Development And Application Of Free Support Tool-truss Composite Slab

Concrete composite slab is one of the most common structural forms in modern industrial building slab structure system,which is composed of prefabricated bottom slab and upper post-poured concrete.However,due to the common concrete precast slab thinner（usually only 50～70 mm）,even in the hoisting and transportation links dedicated support measures is difficult to completely avoid precast slab cracking problem easily,especially during the period of construction,relying on precast slab harder to bear their own all kinds of construction load,so as to make the composite slab as cast-in-place construction site on the construction site,a large number of vertical supports still need to be erected,which not only increases the construction cost and prolongs the construction period,but also does not conform to the characteristics of assembly construction.In order to solve the above problem,this paper puts can turnover of steel tube concrete slab upside and through the fittings are connected to the backplane,developed from free support tool-truss composite slab（FSTCS）.The integrated use of the model test,numerical analysis and theoretical calculation of the means of combining explore FSTCS bending performance and working mechanism,the numerical model and theoretical calculation method of flexural stiffness and deflection of FSTCS are established and verified.The research results provide reference basis and calculation method for the application of FSTCS.The main work and conclusions are as follows:（1）The slab type and structural measures of FSTCS are optimized and determined.The influences of span,slab width and steel tube on the flexural performance of the FSTCS were investigated.Through carrying out static load test,the experimental phenomenon and mid-span bending moment deformation curve of FSTCS are analyzed,and the oblique bar lashing and lapping FSTCS are optimized.Further analysis found that the fracture distribution characteristics closely related to the span,small span specimen fracture distribution in oblique bar coverage on both ends,all across the region,and free of cracks with the increase of span has evolved into cross crack distribution in the regional characteristics,and the width of crack propagation is affected,the thickness of the steel tube will significantly influence on the flexural performance of FSTCS.（2）The flexural performance and working mechanism of FSTCS in each stage are revealed.By analyzing the mid-span moment deformation curves,the measured strains of upper steel tube and lower concrete bottom slab,and the measured strains of vertical connecting rod and inclined tendons,it is proved that the flexural performance of FSTCS has obvious two-stage characteristics with cracking as the boundary.The bending stiffness is provided by the bending deformation of the upper steel tube,the lower concrete slab and the shear deformation of the trusses between them.The maximum mid-span deflection of each specimen under construction load is about 1/263 of the span of the slab,which meets the requirements of the code and can realize slab bottom free support.Reveals the FSTCS work mechanism for cracking in the backplane across area can produce significant tensile deformation,the deformation of the upper pole is lesser,connected to both the vertical rod,oblique bar shear deformations and provides the bending stiffness,if not seen obvious crack across the area,negligible shear deformation on the contribution of bending stiffness.（3）Based on ABAQUS numerical analysis software,the fine numerical simulation of FSTCS are realized.Through the establishment of FSTCS model of refinement of different parameters such as span,width and depth of steel tube to carry out the numerical analysis,can be consistent with the measured values change trend and inosculation of high numerical solution（decision coefficient of minimum value of 0.988）,for the expansion of FSTCS stress performance analysis provides an effective research tool.It also shows the vulnerable area of stress directly,which provides a reference for the design and manufacture of FSTCS.（4）A theoretical calculation method for flexural stiffness and deflection of FSTCS is established.The overall flexural stiffness of FSTCS can be superimposed by the bending deformation of the upper steel tube,the lower concrete slab and the shear deformation of the trusses between them.The theoretical calculation method established has high precision and definite physical meaning.The results show that the overall flexural stiffness of FSTCS is characterized by two stages with cracking as the boundary.After cracking,the stiffness decreases rapidly first and then slowly（the average decrease is 18.2%）,and the overall flexural stiffness increases with the increase of the span in the whole loading process.Before cracking,the contribution of S_S,S_M and S_L with different spans to the flexural stiffness is of steel tube>bottom slab>truss.The contribution of steel tube increases with the cracking of concrete slab and increases to 77.3%,68.4%and 59.2%respectively at the peak load.After cracking,the contribution of bottom slab to stiffness decreases,while that of truss shear deformation to stiffness increases,but the increase is less than that of bottom slab.The innovation of this paper is discussed as follows:（1）The influences of structural measures,span,slab width and steel tube thickness on the flexural performance of FSTCS is investigated,and the working mechanism of FSTCS is revealed.（2）The theoretical calculation method of flexural stiffness and deflection of FSTCS is established,which provides theoretical calculation basis for the popularization and application of FSTCS.