Cracking Analysis Of Roof In Subway Station Under Construction And Application Of "Alternative Bay Construction Method"

Since 1980s,great changes have taken place in the development of China’s subway construction.Cracking problems of main structure can be seen frequently during the construction of subways,especially in the roof of subway station.Under normal circumstances,the length of roof in subway station structure can reach more than 150m,and the thickness is generally range 500mm to 1000mm.So it can be attributed to "The Super-length Mass Concrete Slab".On the other hand,it is subjected to greater constraints from its side walls,frame columns and surrounding soil.During the construction of subway stations,cracking problems occur easily for the influences of temperature and the shrinkage of concrete.This paper takes "Xinyang Avenue Station" of Xiamen Metro Line 2 as an example,and analyzes the characteristics of the temperature field and stress state under different conditions.And then we can determine whether it will crack or not.The main contents of the paper include:1.Analyzing the roof crack characteristics by collecting the cracking data of the roof slab in subway stations.As a result,we can get the initial reasons of cracking in subway station roof:temperature and the shrinking of concrete.2.Combined with the method of the main structure construction in the subway station,as well as the help of the finite element analysis software "ANSYS",we can:(1)Simulate the temperature field during concrete hardening of the roof slab in subway stations.The results show that:the maximum temperature inside the structure can be up to 50℃,the maximum temperature difference is up to 18℃ and the temperature field distribution of the subway station structure is slightly different.(2)Analyze the stress field during concrete hardening of the roof slab in subway stations.The results show that:1)At the early stage of construction,the structural temperature stress caused by the thermal difference of hydration heat reaches 1.1 MPa(3d age).Although the safety factor at this moment is up to 1.4,which is greater than 1.15.In case of the lack of margin,structural surface may crack.2)The internal stress of the structure under the combined action of temperature difference and shrinkage in the later stage of construction reaches 2.36MPa.The safety factor is about 0.96 at that moment,which is less than 1.15.And these dangerous areas are concentrated in the junction of the roof and the side wall,and the junction of the roof.Therefore,it will crack easily.3.With the method of theoretical analysis and structural modeling simulation,this paper explores the technical points of the application of "Alternative Bay Construction Method" in the construction of subway station main structure.The results show that:1)Reasonable "Length of alternative bay" should be 20-30m;2)The difference of reasonable pouring time between two adjacent warehouse should be 6-9d;3)"Sub-warehouse seam" should be used rigid waterproof.