Analysis of CFS Purlins and Girts Subjected to Cyclic Loading

Abstract

The design of metal building roof and wall systems is complicated because of thediversity of design variables,
e.g., different purlin and girt cross-sections, simple or continuous span, through-fastened roof (wall) or standing
seam roof, with or without insulation, and uplift (suction) or gravity (pressure) loadings. The overall objective of
this project is to lay the groundwork for a new strength prediction approach that extends the American Iron and
Steel Institute’s North American Specifications, Direct Strength Method (DSM) to all types of metalbuilding wall
and roof systems. The specific focus of this project presented herein is through-fastened simple span purlins or
girts with the laterally unbraced free flange incompression caused by wind suction on a wall or cyclic load on a
roof.
To calculate through-fastened purlin or girt system strength, a factor for including the rotation effect caused by
shear flow and the rotational restraint is needed for DSM. Tian Gao conducted vaccum box test to validate DSM
prediction method since the rotational restraint provided by the roof and wall sheathing plays an important role
in the calculation of the flexural capacity of the through-fastened purlin and girt. Tian Gao research study not
considered orientation of purlin section on the principal rafter which might cause changes in the buckling
behaviour and buckling load capacity and also not conducted analytical study with the predicted rotational
restraint. To account for the shear flow effect, and a rotational spring is applied in CUFSM version 4.0 to
calculate thecritical buckling loads including the restraint from the sheathing. The same analytical study done in
GBTUL version 2.0 software. In total 49 specimens including mainly Zed sections and Channel sections having
different cross section dimension, thickness, yield stress chosen from research study conducted by Tian Gao.
Buckling behaviour with considering rotational restraint in analytical study done with CUFSM version 4.0 and
GBTUL version 2.0 are studied and load capacity calculated. Comparisons made between CUFSM and GBTUL
analytical results and between load capacity from DSM approach in AISI S100-2007 North American Code and
Experimental load capacity given by Tian Gao in his research work.