Patent Publication Number: US-9834949-B2

Title: Prestressed concrete roof for cylindrical tank

Description:
This application is a continuation application based on a PCT Patent Application No. PCT/JP2014/073482, filed on Sep. 5, 2014, whose priority is claimed on Japanese Patent Application No. 2013-187391, filed on Sep. 10, 2013. The contents of both the PCT Application and the Japanese Application are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a prestressed concrete roof for a cylindrical tank. 
     BACKGROUND ART 
     As is well known, prestressed concrete (PC) construction methods for introducing prestress are used in large cylindrical concrete structures such as ground tanks for storing liquefied natural gas (LNG) or liquefied petroleum gas (LPG). The PC construction methods include methods of using bonded PC steel members as PC steel members and methods of using unbonded PC steel members as PC steel members. The PC construction method using the bonded PC steel members has the following processes. First, sheaths are arranged before concrete is poured. Bonded PC steel members such as PC steel rods, PC steel wires, or PC steel stranded wires (PC steel strands), are inserted into the sheaths before or after the concrete is cured, and are strained when the concrete reaches the desired strength. Afterwards, a material such as cement milk enters the sheaths under pressure in order to perform an anti-corrosion treatment and to bind and integrate the bonded PC steel members and the concrete. On the other hand, in the PC construction method using the unbonded PC steel members, grease is applied to PC steel members, and their surroundings are covered with sheaths. 
     As a prestressed concrete roof for a cylindrical tank, as disclosed in, for instance, Patent Documents 1 and 2, a technique for arranging PC steel members at an outer circumferential side of a discoid roof in a radial direction and thereby supporting strength against bending moment generated adjacent to the outer circumference side of the roof is known. 
     CITATION LIST 
     Patent Documents 
     Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2011-251741 
     Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H07-4111 
     SUMMARY 
     Technical Problem 
     In the related art, a radial compressive force caused by PC steel members may be applied to an outer circumferential side of the roof, whereas a radial tensile force opposite to the radial compressive force may be applied to an inner circumferential side positioned inside of the PC steel members in the roof. Since the opposite force is applied in this way, stress tends to be concentrated around inner circumferential ends of the PC steel members in the roof. To relieve the concentration of the stress, a technique for alternately arranging long PC steel members and short PC steel members in a circumferential direction is known. 
     However, since the stress is still concentrated on an anchoring part of the inner circumferential end of each PC steel member, there is a need to use large anchoring tools or form thick wall parts or reinforced parts around the anchoring parts. As a result, the weight and manufacturing cost of the roof are increased. 
     The present disclosure has been made in view of the aforementioned problems, and an object of the present disclosure is to provide a prestressed concrete roof for a cylindrical tank, which minimizes increases in the weight and manufacturing cost of the roof and inhibits stress from being concentrated on anchoring parts of PC steel members arranged at an outer circumferential side of the roof. 
     Solution to Problem 
     According to first aspect of the present disclosure, there is provided a prestressed concrete roof for a cylindrical tank, which includes a plurality of prestressed concrete (PC) steel members which are disposed side by side in a circumferential direction at an outer circumferential side of a discoid roof and each of which extends in a radial direction, in which the plurality of PC steel members are made up of both arm parts of folded PC steel members that are folded so as to be open to the outer circumferential side of the roof in a plan view. 
     Advantageous Effects 
     According to the present disclosure, in the prestressed concrete roof for a cylindrical tank, it is possible to limit an increase in the weight and manufacturing cost of the roof and inhibit stress from being concentrated on the anchoring parts of the PC steel members arranged at the outer circumferential side of the roof. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a vertical sectional view of a cylindrical tank in an embodiment of the present disclosure. 
         FIG. 2  is an enlarged view of a part A of  FIG. 1 . 
         FIG. 3  is a plan view showing an arrangement of prestressed concrete (PC) steel members in a prestressed concrete roof of the cylindrical tank in the embodiment of the present disclosure. 
         FIG. 4  is a partially enlarged view of  FIG. 3 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. In the present embodiment, a cylindrical prestressed concrete (PC) double shell storage tank for storing liquefied natural gas (LNG) will be described as an example. 
     The construction of the cylindrical tank (PC structure) of the present disclosure will be described with reference to  FIG. 1 . First, a discoid base (bottom part of an outer tank)  1  is laid, and a base part  3  for building a PC wall (sidewall of the outer tank)  2  is provided to protrude at an outer circumferential edge of the base  1 . 
     Next, lateral liners (outer tank lateral plates)  2   a  are built on the base  1  along an inner side of the base part  3  and up to an uppermost level of the cylindrical tank. Concrete is poured and cured on the base part  3  along the built lateral liners  2   a , and the PC wall  2  is built sequentially from a lowermost level to an uppermost level according to the number of levels of the lateral liner  2   a.    
     An inner tank sidewall  4 , an inner tank roof  5 , and an outer tank roof  6  are appropriately assembled, and thereby the cylindrical tank acting as the double shell storage tank having an inner tank made of a metal and an outer tank made of prestressed concrete is constructed. In the drawings, a reference sign  2   b  indicates a ring-shaped beam part formed at an upper end of the PC wall  2 , and a reference sign  7  indicates a cold insulator filled between the inner tank and the outer tank. 
     As shown in  FIG. 2 , a thickness of the outer tank roof (hereinafter referred to simply as a “roof”)  6 , which has an approximate disc shape and bulges upward, increases toward an outer circumferential side. Thus, resistance to bending moment caused by a dead load of the roof  6  is increased. Reinforcing bars are appropriately arranged in the roof  6  in radial and circumferential directions of the roof  6 . 
     Hereinafter,  FIG. 3  is also referred. A plurality of PC steel members  11 , each of which extends in a radial direction, are arranged in a circumferential direction at an outer circumferential side excluding a predetermined region of an inner circumferential side in the roof  6 . Thus, compressive prestress in the radial direction is applied to the outer circumferential side of the roof  6 . Each of the PC steel members  11  is inserted into the outer circumferential side of the roof  6  so as to pass an intermediate portion of the roof  6  in a thickness direction of the roof  6 . Each of the PC steel members  11  is inclined relative to a horizontal direction to follow a slope of the outer circumferential side of the roof  6 . It is preferable to arrange the PC steel members  11  closer to an upper surface of the roof  6  since compressive prestress against the bending moment cause by a dead load of the roof  6  can be efficiently given. 
     An inner circumferential end  11   a  of each PC steel member  11  is anchored by a folded part  12   a  to be described below. An outer circumferential end  11   b  of each PC steel member  11  is fixed at an outer circumferential side of the beam part  2   b  of the PC wall  2  using an anchoring tool  11   c.    
     The PC steel members  11  are formed, for instance, by inserting PC steel wires into sheaths. After the inner circumferential end  11   a  of each PC steel wire is anchored, the outer circumferential end  11   b  of each PC steel wire is pulled and tensioned by, for instance, a jack, and each PC steel wire is held and fixed in a tensed state using an anchoring tool  11   c  with a predetermined tension applied. Afterwards, the interiors of the sheaths are grouted to stick the PC steel wires to the roof  6 , and thereby compressive prestress is given to the roof  6 . A PC steel member or arm part is not limited to the PC steel wire, and may be a PC steel rod, a PC steel stranded wire, or the like. 
     The PC steel members  11  include both arm parts of folded PC steel members  12  that are folded so as to be open to the outer circumferential side of the roof  6  in a plan view. To be specific, the folded PC steel members  12  are bent in a U shape in a plan view. A pair of PC steel members  11  formed by both the arm parts of the folded PC steel member  12  and a folded part  12   a  extending between both the arm parts (that is, between the inner circumferential ends  11   a  of the pair of the PC steel members  11 ) are integrally formed. The folded part  12   a  anchors the inner circumferential ends lla of the pair of the PC steel members  11  continuing with the folded part  12   a . The outer circumferential ends  11   b  of the pair of PC steel members  11  pass through the beam part  2   b , and are fixed in a state in which they are pulled toward the outer circumference. 
     The type of the PC steel member  11  includes a short PC steel member (or arm part)  11 S and a long PC steel member (or arm part)  11 L which are different in length in a radial direction. Also, the folded PC steel member  12  includes a short folded PC steel member  12 S including the short PC steel members  11 S, and a long folded PC steel member  12 L including the long PC steel members  11 L. The radial length of the long folded PC steel member  12 L (i.e., including long PC steel member  11 L) is about twice that of the short folded PC steel member  12 S (i.e., including short PC steel member  11 S). 
     Among the plurality of short folded PC steel members  12 S, the short folded PC steel members  12 S that are adjacent to each other in a circumferential direction are disposed such that lateral portions thereof including the short PC steel members  11 S overlap each other. Hereinafter, the overlap portion between the short folded PC steel members  12 S that are adjacent to each other in the circumferential direction is referred to as a short overlap portion  15 S. 
     The folded parts  12   a  (hereinafter referred to as “folded parts  12   a S”) of the plurality of short folded PC steel members  12 S are disposed to be arranged on the same circumference of the roof  6  in a plan view. Thus, a first circle C 1  on which the plurality of folded parts  12   a S stand in line at positions of the inner circumferential ends of the plurality of the short PC steel members  11 S is formed. 
     Among the plurality of long folded PC steel members  12 L, the long folded PC steel members  12 L that are adjacent to each other in a circumferential direction are disposed such that lateral portions thereof including the long PC steel members  11 L overlap each other. Hereinafter, the overlap portion between the long folded PC steel members  12 L that are adjacent to each other in the circumferential direction is referred to as a long overlap portion or second overlapping portion  15 L. Accordingly, as should be apparent from the foregoing, the arrangement includes a plurality of PC steel members  12 , including a plurality of first PC steel members  12 S and a plurality of second PC steel members  12 L, with the first PC steel members shorter than the second. Further, each of the first PC steel members  12 S includes first arm parts ( 11 S) and a first folded part ( 12   a S), and each of the second PC steel members  12 L includes second arm parts ( 11 L) and a second folded part ( 12   a L). In addition, as shown in  FIG. 3 , adjacent first PC steel members overlap to form first overlapping portions (reflected by the adjacent arms  11 S), and similarly, adjacent second folded PC steel members overlap to form second overlapping portions (reflected by the regions between adjacent arm parts  11 L in  FIG. 3 ). See also first ( 15 S) and second ( 15 L) overlapping portions in  FIG. 4 , and the discussion of overlapping portions  15 S and  15 L below. 
     The folded parts  12   a  (hereinafter referred to as “folded parts  12   a L”) of the plurality of long folded PC steel members  12 L are disposed to be arranged on the same circumference of the roof  6  in a plan view. Thus, a second circle C 2  on which the plurality of folded parts  12   a L stand in line at positions of the inner circumferential ends of the plurality of the long PC steel members  11 L is formed. 
       FIG. 4  is also referred. The short overlap portions  15 S (first overlapping portions) and the long overlap portions  15 L (second overlapping portions) are disposed to be alternately arranged in the circumferential direction. Circumferential widths (i.e., angles in  FIGS. 3 and 4 ) H 1  of the short overlap portions  15 S are uniform, and circumferential widths (i.e., angles in  FIGS. 3 and 4 ) H 2  of the long overlap portions  15 L are uniform. Also, the widths H 1  and H 2  are the same as each other. Also, the widths H 1  and H 2  and an interval (i.e., angle in  FIGS. 3 and 4 ) H 3  in a circumferential direction between the long and short overlap portions  15 L and  15 S that are adjacent are the same as each other. Thus, at the outer circumferential side positioned outside of the second circle C 2 , all of the PC steel members  11  are disposed so as to be arranged at regular intervals in the circumferential direction. To be specific, the short PC steel members  11 S at both sides of the short overlap portions  15 S and the outer circumferential sides of the long PC steel members  11 L at both sides of the long overlap portions  15 L are disposed to be arranged at regular intervals in the circumferential direction. 
     A first intersection P 1  at which bent portions of the short folded PC steel members  12 S adjacent in the circumferential direction intersect each other in a plan view is present at the inner circumferential side of each short overlap portion  15 S. A second intersection P 2  at which bent portions of the long folded PC steel members  12 L adjacent in the circumferential direction intersect each other in a plan view is present at the inner circumferential side of each long overlap portion  15 L. A pair of third intersections P 3  at which the folded parts  12   a S and the pair of long PC steel members  11 L intersect each other in a plan view are present at the inner circumferential side of each short folded PC steel member  12 S. Two PC steel members  11  intersect each other at the intersections P 1 , P 2  and P 3  in a state in which positions thereof are different in a thickness direction of the roof  6 . 
     The pair of third intersections P 3  and the first intersections P 1  at both sides thereof are out of alignment in a circumferential direction. The second intersection P 2  is out of alignment to the first intersection P 1  and the third intersection P 3  in a radial direction. That is, three or more of the PC steel members  11  do not intersect one another at each of the intersections P 1 , P 2  and P 3 . Therefore, an increase in thickness of the roof  6  at each of the intersections P 1 , P 2  and P 3  can be limited. 
     As described above, according to the present embodiment, the plurality of PC steel members  11  extending in the radial direction are disposed at the outer circumferential side of the discoid roof  6  side by side in the circumferential direction. The plurality of PC steel members  11  are made up of both the arm parts of the folded PC steel members  12  that are folded so as to be open to the outer circumferential side of the roof  6  in a plan view. Thus, by using the folded parts  12   a  extending between both the arm parts of the folded PC steel members  12  as the anchoring parts of the inner circumferential ends  11   a  of the PC steel members  11  formed by both the arm parts of the folded PC steel members  12 , tension load of the PC steel members  11  can be dispersed to all the folded parts  12   a . For this reason, stress concentration at the anchoring parts of the inner circumferential ends  11   a  of the PC steel members  11  can be limited. As a result, there is no need to use large anchoring tools or form thick wall parts or reinforced parts around the anchoring parts, and an increase in the weight and manufacturing cost of the roof  6  can be minimized. 
     The present disclosure is not limited to the above embodiment. All the shapes and combinations of the means and components represented in the aforementioned embodiment are only examples, and can be variously modified based on design requirements without departing from the spirit and scope of the present disclosure. 
     For example, in the above embodiment, the application to the LNG tank has been described as an example. However, the present disclosure may be applied to various prestressed concrete structures such as an LPG tank in addition to the LNG tank. 
     According to the present disclosure, by using the folded parts extending between inner circumferential ends of both of the arm parts of the folded PC steel members as anchoring parts of inner circumferential ends of the PC steel members formed by both the arm parts of the folded PC steel members, the tension load of the PC steel members can be dispersed to all the folded parts. For this reason, stress concentration at the anchoring parts of the inner circumferential ends of the PC steel members can be limited. As a result, there is no need to use large anchoring tools or form thick wall parts or reinforced parts around the anchoring parts, and increases in the weight and manufacturing cost of the roof  6  can be minimized. 
     Further, radial positions of the anchoring parts of the inner circumferential ends of the short PC steel members (i.e., folded parts of the short folded PC steel members) and radial positions of the anchoring parts of the inner circumferential ends of the long PC steel members (i.e., folded parts of the long folded PC steel members) are out of alignment with each other, and therefore the stress concentration on the anchoring parts of the inner circumferential ends of the PC steel members can be more effectively limited. 
     Further, the short folded PC steel members adjacent in the circumferential direction and the long folded PC steel members adjacent in the circumferential direction overlap each other in the circumferential direction. Thereby, it is possible to give prestress to the entire region in the circumferential direction. Also, the short overlap portions and the long overlap portions are alternately arranged, and all the PC steel members are arranged at regular intervals in the circumferential direction. Thereby, the prestress can be uniformly given. 
     INDUSTRIAL APPLICABILITY 
     According to the present disclosure, in the prestressed concrete roof for a cylindrical tank, an increase in weight and manufacturing cost of the roof can be minimized, and stress concentration at the anchoring parts of the PC steel members arranged at the outer circumferential side of the roof can be limited.