Experimental Study On Mechanical Behavior Of Assembly Steel Support

The commonly used form of foundation pit enclosure is the vertical enclosure combined with the internal support of the foundation pit,in which the internal support is usually reinforced concrete support.With the concept of green construction gradually popularized,assembly steel support gradually emerged.Assembly steel support has the advantages of flexible layout,high strength connection,convenient installation and removal,short construction period,recyclable steel,and small impact on the environment.Although the theory foundation and construction technology of the section steel composite support are gradually improved,the application research of the section steel composite support is still few.The difference between the theoretical stiffness of support and the actual stiffness,the internal and external stability of the support plane,and the influence of the axial pressure change of support on the foundation pit deformation are still needed to be further studied.Based on the research background of zijin zhunqian foundation pit project in hangzhou,this paper makes theoretical analysis and experimental research on the application of assembly steel support in foundation pit engineering.The analytic formula of stiffness coefficient of elastic fulcrum of angle brace is deduced by the theory of line elasticity and deformation coordination formula of compression bar;combined with the stability checking formula,the safety of the support with different sections is checked,and the applicable situation is explained;the measured data of foundation pit were analyzed statistically,and the axial force loading test of assembly steel suppprt was carried out to explore its mechanical characteristics.The main research contents and results of this paper are as follows:(1)The principles of layout and integral analysis of assembly steel support are summarized;the analytical formula of stiffness coefficient of angle brace is derived;combined with the stability checking formula,the safety degree of different support beams is calculated,and the results show that the safety degree of assembly steel support is inversely proportional to the length of support,the confining pressure of foundation pit and proportional to the cross-sectional area of support beams.(2)Based on the monitoring data,the foundation pit deformation during earthwork excavation and underground structure construction is analyzed.A number of monitoring data show that the foundation pit deformation did not reach the early warning value and there was no more dangerous situation.The assembly steel support reflects strong stability and safety during the whole foundation pit construction.(3)The influence on foundation pit deformation was investigated through the insitu axial force loading test,and the test results showed that: the pile displacement increases linearly with the support axial pressure;the deformation of foundation pit presents spatial characteristics,that is,there is spatial effect,and the displacement of the supporting inner retaining pile is less than that of the outer side;the displacement of adjacent supporting inner retaining pile is toward the outside of the pit,while the displacement of outer retaining pile is toward the inside of the pit,but the influence amount is very small.(4)The test data were used to calculate the support stiffness,the calculated results show that the measured stiffness of the support is less than the theoretical stiffness,and the difference between the theoretical stiffness of the support and the actual stiffness accounts for about 14% ? 20% of the theoretical stiffness.The dimensionless coefficient T is introduced,which is defined as the ratio of supporting area to length,and qualitatively represents the relationship between supporting stiffness and supporting cross-sectional area and supporting length.The calculation results show that the greater the value of T,the greater the supporting stiffness(5)In the process of continuous axial pressure loading,the supporting beam deforms in and out of the plane,and further calculation shows that the maximum deformation rate of the supporting beam in and out of the plane can meet the design requirements.The deformation data further indicate that the high-strength bolt connection between the beam and the support beam can limit the lateral deformation of the support beam,at the same time,the beam has a certain limitation on the vertical deformation of the supporting beam.(6)The axial force monitoring data show that the axial force loading test of the support has the greatest influence on the vertical support.As the axial pressure of the support increases,the axial force of the vertical support decreases significantly.At the same time,there is a loss of axial force in the loading test of support,and the difference between the measured axial force and the standard axial force is about 20% ? 30%.