” means that a central axis CC of the leg  82  is away in the boom direction D 1  from the rotational axis CB of the drive tumbler  4   a  (the first wheel). In the embodiment, the central axis CC of the leg  82  serves as a central axis CC of the hydraulic cylinder (i.e., the central axis CC of the rod  84 ) extending in the upward and downward directions. 
     The leg  82  of the right support member  80  (specifically, the central axis CC of the leg  82 ) is located on the right of the crawler frame  1 , and the leg  82  of the left support member  80  (specifically, the central axis CC of the leg  82 ) is located on the left of the crawler frame  1 . 
       FIG. 50  is a cross-sectional view of the beam  81  of the support member  80 , taken along the line XXXXX-XXXXX in  FIG. 48 , in the crawler  3  of the lower traveling body  11  in  FIG. 48 . As shown in  FIG. 50 , the beam  81  of the support member has a closed cross section perpendicularly intersecting the longitudinal direction of the beam  81 . The beam  81  has a top plate  811  extending in the longitudinal direction of the beam  81 , a bottom plate  812  spaced downward from the top plate  811  and extending in the longitudinal direction, and a pair of side plates  813 ,  814  each extending in the longitudinal direction, as shown in  FIGS. 48 to 50 . The one side plate  813  connects right ends of the top plate  811  and the bottom plate  812  with each other, and the other side plate  814  connects right ends of the top plate  811  and the bottom plate  812  with each other. Specifically, the beam  81  further includes a second top plate  815 . The second top plate  815  lies between the top plate  811  and the leg  82 , and connects the top plate  811  and the leg  82  with each other. The second top plate  815  is inclined downward as advancing to the distal end  8 B of the beam  81 . 
     The proximal end  8 A of the beam  81  is formed with an engaged portion for attaching the beam  81  to the crawler frame  1 . In the embodiment, the engaged portion includes a through hole  811   a  formed in the proximal end of the top plate  811 , and a through hole  812   a  formed in the proximal end of the bottom plate  812 . In contrast, the frame main body  1 A of the crawler frame  1  is provided with an engaging portion. The engaging portion includes a through hole  111   a  formed in the top plate  111  of the frame main body  1 A, and a through hole  112   a  formed in the bottom plate  112 . In a state where each of the through holes  811   a ,  812   a  constituting the engaged portion and the corresponding one of the through holes  111   a ,  112   a  constituting the engaging portion face each other, a pin  203  extending in the upward and downward directions is inserted in the through holes. Consequently, the beam  81  is attached to the crawler frame  1  rotatably about the pin  203 . 
     In the embodiment, each of the support members  80  is configured to be detachably attachable to the crawler frame  1 . Specifically, the support member  80  is detachable from the crawler frame  1  by removing the pin  203  from the through holes. 
       FIG. 51  is a sideview of a front portion of the lower traveling body  11  of the mobile crane  10  in  FIG. 48 .  FIG. 52  is a plan view of the front portion of the lower traveling body  11  of the crane  10  in  FIG. 48 . Each of  FIGS. 51 and 52  shows a state of the support member  80  which is not in use. 
     In the embodiment, the support member  80  is arranged to extend diagonally forward from the frame main body  1 A of the crawler frame  1  in use of the support member  80  in the assembly work and the disassembly work of the crane  10  as shown in  FIGS. 48 and 49 . Conversely, in no use of the support member  80 , the support member  80  is accommodatable in an accommodation space provided in the frame main body  1 A of the crawler frame  1  as shown in  FIGS. 51 and 52 . 
     Specifically, in the embodiment, the accommodation space is in the form of a recess defined by the top plate  811 , the bottom plate  112 , and the side plate  113 . The leg  82  is coupled to the distal end  8 B of the beam  81  via a coupling member  87 . The coupling member  87  has a pin  87   a . The leg  82  is attached to the coupling member  87  rotatably about the pin  87   a . The lower end  85  of the leg  82  is detached from the rod  84 , and the beam  81  is rotated about a pin  86 A while the leg  82  is rotated about the pin  87   a  for accommodating the support member  80 . Consequently, the beam  81  and the leg  82  are accommodated in the accommodation space. 
     [Physical Quantity Detector] 
     A physical quantity detector  90  is configured to detect information necessary to safely raise and lower the boom  14  in the assembly work and the disassembly work of the crane  10 . Specifically, the physical quantity detector  90  detects a strain occurring in the beam  81  of the support member  80 . The physical quantity detector  90  is configured to detect a strain occurring in the beam  81  of the support member  80  and corresponding to a moment in a direction of causing the crane  10  to turn over in the boom direction D 1 . 
     In the embodiment, the crane  10  includes a plurality of physical quantity detectors  90 . Specifically, the crane  10  includes four physical quantity detectors  90  respectively provided in four support members  80 . With this configuration, a strain occurring in the beam  81  of each of the support members  80  is detectable. In the embodiment, the four physical quantity detectors  90  have the same configuration, and each of the detectors is provided at the same position in the corresponding crawler frame  1  as shown in  FIG. 3 . Therefore, one of the physical quantity detectors  90  is mainly focused below. 
     In the embodiment, the physical quantity detector  90  is configured to detect a strain occurring in a portion of the beam  81  of the support member  80  between the proximal end  8 A of the beam  81  supported by the crawler frame  1  and a longitudinal center of the beam  81 . However, the physical quantity detector  90  may be configured to detect a strain occurring in a portion of the beam  81  of the support member  80  between the distal end  8 B supporting the leg  82  and the longitudinal center of the beam  81 , or may be configured to detect a strain occurring at the longitudinal center of the beam  81 . 
     The physical quantity detector  90  is preferably arranged in a portion of the beam  81  where a strain is likely to occur. In the arrangement, a strain caused in the beam  81  by the moment is sensitively detectable. Such a portion where the strain is likely to occur may be, for example, a connection portion between the beam  81  and the crawler frame  1  or an adjacent portion that is adjacent to the connection portion, or a connection portion between the beam  81  and the leg  82  or an adjacent portion that is adjacent to the connection portion. 
     The physical quantity detector  90  includes one or more devices for detecting the strain in the beam  81 . Adoptable for this device is the exemplary device described in the first embodiment. 
     As shown in  FIG. 50 , the physical quantity detector  90  in the embodiment includes a plurality of strain gauges (a first strain gauge  90 A and a second strain gauge  90 B in the illustrated example). The first strain gauge  90 A is provided on the top plate  811  defining an upper portion of the beam  81 , and the second strain gauge  90 B is provided on the bottom plate  812  defining a lower portion of the beam  81 . The strain gauge  90 A can detect a strain occurring in the upper portion of the beam  81 , and the strain gauge  90 B can detect a strain occurring in the lower portion of the beam  81 . 
     In the embodiment, the first strain gauge  90 A is located at the width-center of the top plate  811  of the beam  81 , but the location should not be limited thereto, and may be at a position away in an either width direction from the width-center. Similarly, the second strain gauge  90 B is located at the width-center of the bottom plate  812  of the beam  81 , but the location should not be limited thereto, and may be at a position away in an either width direction from the width-center. In the embodiment, each of the strain gauges is attached to an outer surface of the beam  81 , but may be attached to an inner surface of the beam  81 . 
     The physical quantity detector  90  detects a strain occurring in the beam  81  of the support member  80  in the raising operation and the lowering operation by the crane  10 . A detection signal representing the strain and detected by the physical quantity detector  90  is input to the controller  100  shown in  FIG. 2 . Arithmetic processing to be executed by the controller  100  is the same as that executed in the first embodiment, and thus the description therefor is omitted. 
     [Assembly Work and Disassembly Work] 
     Next, the assembly work and the disassembly work of the crane  10  according to the sixth embodiment will be described. As shown in  FIGS. 48 and 49 , the sixth embodiment is equivalent to the first embodiment in that the boom direction D 1  corresponds to one of the forward and rearward directions (the forward direction in the detailed example). Furthermore, the sixth embodiment is equivalent to the second and the fourth embodiments in that the float  85  constituting the lower end of the beam  82  of the support member  80  serves as a reaction force receiving part, and that the physical quantity detector  90  is provided in the support member  80 . From these perspectives, a basic sequence of each of the assembly work and the disassembly work in the sixth embodiment is the same as the sequence described with reference to  FIGS. 8 to 13 and 26 to 31 , and hence detailed description therefor is omitted. 
     Modifications of Sixth Embodiment 
       FIG. 53  is a plan view of a lower traveling body  11  of a mobile crane  10  according to a first modification of the sixth embodiment. The first modification differs from the aspect shown in  FIGS. 48 and 49  in that a beam  81  of a support member  80  includes a plurality of components attachable to and detachable from each other. It should be noted here that the crane  10  according to the first modification has the same configuration as that shown in  FIGS. 48 and 49  except the aforementioned difference. 
       FIG. 54  is a perspective view of a crawler frame  1  and the support member  80  attached to the crawler frame  1  in the crane  10  according to the first modification of the sixth embodiment, and shows a state where a part of the beam  81  of the support member  80  is disengaged from an engaging portion.  FIG. 55  is a perspective view of the crawler frame  1  and the support member  80  in  FIG. 54 , and shows a state where the part of the beam  81  of the support member  80  is engaged with the engaging portion.  FIG. 56  is a sideview of the crawler frame  1  and the support member  80  in  FIG. 54 , and shows a state where the part of the beam  81  of the support member  80  is disengaged from the engaging portion.  FIG. 57  is a sideview of the crawler frame  1  and the support member  80  in  FIG. 54 , and shows a state where the part of the beam  81  of the support member  80  is engaged with the engaging portion. 
     As shown in  FIG. 54 , the beam  81  of the support member  80  in the crane  10  according to the first modification includes a first component  81 A bearing a proximal end  8 A of the beam  81 , and a second component  81 B bearing a distal end  8 B of the beam  81 . The first component  81 A and the second component  81 B are attachable to and detachable from each other. Further, in the first modification, the second component  81 B detached from the first component  81 A may serve as a component of a trans-lifter  70  provided at each of a front axle  2   b  and a rear axle  2   c  of the lower traveling body  11 . 
     As shown in  FIG. 54 , the first component  81 A is attached to the frame main body  1 A of the crawler frame  1 . The first component  81 A has a top plate  811 A, a bottom plate  812 A spaced downward from the top plate  811 A, and a pair of side plates  813 A,  814 A. The one side plate  813 A connects right ends of the top plate  811 A and the bottom plate  812 A with each other, and the other side plate  814 A connects right ends of the top plate  811 A and the bottom plate  812 A with each other. 
     As shown in  FIGS. 54 and 56 , the proximal end  8 A of the first component  81 A is formed with a connected portion for attaching the first component  81 A to the crawler frame  1 . The connected portion includes a through hole formed in a proximal end of the top plate  811 A, and a through hole formed in a proximal end of the bottom plate  812 A. In contrast, the frame main body  1 A of the crawler frame  1  is provided with a connecting portion. The connecting portion includes a through hole formed in the top plate  111  of the frame main body  1 A, and a through hole formed in the bottom plate  112 . In a state where each of the through holes constituting the connected portion and the corresponding one of the through holes constituting the connecting portion face each other, a pin  86 A extending in the upward and downward directions is inserted in the through holes. Consequently, the first component  81 A is attached to the crawler frame  1  rotatably about the pin  86 A. 
     The first component  81 A is provided with a strain detector  90 . Specifically, the first component  81 A has, for example, a closed cross section shown in  FIG. 50  in the same manner as the embodiment. A first strain gauge  90 A is provided on the top plate  811 A of the first component  81 A, and a second strain gauge  90 B is provided on the bottom plate  812 A of the first component  81 A, the top plate  811 A and the bottom plate  812  defining the enclosed section. Here, the strain detector  90  may be provided in the second component  81 B. 
     Each of the pair of side plates  813 A,  814 A of the first component  81 A has an engaging portion at a position opposite to the connected portion. The engaging portion is aimed at attaching the second component  81 B to the first component  81 A. The engaging portion includes a pair of hooks  88 A and a pair of lower through holes  89 A located below the hooks  88 A. 
     The second component  81 B has a top plate SUB, a bottom plate  812 B spaced downward from the top plate  811 B, and a pair of side plates  813 B,  814 B. The one side plate  813 B connects right ends of the top plate  811 B and the bottom plate  812 B with each other, and the other side plate  814 B connects right ends of the top plate  811 B and the bottom plate  812 B with each other. The second component  81 B has, for example, a closed cross section shown in  FIG. 50  in the same manner as the embodiment. 
     The top plate  811 A of the first component  81 A and the top plate  811 B of the second component  81 B constitute the top plate  811  of the beam  81 , and the bottom plate  812 A of the first component  81 A and the bottom plate  812 B of the second component  81 B constitute the bottom plate  812  of the beam  81 . The one side plate  813 A of the first component  81 A and the one side plate  813 B of the second component  81 B constitute one side plate  813  of the beam  81 , and the other side plate  814 A of the first component  81 A and the other side plate  814 B of the second component  81 B constitute the other side plate  814  of the beam  81 . 
     A distal end of the second component  81 B supports the leg  82 . The leg  82  is coupled to the distal end of the second component  81 B via a coupling member  87 A. 
     Each of the pair of side plates  813 B,  814 B of the second component SIB has an engaged portion at a position opposite to the distal end of the second component  81 B. The engaged portion is engageable with the engaging portion of the first component  81 A, the engaged portion includes a pair of upper through holes  88 B, a pair of lower through holes  89 B located below the upper through holes  88 B, and a pair of pins to be inserted in the through holes. One of the pins is inserted in the pair of upper through holes  88 B for fastening in advance. 
     As shown in  FIGS. 54 and 55 , the pin extending in the upper through holes  88 B of the second component  81 B is hooked by the hooks  88 A of the first component  81 A. Further, the other of the pins is inserted in the lower through holes  89 B of the second component  81 B and the lower through holes  89 A of the first component  81 A in a state where the through holes  89 B and the through holes  89 A face each other. Consequently, the second component  81 B can be attached to the first component  81 A. The second component  81 B is detachable from the first component  81 A in a reverse sequence of the above-described attachment work. 
     When the second component  81 B is detached from the first component  81 A, the first component  81 A is accommodated in the accommodation space provided in the frame main body  1 A of the crawler frame  1  while being rotated about the pin  86 A as shown by the broken line in  FIG. 15 . 
     The second component  81 B detached from the first component  81 A may serve as a component of the trans-lifter  70  provided at each of a front axle  2   b  and a rear axle  2   c  of the lower traveling body  11 . Specifically, a pair of proximal end components  70 A having the same configuration as that of the first component  81 A of the support member  80  is rotatably provided at the front axle  2   b , and another pair of proximal end components  70 A having the same configuration as that of the first component  81 A of the support member  80  is rotatably provided at the rear axle  2   c . The second component  81 B detached from the first component  81 A and attached to each of the proximal end components  70 A of the trans-lifter  70  can serve as a part of the trans-lifter  70 . 
     In the detailed example shown in  FIGS. 55 and 57 , the top plate  811 A of the first component  81 A and the top plate  811 B of the second component  81 B are horizontally juxtaposed and in contact with each other in a state where the second component  81 B is attached to the first component  81 A. This arrangement makes it possible to effectively cause a strain in the first component  81 A and the second component  81 B when a moment in a direction of causing the crane  10  to turn over in a first direction D 1  occurs and the leg of the support member  80  receives an upward reaction force caused by the moment from the ground. 
       FIG. 58  is a perspective view schematically showing a second modification of the sixth embodiment. In the second modification shown in  FIG. 58 , a beam  81  further includes a measurement support base  200  (a deformation member) to which a strain gauge (a strain detector) is attached. The measurement support base  200  is located at such a position as to detect a strain occurring in the beam  81  in a state where a distal end  14 B of a boom  14  is away in a first direction D 1  from a proximal end  14 A of the boom  14  in the forward and rearward directions. The measurement support base  200  is preferably arranged in this manner, and thus the location of the measurement support base  200  is not particularly limited. The details of the configuration of the measurement support base  200  is the same as those in the fifth modification of the second embodiment shown in  FIG. 38 , and thus the description therefor is omitted. 
     Seventh Embodiment 
     A crane  10  according to the seventh embodiment has a configuration equivalent to that according to the sixth embodiment described above with reference to  FIGS. 48 to 57 . The seventh embodiment differs from the sixth embodiment in that the physical quantity detector is constituted by a reaction force detector  93  (see  FIG. 6 ) in place of the strain detector  90  in the sixth embodiment. With this configuration in the seventh embodiment, the strain detector  90  shown in  FIGS. 48 to 57  may be excluded. 
     Hereinafter, the difference of the seventh embodiment from the sixth embodiment will be mainly described. 
     A lower traveling body  11  of the crane  10  according to the seventh embodiment shown in  FIG. 48  includes four support members  80  (four receiving members  80 ) in the same manner as the sixth embodiment. Each of the four support members  80  includes a first right support member  80  (a first right receiving member  80 ), a first left support member  80  (a first left receiving member  80 ), a second right support member  80  (a second right receiving member  80 ), and a second left support member  80  (a second left receiving member  80 ) in the same manner as the sixth embodiment. 
     Each of the four support members  80  has a connection part connected with the crawler frame  1  in a frame unit, and a contact part being in contact with the ground. The connection part of the support member  80  is constituted by a proximal end  8 A of a beam  81 , and the contact part of the support member  80  is constituted by a float  85  that is a lower end of a leg  92 . 
     In the seventh embodiment, a boom direction D 1  corresponds to a first direction D (the forward direction) shown in  FIG. 48  in the same manner as the sixth embodiment. A location of the float  85  (the contact part) in the seventh embodiment is the same as the location in the sixth embodiment. 
     [Physical Quantity Detector] 
     In the seventh embodiment, the physical quantity detector  93  serves as the reaction force detector  93  (see  FIG. 6 ) for detecting information necessary to safely raise and lower the boom  14  in the assembly work and the disassembly work of the crane  10 . Specifically, the physical quantity detector  90  detects a physical quantity which changes in accordance with a change in a reaction force received from the ground by the support member  80 . The physical quantity detector  93  is configured to detect a pressure corresponding to a moment in a direction of causing the crane  10  to turn over in one of the forward and rearward directions. In the seventh embodiment, the support member  80  is arranged so that the float  85  (the contact part) that is the lower end of the leg  82  is away from the proximal end  8 A (the connection part) of the beam  81  in the boom direction D 1  in the same manner as the sixth embodiment. 
     A hydraulic circuit included in the crane  10  according to the seventh embodiment is equivalent to that in the crane according to the third embodiment described with reference to  FIG. 39 , except that the lower traveling body  11  of the crane  10  according to the seventh embodiment includes the four support members  80  and each of the four support members  80  includes a hydraulic cylinder  86 . With this configuration in the seventh embodiment, the physical quantity detector  93  further includes a third pressure sensor and a fourth pressure sensor which are unillustrated, in addition to the first pressure sensor  91  and the second pressure sensor  92  shown in  FIG. 6 . 
     A signal representing the pressure detected by the physical quantity detector  93  is input to the controller  100  shown in  FIG. 2 . 
     In the seventh embodiment, a way of calculating the reaction force is the same as the way described in the third embodiment. 
     Besides, in the seventh embodiment, the controller  100  stores in advance a maximal value RFmax (a maximally permissible reaction force) of the reaction force RFt received from the ground by the floats  85  (the contact parts) of the pair of support members  80  in the same manner as the third embodiment. The calculation section  101  shown in  FIG. 2  calculates the supportive reaction force RFt based on the pressures detected by the physical quantity detector  93  using Formulas (5) and (7), or Formulas (6) and (7). 
     The stability determination section  102  compares the supportive reaction force RFt with the maximally permissible reaction force RFmax, and determines whether the crane  10  is in a stable state or an unstable state in the same manner as the third embodiment. 
     When the stability determination section  102  determines that the crane  10  is in the unstable state, the notification control section  103  outputs a notification instruction of notifying in the notification device  110  the operator of the stability information concerning the stability determined by the stability determination section  102 . 
     The operation control section  104  outputs an operation instruction directing an operation required to avoid a decrease in the stability of the crane  10  based on the stability determined by the stability determination section  102  in the same manner as the third embodiment. 
     Other Modifications 
     Although the mobile crane  10  according to each of the embodiments are described heretofore, the present invention should not be limited to the described embodiments. 
     (A) For instance, although the physical quantity detector  90  includes the plurality of strain gauges in the first, the second, the fourth, and the sixth embodiments, the physical quantity detector  90  may include a single strain gauge. 
     (B) Although the tiltable attachment includes the jib  17  in the embodiments, the mobile crane may be adaptable to a crane including no jib. 
     (C) The boom direction corresponds to the forward direction in the first, and the fourth to the seventh embodiments, but should not be limited thereto, and may correspond to the rearward direction. Similarly, the boom direction corresponds to the rightward direction in the second and the third embodiments, but should not be limited thereto, and may correspond to the leftward direction. 
     (D) Although the physical quantity detector  90  in the first embodiment is arranged at one of the opposite ends of the crawler frame  1  where the tumbler bracket  1 B is located, the physical quantity detector  90  may be arranged at the other end (to which the idler  4   c  is attached) opposite to the one end. Furthermore, the physical quantity detector  90  may be arranged at each of the opposite ends of the crawler frame  1 . The physical quantity detector  90  arranged at each of the opposite ends of the crawler frame  1  in this manner can detect a strain caused in the crawler frame by the turning-over moment in either of the forward and rearward directions corresponding to the raising and lowering directions of the tiltable attachment. 
     (E) In the fourth to the seventh embodiments, the crawler frames  1  in the pair are provided with their respective physical quantity detectors  90 . However, only one of the crawler frames  1  may be provided with a physical quantity detector  90 . 
     (F) Although the crane  10  according to each of the first to the seventh embodiments is aimed at detecting information necessary to safely raise and lower the tiltable attachment in the assembly work and the disassembly work of the crane  10 , the crane  10  may be further adoptable for detecting information necessary to safely operate the crane  10  in other works as well as the assembly work and the disassembly work. Such other works include, for example, a work for an overload test related to the mobile crane. The overload test is a test of confirming a hoisting work of hoisting a predetermined hanged load to apply a load exceeding a rated load to the mobile crane while the moment limiter is stopped or a limit of the moment limiter is released without stopping the moment limiter. The operation control section  104  outputs an operation instruction directing an operation required to avoid a decrease in the stability of the crane  10  based on the stability determined by the stability determination section  102  in the work for the overload test. Specifically, in the overload test, a main winch  34  or an auxiliary winch  36  may be configured to suspend or decelerate an operation of lifting the hanged load in accordance with an operation instruction particularly while the hanged load is hoisted and lifted away from the ground. 
     The other works further include an excavation work of excavating the ground by powerfully lowering an unillustrated leading end attachment such as a bucket attached to a leading end of the main rope  50  shown in  FIG. 1  to the ground from a higher position than the ground. Additionally, the other works include an exemplary typical hoisting work of moving the hanged road with use of the crane  10 . As a result, the safety during the hoisting work is improved. 
     (G) Location of Physical Quantity Detector 
     In the crane, the physical quantity detector is sufficiently configured to detect a strain occurring in the beam of the support member, and hence the location of the physical quantity detector is not limited to those described in the embodiments. 
     (H) Number of Support Members 
     Although the crane preferably includes two or more support members to make the posture of the crane stable when a moment in a direction of causing the crane to turn over in one of the leftward and rightward directions occurs, the number of support members should not be limited to those described in the embodiments. 
     (I) Number of Physical Quantity Detectors 
     The physical quantity detector in the crane is sufficiently configured to detect a strain occurring in the beam of at least one of the support members. Thus, the number of physical quantity detectors to be provided is not limited to those described in the embodiments. In other words, the support members  80  may be provided with their respective physical quantity detectors  90 , or a part of the support members  80  may be provided with a corresponding physical quantity detector  90 . 
     (J) Number of Devices Constituting Physical Quantity Detector 
     Each of the physical quantity detectors  90  includes the first strain gauge  90 A (the first device) and the second strain gauge  90 B (the second device) in the embodiments, but should not be limited thereto. The physical quantity detector  90  may include a single device (e.g., a single strain gauge), and may include three or more devices (e.g., three or more strain gauges). 
     (K) Location of Each of Support Members 
     In the crane, the support members are preferably attached to one of the pair of crawler frames. Therefore, the location of each of the support members is not limited to those described in the embodiments. In  FIGS. 22, 36 and 37 , the plurality of support members  80  may be supported by the crawler frame  1  (the second crawler frame  1 ) of the second crawler  3  (the left crawler  3  in the drawings) of the pair of crawlers  3 . 
     (L) Calculation Section 
     The crane may not necessarily include the calculation section  101 . In this case, the crane may include, for example, a storage for storing in advance balance information (a left and right balance of the crane) corresponding to various detection signals output from the physical quantity detector. The notification control section  103  controls the notification device  110 , based on each of the detection signals output from the physical quantity detector, to notify the operator of the information corresponding to the detection signal concerning the left and right balance of the crane. 
     The calculation section in the mobile crane may calculate, based on the physical quantity detected by the physical quantity detector, a ratio between a reaction force which changes in accordance with a change in the physical quantity and the weight of the mobile crane. Consequently, the ratio resulting in a criterion of the stability of the mobile crane is obtainable. The calculated ratio is notified to the operator via the notification device, for example. 
     (M) Configuration of Leg of Support Member 
     The leg of the support member in the crane is sufficiently configured to receive a reaction fore caused by the turning-over moment and permit a strain to occur in the beam. Hence, the leg may not include a hydraulic cylinder having a cylinder main body  83  and a rod  84  unlike the embodiments, and may include another member instead. 
     (N) The physical quantity detector in the first embodiment may include a pressure sensor for detecting a pressure as the physical quantity in place of the strain detector for detecting a strain as the physical quantity. 
     (O) Location of Counterforce Receiving Part 
     The location of the contact part (the float  85 ) serving as the reaction force receiving part in the fourth to the seventh embodiments is preferably away from the rotational axis CB of the first wheel  4   a  in the boom direction D 1 . If the location of the contact part is away from the rotational axis CB of the first wheel  4   a  in the opposite direction (the second direction D 2 ) to the boom direction D 1 , the first wheel  4   a  would undertake a large proportion of the weight of the crane  10 . This would lead to a decrease in the reaction force received from the ground by the contact part, and a reduction in the detection sensitivity of the physical quantity by the physical quantity detector. Moreover, the location of the contact part serving as the reaction force receiving part is preferably visible by the operator. This is because, if the contact part is hidden by the upper slewing body  12 , the car body, the crawler  3  or the like and thus invisible by the operator, the operator may find it difficult to adjust a position of the contact part so that the contact part comes into contact with the ground. 
     (P) The physical quantity detector in the third, the fifth, and the seventh embodiments may include, for example, a loadcell in place of the pressure sensor. The loadcell can detect a physical quantity which changes in accordance with a change in a reaction force received from the ground by the reaction force receiving part. 
     Conclusively, provided is a mobile crane which can detect information necessary to safely raise and lower the tiltable attachment without an overburdened input operation by an operator. 
     The mobile crane includes: a lower traveling body which includes a pair of crawlers each extending in forward and rearward directions and spaced apart from each other in leftward and rightward directions; an upper slewing body supported on the lower traveling body slewably about a slewing axis; a tiltable attachment including a boom tiltably supported on the upper slewing body; and a physical quantity detector, wherein the lower traveling body has a reaction force receiving part for receiving a reaction force from the ground at a position away from the slewing axis in a boom direction in a state where the pair of crawlers is in contact with the ground, the boom direction coinciding with a horizontal component of a direction in which the boom extends from the upper slewing body, and the physical quantity detector is configured to detect a physical quantity which changes in accordance with a change in the reaction force received from the ground by the reaction force receiving part. 
     The mobile crane is attained from the viewpoint of an increase and a decrease in the reaction force received from the ground by the mobile crane in accordance with an increase and a decrease in the moment in a direction of causing the mobile crane to turn over. Specifically, the mobile crane can achieve safe raising operation and lowering operation without an overburdened input operation by the operator owing to the detection of the physical quantity which changes in accordance with the change in the reaction force. Details will be described below. 
     In a specific work accompanied by an occurrence of a large moment in a direction of causing the mobile crane to turn over such as the assembly work, the disassembly work, and the work for the overload test, the moment in the direction of causing the mobile crane to turn over increases as the angle of the boom to the ground decreases. A downward load that the mobile crane applies to the ground increases and an upward reaction force that the mobile crane receives from the ground increases in accordance with the increase in the moment. Here, the reaction force that the mobile crane receives from the ground is not equally distributed over the entirety of the lower surface of the lower traveling body, but distributed biasedly in the boom direction. Accordingly, the reaction force receiving part in the mobile crane is configured to receive the reaction force from the ground at a position away from the slewing axis in the boom direction in the state where the pair of crawlers is in contact with the ground in the specific work. The reaction force receiving part having this configuration can receive a large reaction force from the ground, thereby enhancing the detection accuracy of the physical quantity. The reaction force received from the ground by the reaction force receiving part increases in accordance with the increase in the moment. The reaction force thus can result in a criterion for determining (presuming) whether the mobile crane is in a stable state where the mobile crane is stably balanced or in an unstable state where the mobile crane is unbalanced and is likely to turn over. Accordingly, there is no need of information concerning a combination of the boom length and the jib length for the determination. In this way, the mobile crane can detect the information necessary to safely raise and lower the tiltable attachment in the specific work without an overburdened input operation by the operator. The mobile crane then utilizes the detected information for the safe raising and lowering operations. 
     In the mobile crane, it is preferable that the boom direction corresponds to one of the forward and rearward directions of the lower traveling body, that each of the pair of crawlers includes: a crawler frame extending in the forward and rearward directions; and a first wheel supported on one of a front end and a rear end of the crawler frame and rotatable about a rotational axis, the one end being away from the slewing axis in the boom direction, and that the first wheel serves as the reaction force receiving part. 
     In this aspect, the physical quantity detector can detect the physical quantity which changes in accordance with a change in the reaction force received by the first wheel serving as the reaction force receiving part. In other words, no additional member is required to serve as the reaction force receiving part in this aspect. Besides, the first wheel supported on the one end of the crawler frame that is away from the slewing axis in the boom direction can receive an extremely large reaction force from the ground. Accordingly, the detection accuracy of the physical quantity is further enhanced. 
     In the mobile crane, it is preferable that each of the pair of crawlers includes: a second wheel supported on the other of the front end and the rear end of the crawler frame and rotatable about a rotational axis; and a crawling member endlessly supported by the first wheel and the second wheel and cyclically movable, and that the physical quantity detector is configured to detect, as the physical quantity, a strain that is caused in the crawler frame of at least one of the pair of crawlers by the reaction force which the first wheel receives from the ground via the crawling member. 
     In this aspect, the strain to be detected by the physical quantity detector is the one actually caused in the crawler frame as a result of the raising operation or the lowering operation for the tiltable attachment in the specific work. The strain increases and decreases in correlation with an increase and a decrease in the moment. That is to say, the strain increases as the reaction force increases, and decreases as the reaction force decreases in the specific work. The strain results in a criterion for determining the situation of the mobile crane whether the crane is in the stable state or in the unstable state. 
     In the mobile crane, each of the pair of crawlers may further include a plurality of lower rollers rotatably supported on a lower portion of the crawler frame and arranged at intervals between the first wheel and the second wheel in the forward and rearward directions for guiding the crawling member, and the physical quantity detector is configured to detect the strain on a specific portion of the crawler frame in the forward and rearward directions, the specific portion being away in the boom direction from a rotational axis of a lower roller closest to the first wheel among the plurality of lower rollers. 
     In this aspect, the strain is accurately detected. Specifically, the strain caused in the crawler frame by the gravity acting on the tiltable attachment lowered in the boom direction particularly increases at a position closer to the first wheel serving as the reaction force receiving part in the specific work. In this aspect, the portion of the crawler frame where the strain is to be detected in the forward and rearward directions is away in the boom direction from the rotational axis of a lower roller (a first lower roller) closest to the first wheel. Accordingly, the strain occurring in the crawler frame can be accurately detected. 
     In the mobile crane, it is preferable that each of the pair of crawlers further includes a plurality of lower rollers rotatably supported on a lower portion of the crawler frame and arranged at intervals between the first wheel and the second wheel in the forward and rearward directions for guiding the crawling member, and that the physical quantity detector is configured to detect the strain on a specific portion of the crawler frame in the forward and rearward directions, the specific portion being located in a region between a rotational axis of a lower roller (the first lower roller) closest to the first wheel among the plurality of lower rollers and the rotational axis of the first wheel. 
     In this aspect, the strain is further accurately detected. As described above, the strain particularly increases at a position closer to the first wheel serving as the reaction force receiving part. Specifically, the strain notably occurs in the region between the rotational axis of the first lower roller and the rotational axis of the first wheel. In this aspect, the portion of the crawler frame where the strain is to be detected in the forward and rearward directions is in the aforementioned region. Accordingly, the strain occurring in the crawler frame can be further accurately detected. 
     In the mobile crane, the physical quantity detector may be configured to detect the strain occurring in the one end of the crawler frame that supports the first wheel. 
     In this aspect, the strain is accurately detected. As described above, the strain particularly increases at a position closer to the first wheel serving as the reaction force receiving part. In this aspect, the aforementioned configuration adapted to detect a strain occurring in the one end that supports the first wheel can achieve the accurate detection of the strain. 
     More specifically, in the mobile crane, the crawler frame may include a frame main body extending in the forward and rearward directions, and a bracket attached to an end of the frame main body to thereby constitute the one end of the crawler frame, and the physical quantity detector may be configured to detect the strain occurring in the bracket. 
     In this aspect, the strain occurring in the bracket can be accurately detected. 
     In the mobile crane, it is preferable that each of the pair of crawlers includes a crawler frame extending in the forward and rearward directions, that the lower traveling body includes: a center frame lying between the crawler frame of one of the pair of crawlers and the crawler frame of the other of the pair of crawlers, and connecting the crawler frames with each other; and at least one receiving member, that the crawler frame of the one crawler, the crawler frame of the other crawler, and the center frame constitute a frame unit, that the at least one receiving member has a connection part connected with the frame unit, and a contact part being in contact with the ground, that the connection part is away from the slewing central axis in the boom direction, and that the contact part is away from the connection part in the boom direction and serves as the reaction force receiving part. 
     In this aspect, the connection part is away from the slewing axis in the boom direction, and the contact part is away from the connection part in the boom direction. With this configuration, the contact part serving as the reaction force receiving part can receive a large reaction force from the ground. Therefore, the detection accuracy of the physical quantity is enhanced. 
     In the mobile crane, the at least one receiving member preferably includes a hydraulic cylinder extendable and retractable in upward and downward directions. 
     In this aspect, the relative position of the contact part to the ground is adjustable by extending and retracting the hydraulic cylinder in the upward and downward directions so that the contact part comes into contact with the ground. 
     The mobile crane preferably further includes: a hydraulic pump for discharging hydraulic fluid; a control valve disposed between the hydraulic pump and the hydraulic cylinder, and shiftable between a supply position for supplying the hydraulic fluid discharged by the hydraulic pump to the hydraulic cylinder through a hydraulic path and a suspension position for suspending the supply of the hydraulic fluid discharged from the hydraulic pump to the hydraulic cylinder; and an instruction device for instructing the control valve to shift between the supply position and the suspension position. 
     In this aspect, the hydraulic cylinder can be set at the supply position in an extendable and retractable manner by activating the control valve in response to an instruction from the instruction device. In this way, the length of the receiving member in the upward and downward directions is appropriately adjustable so that the contact part of the receiving member comes into contact with the ground prior to the specific work. For the specific work, the hydraulic cylinder is set at the suspension position by activating the control valve in response to an instruction from the instruction device. This inhibits the hydraulic cylinder from extending and retracting. As a result, the contact part serving as the reaction force receiving part can receive the reaction force correlating with the increase and the decrease in the moment while being in contact with the ground in the specific work. 
     In the mobile crane, the physical quantity detector may include a pressure sensor for detecting, as the physical quantity, at least one of a pressure on a head side of the hydraulic cylinder and a pressure on a rod side of the hydraulic cylinder. 
     In this aspect, the pressure detected by the pressure sensor increases and decreases in correlation with a decrease and an increase in the moment. That is to say, the pressure increases as the reaction force increases, and decreases as the reaction force decreases in the specific work. The pressure results in a criterion for determining the situation of the mobile crane whether the crane is in the stable state or in the unstable state. 
     In the mobile crane, the at least one receiving member may include a beam including the connection part and extending from the connection part in the boom direction or in a direction oblique to the boom direction, and a leg including the contact part and attached to a distal end of the beam. 
     In this aspect, the distance from the connection part to the contact part can be set suitably depending on the beam length. Further, in this aspect, the contact part of the leg connected with the distal end of the beam can receive a large reaction force from the ground. Accordingly, the detection accuracy of the physical quantity is further enhanced. 
     In the mobile crane, the physical quantity detector may be configured to detect a strain occurring in the beam as the physical quantity. 
     In this aspect, the strain detected by the physical quantity detector increases and decreases in correlation with an increase and a decrease in the moment. That is to say, the strain occurring in the beam increases as the reaction force increases, and decreases as the reaction force decreases in the specific work. The strain results in a criterion for determining the situation of the mobile crane whether the crane is in the stable state or in the unstable state. Specifically, a moment in a direction of causing the mobile crane to turn over is caused by the gravity acting on the tiltable attachment lowered in the boom direction, and the leg of the receiving member receives an upward reaction force caused by the moment from the ground in the specific work. This causes a bending moment (a bending stress) in the beam that supports the leg, the bending moment resulting in a strain occurring in the beam. From these perspectives, the strain represents the physical quantity which changes in accordance with a change in the reaction force, and correlates with the moment in the direction of causing the mobile crane to turn over. 
     In the mobile crane, the physical quantity detector preferably includes a first device for detecting a strain occurring in an upper portion of the beam, and a second device for detecting a strain occurring in a lower portion of the beam. 
     In this aspect, the strain occurring in the boom can be sensitively detected. Details will be described below. A moment in a direction of causing the mobile crane to turn over is caused by the gravity acting on the tiltable attachment lowered in the boom direction, and the beam receives a bending load caused by the moment in the upward and downward directions in the specific work. In this case, a larger strain is likely to occur in each of the upper portion and the lower portion of the beam to which the distance from neutral plane of the beam is large. In this aspect, a strain occurring in the beam is sensitively detectable owing to the first device which can detect a strain occurring in the upper portion of the beam and the second device which can detect a strain occurring in the lower portion of the beam. 
     In the mobile crane, the boom direction may correspond to one of the leftward and rightward directions of the lower traveling body, the at least one receiving member may include: a first receiving member attached to the crawler frame of the one crawler; and a second receiving member attached to the crawler frame of the one crawler at a distance from the first receiving member in the forward and rearward directions, and each of the first receiving member and the second receiving member may be arranged so that the contact part is away from the connection part in the boom direction. 
     In this aspect, in the case where the boom direction corresponds to the one of the leftward and rightward directions (i.e., the leftward direction or the rightward direction) in the specific work, the contact part serving as the reaction force receiving part is away from the connection part in the boom direction, and thus can receive a large reaction force from the ground. Accordingly, the detection accuracy of the physical quantity is enhanced. Moreover, in the aspect, the first receiving member and the second receiving member are attached to the crawler frame at a distance therebetween in the forward and rearward directions. This arrangement can make the posture of the mobile crane more stable than a configuration where only a single receiving member is attached to a crawler frame when the moment in the direction of causing the mobile crane to turn over in the one of the leftward and rightward directions occurs. 
     In the mobile crane, the boom direction may correspond to one of the forward and rearward directions of the lower traveling body, the at least one receiving member may include: a first receiving member attached to the center frame; and a second receiving member attached to the center frame at a distance from the first receiving member in the leftward and rightward directions, and each of the first receiving member and the second receiving member may be arranged so that the contact part is away from the connection part in the boom direction. 
     In this aspect, in the case where the boom direction corresponds to the one of the forward and rearward directions (i.e., the forward direction or the rearward direction) in the specific work, the contact part serving as the reaction force receiving part is away from the connection part in the boom direction, and thus can receive a large reaction force from the ground. Accordingly, the detection accuracy of the physical quantity is enhanced. Moreover, in the aspect, the first receiving member and the second receiving member are attached to the center frame at a distance therebetween in the leftward and rightward directions. This configuration can make the posture of the mobile crane more stable than a configuration where only a single receiving member is attached to the center frame when the moment in the direction of causing the mobile crane to turn over in the one of the forward and rearward directions occurs. 
     In the mobile crane, it is preferable that each of the pair of the crawlers includes a wheel supported on one of a front end and a rear end of the crawler frame rotatably about a rotational axis, the one end being away from the slewing central axis in the boom direction, and that each of the first receiving member and the second receiving member is arranged so that the center of the contact part is away in the boom direction from the rotational axis of the wheel of each of the pair of crawlers. 
     In this aspect, the center of the contact part of each of the receiving members is away from the rotational axis of the wheel in the boom direction. Hence, a large proportion of the weight of the mobile crane acts on the ground via the receiving member. As a result, the contact part of the receiving member can receive a large reaction force. 
     In the mobile crane, the boom direction may correspond to one of the forward and rearward directions of the lower traveling body, the at least one receiving member may include: a first right receiving member and a first left receiving member each attached to a first crawler frame that is the crawler frame of the one crawler, and a second right receiving member and a second left receiving member each attached to a second crawler frame that is the crawler frame of the other crawler, the first right receiving member may be arranged so that the contact part of the first right receiving member is away rightward from the first crawler frame, and is away from the connection part of the first right receiving member in the boom direction, the first left receiving member may be arranged so that the contact part of the first left receiving member is away leftward from the first crawler frame, and is away from the connection part of the first left receiving member in the boom direction, the second right receiving member may be arranged so that the contact part of the second right receiving member is away rightward from the second crawler frame, and is away from the connection part of the second right receiving member in the boom direction, and the second left receiving member may be arranged so that the contact part of the second left receiving member is away leftward from the second crawler frame, and is away from the connection part of the second left receiving member in the boom direction. 
     In this aspect, in the case where the boom direction corresponds to the one of the forward and rearward directions (i.e., the forward direction or the rearward direction) in the specific work, the contact part of each of the first right reaction force receiving member, the first left reaction force receiving member, the second right reaction force receiving member, and the second left reaction force receiving member is away from the corresponding connection part in the boom direction, and thus can receive a large reaction force from the ground. Accordingly, the detection accuracy of the physical quantity is enhanced. Further, in this aspect, the right reaction force receiving member and the left reaction force receiving member are attached to the crawler frame. The contact part of the right reaction force receiving member is on the right of the crawler frame. The contact part of the left reaction force receiving member is on the left of the crawler frame. In this arrangement, the left reaction force receiving member and the right reaction force receiving member can support the crawler frame on the left and right sides thereof while keeping a good balance upon occurrence of the moment (that is the moment in a direction of causing the mobile crane to turn over in the forward direction). If only one of the right reaction force receiving member and the left reaction force receiving member is attached to the crawler frame, the one reaction force receiving member cannot satisfactorily support the crawler frame while keeping the good balance in the leftward and rightward directions. In this case, a torsional moment is likely to occur in the crawler frame. In this aspect, however, an occurrence of such a torsional moment can be suppressed in the above-described manner. Accordingly, it is possible to reduce the influence of the torsional moment on a result of the detection by the physical quantity detector. This consequently achieves suppression of a reduction in the detection accuracy of the physical quantity. 
     In the mobile crane, it is preferable that each of the pair of crawlers includes a wheel supported on one of a front end and a rear end of the crawler frame rotatably about a rotational axis, the one end being away from the slewing central axis in the one of the forward and rearward directions, and that each of the first right receiving member, the first left receiving member, the second right receiving member, and the second left receiving member is arranged so that the center of the contact part is away from the rotational axis of the wheel of each of the pair of crawlers in the boom direction. 
     In this aspect, the center of the contact part of each of the receiving members is away from the rotational axis of the wheel in the boom direction. Hence, a large proportion of the weight of the mobile crane acts on the ground via the receiving member. As a result, the contact part of the receiving member can receive a large reaction force from the ground. 
     In the mobile crane, the receiving member may have a configuration to allow at least a part of the receiving member to be disengageably engaged with an engaging portion. 
     In this aspect, the at least a part of the receiving member is engageable with the engaging portion only when it is needed in the specific work of the mobile crane, and the at least a part of the receiving member is disengageable from the engaging portion when it is unneeded in, for example, the typical hoisting work performed on a work site. This prevents, in the typical hoisting work, the receiving member from impeding the typical hoisting work, and further can achieve reduction in the weight of the mobile crane by the weight of the receiving member. 
     In the mobile crane, the at least one receiving member may have a longitudinally extendable and retractable structure. 
     In this aspect, for the specific work, the receiving member is extended to increase the distance to the turning-over fulcrum. The resultant receiving member can more stably support the mobile crane, and the contact part of the receiving member can receive a larger reaction force from the ground in the specific work. In contrast, for the typical hoisting work, the receiving member is retracted to prevent the receiving member from impeding the hoisting work. Additionally, the receiving member in the retracted state can serve as a trans-lifter for lifting the lower traveling body from the ground. 
     The mobile crane may further include a parameter calculation part for calculating, based on the physical quantity detected by the physical quantity detector, a moment in a direction in which the weight of the tiltable attachment causes the mobile crane to turn over. 
     In this aspect, the parameter calculation part calculates the moment based on the physical quantity detected by the physical quantity detector, thereby obtaining the moment of causing the mobile crane to turn over. The calculated moment is notified to the operator via the notification device, for example. 
     The mobile crane may further include a notification device for notifying an operator of stability information concerning a stability of the mobile crane based on the physical quantity detected by the physical quantity detector. 
     In this aspect, the operator can acquire the information concerning the stability of the mobile crane via the notification device. Thus, the operator can maneuver the mobile crane by using the information as a criterion, thereby permitting the mobile crane to safely execute the raising and lowering operations. 
     The mobile crane may further include: a stability determination section for determining the stability based on the physical quantity detected by the physical quantity detector; and a notification control section for outputting a notification instruction of notifying in the notifying device the operator of the stability information concerning the stability determined by the stability determination section. 
     In this aspect, it is possible to provide the operator with the stability information necessary to safely raise and lower the tiltable attachment in the specific work. The operator having acquired the provided information may manipulate the manipulation lever of the mobile crane for an operation (a avoidance operation) required to avoid a decrease in the stability of the mobile crane. Alternatively, the controller in the mobile crane can automatically execute the avoidance operation to be described later in place of the operator who manipulates the manipulation lever. 
     Specifically, the mobile crane may further include an operation control section for outputting an operation instruction directing an operation required to avoid a decrease in the stability of the mobile crane based on the stability determined by the stability determination section. 
     In this aspect, such automatic execution of the avoidance operation in accordance with the operation instruction leads to reduction in the burden on the operator. 
     The mobile crane may further include a parameter calculation part for calculating a first parameter in connection with a first moment caused by the gravity acting on the tiltable attachment based on the physical quantity, wherein the upper slewing body may carry a counterweight at a position away from the slewing central axis in the opposite direction to the boom direction, and the stability determination section may be configured to determine the stability by comparing the first parameter calculated by the parameter calculation part with a second parameter in connection with a second moment caused to oppose to the first moment and prevent the mobile crane from turning over by the gravity acting on the counterweight. 
     In this aspect, the comparison between the first parameter and the second parameter can contribute to the comparison between the first moment of causing the mobile crane to turn over and the second moment of preventing the mobile crane from turning over. Accordingly, the stability of the mobile crane is appropriately determined. The first parameter may be the first moment itself, or may be other parameter which changes in accordance with a change in the first moment. Similarly, the second parameter may be the second moment itself, or may be other parameter which changes in accordance with a change in the second moment. 
     The mobile crane may further include: a ratio calculation part for calculating a ratio between the first parameter and the second parameter, wherein the stability determination section may be configured to determine the stability based on the ratio. 
     In this aspect, the stability determination section can determine the stability based on the ratio calculated by the ratio calculation part.