Study On The Stress States Of The Top Concrete Slab In The Construction Process Of The Large-Span Non-Column Underground Spatial Structure

This thesis is based on the Gangxiabei(GSB)comprehensive transportation hub which is a large-span underground structure.This is the first case that one project with a large span area that 51.2m×48m of the atrium but no column in China.With the method of finite element analysis and on-site monitoring,this thesis mainly analyzes the stress state of the large-area concrete slab located on the top layer of the large-span underground structure.Affecting factors such as hydration heat,concrete pouring scheme,support system conversion and long-term overburden load are considered in the study.Besides,the methods of reinforced concrete cracks control are proposed.The main work and achievements are as follows:(1)Considering factors such as concrete hydration heat,shrinkage and creep,the cracking risk of large-area concrete slab that use the actual concrete ratio are assessed,and the stress state of the large-area concrete slab at early age are analyzed.The results show that the stress and temperature of the concrete slab at the early age change sharply along the slab thickness direction.The first 3 days after the concrete pouring is hydration heating stage,and the maximum temperature reaches 50.5 ℃,and in 3～10 days the maximum stress reaches 2.91 MPa,all the situation appear in the middle of the plate thickness.After concrete pouring for 7days,the risk of cracking in the middle of the slab thickness reach the maximum.However,there is little risk of cracking on the top surface of the slab,the through cracks is hard to form along the slab thickness.(2)Considering the size of pouring zone and the retention time of the post-cast strip,the final pouring scheme suggestion is decided among a variety of schemes.Comparing the stress state of the large-area concrete slab with those pouring schemes,the results show that the length of the pouring zone should not exceed 30 m,and the retention time of the post-cast strip should be within 7-14 days.The final proposed pouring scheme is as follows: the pouring zone is divided by 30 m along the length direction;the post-cast strip retention time is around 10 days;micro-expansion concrete is utilized in the post-cast strip to reduce the risk of cracking.(3)This thesis also analysis the effect to the stress states of the top concrete slab influenced by the support system conversion and the overburden load.The results are as follows: after the overburden load applying for 1 year,the maximum tensile stress of the slab is 0.99 MPa,and the maximum deflection increases from 9.8mm to 43.0mm with growth of 344%;after the overburden load applying for 10 year,the maximum tensile stress of the slab nearly remain the same,however the maximum deflection is 1.37 times than that for 1 year,but still meets the specification stiffness limit.(4)After the on-site monitoring for temperatures and stress,the numerical value at each measuring point are agreed well between the finite element analysis results and the measured value.This proof sufficiently that the analysis results are correct and the pouring scheme is practicable for the large-area concrete slab with non-column in Gangxiabei project.Part of the achievements has already been applied to the Gangxiabei project successfully,the results are appreciating.