The Correlation Between The Stiffness Of Floor Connections And The Axial Force Andits Effect On The Global Behavior Of The Structure

The mechanical capability of floor connections in rack analysis and design can't be neglected,while the traditional method usually sets the floor connection of steel rack as ideal hinge.However,axial force has great influence on it in practical rack engineering,and it is difficult to achieve the ideal hinged condition for floor connections with large number of mechanical connections.This paper researches the correlation between the equivalent stiffness of floor connection and and the axial force and its effect on the global behavior of the structure with commonly used floor connections of practical rack engineering.The main research work of this paper is given as follows:1.Eccentric socketed floor(TJ1),centric socketed floor(TJ2)and end-plate socketed floor(DB)are selected as research objects.Then the delicate finite element models are established with the same load and boundary conditions as the tests in relavent papers.Comparing the finite element model results to tests results,the correction and reliability of finite element model are verified.2.This paper carries out the parametric analysis of axial force based on the established finite element floor models.According to moment-rotation curves under different axial compression,the equivalent stiffness,yield rotation,ultimate moment and ultimate rotation-axial compression curve are achieved.The results reveals that: as the TJ1 floor connection is bended under low axial compression,the bottom plate lifts up which affects the global mechanical capability of it;as axial compression rises,the effect decreases and the stiffness of the TJ1 floor connection is controlled by its bending stiffness until the axial compression is able to prevent the lift-up of the bottom plate.When the TJ2 floor connection loads,the bottom plate would not lift up.This floor connection has low equivalent stiffness and is less influenced by axial compression,which is most close to ideal hinge.Similar to TJ2,there is no lift-up and complex nonlinear mechanical characteristic when the DB floor connection loads.Its stiffness is least influenced by axial compression and it has larger equivalent stiffness than TJ2.Besides,this paper analyzes the effect between axial tension and equivalent stiffness and ultimate rotation of three floor connections as well,finding that axial tension weakens equivalent stiffness to different degrees,but TJ1 is more affected by axial tension than others.After that,this paper carries out the correlation analysis between axial force and stiffness of three floor connections,and then gives the PM? surface.Based on floor equivalent stiffness,the formula of PM? is further simplified and the bilinear M-? constitutive relation of the column foot is established.3.The equivalent stiffness which is associated with axial force is the key variable of the M-? constitutive relation of the foot connections.It has a great influence on the steel rack design.This paper focuses on the equivalent of the TJ1 foot connection with the greatest influence on the axial compression.The stiffness of the column,the thickness of the bottom plate and the thickness of the joint are the key influencing factors of the equivalent stiffness,and the parameterization analysis of the three factors is carried out.This paper reveals the critical axial compression ratio of the TJ1 foot connection.When the axial compression ratio is less than the critical axial compression ratio,the bottom plate will be lifted up,and the bending stiffness of the bottom plate and the column will affect the equivalent stiffness of the foot connections;When the axial compression ratio exceeds the critical axial compression ratio,the bottom plate will not be lifted up.The bending stiffness of the column and the connecting member determines the equivalent stiffness of the column,and the influence of the column on the equivalent stiffness is greater than that of the connecting member.Finally,the formula for calculating the equivalent stiffness of the TJ1 foot connection under axial compression is established,and the reliability of the formula is proved by the modal test results of the low-rise frame and the finite element example.4.The effect of the correction between floor connections stiffness and the axial force on the global behavior of structure is investigated.First,according to the common type of rack structures,the high-rise with back support and multi-rack withoutd back support structure model are established.Then,a static calculation iterative algorithm which can dynamically consider the change of floor connections stiffness is established.The spring constitutive is based on the equivalent stiffness K0-? fitting formula of the three floor connections established in the previous equation.Finally,a parametric analysis of the two type of rack is carried out.The results show:(1)The high-rise racks are no sway frame,and the lateral displacement curves of each model are close to the “straight line” type,and the stiffness of the foot connections has less affected on lateral drift.The multi-racks are sway frame with weak lateral resistance.The lateral displacement curve are close to the "parabolic" type,the stiffness of the foot connections has a greater influence on lateral drift and a less influence on the bending moment and deflection of the beams;(2)If the correlation between the stiffness of floor connections and the axial force is not considered,the bending moment of the foot connections and the stress of the bolt hole of the anchor bolt,the bottom plate and the connecting piece are underestimated,and there is a large safety hidden danger;(3)Compared with the column foot setting,the stress ratio of the first floor column of the rack and the lateral displacement are much lower with considering the correlation,and the multi-racks is more prominent than the high-rise racks.At the same time,the increase in the seismic action of the multi-racks is relatively larger,and the impact of the high-rise racks with back support is less.In conclusion,the hinge foot connections make the analysis of the shelf structure macroscopically conservative,and the details are unsafe.Considering the correlation between the stiffness of floor connections and the axial force can improve the safety and economy of the rack design and promote the development level of the logistics warehousing structure.