Buckling Study On C-section Cold-formed Thin-walled Steel Purlin

Cold-formed thin-walled steel members have been widely used in construction industry due totheir advantages such as light weight, good mechanical performance, short production cycle, easilyto be assembled and so on. C-section purlin as a common section style in cold-formed steel roofingsystem is necessary to be researched on its stability. In this paper, pure compression and purebending tests are employed to obtain the critical buckling stress and ultimate load of C-sectioncold-formed thin-walled steel purlins with different cross-sections and length. Finite elementsimulation on that of purlins under uniformly distributed load with different lengths is implementedalso.In chapter two, axle pressure tests are implemented on C-section purlins with nine kinds ofcross-sections. The experimental results showed that ultimate bearing capabilities obtained by testsare larger than the results obtained by direct strength method (DSM). Ultimate load increases atfirst and then decreases. but post-buckling strength increases with the increase of the height of theweb. The variation rule of ultimate load with width of flange is similar to the rule of that withheight of the web, but post-buckling strength decreases with the increase of the width of flange.Within a certain range, the larger the length of lip is, the larger the ultimate load will be, while post-buckling strength decreases with the increase of the length of lip. At the same time, the resultsobtained by digital speckle technique is very close to the result obtained by using strain gauges. Itmeans the test data is correct.In chapter three, pure bending tests on six kinds of C-section purlins with different cross-sections are conducted. The experimental results showed that ultimate moments obtained byexperiments are smaller than that from calculation by direct strength method. Ultimate momentincreases with the increase of the height of web, but critical distortional buckling stress decreases atfirst and then increases, and post-buckling strength is quite the reverse, it increases at the first andthen decreases. Ultimate moment increases with the increase of the width of flange, but criticaldistortional buckling stress and post-buckling strength decreases with the increase of the width offlange. Within a certain range, the larger the length of lip is, the larger the ultimate moment will be,also post-buckling strength decreases with the increase of the length of lip.The fourth chapter presents the nonlinear finite element simulation on C-section purlin underuniformly distributed loading. The cross-section is height of web is120mm, width of flange is50mm, length of lip is15mm and thickness is1.5mm. Purlin lengths are chosen as4000mm,7000mm and10000mm. The influence of the initial geometric imperfection caused by gravity and 5%deformation from the first order modal is considered in nonlinear analysis of ultimate bearingcapacity. And the effects of the rotational constraint provided by the roof sheeting are alsoconsidered to analyze the ultimate bearing capacity. The constraints supplied by roof sheeting arefull lateral constraint and alterable rotational constraint taken as0N·m/rad,3N·m/rad and7.5N·m/rad. The analysis results show that impacting of the rotational stiffness on the bearingcapacity of purlin is the most significant, the greater the rotational spring stiffness is, the strongerthe bearing capacity will be. The longer the purlin is, the smaller the bearing capacity will be. Theeffect of the initial geometric imperfection caused by gravity on ultimate bearing capacity is small,but it has a big influence on the lateral displacement of the purlin. As the initial geometricalimperfection5%deformation from the first order modal mainly affects the ultimate bearingcapacity of purlin. It has remarkable effect on ultimate bearing capacity of short purlin and theeffect of this imperfection on the ultimate bearing capacity decreases with the increasing of thelength.