A Calculation Model For Wind-induced Buckling Capacity Prediction Of Cold-formed Purlins And Its Application On The Condition Assessment After Wind Hazards

The purlin-sheeting systems,which are made of cold-formed purlins,corrugated sheetings and screws,are widely used in light weight steel industrial buildings.When subjected to wind suction,the upper flange of the purlin is pulled with continuously restrained by the sheeting while the bottom flange of the purlin is compressed with less constrained.Moreover,the asymmetry of the constrained effect of the purlins coupled with warping torsion effect induced by the load eccentricity leads to the complex stress distributions of the purlins.This effect will furthermore change the buckling mode and buckling strength of the purlins.In addition,the screw fasteners of the purlin-sheeting systems will suffered wind-induced fatigue problems during the service period,which will reduce the strengthen effect provided from the sheeting to the buckling strength of the purlin.Therefore,it is of great importance to carry out the post-disaster condition assessment of the purlin sheeting systems.Then,establishing a reasonable and sound calculation model for the purlins which can be employed to the design and condition evaluation become the key scientific issues for the cold-formed purlins.In this thesis,the simply-supported cold-formed C/Z-section purlins are taken as the research object to carry out the study.Based on the traditional structural mechanics theory of thin-walled members,a differential equation for the simply-supported purlins considering the diaphragm effect is proposed.Combined with the Direct Strength Method,the refined calculation model of the purlins under wind uplift is provided.This model can be used to conduct the condition assessment of the purlin-sheeting systems after wind hazard in probability of category.The whole framework will lay the foundation of the performance-based wind-resistalce design of light steel structures.The main content can be divided into the following parts:Firstly,it summarizes the development history and research status of the wind-resistance design of the cold-formed purlins in the past several decades and concludes the characteristics of various calculation models of the purlins.Secondly,the wind-induced damage of the typical steel structures in the southeast coast of China was investigated after typhoon and tornadoes,especially the damage circumstance of the damage of wind-induced roof systems.Based on the survey results and existing research method,the concept analysis was also carried out.Thirdly,based on the structural mechanics theory of thin-walled members,the diaphragm effects on the mechanical properties of the purlins,which may included the stress distributions,the displacement and buckling strength are investigated.To obtain the whole failure process of the purlins,the thesis also proposed a nonlinear twisting model made of differential equations of the simply-supported purlins which can account for the diaphragm effects,the load eccentricity effect and the out-of-plane stability effects.The algorithm was also employed to capture the buckling mode and buckling strength of the purlins with the help of the Finite Strip Method program CUFSM and the proposed modified Direct Strength Method.The accuracy of the framework was also verified by the test data obtained around the world in past several decades.Then,through the proposed refined prediction framework for the buckling capacity of the cold-formed purlins,the influence of the restraining stiffness of the sheeting on the purlins and the position of the screws was also studied.The variation rule of the buckling capacity of the purlins with respect to the restrained stiffness of the sheeting are also provided which can be used as a foundation rule for the condition assessment of the purlin-sheeting systems after wind-hazards.Finally,based on the proposed buckling prediction framework of the cold-formed purlins,and combined the time-dependent reliability theory,the variation law of the time-varying reliability of the purlins after single or multiple typhoons is also obtained.The life-cycle prediction and damage evaluation were also provided based on the time-dependent fragility analysis through setting the reasonable damage states.Thereafter,the corresponding mathematical formula and fragility curves were also provided which will provide a solid theoretical basis for the post-disaster damage assessment of the purlin-sheeting systems.