Mechanical Performance Of Prestressed Concrete Composite Slabs With Multi-steel Bar Trusses And Concrete Flanges

In recent years,China has been vigorously developing assembled buildings.In the assembled concrete building,the most commonly used prefabricated component is the composite slab.In the practical projects,the cost of composite slabs is higher than that of castin-place slabs,which is a common problem in composite slabs,the main reasons is the support mode of the prefabricated bottom slab in the construction stage is complex.If the construction stage is fully considered in the process of designing composite slabs,and the use of unsupported composite slabs will to some extent reduce costs and improve construction efficiency.Prestressed composite slab with multi-steel bars truss and concrete ribs is a new type of composite floor structure,which is composed of lattice girder,concrete ribs and concrete bottom plate.The concrete rib and bottom plate are connected by lattice girder.This kind of composite slab has large bending rigidity and bearing capacity,and it is convenient to lay water pipelines during construction,and it can realize construction without support if properly designed.In order to study the flexural performance of this composite slab,the following work has been done in this paper:(1)The bending stiffness,bearing capacity,crack resistance and deflection of the prestressed concrete composite slab with multiple steel bar trusses are analyzed theoretically,the relevant design formulas are summarized,and the stability checking formula of the steel bar truss diagonals member not involved in the code is proposed to perfect the design method.According to the design method,the control index of the free-support design of the prestressed concrete composite slab with multiple steel bar trusses under different spans and live loads is obtained as the crack resistance calculation of the bottom slab in the construction stage.(2)The bearing capacity,cracking load,crack distribution,and deflection change of the slabs under distributed load are studied through the test of 7 prefabricated bottom slabs and 5composite slabs.The fabrication process of the test composite slab elements simulated the actual stress state of the construction without support.The results show that the prefabricated bottom slab with a span of 3.3m can meet the requirement of free support during the construction stage,and the composite layer and prefabricated bottom slab can cooperate in the use stage,and the flexural performance of the bottom slab,composite slab in the span direction and vertical direction meets the design requirements.(3)The finite element analysis software,ABAQUS,is used to establish the model of prestressed composite slab with multi-steel bars truss and concrete ribs and carry out parameter analysis,mainly to study the stress performance of the precast bottom slab in the nonprestressed direction and the setting position of the lifting point of the bottom slabs.It can be seen from the analysis that the number of steel trusses has influence on the flexural stiffness of precast bottom slab after cracking,and the number of trusses has more stiffness after cracking;the reasonable dimensional parameters of concrete flange and steel truss position in the direction of vertical prestressing tendons are determined through calculation and analysis;When the cantilever length of the prefabricated bottom slab is not more than 400 mm and there are two steel trusses under the rib,the concrete stress in the non-prestressed direction is small and there will be no cracks parallel to the span direction of the bottom slab;When the cantilever length of the bottom plate is 300 mm or 350 mm,the rib spacing shall not exceed 800mm;when the cantilever length of the bottom plate is 400 mm,the rib spacing shall not exceed 900mm;The bottom slab stress which lifting point is set on the concrete flange is smaller than the lifting point is set on the bottom plate.Lifting point which set on the concrete flange is more difficult to crack.(4)Two key issues of the design of the composite slabs without support are summarized.One is to design as a one-way slab,and the other is to reasonably controlling the stiffness of the bottom slab.Further proposing the design process of unsupported composite slabs.Firstly,the appropriate type of composite slab should be selected,and then the reinforcement and key size parameters of the composite slab(thickness of the bottom slab,thickness and width of the concrete rib,etc.)should be determined through iterative calculation.The engineering case design was completed by using the design method and process proposed in this article.