Research On Mechanical Properties Of Reinforced Concrete Slab-Column Connections With Pallet Under Fires

This thesis mainly studied the punching resistance of reinforced concrete slab-column connections with pallets,summarizes the mechanical and thermal properties of steel and concrete,and studies the effects of changing the concrete strength grade and pallet thickness on the bearing capacity of connections at room temperature.The propagation of cracks is followed by the temperature field distribution of the ISO-834 simulated component under fire for 3 hours.The load-displacement curves under different pallet thicknesses are studied at high temperatures,and the expressions of load-displacement are fitted by Gaussian functions.The influence of different pallet thicknesses and column cross-section dimensions on its ultimate bearing capacity under the same fire time,as well as the displacement changes of the connections under different fire time,the following conclusions are as follows:(1)By analyzing the Mises stress contour plot and the maximum principal stress contour plot of concrete at room temperature,it is concluded that the compressive strength and tensile strength of concrete of different strength levels exceed the standard values in concrete specifications.It can be considered that concrete materials have begun to fail.It can be seen from the equivalent plastic strain contour plot of steel bars and concrete that the PEEQ values at the connections are greater than zero,so both the steel bars and concrete have yielded,the connections have undergone punching failure,and the contour plot with tensile damage can simulate the cracks around the connections to the surrounding diffusion,eventually forming an elliptical failure surface.(2)Changing the strength grade of concrete has a certain limit to increase the punching shear capacity of the component.When C25 concrete is increased to C40,its impact resistance is significantly improved.When C30 concrete is increased to C40,its impact resistance is improved.The impact of bearing capacity is not obvious.It is recommended that the concrete of slab-column connections be generally C30;when the thickness of the pallet is from 100 mm to 200 mm,the impact resistance of concrete can be greatly increased,and the maximum can be increased by 71.70%.Therefore,in actual engineering,we should give priority to the beamless floor structure with thicker pallets.(3)It can be known from the simulation of the concrete temperature field that as the thickness of the supporting plate increases,the temperature of the internal concrete at the slab-column connection will decrease.The temperature in the column is distributed in the form of an therm with an obvious temperature gradient.Compared with experiments,ABAQUS finite element software can accurately analyze the temperature values of each section and connection under different fire time.(4)With the increase of fire time,the ultimate bearing capacity of the structure gradually decreased,and the poorer the punching resistance,the punching capacity decreased by 49.93%,and the maximum displacement at the joint reached 50 mm.Load-displacement curves under different fire times were fitted with Gaussian functions to obtain the expression of the load-displacement function.(5)Under fire,when the thickness of the supporting plate is 100 mm to 150 mm,the ultimate bearing capacity of the connection can be increased by 28%,and the ultimate bearing capacity of the connection can be increased by 161% when the column section size is from 300 × 300 to 600 × 600.Therefore,in the engineering design,the column cross section of the slab column structure should use a large section and increase the pallet as much as possible,and the thickness of the pallet is recommended to be more than 150 mm.