Effects of Group Arrangement on the Ultimate Strength of Stud Shear Connection

Transcription

1 Effects of Group Arrangement on the Ultimate Strength of Stud Shear Connection Chang-Su Shim Department of Civil Engineering, Chung-Ang University Ansungsi, Korea Pil-Goo Lee Civil Engineering Research Department, RIST Hwasung, Korea Dong-Wook Kim Department of Civil Engineering, Chung-Ang University Ansungsi, Korea Chul-Hun Chung Department of Civil Engineering, Dankook University Yonginsi, Korea ABSTRACT For the design of shear connection for region of highly concentrated shear force in steelconcrete composite bridges, connection details are the most important design issues. Failure modes of the shear connection govern the ultimate strength for the design and governing design parameters can be changed. Instead of rigid shear connectors, this paper deals with the group stud shear connection for the precast decks. Shear pockets for stud connectors arise difficulty in the details of precast decks. Push-out tests were conducted to evaluate the ultimate strength according to the expected failure modes. Main parameters of the test were stud spacing, reinforcement details and stud diameter. Test results showed that current design provisions for the stud connectors can be used for the design of group stud shear connection when the design requirements on the minimum spacing of studs are satisfied and the splitting failure of concrete slab is prevented. An empirical equation was proposed to consider the effect of stud spacing when the spacing is less than the minimum requirement. Fatigue tests showed that the group stud connectors with spacing of more than three times of diameter has similar fatigue life with current design codes. Based on the test results, design recommendations for shear connection in a precast deck bridge were derived.

2 INTRODUCTION There are various short and medium span composite bridges varying in section, connection details and method of construction. For these steel-concrete composite or hybrid girders, proper shear connection details are required. Connectors are embedded in a concrete medium and impart highly concentrated forces onto the concrete element. This concentrated load can cause the concrete to fail in tension by embedment cracking, ripping, shear and splitting resulting in the decrease of ultimate strength of shear connection. Failure modes of the shear connection can be guided according to the ratio of the shear strength of mechanical connectors and concrete strength. Stud shear connectors are the most common type of mechanical connector used and need to be arranged according to the design provisions on minimum and maximum spacing requirements. Due to the constraint, rigid connection such as perfobond connectors is frequently used for the high shear regions with small area for the connectors. It is difficult to utilize the rigid connector for the shear connection of precast decks and composite truss joints because reinforcement details cannot be easily accommodated in the pocket area or in the narrow joint area. Furthermore, the strength of the connection is usually governed by the strength of the concrete slab. In order to resolve these limitations, group stud shear connection with relatively large studs is proposed and details for the connection have been investigated (Shim et al., 2004). Figure 1 shows the typical examples of the shear connection detail dealt in this paper. Pavement Mortar Stud Precast Deck Rubber Strip Steel Girder (a) shear connection Transverse reinforcement Shear pocket Longitudinal reinforcement Stud connector (b) details of precast deck Sheath for longitudinal tendons Fig 1- Shear connection for precast deck

3 Stud shear connectors are influenced by several parameters according to previous researchers, with major factors categorized into shank diameter, height and tensile strength of studs, compressive strength and elastic modulus of concrete, and direction of concrete casting. In addition for shear connection in precast deck bridges, material properties of filling material and bedding height must also be considered for the evaluation of structural performance of stud shear connection (Shim, 2000; Shim, 2001). Large studs up to 31.8mm diameter were experimentally investigated and availability of the current design provisions on stud shear connectors was verified based on the test results (Badie et al., 2002, Shim et al., 2004). Hanswille et al. (2007) examined the effects of the loading sequence and damage accumulation on the fatigue life and showed early reduction of the static strength from the damage. The failure modes of shear connection can be categorized according to relative strength of surrounding concrete and stud connectors. Mode-1 is defined as stud failure without considerable concrete damage. Mode-3 means the concrete failure without stud failure. When the connectors are failed after considerable concrete damage, it can be defined as Mode-2. In this paper, two series of group arrangement of stud connectors were dealt with. One is the shear connection with 25mm studs and relatively stronger concrete slab resulting in stud failure. The other is the group stud connection for precast decks including internal and external reinforcements to increase the bearing strength and the splitting strength of the concrete slab, respectively. Based on the failure modes, three design equations from current design codes were verified and proper adjustment factors were suggested. General design guidelines for the group stud connectors were discussed. EXPERIMENTAL PROGRAM In order to evaluate the effect of stud spacing on the static and fatigue strength of shear connection, push-out specimens with group arrangement were fabricated. Nine 25mm and 22mm studs were welded at each flange with different stud spacings, such as 5d, 4d and 3d. In order to strengthen the shear strength of the concrete slab, additional reinforcements were placed inside and outside of the shear pockets. Table 1 summarizes the test specimens. Compressive strength of concrete was designed to have 35MPa. Figure 2 shows the push-out specimen for precast decks and CIP slab. Precast decks with 250mm thickness were prefabricated and were combined with steel beam by filling non-shrink mortar in shear pockets. Nine studs were arranged at each face by stud welding gun. External reinforcements were placed before casting concrete of the precast decks. Internal reinforcements were put in the shear pockets after placing the slab on the steel beam. Dimensions of the shear pocket were the same for all the specimens. The stud spacing less than 3d may have problem on workability of welding gun. Two additional test results were used from the previous tests on shear connection in precast decks [Shim, 2000] and six specimens on shear connection in cast-in-place concrete slab were referred [Shim, 2004]. According to the failure modes of shear connection, a proper design equation needs to be used. The effect of stud spacing on the static strength can be expected when the surrounding concrete has some damage under relatively low shear load. Therefore, the stud failure after concrete crushing was intended for some specimens in order to estimate the effect of stud spacing. Generally, stud spacing is decided from fatigue design. Four specimens were fabricated to evaluate the effect of group arrangement on the fatigue endurance and were compared with previous test results [Shim, 2000 & Shim, 2004]. The stud spacing was smaller than the current minimum requirement in design codes [Eurocode-4, 1997]. Fatigue endurance of group stud shear connectors should be verified for the enhanced details of shear connection for precast

4 decks. In relatively low stress level, the effect of group arrangement is expected to have little effect. Therefore, the stress level was decided to have low cycle fatigue judging from the previous test results. This assumption can provide more severe condition to the shear connection with group arrangement. The fatigue endurance of group stud shear connection was compared with that of previous tests. Table 1- Static test specimen for precast decks Compressive strength of Specimen mortar Compressive Shank Strength of Bedding Stud diameter concrete height spacing (mm) 2 2 (mm) (mm) ( N / mm ) ( N / mm ) Reinforcement G25NS No additional r.f. G25OS d External r.f.(d16) G25IS Internal r.f.(d10) G25OS d External r.f.(d16) G25NS No additional r.f. G25OS d External r.f.(d16) G25IS Internal r.f.(d10) G22OS External r.f.(d16) 4d G22IS Internal r.f.(d10) G22OS External r.f.(d16) 3d G22IS Internal r.f.(d10) CIP25A d No additional r.f. CIP25A d No additional r.f. CIP25A d No additional r.f. CIP25B d No additional r.f. CIP25B d No additional r.f. CIP25B d No additional r.f. S22A d No additional r.f. S22B d No additional r.f. (a) G-series specimen

5 (b) CIP series specimen (c) S series specimen Fig. 2- Test Specimen Table 2- Fatigue tests for shear connection Compressive Specimen strength of mortar(mpa) Compressive strength of concrete (MPa) Stud spacing Stress range (MPa) FG25OS d 130 FG25OS d 150 FG25OS d 130 FG25OS d 150 F150A d F170A d F130B d F150B d F180B d F130C d F150C d F180C d F130C d 151.1

6 STATIC STRENGTH OF GROUP STUD SHEAR CONNECTION Test specimens showed different failure modes according to relative strength ratio between concrete slab and stud connectors. For group stud connection of precast decks, closer spacing reduced the shear strength up to 30% when the failure mode is stud failure after concrete cracking of the slab. External reinforcements increased post-cracking strength of the concrete slab while internal reinforcement increased bearing strength a little. Therefore, it is important to strengthen the concrete slab when group stud connectors are used. When the failure mode is splitting failure of the concrete slab without stud failure, current design provisions on shear strength of concrete slab are appropriate to evaluate the strength of the connection. In order to allow the particular design situation of closer stud spacing than the design requirement, it is necessary to provide an empirical equation for the shear connection in precast decks. Based on test results including the previous research [Shim et. al 2000, 2001], an empirical equation (1) for the reduction factor of stud spacing is proposed by linear regression analysis as in Figure 3. When the stud spacing is smaller than three times of stud diameter, the ultimate strength of the shear connection can be evaluated using the equation. The equation considers stud failure after concrete cracking. Although this equation needs improvement by more tests, it showed clear view on the effect of stud spacing. In the equation, the d s can be used as 5 when the stud spacing is greater than five times of shank diameter of stud. βs = 0.174d βs = 1 s for 3 d for d s s 5 > 5 where, d s is the stud spacing multiplier to stud diameter. When the failure mode of the shear connection is the concrete slab failure, the ultimate strength of the shear connection should be the shear strength of the concrete slab. Local strengthening reinforcements need to be considered in the evaluation of the shear strength of concrete slab. For the group arrangement of stud connectors, it is necessary to estimate the shear strength of shear connection and concrete slab. For the shear connection in precast decks, the internal strengthening is less effective than the external strengthening. (1) reduction factor stud spacing(multiply by stud diameter) Fig. 3 Effect of stud spacing on the strength reduction

7 FATIGUE STRENGTH OF GROUP STUD SHEAR CONNECTION Normally, the stud spacing is determined from the fatigue design. In order to verify the fatigue endurance of the shear connection with group arrangement, fatigue tests were conducted for shear connection precast decks. When the static failure mode of the shear connection is stud failure with negligible damage of concrete slab, fatigue endurance of the shear connection with group arrangement was greater than the design value. Figure 4 shows the S-N curves of the shear connection. For the shear connection in precast decks which showed stud failure with shear cracking of concrete slab, fatigue endurance is similar with the design value from Eurocode-4 and also with previous test results of the shear connection for precast decks without group arrangement. From this result, the group arrangement of stud connectors in high shear region can be adopted effectively. Even though the ultimate strength of the shear connection is reduced, the fatigue endurance will be similar with current design value. Fig. 4 Fatigue endurance (a) steel part (b) concrete part Fig. 5- Fatigue failure mode

8 CONCLUSION In high shear region, we need stronger shear connection. Group stud shear connection is dealt with in this paper in terms of static strength and fatigue endurance. Push-out tests were conducted for the shear connection in precast decks. For the specimens of precast deck bridges, the effect of the stud spacing and confining reinforcements was clearly observed. Decreasing the stud spacing resulted in lower ultimate strength of the shear connection. The confining reinforcements inside and outside of the shear pocket can enhance the shear strength of the shear connection. The requirement of the minimum spacing for the stud connectors needs to be revised for precast decks. However, the shear connection with smaller spacing should have adequate reinforcement details to increase the failure load of concrete slab. In this paper, the empirical equation was proposed and fatigue endurance of the shear connection with group arrangement was verified. REFERENCES Badie, S.S. Tadros, M.K. Kakish, H.F. Splittgerber, D.L. Baishya M.C. 2002, Large Shear Studs for Composite Action in Steel Bridge Girders, J. of Bridge Engineering, Vol. 7, No. 3, May, pp Eurocode 4, 1997, Design of composite steel and concrete structures, Part 2 : composite bridges (ENV ), CEN Hanswille, G. Porsch, M. Ustundag, C Resistance of headed studs subjected to fatigue loading Part I: Experimental study, Journal of Constructional Steel Research, 63, pp Minami, H. Yamamura, M. Taira, Y. Furuichi, K. 2004, Design of the KINOKAWA Viaduct Composite Truss Bridge, Proceedings 1st Congress, Composite Structures, pp Oehlers, D.J. Bradford, M.A Composite Steel & Concrete Structural Members; Fundamental Be-havior, PERGAMON. Shim, C.-S. Lee, P.-G. Yoon, T.-Y Static behavior of large stud shear connectors, Engineering Structures, 26(12), pp Shim, C.S. Kim, Chung, C.H. and Chang, S.P The Behavior of Shear Connection in Composite Beam with Full-Depth Precast Slab, Structures and Buildings, The Institution of Civil Engineers, Jan., Vol. 140, pp Shim, C.S. Chang, S.P. Lee P.G Design of Shear Connection in Composite Steel and Concrete Bridges with Precast Decks," Journal of Constructional Steel Research, 57 pp Shim, C.S. Park, J.S. Jeon, S.M. Kim, D.W Experimental Study on Group Stud Shear Connection, Proceeding of 5th International Conference on Advances in Steel Structures, pp Tsujimura, T. Shoji, A. Noro, T. Muroi, S Experimental Study on a Joint in Prestressed Concrete Bridge with Steel Truss Web, Proceedings 1st Congress, Composite Structures, pp