Patent Publication Number: US-11383770-B2

Title: Vehicle-body structure of vehicle

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to a vehicle-body structure of a vehicle that can withstand a collision from a side of the vehicle. 
     Background Art 
     In general, if a B pillar (center pillar) of a vehicle such as an automobile intrudes into a passenger compartment of the vehicle under a side collision load that acts on a side sill at a side of the vehicle due to another vehicle colliding with the side of the vehicle or the side of the vehicle colliding with an obstacle, an occupant will be injured. Thus, a cross member extending in a vehicle width direction of the vehicle has been used to receive the side collision load acting on the side sill and prevent the B pillar from intruding into the passenger compartment. 
     In vehicles such as front-engine rear-drive (FR) vehicles and four-wheel drive (4WD) vehicles, a floor tunnel extending in a front-rear direction and having a huge cross-section is provided to dispose a propeller shaft that transmits power from a drive source located frontward to drive wheels located rearward. It is important to prevent the side collision load acting via the cross member from concentrating on the floor tunnel with such a huge cross-section. 
     For example, Japanese Patent Laid-Open No. 2015-134520 discloses a structure in which a cross member abuts on an upper member extending in the front-rear direction between a side sill and a floor tunnel and also abuts on a connecting member provided at a side wall portion of the floor tunnel. 
     Japanese Patent Laid-Open No. 2019-014354 discloses a lower vehicle-body structure in which, in order to increase both of the rigidity of a side collision load transmission path and the rigidity of a seat mounting portion, a seat mounting bracket mounted to a cross member is joined via a floor tunnel to a tunnel reinforcement provided at an underside of the floor tunnel. 
     However, the vehicle-body structure disclosed in Japanese Patent Laid-Open No. 2019-014354 has the following problem. When the vehicle includes a floor tunnel with a huge cross-section like the aforementioned FR vehicle, the tunnel reinforcement itself becomes large, and thus the side collision load, which is transmitted from the cross member to the tunnel reinforcement via the seat mounting bracket and the floor tunnel, cannot be received by the tunnel reinforcement alone. This renders the floor tunnel prone to deformation. 
     SUMMARY 
     The present disclosure has been made in view of the above conventional problem and aims to provide a vehicle-body structure of a vehicle that can prevent deformation of the floor tunnel caused by the side collision load transmitted from the cross member to the floor tunnel via the seat mounting bracket. 
     Accordingly, the present disclosure is a vehicle-body structure of a vehicle, the vehicle-body structure including a pair of left and right side sills extending in a front-rear direction of a vehicle; floor panels extending from vehicle-width-direction inner side ends of the side sills toward a vehicle-width-direction inner side; a floor tunnel connecting vehicle-width-direction inner side ends of the floor panels and having a shape protruding upward; a pair of left and right cross members connecting the side sills and the floor tunnel; a seat mounting bracket mounted to both ends in a vehicle width direction of the cross member; a pair of left and right backbone frames facing a top and a side of the floor tunnel and fixed to the floor tunnel, the pair of left and right backbone frames extending in a vehicle front-rear direction; and a tunnel reinforcement having a front projection, an intermediate projection, and a rear projection abutting on an underside of the floor tunnel. The backbone frame includes a plurality of beads abutting on the floor tunnel, and the tunnel reinforcement is fixed at the front projection, the intermediate projection, and the rear projection to the seat mounting bracket via the floor tunnel and is also fixed to the plurality of beads of the backbone frame via the floor tunnel. 
     Ridges of the front projection, the intermediate projection, and the rear projection of the tunnel reinforcement and ridges of the beads of the backbone frame are in contact with each other. 
     The vehicle-body structure includes a first front abutting portion, a first intermediate abutting portion, and a first rear abutting portion abutting on the plurality of beads of the backbone frame respectively at the front projection, the intermediate projection, and the rear projection, and includes a second front abutting portion, a second intermediate abutting portion, and a second rear abutting portion abutting on the seat mounting bracket. 
     The vehicle-body structure includes a front recess and a rear recess protruding in a direction away from the underside of the floor tunnel respectively between the front projection and the intermediate projection and between the intermediate projection and the rear projection. 
     The tunnel reinforcement includes an extended portion rearward of the rear abutting portion, and the extended portion includes, at a rear end thereof, a rear end projection fixed to an additional bead of the backbone frame. 
     The vehicle-body structure further includes a rear end recess protruding in a direction away from the underside of the floor tunnel between the rear projection and the rear end projection. 
     According to the disclosure, the tunnel reinforcement is fixed at the front projection, the intermediate projection, and the rear projection to the seat mounting bracket via the floor tunnel and is also fixed to the multiple beads of the backbone frame via the floor tunnel. This allows the side collision load input from the cross beam to the seat mounting bracket in the event of a side collision to be transmitted to the multiple beads of the backbone frame via the front projection, the intermediate projection, and the rear projection of the tunnel reinforcement, and this restrains deformation of the tunnel reinforcement as well as the floor tunnel. 
     According to the disclosure, ridges of the front projection, the intermediate projection, and the rear projection of the tunnel reinforcement are in contact with ridges of the beads of the backbone frame. This allows the side collision load to be surely transmitted to the multiple beads of the backbone frame via the front projection, the intermediate projection, and the rear projection of the tunnel reinforcement. 
     According to the disclosure, the vehicle-body structure includes a first front abutting portion, a first intermediate abutting portion, and a first rear abutting portion abutting on the multiple beads of the backbone frame respectively at the front projection, the intermediate projection, and the rear projection, and includes a second front abutting portion, a second intermediate abutting portion, and a second rear abutting portion abutting on the seat mounting bracket. This allows for easy and reliable joining of the tunnel reinforcement and the backbone frame and of the tunnel reinforcement and the seat mounting bracket via these abutting portions. 
     According to the disclosure, the vehicle-body structure includes a front recess and a rear recess protruding in a direction away from the underside of the floor tunnel respectively between the front projection and the intermediate projection and between the intermediate projection and the rear projection. This increases the rigidity of the tunnel reinforcement with these continuous projections and recesses. 
     According to the disclosure, the tunnel reinforcement includes an extended portion rearward of the rear abutting portion, and the extended portion includes, at a rear end thereof, a rear end projection fixed to an additional bead of the backbone frame. This increases the rigidity of the floor tunnel and surely restrains deformation of the floor tunnel even when the side collision load is input from the seat mounting bracket to a front portion of the tunnel reinforcement. 
     According to the disclosure, the vehicle-body structure includes a rear end recess protruding in a direction away from the underside of the floor tunnel between the rear projection and the rear end projection. This increases the rigidity of the tunnel reinforcement with these continuous projections and recess. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a lower vehicle-body of an automobile having a vehicle-body structure according to an embodiment of the present disclosure; 
         FIG. 2  is a perspective view of a first cross member and a second cross member of the lower vehicle-body as viewed from a vehicle front side toward a vehicle rear side; 
         FIG. 3  is a sectional view of the first cross member in a vehicle width direction; 
         FIG. 4  is an enlarged exploded perspective view showing components of the first cross member; 
         FIG. 5A  is a perspective view showing an example of a fragile portion at a portion where a first member of the first cross member is mounted to a side sill,  FIG. 5B  is a perspective view showing a modification of the fragile portion, and  FIG. 5C  is a perspective view showing another modification of the fragile portion; 
         FIGS. 6A-6C  are plan views showing a procedure for mounting the first cross member; 
         FIG. 7  is a sectional view of the second cross member in the vehicle width direction; 
         FIG. 8  is an enlarged perspective view of a joining portion of a tunnel-side seat mounting bracket of the second cross member and a floor tunnel; 
         FIG. 9  is an enlarged exploded perspective view showing how the tunnel-side seat mounting bracket of the second cross member is connected to the floor tunnel, a tunnel reinforcement, and a backbone frame; 
         FIG. 10A  is a sectional view along the line Xa-Xa in  FIG. 9 , and  FIG. 10B  is a sectional view along the line Xb-Xb in  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present disclosure will be described below with reference to the accompanying drawings. Throughout the drawings, reference characters Fr and Rr respectively refer to a front side (or frontward) and a rear side (or rearward) of an automobile, and reference characters OUT and IN respectively refer to an outside (or outward) and an inside (or inward) of the automobile in the vehicle width direction. 
     &lt;Overall Structure of Lower Vehicle-Body&gt; 
       FIG. 1  shows a lower vehicle-body  1  of the automobile having a vehicle-body structure of a vehicle according to an embodiment of the present disclosure. The lower vehicle-body  1  includes a pair of left and right side sills  2  extending in a vehicle front-rear direction, a floor tunnel  3  disposed at a center in the vehicle width direction between the pair of left and right side sills  2  and extending in the vehicle front-rear direction, and floor panels  4  extending from vehicle-width-direction inner side ends of the pair of left and right side sills  2  toward a vehicle-width-direction inner side up to the floor tunnel  3 . 
     A pair of left and right floor frames  5  each extending in the vehicle front-rear direction between a corresponding one of the pair of left and right side sills  2  and the floor tunnel  3  are disposed in the lower vehicle-body  1  each as a reinforcement member. A first cross member  6  and a second cross member  7  laid between each one of the pair of left and right side sills  2  and the floor tunnel  3  are disposed in this order toward the vehicle rear side. 
     An oblique frame  8  extending obliquely rearward from each one of the pair of left and right side sills  2  to a corresponding one of the pair of left and right floor frames  5  is disposed frontward of the first cross member  6 . A third cross member  9  laid in the vehicle width direction between each one of the pair of left and right side sills  2  and the floor tunnel  3  is disposed rearward of the second cross member  7 . 
     Meanwhile, the lower vehicle-body  1  includes, in its front portion, a dash panel  10   a , a dash panel cowl  10   b , a pair of left and right front side frames  11 , a bumper reinforcement  12 , a front cross member  13 , apron members  14 , a suspension housing  15 , and the like. The lower vehicle-body  1  includes, in its rear portion, a pair of left and right rear side frames  16  extending from respective rear ends of the pair of left and right side sills  2  to the vehicle rear side, a rear floor panel  17  extending in the vehicle width direction between the pair of left and right rear side frames  16 , and a rear cross member  18 . Detailed description of the oblique frame  8 , the third cross member  9 , and the front and rear components of the lower vehicle-body  1  has been omitted as they do not relate directly to the present disclosure. 
     &lt;Side Sill&gt; 
     As shown in  FIG. 3 , each of the pair of left and right side sills  2  is composed of a side sill inner panel  20  with a cross-section of a hat-shaped profile opening toward the vehicle-width-direction outer side and a side sill outer panel  21  with a cross-section of a hat-shaped profile opening toward the vehicle-width-direction inner side. 
     The side sill inner panel  20  includes a top face  22 , a side face  23  extending downward from a vehicle-width-direction inner side end of the top face  22 , a bottom face  24  extending from a lower end of the side face  23  toward the vehicle-width-direction outer side, an upper ridge portion  25  between the top face  22  and the side face  23 , and a lower ridge portion  26  between the bottom face  24  and the side face  23 . A reinforcement plate  27  is fixed inside the side sill inner panel  20  at multiple positions in the vehicle front-rear direction, particularly positions overlapping the first cross member  6  and the second cross member  7  in the vehicle front-rear direction. 
     The side sill outer panel  21  has a shape similar to that of the side sill inner panel  20  and facing the side sill inner panel  20 . The side sill inner panel  20  and the side sill outer panel  21  are joined so as to form a substantially rectangular closed cross-section in the vehicle width direction. Lower portions of a hinge pillar supporting a front door and a center pillar supporting a rear door are joined to the side sill  2 , though not shown in the figures. 
     &lt;Floor Tunnel&gt; 
     As shown in  FIG. 1 , the floor tunnel  3  extends in the vehicle front-rear direction from the dash panel  10   a  to the third cross member  9 . As shown in  FIG. 2 , the floor tunnel  3  has a hat-shaped profile opening downward and protruding upward in its cross-section in the vehicle width direction. Specifically, as shown in  FIG. 3 , the floor tunnel  3  is composed of a flange portion  30  joined to the floor panel  4 , a wall rising portion  31  rising upward from the flange portion  30 , a raised portion  32  extending from the wall rising portion  31  toward the vehicle-width-direction inner side and positioned one step higher than the flange portion  30 , an inclined wall portion  33  obliquely rising from the raised portion  32  toward the center in the vehicle width direction, and a top plate portion  34 . 
     A pair of left and right backbone frames  35  is mounted on a top side of the floor tunnel  3 . Each of the pair of left and right backbone frames  35  has an inverted L-shaped cross-section in the vehicle width direction so as to form a closed cross-section with the top plate portion  34  and the inclined wall portion  33  of the floor tunnel  3  which the backbone frame  35  faces. An upper flange portion  36  and a lower flange portion  37  are provided at respective ends in the vehicle width direction of the backbone frame  35 . The upper flange portion  36  and the lower flange portion  37  are respectively joined to the top plate portion  34  and the inclined wall portion  33  of the floor tunnel  3 . In particular, as shown in  FIG. 9 , the lower flange portion  37  of the backbone frame  35  is formed with multiple beads  38  (linearly extending recesses or projections formed by pressing a material) extending in the vehicle width direction at its portions joined to the floor tunnel  3  at positions thereof overlapping the second cross member  7  in the vehicle front-rear direction. The multiple beads  38  are joined to a tunnel reinforcement  100  (described later) through the floor tunnel  3 . 
     A mount member  39   a  for supporting a rear portion of a transmission (not shown) is mounted to an underside of the raised portion  32  on both sides of the floor tunnel  3  in the vehicle width direction, and this restrains the decrease in vehicle-body rigidity that would otherwise occur due to the first cross members  6  on both sides being separated by the floor tunnel  3 . 
     &lt;Floor Panel&gt; 
     Returning to  FIG. 3 , the floor panel  4  extends from a vehicle-width-direction inner side end of the side sill  2  toward the vehicle-width-direction inner side. In other words, a vehicle-width-direction outer side end of the floor panel  4  is joined to the side face  23  of the side sill inner panel  20 , and a vehicle-width-direction inner side end of the floor panel  4  is joined to the flange portion  30  on both sides of the floor tunnel  3  in the vehicle width direction. Also, the floor panel  4  extends in the vehicle front-rear direction from the dash panel  10   a  to the third cross member  9  in  FIG. 1 . 
     The floor panel  4  includes a step portion  40  provided at an approximate center between the side sill  2  and the floor tunnel  3  and extending in the vehicle front-rear direction, a first floor panel  41  on the vehicle-width-direction inner side of the step portion  40 , and a second floor panel  42  on the vehicle-width-direction outer side of the step portion  40 . The step portion  40  is provided with a floor frame  5  described later. The second floor panel  42  is formed at a position lower than the first floor panel  41 . In other words, the second floor panel  42  located between a floor upper frame of the floor frame  43  and the side sill is formed at a position lower than the first floor panel located between the floor upper frame and the floor tunnel  3 . This allows a rectifier (not shown) to be only provided below the first floor panel  41  and eliminates the need for providing the rectifier below the second floor panel  42 , helping reduce the area of the rectifier. 
     Each of the pair of left and right floor frames  5  is provided such that the step portion  40  of the floor panel  4  is interposed in the floor frame  5  in an up-down direction, and extends in the vehicle front-rear direction. The floor frame  5  is composed of a floor upper frame  50  forming a closed cross-section jointly with a top side of the step portion  40  of the floor panel  4 , and a floor lower frame  51  forming a closed cross-section jointly with an underside of the step portion  40  of the floor panel  4 . 
     The floor upper frame  50  includes a frame top face  50   a  facing the first floor panel  41 , a frame side face  50   b  extending from a vehicle-width-direction inner side end of the frame top face  50   a  to the first floor panel  41 , and a frame side face  50   c  extending from a vehicle-width-direction outer side end of the frame top face  50   a  to the second floor panel  42 . The frame top face  50   a  of the floor upper frame  50  is provided at a position higher than a center position P between the upper ridge portion  25  and the lower ridge portion  26  of the side sill  2  in the up-down direction. A lower end of the frame side face  50   b  on the vehicle-width-direction inner side is joined to a top side of the first floor panel  41 , and a lower end of the frame side face  50   c  on the vehicle-width-direction outer side is joined to a top side of the second floor panel  42 . 
     The floor lower frame  51  includes a frame bottom face  51   a  facing the first floor panel  41 , a frame side face  51   b  extending from a vehicle-width-direction inner side end of the frame bottom face  51   a  to the first floor panel  41 , and a frame side face  51   c  extending from a vehicle-width-direction outer side end of the frame bottom face  51   a  to the second floor panel  42 . The frame bottom face  51   a  of the floor lower frame  51  is provided at substantially the same position as the second floor panel  42 . An upper end of the frame side face  51   b  on the vehicle-width-direction inner side is joined to an underside of the first floor panel  41 , and an upper end of the frame side face  51   c  on the vehicle-width-direction outer side is joined to an underside of the second floor panel  42 . 
     &lt;First Cross Member&gt; 
     The first cross member  6  is composed of a first member  61  on a side sill-side and a second member  62  on a floor tunnel-side. The first member  61  is mounted with a sill-side seat mounting bracket  60 , and the second member  62  is mounted with a tunnel-side seat mounting bracket  63 . 
     The first member  61  extends from the side sill  2  to the frame top face  50   a  of the floor upper frame  50 . As shown in  FIG. 4 , the first member  61  has a hat-shaped cross-section in the vehicle front-rear direction and includes a top face  64  facing the floor panel  4  and a front face  65  and a rear face  66  extending from respective ends in the vehicle front-rear direction of the top face  64  to the floor panel  4 . The front face  65  and the rear face  66  respectively includes a front lower portion  65 A and a rear lower portion  66 A at positions lower than the first floor panel  41 . 
     Flanges  65   a ,  66   a  are provided to respective vehicle-width-direction outer side ends of the front face  65  and the rear face  66  of the first member  61 , and the flanges  65   a ,  66   a  are joined to the side face  23  of the side sill  2 . Flanges  65   b ,  66   b  are provided to respective lower ends of the front face  65  and the rear face  66  of the first member  61 , and the flanges  65   b ,  66   b  are joined to the second floor panel  42 . Flanges  65   c ,  66   c ,  65   d ,  66   d  are provided to vehicle-width-direction inner side ends of the front face  65  and the rear face  66  of the first member  61 , and the flanges  65   c ,  66   c  are joined to the frame side face  50   c  of the floor upper frame  50  and the flanges  65   d ,  66   d  are joined to the frame top face  50   a  of the floor upper frame  50 . 
     The top face  64  of the first member  61  includes, at its vehicle-width-direction outer side end, a fragile portion  67  formed of a cutout. The fragile portion  67  functions to transmit a side collision load to the top face  64  of the first cross member  6  in the event of a side collision by cutting a part of the vehicle-width-direction outer side end of the top face  64  of the first member  61  as shown in  FIG. 5A  and thereby allowing the vehicle-width-direction outer side end of the top face  64  to be preferentially crushed under the side collision load. Besides the cutout, the fragile portion  67  may be a bead or a hole extending in the vehicle front-rear direction as shown in  FIGS. 5B and 5C ; in short, the fragile portion  67  is only required to weaken the rigidity in the vehicle width direction of the vehicle-width-direction outer side end of the top face. 
     A recessed bead  68  extending in the vehicle width direction is formed in each of the front face  65  and the rear face  66  of the first member  61 . Preferably, the beads  68  are provided above the second floor panel  42  and below the aforementioned center position P between the upper ridge portion  25  and the lower ridge portion  26  in the up-down direction of the side sill  2 . The beads  68  increase the rigidity of the front face  65  and the rear face  66  of the first member  61  against the side collision load. 
     The second member  62  extends from a vehicle-width-direction inner side end of the first member  61  to the floor tunnel  3 . As shown in  FIG. 3 , the second member  62  has a hat-shaped cross-section in the vehicle front-rear direction and includes a top face  69  facing the floor panel  4  and a front face  70  and a rear face  71  extending from respective ends in the vehicle front-rear direction of the top face  69  to the floor panel  4 . 
     A vehicle-width-direction outer side end of the top face  69  of the second member  62  is positioned below a vehicle-width-direction inner side end of the top face  64  of the first member  61  and, as shown in  FIG. 3 , is overlapped with and joined to the vehicle-width-direction inner side end of the top face  64  of the first member  61  to form a high-strength portion  72  that has a larger plate thickness than the front lower portion  65 A and the rear lower portion  66 A of the first member  61  and the front face  70  and the rear face  71  of the second member  62  and is given a high strength. Flanges  70   a ,  71   a  are provided to respective lower ends of the front face  70  and the rear face  71  of the second member  62 , and the flanges  70   a ,  71   a  are joined to the second floor panel  42 . The vehicle-width-direction outer side end of the top face  69  of the second member  62  extends to a joined portion of the first member  61  and the sill-side seat mounting bracket  60 , and this gives the high-strength portion  72  a higher strength than a high-strength portion  86  (described later) between the second member  62  and the tunnel-side seat mounting bracket  63 . 
     A recessed bead  73  extending in the vehicle width direction is formed in the top face  69  of the second member  62 . The bead  73  is provided so as to overlap beads  78 ,  80  (described later) of the sill-side seat mounting bracket  60  in the vehicle front-rear direction. 
     The sill-side seat mounting bracket  60  is a fixing member for fixing a seat rail of the vehicle and is also a reinforcement member for reinforcing a sill-side end of the top face  64  of the first member  61  of the first cross member  6 . The sill-side seat mounting bracket  60  includes a top face  74  facing the top face  64  of the first member  61  and a front face  75  and a rear face  76  extending from respective ends in the vehicle front-rear direction of the top face  74  to the floor panel  4 . 
     A seat rail mounting portion  77  protruding upward is formed at a center in the vehicle width direction of the top face  74  of the sill-side seat mounting bracket  60 . A recessed bead  78  extending in the vehicle width direction is formed at an end of the top face  74  on the outside of the seat rail mounting portion  77  of the sill-side seat mounting bracket  60 . A protruding piece  79  protruding to the vehicle-width-direction outer side is formed at a vehicle-width-direction outer side end of the top face  74  of the sill-side seat mounting bracket  60 , and the protruding piece  79  abuts on and is joined to the top face  22  of the side sill  2 . An upward projecting bead  80  extending in the vehicle width direction is formed at an inner end of the top face  74  of the sill-side seat mounting bracket  60 , and on both sides of the bead  80 , the sill-side seat mounting bracket  60  abuts on and is joined to the top face  64  of the first member  61 . 
     Multiple projecting beads  81  extending in the up-down direction are formed in each of the front face  75  and the rear face  76  of the sill-side seat mounting bracket  60 , and on both sides of each bead  81 , the front face  75  and the rear face  76  abut on and are joined to the front face  65  and the rear face  66 , respectively, of the first member  61 . Flanges  75   a ,  76   a  are formed on respective vehicle-width-direction outer side ends of the front face  75  and the rear face  76  of the sill-side seat mounting bracket  60 , and the flanges  75   a ,  76   a  abut on and are joined to the side face  23  of the side sill  2 . 
     The tunnel-side seat mounting bracket  63  is a fixing member for fixing the seat rail of the vehicle. The tunnel-side seat mounting bracket  63  extends from a vehicle-width-direction inner side end of the floor panel  4  to the inclined wall portion  33  of the floor tunnel  3 . A vehicle-width-direction outer side half of the tunnel-side seat mounting bracket  63  has a hat-shaped cross-section in the vehicle front-rear direction formed by a top face  82 , a front face  83 , and a rear face  84 . A seat rail mounting portion  85  protruding upward is formed on a top side of a vehicle-width-direction inner side half of the tunnel-side seat mounting bracket  63 . 
     A vehicle-width-direction outer side end of the tunnel-side seat mounting bracket  63  is positioned below a vehicle-width-direction inner side end of the top face  69  of the second member  62  and is overlapped with and joined to the vehicle-width-direction inner side end of the top face  69  of the second member  62  to form, as shown in  FIG. 3 , a high-strength portion  86  that has a larger plate thickness than the front lower portion  65 A and the rear lower portion  66 A of the first member  61  and the front face  70  and the rear face  71  of the second member  62  and is given a high strength. 
     A flange  82   a  is provided to a vehicle-width-direction inner side end of the tunnel-side seat mounting bracket  63 , and the flange  82   a  abuts on and is joined to the inclined wall portion  33  of the floor tunnel  3 . 
     Flanges  82   b ,  82   c  are provided to each end in the vehicle front-rear direction of the vehicle-width-direction inner side half of the tunnel-side seat mounting bracket  63 , and the flanges  82   b ,  82   c  abut on and are joined to the inclined wall portion  33  and the raised portion  32 , respectively, of the floor tunnel  3 . Flanges  83   a ,  84   a  are provided to the front face  83  and the rear face  84 , respectively, of the vehicle-width-direction outer side half of the tunnel-side seat mounting bracket  63 , and the flanges  83   a ,  84   a  are joined to a vehicle-width-direction inner side end of the first floor panel  41 . 
     In assembling the first cross member  6 , as shown in  FIGS. 6A-6C , the tunnel-side seat mounting bracket  63  is first mounted to the floor tunnel  3 , and then the second member  62  is mounted so as to overlap a vehicle-width-direction outer side end of the tunnel-side seat mounting bracket  63  ( FIG. 6A ). Then, a vehicle-width-direction inner side end of the first member  61  is overlapped with and mounted to a vehicle-width-direction outer side end of the second member  62 , and a vehicle-width-direction outer side end of the first member  61  is mounted to the side sill  2  ( FIG. 6B ). Finally, the sill-side seat mounting bracket  60  is overlapped with and mounted to the first member  61 , and a vehicle-width-direction outer side end of the sill-side seat mounting bracket  60  is mounted to the side sill  2 . ( FIG. 6C ). Two adjacent members or all members may be sub-assembled in advance. 
     &lt;Second Cross Member&gt; 
     Similarly to the first cross member  6 , the second cross member  7  is composed of the first member  61  on the side sill-side and the second member  62  on the floor tunnel-side as shown in  FIG. 7 , and the first member  61  is mounted with the sill-side seat mounting bracket  60  and the second member  62  is mounted with the tunnel-side seat mounting bracket  63 . 
     The second cross member  7  has a similar structure to that of the first cross member except that the second cross member  7  is positioned generally lower than the first cross member  6 , the tunnel-side seat mounting bracket  63  is composed of a lower bracket  87  and an upper bracket  88 , and a tunnel reinforcement  100  is provided inside the floor tunnel  3  to which the tunnel-side seat mounting bracket  63  is mounted. Accordingly, description of equivalent parts has been omitted and only differences will be described below. 
     Because the second cross member  7  is positioned generally lower than the first cross member  6 , a flange  74   a  protruding upward is provided to a vehicle-width-direction outer side end of a top face of the sill-side seat mounting bracket  60 , and the flange  74   a  abuts on and is joined to the side face  23  of the side sill  2 . 
     The tunnel-side seat mounting bracket  63  of the second cross member  7  is composed of the lower bracket  87  and the upper bracket  88 . 
     The lower bracket  87  extends from the raised portion  32  of the floor tunnel  3  to the vehicle-width-direction inner side end of the floor panel  4 . As shown in  FIG. 8 , a vehicle-width-direction outer side half of the lower bracket  87  has a hat-shaped cross-section in the vehicle front-rear direction formed by a top face  89 , a front face  90 , and a rear face  91 . 
     Via the raised portion  32  of the floor tunnel  3 , the lower bracket  87  is bolted to both ends of a tunnel member  39   b  that connects both ends in the vehicle width direction of the floor tunnel  3 , and this restrains the decrease in vehicle-body rigidity that would otherwise occur due to the second cross members  7  on both sides being separated by the floor tunnel  3 . 
     A vehicle-width-direction outer side end of the lower bracket  87  is positioned below a vehicle-width-direction inner side end of the top face  69  of the second member  62  and is overlapped with and joined to the vehicle-width-direction inner side end of the top face  69  of the second member  62  to form a high-strength portion  92  that has a larger plate thickness than the front lower portion  65 A and the rear lower portion  66 A of the first member  61  and the front face  70  and the rear face  71  of the second member  62  and is given a high strength. 
     Flanges  90   a ,  91   a  are formed at respective ends in the vehicle front-rear direction of the front face  90  and the rear face  91  in the vehicle-width-direction outer side half of the lower bracket  87 , and the flanges  90   a ,  91   a  abut on and are joined to the vehicle-width-direction inner side end of the floor panel  4 . An inner end and front-rear direction ends in the vehicle-width-direction inner side half of the lower bracket  87  abut on and are joined to the raised portion  32  of the floor tunnel  3 . 
     The upper bracket  88  is disposed above the lower bracket  87 . The upper bracket  88  is formed on its top side with a seat rail mounting portion  93  protruding upward. A flange  88   a  is provided to a vehicle-width-direction outer side end of the upper bracket  88 , and the flange  88   a  abuts on and is joined to the top face  89  of the lower bracket  87 . 
     Flanges  88   b ,  88   c ,  88   d  are provided to a vehicle-width-direction inner side end of the upper bracket  88 , and these flanges  88   b ,  88   c ,  88   d  abut on the inclined wall portion  33  of the floor tunnel  3  and are joined to the tunnel reinforcement  100  (described later) via the inclined wall portion  33 . Also, flanges  88   e ,  88   f  are provided to respective ends in the vehicle front-rear direction of the upper bracket  88 , and these flanges  88   e ,  88   f  abut on and are joined to the raised portion  32  of the floor tunnel  3 . 
     &lt;Tunnel Reinforcement&gt; 
     A tunnel reinforcement  100  is mounted to an underside of the floor tunnel  3  to which the tunnel-side seat mounting bracket  63  of the second cross member  7  is joined. The tunnel reinforcement  100  is substantially saddle-shaped with its cross-section in the vehicle width direction being in the form of an inverted U-shape opening downward. 
     As shown in  FIGS. 10A and 10B , the tunnel reinforcement  100  includes a front projection  101 , a front recess  102 , a rear projection  103 , a rear recess  104 , and a rear end projection  105  disposed in this order in the vehicle front-rear direction, and these are provided so as to extend in a circumferential direction. The rear projection  103  is formed, at its each end in the vehicle width direction, a wide portion  106  that is wider than an intermediate portion of the rear projection  103 . The front recess  102  includes, on its each side in the vehicle width direction, an intermediate projection  107  that is provided so as to extend in the up-down direction along the front projection  101  and the rear projection  103 . A first front recess  102   a  and a second front recess  102   b  are provided to respective ends in the vehicle front-rear direction of the intermediate projection  107 . When viewed from inside the tunnel reinforcement  100 , the front projection  101 , the first front recess  102   a , the intermediate projection  107 , the second front recess  102   b , and the rear projection  103  form a substantially W-shaped cross-section in the vehicle front-rear direction. Also, the rear projection  103 , the rear recess  104 , and the rear end projection  105  form a substantially U-shaped cross-section in the vehicle front-rear direction. 
     The tunnel reinforcement  100  is joined to the underside of the floor tunnel  3  and, as explained below, also joined to the backbone frame  35  and the tunnel-side seat mounting bracket  63  via the floor tunnel  3 . 
     That is, as shown in  FIG. 10A , the front projection  101 , the intermediate projection  107 , the rear projection  103 , and the rear end projection  105  are respectively provided with a first front abutting portion  101   a , a first intermediate abutting portion  107   a , a first rear abutting portion  103   a , and a first rear end abutting portion  105   a  that abut on and are welded to the respective beads  38  of the backbone frame  35  via the floor tunnel  3 . 
     Also, as shown in  FIG. 10B , the intermediate projection  107  and the rear projection  103  are respectively provided with a second intermediate abutting portion  107   b  and a second rear abutting portion  103   b  that abut on and are welded to the flange  88   b  of the tunnel-side seat mounting bracket  63  via the floor tunnel  3 . Also, the front projection  101  and the wide portion  106  of the rear projection  103  are respectively provided with a second front abutting portion  101   b  and a third rear abutting portion  106   a  that respectively abut on and are welded to the flanges  88   c ,  88   d  of the tunnel-side seat mounting bracket  63  via the floor tunnel  3 . 
     The tunnel reinforcement  100  is provided with an extended portion  108  rearward of the tunnel-side seat mounting bracket  63 , and accordingly the length of the tunnel reinforcement  100  in the vehicle front-rear direction is longer than that of the tunnel-side seat mounting bracket  63  of the second cross member  7 . 
     &lt;Functions of Cross Members in the Event of Side Collision&gt; 
     Below a description will be given of functions of the first cross member  6  and the second cross member  7  of the above-configured lower vehicle-body  1  according to the present embodiment in the event of a side collision. 
     In the lower vehicle-body  1  of the present embodiment, the first cross member  6  and the second cross member  7  extending in the vehicle width direction receive a side collision load F that acts on the side sill  2  as shown in  FIG. 1  when another vehicle collides with a side of the vehicle or when the side of the vehicle collides with an obstacle. This prevents a pillar (not shown) from intruding into a passenger compartment. 
     That is, as shown in  FIG. 3 , in the first cross member  6  in the lower vehicle-body  1  of the present embodiment, the top face  64  of the first member  61  is joined to the side sill  2  via the sill-side seat mounting bracket  60 , the vehicle-width-direction inner side end of the top face  64  of the first member  61  is joined to the vehicle-width-direction outer side end of the top face  69  of the second member  62 , and the vehicle-width-direction inner side end of the top face  64  of the second member  62  is joined to a vehicle-width-direction outer side end of the top face  82  of the tunnel-side seat mounting bracket  63 , and the tunnel-side seat mounting bracket  63  is joined to the raised portion  32  and the inclined wall portion  33  of the floor tunnel  3 . Thus, the top faces  64 ,  69 ,  74 , and  82  of the first cross member  6  are joined to the side sill  2  and the floor tunnel  3  (the part A in  FIG. 3 ). 
     Also, in the first cross member  6 , the vehicle-width-direction outer side ends of the front face  65  and the rear face  66  of the first member  61  are joined to the side sill  2 , and the vehicle-width-direction inner side ends of the front face  65  and the rear face  66  of the first member  61  are joined to the frame top face  50   a  of the floor upper frame  50  at a position higher than the center position P of the side sill  2  in the up-down direction (the part B in  FIG. 3 ). 
     Thus, the first cross member  6  can counteract a side collision load in the event of a side collision with its first bracing effect provided by the part A in  FIG. 7  of the top faces  64 ,  69 ,  74 , and  82  of the first cross member between the side sill  2  and the floor tunnel  3  and with its second bracing effect provided by the part B in  FIG. 7  of the front face  65  and the rear face  66  of the first member  61  between the side sill  2  and the frame top face  50   a  of the floor upper frame  50 , resisting the inwardly falling motion of the side sill  2  toward the vehicle-width-direction inner side as shown by the arrow M in  FIG. 3 . The same applies to the second cross member  7 . Thus, even though the vehicle has a lowered floor including the second floor panel  42  lower than the first floor panel  41 , the first cross member  6  and the second cross member  7  can effectively distribute and transfer the side collision load F in the vehicle width direction from the side sill  2  to the floor upper frame  50  and the floor tunnel  3 . 
     Further, the bead  68  extending in the vehicle width direction is provided in each of the front face  65  and the rear face  66  of the first member  61  of the first cross member between the side sill  2  and the frame side face  50   b . This prevents out-of-plane deformation (buckling) of the front face  65  and the rear face  66  and allows the side collision load F to be effectively supported by the bracing between the side sill  2  and the floor upper frame  50  in the event of a side collision, helping avoid stress concentration on the floor tunnel  3  that may occur when the side collision load F is mainly supported by any bracing between the side sill  2  and the floor tunnel  3 . 
     Additionally, the top faces  64 ,  69 ,  74 , and  82  of the first cross member  6  connect the raised portion  32  of the floor tunnel  3  and the side sill  2 , and the front face  65  and the rear face  66  of the first member  61  connect the wall rising portion  31  of the floor tunnel  3  and the side sill  2 . This allows the side collision load F to be received between the side sill  2  and each of the wall rising portion  31  and the raised portion  32  of the floor tunnel  3 , helping avoid stress concentration on the inclined wall portion  33  and the top plate portion  34 , which constitute a main body of the floor tunnel  3 . 
     &lt;Functions of High-Strength Portions of Cross Member&gt; 
     In the lower vehicle-body  1  of the present embodiment, the high-strength portions  72 ,  86  having a larger thickness than the front lower portion  65 A and the rear lower portion  66 A of the first member  61  and given a high strength are provided to the first cross member  6  and the second cross member  7  at the overlapped portion of the sill-side seat mounting bracket  60  and the top face  64  of the first member  61 , at the overlapped portion of the top face  64  of the first member  61  and the top face  69  of the second member  62 , and at the overlapped portion of the top face  69  of the second member  62  and the tunnel-side seat mounting bracket  63 . This allows the side collision load F, which is input to the first cross member  6  and the second cross member  7  so as to cause the side sill  2  to fall inward to the inside of the vehicle cabin in the event of a side collision, to be surely transmitted in the vehicle width direction via the high-strength portions  72 ,  86 . This can restrain deformation of the front face  65  and the rear face  66  of the first member  61  facing the lowered second floor panel  42 . 
     Since the high-strength portions  72 ,  86  are portions made by overlapping multiple members including the first member  61 , the second member  62 , the sill-side seat mounting bracket  60  and the tunnel-side seat mounting bracket  63 , the high-strength portions can be configured with a simple configuration. 
     The sill-side seat mounting bracket  60  and the tunnel-side seat mounting bracket  63  are respectively mounted to the first member  61  and the second member  62  having the high-strength portions  72 ,  86 , and this restrains deformation of the sill-side seat mounting bracket  60  and the tunnel-side seat mounting bracket  63  and reduces impact on an occupant sitting in the seat in the event of a side collision. 
     The sill-side seat mounting bracket  60 , the first member  61 , the second member  62 , and the tunnel-side seat mounting bracket  63  are fixed to each other such that one closer to the side sill  2  lies on top of another adjacent one closer to the floor tunnel  3 . Accordingly, when the side collision load F causing inward falling deformation of the side sill  2  is input to the first cross member  6  and the second cross member  7 , compressive force in the surface direction acts on the overlapped portion of each member, which can restrain peeling of the overlapped portion. 
     As shown in  FIG. 2 , the bead  73  on the top side of the second member  62  is provided so as to overlap the beads  78 ,  80  of the sill-side seat mounting bracket  60  in the vehicle front-rear direction. This allows the side collision load F on the side sill  2  to be transmitted to the vehicle-width-direction inner side via the beads  78 ,  80  of the sill-side seat mounting bracket  60  and the bead  73  of the second member  62 . 
     &lt;Functions of Fragile Portion in Cross Beam&gt; 
     In the lower vehicle-body  1  of the present embodiment, the fragile portion  67  formed of a cutout extending in the vehicle front-rear direction is provided to a side-sill side end of the first member  61  at a position overlapping the sill-side seat mounting bracket  60  in each of the first cross member  6  and the second cross member  7 . 
     In the event of a side collision, due to the side collision load F being applied to the side sill, a moment M acts on the side sill  2  so as to cause it to fall inward. Under this bending moment M, first, a proximal end of the protruding piece  79  of the sill-side seat mounting bracket  60  joined to the top face  22  of the side sill  2  experiences out-of-plane deformation. Because the seat mounting bracket  60  on the vehicle-width-direction inner side of the protruding piece  79  has high rigidity by virtue of its hat-shaped cross-section, the side collision is transmitted to the fragile portion  67  at the vehicle-width-direction outer side end of the top face  64  of the first member  61 . 
     Thus, the fragile portion  67  of the first member  61  crushes and thereby absorbs energy of the side collision, restraining buckling deformation of the first cross member  6  and the second cross member  7  from occurring in their portions where such deformation is not intended to occur. Further, guiding the side collision load F preferentially in the direction in which the fragile portion  67  crushes allows to effectively distribute and transfer the side collision load F in the vehicle width direction. 
     Also, the sill-side seat mounting bracket  60  as the reinforcement member is provided to the side sill-side end of the top face  64  of the first member  61  of each of the first cross member  6  and the second cross member  7 . Thus, the sill-side seat mounting bracket  60  as the reinforcement member can receive a region including the crushed fragile portion  67  and thereby allows the side collision load F after crushing the fragile portion  67  to be transmitted in the vehicle width direction via the sill-side seat mounting bracket  60  as the reinforcement member as well as the first member  61  and the second member  62 . 
     The fragile portion  67  is selected from a cutout, a bead, and a hole. Hence, the fragile portion  67  can be formed by simple processing and also its deformation behavior in the event of a side collision can be controlled. 
     The reinforcement member is the sill-side seat mounting bracket  60 , and the fragile portion  67  is positioned on the vehicle-width-direction outer side of the seat rail mounting portion  77  of the sill-side seat mounting bracket  60 . Hence, even when the fragile portion  67  is crushed and deformed in the event of a side collision, it does not reach the seat rail mounting portion  77  of the sill-side seat mounting bracket  60 , and thus impact on an occupant sitting on the seat can be reduced. 
     The fragile portion  67  is provided only on the top face  64  at the side sill-side end of the first member  61 . Hence, ridge portions between the top face  64  and the front face  65  and between the top face  64  and the rear face  66  remain uncrushed in the event of a side collision, allowing the side collision load F to be transmitted in the vehicle width direction. 
     &lt;Functions of Tunnel Reinforcement&gt; 
     In the lower vehicle-body  1  of the present embodiment, as shown in  FIG. 9 , the tunnel reinforcement  100  is fixed at its second front abutting portion  101   b , second intermediate abutting portion  107   b , second rear abutting portion  103   b , and third rear abutting portion  106   a  to the tunnel-side seat mounting bracket  63  via the floor tunnel  3 . Also, the tunnel reinforcement  100  is fixed at its first front abutting portion  101   a , first intermediate abutting portion  107   a , and first rear abutting portion  103   a  to the multiple beads  38  of the backbone frame  35  via the floor tunnel  3 . Thus, the side collision load F input from the second cross member  7  to the tunnel-side seat mounting bracket  63  in the event of a side collision is transmitted to the tunnel reinforcement  100  via the second front abutting portion  101   b , the second intermediate abutting portion  107   b , the second rear abutting portion  103   b , and the third rear abutting portion  106   a  of the tunnel reinforcement  100  and further can be transmitted to the multiple beads  38  of the backbone frame  35  via the first front abutting portion  101   a , the first intermediate abutting portion  107   a , and the first rear abutting portion  103   a  of the tunnel reinforcement  100 . This restrains deformation of the tunnel reinforcement  100  as well as the floor tunnel  3 . 
     Additionally, ridges of the first front abutting portion  101   a , the first intermediate abutting portion  107   a , and the first rear abutting portion  103   a  of the tunnel reinforcement  100  and ridges of the beads  38  of the backbone frame  35  are in contact with each other. This allows the side collision load F to be surely transmitted to the multiple beads  38  of the backbone frame  35  via the first front abutting portion  101   a , the first intermediate abutting portion  107   a , and the first rear abutting portion  103   a  of the tunnel reinforcement  100 . 
     The tunnel reinforcement  100  includes, respectively in the front projection  101 , the intermediate projection  107 , and the rear projection  103 , the first front abutting portion  101   a , the first intermediate abutting portion  107   a , and the first rear abutting portion  103   a  that abut on the respective multiple beads  38  of the backbone frame  35 , and includes the second front abutting portion  102   a , the second intermediate abutting portion  107   b , the second rear abutting portion  103   b , and the third rear abutting portion  106   a  that abut on the seat mounting bracket  63 . This allows for easy and reliable joining of the tunnel reinforcement  100  and the backbone frame  35  and of the tunnel reinforcement  100  and the seat mounting bracket  63  via these abutting portions. 
     The tunnel reinforcement  100  includes, respectively between the front projection  101  and the intermediate projection  107  and between the intermediate projection  107  and the rear projection  103 , the front recess  102   a  and the rear recess  102   b  protruding in a direction away from the underside of the floor tunnel  3 . The tunnel reinforcement  100  further includes the rear end recess  104  protruding in the direction away from the underside of the floor tunnel  3  between the rear projection  103  and the rear end projection  105 . These continuous projections and recesses can increase the rigidity of the tunnel reinforcement  100 . 
     The tunnel reinforcement  100  includes the extended portion  108  rearward of the rear abutting portion  103 , and the extended portion  108  includes, at its rear end, a rear end projection  105  fixed to the additional bead  38  of the backbone frame  35 . This tunnel reinforcement  100  elongated in the vehicle front-rear direction by virtue of its extended portion  108  increases the rigidity of the floor tunnel  3 , which as a result allows to surely restrain deformation of the floor tunnel  3  even when the side collision load F is input from the seat mounting bracket  63  to the front portion of the tunnel reinforcement  100 . 
     The present disclosure is not limited to the above embodiment and may be modified in various ways without departing from the scope of the present disclosure. 
     For example, although the tunnel-side seat mounting bracket  63  of the second cross member  7  is composed of the lower bracket  87  and the upper bracket  88 , it may be composed of a single component similarly to the tunnel-side seat mounting bracket  63  of the first cross member  6 . Conversely, the tunnel-side seat mounting bracket  63  of the first cross member  6  may be composed of the lower bracket  87  and the upper bracket  88  similarly to the tunnel-side seat mounting bracket  63  of the second cross member  7 .