CHAPTER 5 Strength of steel-concrete-steel sandwich composite beams with ultra
5.5 Beam test results and discussion
Test results of the SCS sandwich composite beams are described by 1) Load-deflection behaviour; 2) failure modes and deformed shape; 3) maximum load carrying capacity; 4) effects of the investigated parameters.
5.5.1 Load deflection behaviour
Load-deflection curves of SCS sandwich beams designed with different parameters are shown in Fig. 5.14. From the load-deflection curves, it exhibits three main types of behaviors corresponding to different failure modes. The first type of behavior of load- deflection curve occurred to beams J7, J2-3, B7 and B2-3. From the curve, it can be seen that the strength increases linearly to around 70% of the maximum value as the central deflection increases. After that, the increasing rate of the load carrying capacity decreases significantly but the strength still keeps increasing with large deflection developed. The central deflections even achieve extremely high values of about 70mm. The load- deflection curves exhibit a plateau. The second type of load-deflection curves combine the first and second types that occurred to beams J6 and B6. These three types of load- deflection curves correspond to different types of failure modes. The third typical load- deflection curve occurred to the rest beams. The strength of the beam increases almost linearly up to the maximum value as deflection increases. Then, the curve exhibits a sharp drop after the maximum value.
5.5.2 Failure modes & maximum loads
Maximum loads and failure modes observed from the test are listed in Table 5.9. Three main failure modes observed from the tests were flexural failure, vertical shear failure and connector failure. Typical flexural failure and vertical shear failure are shown in Fig. 5.15
‐ 196 - and Fig. 5.16, respectively. Connector failure modes are shown in Fig. 5.17. In the flexural failure, micro vertical cracks were observed to be developed from the bottom flange to the top flange. The tension steel plate yielded and the structure exhibited a large deformation. For the vertical shear failure, the diagonal shear cracks were observed in the core material linking the bottom and the loading point. At the final stage, the main shear cracks were observed to link the loading point and the support. For the combined failure mode, micro cracks were firstly developed in the pure bending region from the bottom flange to the top. After that, the structure exhibited a certain degree of ductility with a large deflection developed. Finally, the beams failed in shear modes.
5.5.3 Effect of shear span-to-beam thickness ratio
Three types of shear span-to-slab thickness ratios (L/H, L is span; H is depth) were used for beams both in B series and J series. The investigated L/H ratios were 2.3, 3.4 and 6.1 for beams B2-1, B6 and B7 respectively. The same ratios were set to J2-1, J6 and J7 respectively. The load carrying capacities of the beams are listed in the Table 5.9. The effect of the shear span on the strength of the beam is shown in Fig. 5.14(a).
The section of the beam is usually under combined bending moment and vertical shear force when the concentrated load is applied to the beam. The bending moment acted on the section is greatly influenced by the shear span due to its changing on the level arms of the force. From Fig. 5.14(a), it can be seen that the failure modes of the beams in both B series and J series change from typical shear failure to the flexural failure when the L/H ratio increases from 2.3 to 6.1. For beams B6 and J6, a combined shear and flexural failure occurred to them due to their intermediate shear span-to-beam thickness ratios.
‐ 197 - 5.5.4 Effect of thickness of the steel face plate
Steel plates with three different thicknesses 4, 6, and 12 mm were used for beams B1, B2- 1, and B3 respectively. Steel plates with the same thicknesses were used for beams J1, J2- 1 and J3 respectively. The material properties of the steel plates are given in Table 5.1.
The load-deflection curves of the beams investigating this influence are plotted in Fig.
5.14(b) & (c). The ultimate strength and failure modes of these beams are illustrated in Table 5.9.
Based on these information in Figs. 5.15 (b) and (c) and Table 5.9, several observations and remarks are given as follows:
1) Using higher thickness steel plate is an effective way to improve the shear strength of the structure. This can be explained by that the thickness of the steel plate increases the effective depth of the cross section. The effective depth of the beam can be calculated by Eqn. (5.19b). According to this formula, the relationship between the ultimate strength of the beam and effective depth of the beam section is linear. This linear relationship is shown in Fig. 5.14 (d).
2) The load carrying capacity of the beam increases by about 10% when the thickness increases from 4 to 6 mm for beams with headed shear connectors whilst this increment is around 20% for beams with J-hook shear connectors. When the thickness of the surface plates increase from 6 to 12 mm, the load carrying capacities of the beams increase by 90% and 59% for beams with headed shear studs and J-hook connectors, respectively. All these increments in shear strength of the beam were caused by the thickness of the steel face plates that increased the effective depth of the beams.
5.5.5 Effect of core material strength
Limited by the single strength of the ULCC, in order to investigate the influence of
‐ 198 - strength of the in-filled core material to the load carrying capacity, three different grades but different types of core material were used in the SCS sandwich beams that are ULCC (fck =60 MPa), a lightweight aggregate concrete (LWC) (fck =25 MPa) and HPC (fck =160 MPa). The influence of the core material on the global load-deflection curves is shown in Fig. 5.14 (e).
From Fig. 5.14(e), it can be seen that increasing strength of the core material not only increases the load carrying capacity of the beams but also improves the ductility (see load-deflection curves of beams B2-3 and J2-3 in Fig. 5.14(e). This phenomenon can be explained by that the higher strength core material leads to higher tensile strength and compressive strength. These higher strengths increase higher longitudinal shear strength and tensile strength of the connectors. Moreover, higher strength concrete also provides higher compression force of the concrete that leads to larger bending moment capacity of the beam section. All these increments finally increase both shear resistance and bending moment capacity of the structure. These increased strengths change the failure modes of the beam from the shear failure to flexural bending failure. From Fig. 5.14(e), it can be observed that the ductility of the beam with HPC is increased by more than five times compared with the beam with ULCC.
The relationship between the ultimate shear strength and the strength of the core material is shown in Fig. 5.14(f). From this figure, it is found that the shear strength of the beam is proportional to 0.5 times of the core material’s compressive strength. This observation is consistent with the design specifications in ACI 318 where it also specified that the shear strength is also proportional to 0.5 times of the cylindrical compressive strength of the concrete.
‐ 199 - 5.5.6 Effect of spacing of the shear connectors
Beams with connectors’ spacing of 100 mm, 150 mm, and 200 mm are tested in order to investigate this influence. B2-1, B4, and B5 in B-series and J2-1, J4, and J5 in J-series beams were set for the influence of the spacing of the connectors. The test results are shown in Table 5.9 and the load-deflection curves are plotted in Fig. 5.14 (g) & (h).
Specimens with connectors in 100 mm spacing exhibited close ultimate load carrying capacities to the specimens with 150 mm spacing of connectors (See Table 5.9). This is because that the quantity of shear connectors is the same in these two beams that contributes to the shear resistance within each shear span (i.e. from support to load point).