Study on structural behavior characteristics of concrete filled steel tube girder bridges

Transcription

1 Tailor Made Concrete Structures Walraven & Stoelhorst (eds) 28 Taylor & Francis Group, London, ISBN Study on structural behavior characteristics of concrete filled steel tube girder bridges Won Jong Chin, Jae Yoon Kang, Eun Suk Choi & Jung Woo Lee Korea Institute of Construction Technology, Goyang, Republic of Korea ABSTRACT: The new bridge system described in this paper uses concrete-filled steel tube girders instead of conventional girders. A new type of shear connector attached to the tube that is -shaped perfobond rib provides the means for developing composite action between the steel tube and the concrete slab deck. Experimental investigations were carried out to examine the flexural behavior of the proposed concrete filled steel tube girder. The experimental investigation consisted of designing and constructing a test specimen and loading it to collapse in bending to understand the ultimate capacity of the system. Test results showed that concrete filled steel tube girder has good ductility and maintains its strength up to the end of the loading. Results of this investigation demonstrated the potential for using a concrete filled tube as bridge girder. Additional experimental investigations were carried out to examine the fatigue behavior of the concrete-filled steel tube girder and revealed also that the new type of girder bridge exhibits good fatigue durability. 1 INTRODUCTION Concrete filled steel tube structures (CFT) is a structure composed by filling a steel tube with concrete in which the concrete filling material prevents buckling of the steel tube under compression and the steel tube improves the material strength by restraining the filling material. Previous studies already demonstrated that the CFT member is developing load bearing capacity and deformation performance significantly superior to structural members composed of a unique material. However, most of these studies limited their scope to column members subjected to compressive forces. Even if Japan, China and Europe started recently to attempt the construction of bridges using CFT structures, experimental studies are still insufficient to reflect at the most their behavioral characteristics as flexural members in the design. Accordingly, the current design does not consider the increase of load bearing capacity brought by the composition between the filling inside the steel tube and the deck, and ignores the stiffness of the filling material in positive moment zone or the composition with the deck in negative moment zone. Such design is thus inefficient because disregarding the advantages and behavioral characteristics of the CFT member. Previous steel girder bridges present poor economic efficiency and are subjected to fatigue due to the numerous welding spots necessary for their fabrication, are relatively expensive because of the increase of weight, provoke environmental problems due to vibration and noise and, exhibit poor constructability for concrete girders. The application of CFT members as bridge girder may represent an alternative that can remedy such problems to some extent. CFT girder may also contribute to the reduction of construction and maintenance costs, durability and economic efficiency of the structure, enhancement of construction quality, and may secure larger clearance by diminishing the height of the girder. Substantive and experimental studies shall thus be primarily conducted on the behavioral characteristics of composite girder bridges using CFT girder so as to develop adequate design method and promote their practicability. Previous researches implemented numerous tests on reduced-scale prototypes to investigate the flexural behavior of CFT member and already demonstrated its remarkable structural effects. This study focuses on the examination of the behavioral characteristics and fatigue endurance according to the connection method of the CFT girder bridge. Specimens were manufactured considering various variables affecting the design, and test results were exploited to derive alternatives for the effective reflection of the remarkable load bearing capacity of the CFT member in the design. Figure 1 depicts the structural shape and schematic concept of the composite bridge using CFT girder. 821

2 2 DEVELOPMENT OF NEW SHEAR KEY Figure 2 shows the perfobond plate, a newly developed shear key offering constructability, structural safety and economic efficiency. Korean research teams conducted studies on the performance of this new shear key (Chung, 24; BBM Korea, KICT, 24; Lee, 26) and revealed that the proposed -shaped perfobond rib exhibit remarkable static ultimate bearing capacity. This key can be applied effectively for girders subjected to large horizontal shear forces, especially for composite bridges like CFT girder for which the construction of stud shear connector is very difficult. 3 BEHAVIORAL CHARACTERISTICS ACCORDING TO THE CONNECTION METHOD OF CFT GIRDER BRIDGE Field or shop welding was mainly applied for the previous CFT columns or CFT girder bridges. However, field welding is very sensitive to the working conditions like weather and humidity, and requires additional costs due to the involved manpower. To overcome such problems, Japan already carried out experimental studies on the bolted connection of the steel tubes and found out that common bolts can be applied instead of rounded bolts for steel pipe diameters larger than 8 mm. This study examined the adoption of bolts instead of welded connections through flexural tests. 3.1 Connection method of precast CFT members Specific connection is required to connect precast CFT members without loss of section or stiffness. Figure 3(b) illustrates the selected connection method to be used for the application of bolts instead of welded connection. Figure 3(a) presents connection on which concrete is poured after fastening of steel rods and bolt connection with the connection plate. This solution offers the remarkable structural advantage through the introduction of compressive force by the steel rods. However, disadvantage resides in the need for larger space for post-cast concrete within the whole section due to additional lengths of 2 m required at both sides of the connection to secure working space for the fastening of steel rods. Figure 3(b) shows the connection poured with concrete immediately after its connection by bolts. Here, even if there is no compressive force introduced by the steel rods, the shear keys inside the connection are fastening completely concrete. Since this connection method does not require separate space for the tensioning works, this methods appears to be the most optimal for connection in bridges of which scale does not need compression of the connection. Various specimens were manufactured to evaluate comparatively the behavior according to the connection method of steel tubes. These specimens were D78-W-S, the most common connection through welding of the tubes; D78-B-S, bolted connection; D78-BF-S, bolted connection filled with concrete; and, D78-N-S, tubes without connection. The proposed connection is illustrated in Figure 5. The specimens presented total length of 6 m, external diameter (d) of 78 mm, and thickness (t) of 14 mm so as to be manufactured with a d/t ratio of This ratio falls within the range featuring compact section based on previous studies. All the specimens were filled in their end sections with length of 1.6 m from the supports. The bolted specimens were manufactured by classifying them into filled and non-filled specimens according to the eventual filling of the central connection. Filling was performed with ordinary concrete presenting strength of 21 MPa. Ordinary concrete with strength of 27 MPa was poured to manufacture the deck. The test method shown in Figure 6 proceeded Figure 1. bridge. Structural and sectional shapes of CFT girder Figure 3. Connection methods of precast CFT members. Figure 2. Proposed type of shear key ( -shaped perforbond). Figure 4. Drawing of the bolted connection. 822

3 by 3-point loading on the simply supported specimen using an actuator with capacity of 3,5 kn. Table 1 summarizes the connection method of each specimen and their dimensions yield of steel tube at kn yield of steel tube at kn Behavioral characteristics according to the connection method of CFT members For the no-connection specimen shown in Figure 7(a), the maximum load was 3,45 kn corresponding to the maximum capacity of the actuator, the maximum deflection was approximately 37 mm, and the residual deformation was about 11 mm. The specimen developed resistance all along the loading process without clear sign of stiffness degradation. In the cases of the welded connection and bolted connection specimens in Figure 7(e), even if the maximum deflection increased, the specimens exhibited sufficient serviceability without clear degradation of the stiffness. 5 residual deflection = 11.mm (a)load-displacement curve of D78-N-S slip at the bolt connections yield of steel tube at kn 5 residual deflection = 19.3 mm (c)load-displacement curve of D78-B-S residual deflection = 9.5 mm (b) Load-displacement curve of D78-W-S. D78-BF-S yield of steel tube at kn residual deflection = 14.8 mm (d) Load-displacement curve of D78-BF-S. D78-B-S Applied Load (kn) D78-N-S D78-W-S (e) Load-displacement curves according to connection method. Figure 5. View of bolted connection. Figure 7. Static loading test results relative to the connection method of CFT members. Figure 6. Loading of the specimens. As shown in Figure 7, the welded and bolted connections presented similar initial stiffness up to about 94 kn. However, the bolted connection specimens (D78-BF-S, D78-B-S) exhibited tendency to loss stiffness due to slip occurring in the tolerance of the bolts. Especially, the specimen without filling of the connection (D78-B-S) exhibited continuous degradation of stiffness since the occurrence of slip in the bolts Table 1. Dimensions of the prototypes. Total Length No. Ext. length of of steel of diam. Thick. specimen tube tubes Fabrication (d) (t) (L) (L) (ea.) Designation conditions D78-N-S no connection D78-W-S welding D78-W-S bolt D78-BF-S bolt + filling 823

4 Figure 1. Slip of gusset plate in bolt connection. 4 Figure 8. View of tests on no-connection specimen and bolted connection specimen. Figure 9. Rebar arrangement of deck. according to the increase of load. On the other hand, the specimen of which connection was filled (D78BF-S) presented a tendency to resist against the loss of stiffness through strengthening effect of the filling material. Accordingly, when bolted connection is to be applied in actual bridge, filling of the bolted connection appears to prevent the loss of serviceability while exhibiting initial stiffness and load bearing capacity similar to non connected steel tubes. EVALUATION OF FATIGUE ENDURANCE OF CFT GIRDER BRIDGE Fatigue loading tests were conducted on CFT girderdeck composite sections to examine the fatigue behavior of the CFT girder bridge. The loaded specimens presented identical dimensions and properties to the ones used in the static loading for the comparison of the connection methods. Comparative analysis was performed for the fatigue bearing capacity according to the connection method as well as for the fatigue behavior of the CFT girder-deck sections. Loading of 25 kn was set up considering the impact factor (i =.3) for the rear wheel load of DB-24 service load. Loading was applied from 5 kn to 3 kn for fatigue test purpose (stress ratio of about.166). For the evaluation of the load bearing capacity according to the loading cycles, the displacement of the actuator and the deflection at the bottom of the center of the tube by applying a static load of 3 kn at determined loading cycles, and the residual displacement of the actuator was also measured at about 5 kn after removal of the static load. Loading was applied using MTS actuators with capacity of 5 kn and 1, kn. The velocity of fatigue loading was fixed to 3 Hz. From the fatigue test results, clear tendency of the maximum displacement to increase was not observed until 2 million loading cycles regardless of the connection method and in any specimen, and fatigue damage was not detected at service load level. The maximum deflection at each loading cycle remained below 2.5 mm, showing that the deflection is extremely small at service load level. After removal of the load, the residual deflection also exhibited extremely small value below 1 mm, revealing that fatigue damage did not occur anyway. An observation of the average maximum displacement of each specimen in the following graphs reveals that the maximum deflections showed practically no variation according to the specimens that about 2.24 mm for the no-connection specimen, about 2.74 mm for the welded specimen, about 2.4 mm for 824

5 Max average deflection = 2.24mm Max average deflection = 2.41mm Figure 11. Maximum deflection of specimen D78-N-F Figure 14. Maximum deflection of specimen D78-BF-F 3.15 the bolted specimen and, about 2.41 mm for the bolted and filled specimen. Max average deflection = 2.74mm Figure 12. Maximum deflection of specimen D78-W-F Max average deflection = 2.4mm Figure 13. Maximum deflection of specimen D78-B-F 5 CONCLUSIONS Flexural loading tests were conducted to evaluate the behavioral characteristics and load bearing capacity according to the connection method of CFT girders. Test results revealed that the non-connected CFT members and the welded and bolted connections exhibited similar performance in terms of initial stiffness and load bearing capacity. Regard to ductility, the method using bolt connection appeared to have superior behavior. Even if the welded connection behaved similarly to the non-connected CFT members, necessity remains to solve the disadvantages in time and cost consumption produced by field welding. The method adopting bolted connection being an alternative remedying such problems exhibited remarkable load resistance and stiffness together with improvement of serviceability in terms of vibration and noise of the CFT members through the filling of the connection. In addition, increase of the ductility of the CFT members could also be obtained by this method. Accordingly, for the application of CFT members for long-span bridges and, 2-span or 3-span continuous bridges, the adoption of bolted connection appears to be more effective than field welding in view of the time and economic disadvantages that can be produced by the latter. Moreover, fatigue tests were also performed to evaluate the fatigue performance of the CFT girder bridge. Results showed no clear increase of the maximum displacement until 2 million loading cycles in all the specimens and regardless of the connection method. Fatigue damage was also not detected at service load level. Therefore, the CFT girder bridge can be seen 825

6 to also have remarkable behavioral characteristics in terms of fatigue endurance. ACKNOWLDEGEMENTS This work is part of the Development of composite bridge structures using CFT girders, a R&D project of the Ministry of Construction and Transportation. The authors acknowledge for the support. REFERENCES Chin, W.J., Kang, J.Y., Choi, E.S., Lee, J.W., and Kim, B.S. 27. Experimental Study on Flexural Behavior of Concrete Filled Steel Tube Girder Bridges, IABSE Symposium 27 Weimar, Vol. 93: Leonhardt, F., Andrae, W., and Harre, W New advantageous shear connector with high fatigue strength for composite structures (Neues vorteilhaftes Verbundmittel fur Stahlverbundtragwerke mit hoher Danerfestigkeit), Beton-und Stahlbetonbau, Vol. 12: Hosaka, T., Nakamura, S., Umehara, R., and Nishiumi, K Design and experiments on a new railway bridge system using concrete filled steel pipes. International Conference of IABSE in Innsbruck. Nakamura, S. 2. New Structural forms for steel/concrete composite bridges. Structural Engineering International, Journal of IABSE. Vol. 1(1): Matsumura, T., Hosaka, T., Hiraoka, C., and Nichiumi. K. 23. Practical application of composite bridge for Shinkansen using CFT. IABSE Symposium in Antwerp. Vol. 87: Nakamura, S., Hosaka, T., and Nishiumi, K. 24. Bending behavior of steel pipe girders filled with ultralight mortar, Journal of Bridge Engineering. Vol. 9, No.3: