Research On Seismic Performance Of Concrete Composite Slabs With Preecast Ribbed Bottom Pabels

In the paper,14geometrically identical full-size square RC panels were constructed and tested under quasi-static reversed cyclic lateral loading, in order to investigate the secismic performance of composite slab with precast ribbed bottom panels. In this test, no vertical load was applied to specimens. No surface finishing was conducted on the precast ribbed panels before the concrete topping was cast. Among14specimens, two of which have not boundary beams on both sides:one is a composite slab with precast ribbed panels and the other is a comparing cast-in-situ RC slab. Twelve specimens have boundary beams on both sides, including two comparing cast-in-situ RC slabs and ten composite slabs with precast ribbed bottom panels. Among ten composite slabs, two of which were standard composite slabs reinforced according to GanSu standard design; one of which was not reinforced to resist structural negative moment, and one of which was reinforced doubly to resist structural negative moment; two of which were loaded in the direction parallel to slab joints of precast ribbed panels:one specimen was reinforced the same as standard composite slabs, the other was reinforced doubly to resist structural negative moment; one of which was not reinforced in transverse direction, and one of which was reinforced doubly in transverse direction; and the other two are specimens which was, respectively, subjected to low-frequency repeated horizontal loading and reinforced along the slab joints between precast ribbed panels.According to test results, the crack of composite slab are less in number than that of cast-in-situ slab and mainly distributed over the areas below one third height of center composite slab. Several splayed cracks are observed as the loading direction is prependicular to the slab joints while several horizontal cracks are observed as the loading direction is parallel to the slab joints. The boundary beams can improve the distributed areas of cracks and increase the plastic zones at the bottom of composite slabs. The cracks distributed over larger areas with more structural negative moment reinforcements and transverse reinforcements. Three failure modes was observed for composite slabs with precast ribbed bottom panels. One was that several longitudinal cracks at the connections between composite slab and bottom beams were observed for most specimens. The second was that the slab joints cracked seriouly as the loading direction is parallel to slab joints. The third was that some lognitudinal cracks on cast-in-situ concrete layer was observed along the rib of precast slab, mainly due to poor manufacture quality.Some longitudinal reinforcements in boundary beams were tensed to failure and concrete at the bottom of boundary beams was crushed to failure. The failure mode of composite slabs is controlled by bending moment at the bottom of composite slab, also with a small amount of shear deformation.The hysteretic curves and skeleton curves of specimens were drawn according to test results. Both for composite slabs and cast-in-situ slabs, the hysteretic curves were developed to Z shape at ultimate state which indicates pinch phenomenon of hysteretic curves. It is known that composite slabs still remain much potential for deformation after cracking state; the cracking load is almost the half of peak load; the energy dissipation capacity of composite slabs is as good as that of cast-in-situ slabs. It is also shown that the boudary beams and structural negative moment reinforcements can both increase the deformability, load-carrying capacity and energy dissipation capacity for composite slabs.A mechanical model based on the strut-and-tie theory was proposed to calculate the load-capacity of composite slabs. Some finite element models for specimens are created to conduct parameter analysis. A modified formula of the depth of strut for composite slabs with precast ribbed bottom panels was proposed on the base of experiment and finite element simulation results. It was indicated that the theoretical and experimental load-carrying capacity agreed very well.A formula of initial stiffness of the composite slabs with precast ribbed bottom panels was proposed and the relationships between the stiffness of connection between adjacent feature points and initial stiffness was determined on the basis of degenerate four-linear skeleton curves. Then the relationships between crack bearing capacity, yield bearing capacity, ultimate bearing capacity and peak bearing capacity were respectively established. A mothod for calculating the top displacements of specimens was then proposed. The skeleton curves and hysteretic curves of specimens were finally calculated and both were in good agreement with test ones.Several damage indexes were evaluated and the mixed damage index between displacement and energy dissipation was better to reflect the damage of the composite slab with precast ribbed bottom panels. The results showed that the boundary beams, structural negative moment reinforcements and transverse reinforcements can reduce the damage of composite slabs.At last, a method for calculating seismic bearing capacity and deformation of the composite slab with precast ribbed bottom panels has been proposed. And some details of seismic design has been put forward.According to experimental and theoretical study, composite slabs with precast ribbed bottom panels demonstrate good integrity. The application of composite slabs with precast ribbed bottom panels is the same with that of cast-in-situ slabs, which indicates that composite slabs precast ribbed bottom panels should not be limited on buildings with height below50meters.