Finite element analysis on steel-concrete-steel sandwich composite beams with J-hook shear connectors

Transcription

1 Finite element analysis on steel-concrete-steel sandwich composite beams with J-hook shear connectors Jia-bao Yan 1), J.Y.Richard Liew 2), Min-hong Zhang 3), and Zhenyu Huang 4) 1)~4) Department of Civil and Environmental Engineering, National University of Singapore, Singapore ) 2) ABSTRACT Steel-concrete-steel (SCS) sandwich composite structure with J-hook connectors have been developed and studied in National University of Singapore. In SCS sandwich composite structures, J-hook connectors are used to achieve composite action between the steel and concrete. These J-hook connectors in the structure complex the analysis of the strength of the SCS sandwich composite beams. In this paper, a three dimensional (3D) finite element (FE) model was developed to analyze the structural behaviour of the SCS sandwich beams subject to static loading. This developed FE model can properly simulate the shear-interfacial slip behavior and tension-elongation behavior of the J-hook connector. The FE simulations on these behaviours were confirmed through verifications against the push- and pull-out tests on the J-hook connectors. Finally, the FE model of the SCS sandwich beams was verified by six tests on SCS sandwich beams with the J-hook connectors. Through the verifications, it can be observed that the FE model offered reasonable predictions on the ultimate strength behaviour of the SCS sandwich composite beams with J-hook connectors. 1. INTRODUCTION Steel-concrete-steel sandwich composite structure consisting of concrete core sandwich by two steel face plates has been developed and widely studied in recent decades. Due to its excellent cost-strength performances, it exhibits versatile potential applications in the building and offshore structures as building core, floors, submerged tunnels, offshore decks, ship hulls, and oil containment. Generally, cohesive material (e.g. epoxy) and mechanical shear connectors are the common measures to bond the 1) Research Fellow 2) Professor 3) Professor 4) Graduate student 373

2 steel and concrete. Compared with cohesive materials, the mechanical shear connectors exhibited advantages in providing transverse shear resistance to the structure (Solomon, 1976). Many types of connectors have been developed and used in the SCS sandwich composite structure. SCS sandwich structure with overlapped headed shear studs i.e. double skin structure was originally proposed for submerged tunnels (Narayanan et al., 1987). However, the pullout resistance of headed studs was smaller compared with bar connectors in bi-steel structures. Though bi-steel structures could provide high resistance, the equipment for the friction welding of connectors in bisteel structure limited its thickness within.2 m to.7m and the connectors to a constant diameter of 25 mm (Foundoukos, 5). J-hook connectors could overcome this disadvantage and have no limit on the thickness of the SCS sandwich structure that can be used in the slim deck structure (Liew et al., 8). Moreover, the interlocked J-hook connectors worked in pairs and could provide certain degree of tensile strength even without confinement of the concrete compared with the headed studs in double skin structure. Lightweight concrete was used in the SCS sandwich composite structure to reduce the self weight. Lightweight concrete with a compressive strength of 3 MPa and a density of 145 kg/m 3 was used in the SCS sandwich composite structure as the core material (Sohel, 8; Dai, 8). Ultra-lightweight cement composite with the density of 145 kg/m 3 but higher 6 MPa compressive strength has been developed and was used in the SCS sandwich structure (Yan et al., 12). SCS sandwich composite structure with J-hook connectors and ULCC has been developed by Yan (12). Since various types of SCS sandwich composite structures have been developed. In recent years, extensive researches have been carried out on finite element analysis (FEA) on this type of structure (Shanmugam et al., 2; El-Lobody and Lam, 3; Foundoukos and Chapman, 8; Mirza and Uy, 1). However, these works were mainly developed for SCS sandwich composite structure with headed shear studs. In this paper, a three-dimensional (3D) finite element (FE) model was proposed to describe the structural behavior of the SCS sandwich composite beams with the J-hook connectors. The developed 3D FE model was firstly validated against the push-out tests and tensile tests of the J-hook connectors to confirm its simulation on basic components of the SCS sandwich structure with J-hook. Then, the FE model was further validated by six static tests on the SCS sandwich beams. 2. Experimental works Push-out and tensile tests were usually used to obtain shear and tensile behavior of the J-hook connectors. In this report, six push-out specimens and tensile test specimens were prepared to investigate the shear and tensile strength of the J-hook connectors in the ultra-lightweight cement composite (ULCC), respectively. Six steelconcrete-steel (SCS) sandwich composite beams were prepared to investigate their structural behaviors. The details of the specimens for push-out and tensile tests are listed in Table 1 and Table 2, respectively. Details of the six SCS sandwich composite beams with J-hook connectors are listed in Table

3 Specimen t mm Table 1 Details of push-out test specimens d mm h c mm h s mm B mm u MPa f ck MPa E c GPa w kg/m 3 PU PU PU PU PU PU *ULCC= ultra-lightweight cement composite; σ u = ultimate strength of connector; f ck = compressive strength of concrete cylinder; B, d, h c, h s, and t are as shown in Fig. 1; E c = elastic modulus; w= density of concrete. Table 2 Details of the tensile test specimens Specimen t h s h c d Fiber by σ y σ u f ck (mm) (mm) (mm) (mm) volume MPa MPa MPa TU TU TU TU % TU TU T hc d h c T Elevation View Top View Fig. 1 Push-out test of J-hook connectors Fig. 2 Tensile test of J-hook connectors Table 3 Details of steel-concrete-steel sandwich composite beam Specim en t c &t s (mm) d (mm) S x (mm) L (mm) σ u (MPa) f y (MPa) f c (MPa) E c (GPa) J J J J J J *t c, t t = thickness of the steel plate under compression and tension, respectively; d =diameter of the connectors; S =spacing of the connectors in the beam; L=span of the beam; E c =elastic modulus of concrete; σ u =ultimate strength of the connectors; f y =yield strength of steel plate. 375

4 The test setup and geometry of these six beams are shown in Fig. 3. (a) Test setup F for beam J1~4 (b) Test setup for J5 (c) Test setup for beam J6 Fig. 3 Test setup of the nine sandwich beams 3. Finite element model General commercial software package ABAQUS was used for the developed finite element model. Eight-node brick element (C3D8R) with reduced integration stiffness and hourglass controlled was used to model the steel plates, core material-ulcc, and connectors. In order to simplify the geometry of the J-hook connectors, three-dimensional nonlinear spring element was used to simulate the tension-elongation behavior of the J-hook connectors. This simplification is shown in Fig. 4. Spring Spring Symmetrica l restraints Fig. 4 Simplifications of the J-hook Fig. 5 FE model for push-out test The FE model for the push-out test is shown in Fig. 5. Considering the symmetry of the structure, only half of the specimen is modeled. The symmetrical restraints were applied to the surface as shown in Fig

5 The FE model for the SCS sandwich beams is shown in Fig. 6. Considering the symmetry, only one fourth of the model is built. All the interlocked J-hook connectors were simulated two straight connectors that were linked by the spring element as shown in Fig. 6. Fig. 6 Finite element model for the SCS sandwich beam An elastic-plastic isotropic model was assigned to the steel plates, connectors, and load cell. The stress-strain relationship is as described in Fig. 7. Concrete damage model was used for the ULCC. The compressive and tensile stress-strain behaviors of the ULCC and steel are shown in Fig. 7 and 8, respectively. Fig. 7 Stress-strain model of steel Fig. 8 Stress-strain behaviour of ULCC Hard contact in the normal direction to the steel-concrete interaction surface and friction penalty contact along the interaction surface were defined to simulate the 377

6 interaction between the steel face plates and concrete core, and between the shear connectors and ULCC. AQAQUS standard solver is used for the solution. Static displacement loading is applied as shown in Fig. 5 and 6 for push-out test and beam test, respectively. 4. Validation of the finite element model 4.1 Validation against the push-out and tensile test The finite element model (FE) was validated against the six push-out tests as listed in Table 1. The load-slip curves by the FE model were compared with the experimental curves in Fig. 9. The tension-elongation behaviors of the J-hook connector obtained by the FE model were compared with the test ones in Fig PU1 Test PU1 FE (a) PU3 Test PU3 FE (c) PU2 Test PU2 FE (b) PU4 Test PU4 FE (d) 378

7 PU5 Test PU5 FE (e) (f) Fig. 9 Validation of the load-slip curves for FE model PU6 Test PU6 FE TU1 TU3 TU4 TU5 FE model 1 3 Elongation (mm) Fig. 1 Comparison of load-elongation curve between the tests and FE analysis Table 4 Comparisons between FE predictions and test results Item P Test (kn) P FE (kn) P Test /P FE PU PU PU PU PU PU Mean 1.2 Cov.5 From the above comparisons, it can be seen that the load-slip curves of the FE model resembles well the experimental ones. The average test-to-prediction ratio of the ultimate shear strength is 1.2 with a coefficient of variance (Cov) of.5. Moreover, the axial tension-elongation of the FE model agrees well with the test curves. Therefore, 379

8 it can be concluded that the developed FE model is capable of describing the structural behavior of the J-hook connectors under shear and axial tensile forces. 4.2 Validation against the beam tests The load-central deflection curves of the beams obtained by the FE model are compared with the corresponding experimental ones in Fig. 11a~d. From these figures, it can be seen that the load-central deflection curves by the FE model are close to the test ones both in displacements and ultimate strength. (a) (b) (c) (d) Fig. 11 Comparison of load-deflection curves between test and FE analysis 38

9 The ultimate strengths of the FE results are compared with the experimental ones in Table 5. From this table, it can be found that the average FE prediction-to-test ratio is.95 with a standard deviation of.11. Table 5 Comparisons between the FE prediction and the test results Specimen J1 J2 J3 J4 J5 J6 Mean Cov P FE P Test P FE /P Test The FE predicted deformed shapes as well as the cracks in the core material are compared with the experimental ones in Fig. 12. Most of the main cracks in the core material observed from the tests were identical to those found in the FE analysis results. (a) Comparisons of cracks between the test and FE of specimen J6 (b) Comparisons of cracks between the test and FE of specimen J5 (c) Comparisons of cracks between the test and FE of specimen J2 381

10 (d) Comparisons of cracks between the test and FE of specimen J1 Fig. 12 Comparisons of the cracks in the core material between the FE model and test The FE model offers an effective mean on predicting ultimate strength, describing load-deflection behaviours, and predicting shear cracks in the concrete for the SCS sandwich beams with J-hook connectors. 5. Conclusions This paper presented a three-dimensional finite element model to describe the structural behavior of the steel-concrete-steel sandwich composite beams with J-hook connectors. This 3D nonlinear FE model simplified complex J-hook connectors by using the three-dimensional spring element. This simplification was validated by the push-out tests and tensile tests that are used to obtain the shear and axial tension behaviors of the J-hook connectors. The validation proved that this simplification in the proposed FE model can capture the structural behavior but reduce the quantity of the elements, simplify the contact and avoid convergence problems. The FE model for the SCS sandwich beams were validated by six beam tests. Through the verifications of the FE analysis results against the test ones, it was observed that the FE model can offer useful means on predicting ultimate strengths, shear cracks in the core material, deflections and deformed shapes of the SCS sandwich beams with J-hook connectors. All these imply that this proposed FE model is an effective method to the nonlinear analysis of the SCS sandwich composite beams with J-hook connectors under static loading. Acknowledgement The research described herein was funded by the Maritime and Port Authority of Singapore, American Bureau of Shipping (ABS) and National University of Singapore under research project titled Curved steel-concrete-steel sandwich composite for Arctic region (Project No. R ). The authors gratefully express their gratitude for the supports. REFERENCES Dai, X.X. (8), Fatigue analysis and design of steel-concrete-steel sandwich composite structures, Ph.D thesis, National University of Singapore, Singapore. 382

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