Study On Longitudinal Seismic Performance Of Medium And Heavy Multi-Tiered Braced Steel Structure

Column bracing is the core component of the longitudinal seismic resistance of the plant frame structure.In tall building,,multi-tiered brace was often used to reduced the brace slenderness ratio.At the same time,the intersection of the brace and the column can also be used as an effective support point outside the plane of the columns,made it easier for members to met the slenderness ratio limit and reduced the amount of steel used.However,when the structure is subjected to longitudinal rare earthquakes,due to the different bearing capacity of each layers,the brace failure may only occur in the relatively weak brace layer.The weak lateral resistance of the weak layer causes the displacement angles between layers to appear large differences,which causes the column to bend in the brace plane.However,the code stipulates that the column is designed as a unidirectional bending member.This is obviously inconsistent with the actual situation,which may lead to the failure of the column designed according to the code assumption in this case,causing structural damage.In response to the above problems,this paper designed three medium-sized and eleven heavy-duty multi-tiered brace steel structures.Through static pushover analysis,the seismic performance and failure mechanism of the structure under longitudinal load are studied.At the same time,the effect of the brace slenderness ratio on the longitudinal seismic performance and failure mechanism of the multi-tiered brace structure was studied.The analysis results show that there is concentration of displacement angle between layers,and as the lateral displacement of the structure increases,the column bends in the support plane.To avoid this situation,it is proposedthat the internal forces of the column design should consider the additional bending moment caused by the brace.As the slenderness ratio of the brace increases,the horizontal component before compression yield decreases,the value of the column bending moment caused by the unbalanced shear decreases,and the structural bending failure is reduced.When the height of the weak layer is smaller than the height of the structure,the concentration of lateral shift is more serious.After the column section is designed according to bidirectional compression and bending,the time history analysis of the whole plant under the action of longitudinal earthquake is conducted to analyze the influence of the combination of different brace forms,the number of brace layers and other parameters.The analysis results show that:under frequent earthquakes,the structure longitudinal displacement is small,which meets the limits required by the code;Under rare earthquakes,moderate damage occurred in some structures,severe damage occurred in some cases,and the weak layer braces buckled.The new designed component can effectively prevent instability and destruction of the column.Therefore,design suggestions are made:(1)When the height of the weak layer is smaller than the height of the structure,the concentration of lateral shift is more serious.If the design conditions allow,a layer of brace height can be added appropriately.(2)Because the bracet shear is unbalanced,the column was subjected to bending moments in the brace plane,and the design needs to be considered according to bidirectional bending.When the longitudinal frame of the factory building is calculated and analyzed by the parameters of frequent earthquakes,the tensile brace does not yield,and the column does not need to be checked by additional bending moments;under the action of rare earthquakes,the column may bend in two directions.Consider the bending moment in the brace plane caused by the difference in displacement angle between layers.(3)Increasing the brace slenderness ratio can avoid bending instability and damage.It is better to use a slender and flexible slenderness ratio,or to control it by less than 65.(4)Considering different brace combinations,the X-shaped brace combination has the best resistance to lateral displacement;when the cross-sectional area is the same,the performance of the angle steel is poor.