EXPERIMENTAL STUDY ON A NEW JOINT FOR PRESTRESSED CONCRETE COMPOSITE BRIDGE WITH STEEL TRUSS WEB

Transcription

1 EXPERIMENTAL STUDY ON A NEW JOINT FOR PRESTRESSED CONCRETE COMPOSITE BRIDGE WITH STEEL TRUSS WEB Kousuke Furuichi**, Masato Yamamura*, Hiroyuki Nagumo* and Kentaro Yoshida** *Civil Engineering Design Department, Kajima Corporation, Japan ** Technical Research Institute, Kajima Corporation, Japan Abstract A composite truss bridge has been developed that comprises concrete upper and lower slabs and steel truss as the web. This structure rationalizes structural performances, reduces weight and labor costs. A structure is proposed to simplify the cantilever construction, and to utilize both steel and concrete effectively to realize a new composite joint structure for a composite truss bridge. This joint structure is achieved by inserting the diagonal tubular steel into a box-shaped steel structure called a steel BOX made from welded perforated steel plates. The force flow in this joint structure was expected to be complicated. Therefore, static destructive tests were carried out using scale models to conceive their force propagation conditions and ultimate strengths, and to obtain basic data for designing. These tests confirmed that the force was propagated in the steel BOX until shear cracks occurred at the joint. At this point, re-bars in the joint carried some of the forces. It was thus confirmed that varying both quantity and placement of re-bars could control the ultimate strength at the joint. 1. Introduction Hybrid bridge structures have recently been developed that utilize the advantages of both steel and concrete 1), 2). These structures have enabled cost saving by rationalizing structural performances, and reducing bridge weight and labor costs. An example of this type of hybrid structure is shown in Fig. 1. Fig.1 PC Composite Bridges with Steel Truss Web 125

2 引張斜材 Tensile Diagonal Member 圧縮斜材 Compressive Diagonal Member Joint of Compressive Diagonal Member Rings of Rebar Concrete Slab Steel BOX Main Re-bars Stirrups Fig.2 Joint Structure Fig. 3 Object Bridge This is a composite truss bridge comprising concrete upper and lower slabs and steel truss as the web. This structure draws a bead on decreasing bridge weight while maintaining high rigidity against live loads, and reducing labor costs. The concrete web is replaced with steel truss, because it contributes little to the resisting moment in a PC box girder. Applications of this structure are found in the Albore Bridge, the Loars Bridge, etc. However, in this type of bridge, there is a possibility of brittle failure of the total system due to breakdown of the joints between concrete slabs and steel truss members. Therefore, it is necessary to develop the joint structure that can propagate the required forces. Various joint structures have been proposed, but authors have developed a new joint structure 5), 6), 7), 8) which is compact and has sufficient strength, as shown in Fig. 2. This joint is easy to construct during the cantilever construction, and effectively utilizes steel and concrete to produce joint with a composite structure. This joint structure is achieved by inserting tensile and compressive tubular steel diagonal members into a box-shaped steel structure called a steel BOX. The steel BOX is made of welded perforated steel plates. The steel BOX and the tensile diagonal members are welded to form an integrated structure. The compressive diagonal members and the concrete in the steel BOX are unified by the effect of bonding of the re-bars, which are welded along the internal perimeter of the tubular steel. This paper reports static loading tests using scale models of the joint to conceive the force propagation conditions and the ultimate strengths of the joint structure, and to establish a concept of designing. And further more, it proposes the equation of shear strength derived from test results. 5 2@8.5 5 D @81.5= @65=325 D D1 D1 D13 Tensile Diagonal Member φ=165.2 t= @125= Steel BOX t=6. Hall of Diameter 25. Pith of Hall 5. Compressive Diagonal Member φ=165.2 t= @8.5 5 D @75= @65= @32= @125=75 6@125=75 1 D1 4@65=26 4@65=26 D1 D13 Tensile Diagonal Member φ=165.2 t=7.1 Steel BOX t=6. Hall of Diameter 25. Pith of Hall 5. Compressive Diagonal φ=165.2 t=7.1 a) No.1 Specimen b) No.2 Specimen Fig.4 Aspect of Specimen 1251

3 2. Test outline Specimens Dimensions of specimens and applied design loads for the specimens were determined to simulate the joint with a 1:2 scale model of the bridge, as shown 111 φ25. φ25. t=6,sm49 in Fig. 3. Fig. 4 and 5 show the configurations of the specimens and the steel BOX, respectively. Two specimens were tested. The first specimen (No.1) was used to evaluate the shear strength of the joint. In the second specimen (No.2), the quantity 152 5@5= of re-bars in the joint is approximately doubled to Fig.5 Steel BOX prevent failure in the joint before failure in the other members. The diagonal tubular steel has an outside diameter of mm and a wall thickness of 7.1 mm (diameter-thickness ratio 23). To unify the joint of the compressive diagonal members with the slab, re-bars with diameter of 6 mm were welded along the perimeter of the steel pipe at 3 mm pitch in three stages. This design referred to results of previously implemented axial compression element tests. 24 perforations of 25. mm diameter were aligned at 5. mm spacing on the side steel plates of the steel BOX taking into an account of the integration with the slab concrete and of filling capability during the concrete placement. Table 1 shows the mechanical properties of the diagonal tubular steel and the steel plates of the steel BOX. The slab concrete was determined as 42.5 cm high and 42.5 cm wide. For the 1/2 scale model, the maximum grain diameter of coarse aggregate was set to be 1 mm. Table 2 shows the material properties of the concrete. Re-bars with diameter of 1. mm and 16. mm were used in the slabs. Table 3 shows the material properties of the re-bars @5= Table 1 Material Properties of Steel Tubular and Steel BOX Tensile Strength Yield Point Elastic Modulus Elongation N/mm 2 N/mm N/mm 2 % Steel Tubular Steel BOX Table 2 Material Properties of Concrete No.1:21days No.2:25days No Compressive Strength Elastic Modulus N/mm N/mm 2 No.1 Specimen No.2 Specimen No No.1 Specimen No.2 Specimen Table 3 Material Properties of Reinforcement D1 D13 Tensile Strength Yield Strength Elastic Modulus Tensile Strength Yield Strength Elastic Modulus N/mm 2 N/mm N/mm 2 N/mm 2 N/mm N/mm Test method A horizontal load was applied at the end of the concrete slab using the apparatus shown in Fig.6, so that it was precisely transferred to the joint, tensile diagonal member and compressive diagonal member. In order to determine the horizontal load P, the linear frame 1252

4 反力295 壁 Hydrauric Jack 球座 2kN cell Specimen Result of Linear Analysis Maximum 931.4kN Displacement 31.3mm of Serviceability Condition 3(P=663kN) of Ultimate Condition (P=377kN) of Serviceability C (P=221kN) Shear P=839.kN Displacement 59.4mm Maximum 983.5kN Displacement 51.1mm Specimen No.2 Specimen No δ(mm) Fig.6 Side View of Carrying Fig.7 -Displacement Relationship analysis was conducted separately to evaluate the relationship between the axial forces in the diagonal members and the axial forces in the subject bridge. The following static repeated loads were applied: 1) the design load (P=221kN), 2) the design ultimate load (No.1: P=584kN, No.2: P=671kN), and 3) the maximum load and load conformable to maximum load to confirm the conditions of developed displacements. The design load and design ultimate load were specified to originate the axial force in the tensile diagonal member identical to the axial force for each loading state based on the separately conducted linear frame analysis. The measured parameters were horizontal load (load cell), specimen displacement (displacement meter), open width between concrete and diagonal tubular steel (cantilever type displacement meter), diagonal tubular steel strain (strain gauge), slab concrete strain (strain gauge and mold gauge), and re-bar strain (strain gauge). 3. Test Results and discussions 3.1 Horizontal load horizontal displacement Fig. 7 shows the relationship between horizontal load P and horizontal displacement δ for the specimen No.1 and No.2. Both specimens showed an increase in displacement of about 2.8 mm at the beginning of the loading. This is caused by the margin at the hinge that fixed the specimen on the platform. Thereafter, the gradient of P-δ (rigidity) became larger and the relationship was linear up to P=55 kn, which was larger than the design ultimate load (P=377 kn). The gradient (rigidity) up to that point was almost the same as that of the gradient obtained from the linear frame analysis, which took into account the rigid connection of nodes and the eccentricity of the joint. In the process, both specimens showed a similar trend up to P=931.4 kn (the maximum load of specimen No.1), indicating that increase in displacement was gradually enhanced. For specimen No.1, the width of the shear crack at the joint increased and at the same time the load started to decrease at the maximum load of P=931.4 kn (δ=31.1 mm). The load rapidly decreased after P=839. kn (δ=59.4 mm) due to the shear failure, and thereafter specimen No.1 reached its ultimate strength. For specimen No.2, the load increased beyond the maximum load for specimen No.1 (P kn). It reached a maximum at P=983.6 kn (δ=51.1 mm), and specimen No.2 reached its 1253

5 upper view upper view side view side view lower view lower view a) Specimen No.1 b) Specimen No.2 Fig.8 Specimen Cracking after ing ultimate strength due to full plastic buckling of the compressive diagonal member. 3.2 Cracking patterns Shear cracking occurred in both specimens at a force P of about 6 kn. There were no signs of cracking at the design load and design ultimate load. Fig.8 shows the cracking patterns observed at the completion of tests. In specimen No.1, cracking occurred on the side face of the joint in an angle of 45 degrees. The crack development was also observed along the main re-bars. On the upper side, the crack extended from the center to the periphery of the steel BOX. This is probably caused by the fact that the compressive diagonal member was trying to extrude the steel BOX to the upper side. In specimen No.2, shear cracking occurred on the side face of the joint as for specimen No.1. However, unlike specimen No.1, the cracks were concentrated on the side face of the steel BOX. More cracks induced by the diagonal tubular member were found on the lower side of specimen No.2 than that of specimen No.1. The joint in specimen No.2 has higher shear strength and shear rigidity owing to the increased number of re-bars. As a result, the larger forces were converged at the joint between the tubular steel and the concrete slab for specimen No.2 than that of specimen No.1 causing more cracks at that point. 3.3 Strains in tubular steel Fig. 9 shows the axial strains and the bending strains of the tubular steel. It is found that no portion of either specimen reached yield strain at design ultimate load (P=377 kn). Strains of the two specimens were almost identical up to the maximum load of specimen No.1. For specimen No.2, it was observed that at around the maximum load of specimen No.1, the middle-stage bending strain of the compressive diagonal member was released and the other strains were greatly increased. Therefore, it was possible to confirm that the full plastic buckling in the compressive tubular member occurred around this point. The smaller strains took place in the upper stage than in the other stages because the upper stage was situated inside of the slab concrete and the force was propagated through the concrete in the tubular steel. 1254

6 1 Allowable Strain (667μ) 2 Nominal Yield Strain (1143μ) Actual Yield Strain (1916μ) of Serviceability Condition (P=221kN) 2 1 Positive of Serviceability Condition (P=221kN) a) No.1 Specimen (Axial Strains of Tensile Tubular Steel) b) No.1 Specimen (Bending Strains of Tensile Tubular Steel) of Serviceability Condition 3 (P=663kN) of Serviceability Condition (P=221kN) of Serviceability Condition 3 (P=663kN) of Serviceability Condition 2 (P=221kN) Positive c) No.1 Specimen (Axial Strains of Comp. Tubular Steel) d) No.1 Specimen (Bending Strains of Comp. Tubular Steel) of Serviceability Condition (P=221kN) of Serviceability Condition 2 2 (P=221kN) Positive a) No.2 Specimen (Axial Strains of Tensile Tubular Steel) b) No.2 Specimen (Bending Strains of Tensile Tubular Steel) of Serviceability Condition (P=221kN) of Serviceability Condition 2 2 (P=221kN) Positive c) No.2 Specimen (Axial Strains of Comp. Tubular Steel) d) No.2 Specimen (Bending Strains of Comp. Tubular Steel) Fig.9 -Steel Tubular Strain Relationship 1255

7 a) Main Re-bar b) Stirrups Fig.1 Strain Distribution of Reinforcements on the Joint 3.4 Reinforcing bar strains Fig.1 shows the strain distributions of the main re-bars and stirrups in the joint when the load equivalent to the design load (P=2 kn) was applied and after cracking took place (P=9 kn). Neither shear cracking nor strain was observed in the re-bars at P=2 kn for either specimen. However, strains were observed mainly at the joint in the main re-bars and in the stirrups at P=9 kn. In specimen No.2, the strains in the re-bars were small and were more evenly distributed than in specimen No.1. The strains in the main re-bars were found to be evenly distributed over the height of the steel BOX and the strains in the stirrups were found to be concentrated near the intersection with the centroid of the tension 引張 diagonal member. compression 圧縮 3.5 Steel BOX strains Fig.11 shows the principal strains (P=2 kn and P=9 kn) occurring in the steel BOX. On the side face of the steel BOX, diagonal compressive strains and diagonal tensile strains occurred at an angle of 45 degrees at P=2 kn. At P=9 kn, the principal tensile strains were lateral. Furthermore, the principal tensile strains wee vertical at near the compressive diagonal member. This is considered to be the result of the constraint of the concrete in the steel BOX and the resistance against slipping-out of the compressive diagonal member to the upper face. It is noted that the principal strains on the side face of No.1 試験体 (2KN) 5μ μ 5μ μ 引張引張圧縮圧縮 a) No.1 Specimen (P=2kN) b) No.1 Specimen (P=9kN) tension 引張 compression 圧縮 No.1 試験体 (9KN) 5μ μ c) No.2 Specimen (P=2kN) d) No.2 Specimen (P=9kN) No.2 試験体 (2KN) No.2 試験体 (9KN) Fig.11 Principal Strains of Steel BOX 1256

8 the steel BOX were smaller than the yield strain even at P=9 kn. On the upper side of the steel BOX, tensile strains were directed toward the center of the tensile diagonal member, because the steel Box was welded to the tensile diagonal member. Although the principal tensile strains exceeded the yield strain at P=9 kn, they were smaller than the allowable strain for three times the design load (P=663 kn). In the steel member connected to the side face of the steel BOX, the tensile strains were about 45 μ and 2μ on the tensile side and the compressive side, respectively. The steel member on the tensile side had larger strains. 4. Discussions and points to be clarified for joint design method 4.1 Force distribution in joint The strain distributions of the re-bars (main re-bars and stirrups) placed in the joint, and of the steel Box showed a difference between the force distributions of the joints before and after cracking when design load was applied. With the design load, almost no reinforcement strains were observed. The force is considered to be propagated mainly in the steel BOX. In contrast, the re-bar strains increased after cracking took place. At this point, the surface of the steel BOX would be contributing to the strength. The following re-bar strain distributions were observed at that time. The main re-bar strains ranged over the height of the steel BOX and, the stirrup strains occurred at the intersection of the centroids of the diagonal members. It is considered that this can be used as a reference to determine the range of the contributing re-bar when estimating the ultimate shear strength of the joint. The principal strains on the side face of the steel BOX resulting from the shear are decisive with the design load. As the load level increases after shear cracking, the rigidity of the joint decrease and the amount of deformation increases. It is therefore considered that, as well as the effect of shear, the constraint of the concrete inside the steel BOX, slipping-out of the tensile diagonal member and the rotation effect due to the extrusion of the compressive diagonal member influenced this behavior. Therefore, additional analyses or experiments are necessary to clarify the ambiguities for further improvement in the detailed designing for the configuration of the steel BOX, arrangement and size of the perforations, etc. 5. Equation of Shear Strength 5.1 Relationship between the applied horizontal force and shear force at the joint To formulate the equation of shear strength, the relationship between the applied horizontal force and shear force at the joint is calculated by using the frame analysis. From results of the frame analysis, an applied horizontal force of 1kN results in 13kN of force at the joint. Fig.12 Shear-Strain Diagram of Steel BOX 1257

9 5.2 Formulating the equation of shear strength Fig.12 shows the shear-strain diagram of a steel BOX with applied horizontal force of 931kN. All gages attached on the surface of the steel BOX show that all of the strains exceeds the shear yield strain of 278μ. As a result, the shear-stress distribution may be assumed as a rectangular shape. Fig.1-b) shows the transition of strains measured from the gages attached on stirrups. Consequently, if the applied horizontal force is large enough to bring the steel plate to the point of shear yield, stirrups would also yield. The shear strength at the joint is sum of shear strengths of the concrete, stirrups, and steel BOX. Therefore, the equation of shear strength at the joint can be described as Vnd: Vcd: Vnd = Vcd + Vstd + Vssd nominal shear strength at joint nominal shear strength provided by concrete at joint Vcd = βd βp βn fvcd bw d fvcd =.2 3 fcd βd = 4 1/d βp = 3 1pw βn = 1. fcd: compressive strength of concrete bw: web width d: distance from extreme compression fiber to centroid of tension reinforcement pw = As/(bw d) As: area of tension reinforcement Vstd: nominal shear strength provided by stirrup at joint Vstd = Aw fyd z / s s: pitch of spiral reinforcement Aw: total amount of area of shear reinforcement over the interval z: distance from compression resultant to centroid of tension reinforcement fyd: design yield strength of tension reinforcement Vssd: nominal shear strength provided by steel BOX at joint Vstd = 2 t h σsy / 3 t: thickness of surface steel plate h: height of surface steel plate σsy: allowable tension stress Table 4 shows both experimental and calculated shear strengths of specimen No.1 and 2. Table 4 Comparison of Experimental and Calculated Shear Strengths Unit:kN No.1 Specimen No.2 Specimen Vcd Vstd Vssd Vnd Experimental Value: Vn Vn / Vnd Comparing with experimental values, calculated values are fairly good and are 8% of that of experimental values. Farther analysis such as a non-linear analysis would be carried out in a 1258

10 future to clarify both shear transfer capacity and system of applied load endurance, and to decide whether using equation to calculate the shear strength is proper way to do. The restraint of the concrete in steel BOX makes the actual shear strength provided by the concrete larger than that of the calculated one. 6. Concluding remarks Destructive tests were conducted using scale models. The object of the tests was the joint structure made by inserting tensile diagonal members and compressive diagonal members into a steel BOX made of welded perforated steel plates. The acquisitions obtained from the tests are summarized as follows: a. By actually destroying the joint, properties of shear failure in the joint were conceived. b. No abnormalities such as shear cracking were found under the design loading. c. It was confirmed that the joint was safe at over three times the design load. d. The force propagation conditions in the joint were conceived from the strain distributions in the re-bars (main re-bars and stirrups) of the joint and in the steel BOX. e. The force propagation conditions in the joint of the compressive diagonal members were conceived. f. The equation of shear strength was derived from the tests results. It is planned to conduct additional analysis and tests to confirm the current test results and to improve and refine the designing method of full-scale joint structures. APPENDIX. REFERENCES 1) Keiichiro Sonoda, Hybrid Structures, Bridge and Foundation, pp23-29, ) Atsuo Ogawa, and Norio Terada, Composite Bridges in J.H., Bridge and Foundation, pp48-55, ) Express Highway Research Foundation of Japan, Report on Investigation of Prestressed Concrete Composite Bridges, ) Yasuo Inokuma, et al., Scheme of Tomoe-gwa (Prestressed Concrete Composite Bridge), Proceedings of the 51 st Annual Conference of the JSCE, pp , ) Yasuo Inokuma, et al., Analytical/Experimental Study of a joint in Prestressed Concrete Composite Bridges with Steel Truss Web, Proceedings of the Prestressed Concrete Symposium, pp73-78, ) Hiroshi Miwa, et al., Experimental Study on the Mechanical Behavior of Panel Joints in PC Hybrid Truss Bridges, Journal of Structural Engineering, pp , ) Masaaki Hoshino, et al., Experimental Study of a Panal Joint in Prestressed Concrete Hybrid Truss Bridges, Journal of Structural Engineering, pp , ) Kyoji Niitani, et al., Experimental Study on a Joint in Prestressed Concrete Composite Bridges with Steel Truss Web, Journal of Structural Engineering, pp , ) Youhei Taira, et al., Investigation of Connecting Steel Pipe and RC member Using Dubel, Proceedings of the 54 th Annual Conference of the JSCE, pp ,