Experimental Study On Deformation And Stress Of Steel Truss Floor Slab During Concrete Pouring And Setting Hardening Stage

The steel truss floor board is composed of prefabricated steel truss,galvanized steel plate bottom mold and post-casted concrete.The self-weight and construction load in concrete pouring stage are carried by steel truss and galvanized steel plate which is used as formwork as well.Recently,with the great development of prefabricated buildings,steel truss floor board has been widely used by its advantages such as no site support,easy installation,good overall connectivity,multi-floor construction simultaneously,obvious reduction of construction period and economic cost.According to the previous experience of the construction,the floor deformation is large and the cracking is usual,but there are few researches on this aspect.Based on the actual project,simulation test has been executed by the deformation and stress of four-span continuous steel truss slab at concrete pouring stage.Deriving from simulation test,under same condition,deformation and stress research is conducted in two four-span continuous steel truss slabs concrete pouring and hardening.In these samples,one of them has been added supports while another one has not.This paper analyzes the stress and deformation of concrete slab during concrete pouring,also the change of structural deformation and stress state caused by the change of structural stress system when the concrete set and certain strength has been acquired initially.The main research results are as follows:1.The results of simulating concrete pouring construction process demonstrate that: Under the action of concrete self-weight and construction load,floor board and steel secondary beam have large deformation.The mid-span deformation of side slab belt with large span exceeds the allowable limit of the standard.For same span length,the deformation of side slab belt is much larger than that of internal slab belt.After uninstalling load,some formworks of slabs have large residual deformation,which indicates that the stress in advance exists in galvanized steel plate or truss steel bar.Reasonable construction sequence may reduce the harmful deformation of plate belt to some extent.In the process of concrete pouring,steel secondary beam should be regarded as elastic support,which could demonstrate the deformation and stress of steel truss floor board factually.2.The test results of deformation and stress of steel truss floor board during concrete pouring and concrete setting and hardening show that:(1)Without the support plate belt,the deformation and stress in the concrete pouring construction process are basically consistent with the simulation test results.The floor board has a large deflection deformation,which is one of the main reasons of concrete cracking;(2)With support plate belt,the floor flexural deformation is smaller than the one without supportplate belt obviously in the concrete pouring construction process.Erection of support can effectively control the cracking of floor board concrete;(3)In the process of concrete setting and hardening,as the concrete strength increases,concrete,steel truss and the steel beam form a synchronous stress system.Meanwhile,the structural stiffness increases,the deformation of the floor board and the strain of the steel truss and the steel beam reduce as well.These outcomes show that there are essential differences in structural stiffness and the stress mode between the construction stage and the service stage.In the structural design,the stress in different stages should be considered to ensure the safety of the structure at each stage;(4)After a certain strength of concrete has reached,the vertical support is gradually removed from concrete,and the increased axial force on both sides of the support is obviously less than the original axial force of the dismantled support.Therefore,this operation indicates that with the improvement of concrete strength,the load-transferring mode of floor board is changing.Choosing a reasonable bracing and demolition time can save the formwork support and guarantee the construction quality and safety of floor board.3.The influence from significant changes in the stiffness of secondary beams and floor board on the stress state of the structure is analyzed and calculated during the concrete pouring and the setting and hardening process.Calculation consider: in concrete pouring process,only the steel beam has the stiffness and the strength contribution,the secondary beam support stiffness is small(similar to the elastic support).After the pouring is completed,as the concrete strength increases,the composite structure would be formed finally,the structural stiffness significantly increases.The support of secondary beam changes from the elastic to rigid.Through comparative analysis,the calculation method of flexural stiffness of floor board section before concrete setting and hardening is determined,and the calculation method of flexural stiffness of floor board section before concrete setting and hardening is determined at the same time.Meanwhile,the finite element model of steel bar truss plate at concrete pouring stage is established,finite element deformation results matched with the measured deformation results.A simply supported reinforced truss condensed plate finite element model simulating the concrete hardening process is built as well,the result shows the "steel" and "concrete strain lag" phenomenon apparently.This paper discusses the deformation of slab in the process of pouring concrete due to the influence of span then come up with a span range of the slab that needs to be erected for support.Eventually this research optimizes the number of supports and the time of dismantling,effectively controls the cracking of the slab.Thus the technical theory and support for the design and construction of the steel truss slab is provided.