Moment redistribution in cold-formed steel purlin systems
File(s)
Author(s)
Hui, Chi
Type
Thesis
Abstract
The external envelope of steel framed industrial buildings normally involves the use of
purlins and rails spanning between the main hot-rolled frames to support the
roofing/cladding. These purlins are typically light-gauge cold-formed steel members of
complex shape for which the thinness of the material means that local instabilities will
significantly influence their structural behaviour. In this thesis, the finite element (FE)
method (ABAQUS) has been used to develop numerical analyses to study the buckling
behaviour and degree of moment redistribution in continuous and sleeved cold-formed
steel 2-span purlin systems. Five types of nonlinear FE analyses have been validated
against reported physical tests: (i) continuous 2-span beams subjected to uniformly
distributed load (UDL), (ii) single span beams subjected to a moment gradient, (iii) single
span beams subjected to pure bending (iv) sleeved 2-span beams subjected to a UDL and
(v) single span sleeved sections subjected to a moment gradient. The FE analyses were
used to generate a large portfolio of new results for gravity and uplift loading for
continuous and sleeved 2-span arrangements covering a wide range of cross-sections by
varying the flange and web dimensions and material thickness. The effects of local and
distortional buckling and limited rotational capacities for single span FE models were
investigated. The 2-span FE results formed the basis for a simple modification to
conventional plastic design that recognises the possibility of a reduction in moment with
increasing rotation in the interior support region. The assumption of full moment
redistribution for gravity loading was found to be only valid for stocky sections but not
for slender sections. For uplift loading in addition to the potential reductions in moment at
the interior support, limitations in the span moment due to lateral torsional buckling
(LTB) for slender members were also accounted for. Based on the FE results, an α-
reduction framework was established to predict the collapse load for continuous and
sleeved 2-span systems. It was assumed that the cross-section or LTB resistance was achieved in the span while a reduced cross-section resistance allowing for the post-peak
fall in capacity was achieved at the interior support. The accuracy of the proposed design
method was compared against elastic and full plastic design cases by considering their
ultimate load carrying capacities. Whereas the elastic design method provides overly-conservative
results and plastic design overestimates the capacity of slender sections, the
proposed design method gave accurate predictions of the failure load with minimal scatter
for all cases. The developed α-reduction framework provides a foundation for allowing
the use of other purlin sections and interior support connections by inserting alternative
cross-sectional moment capacity inputs obtained from several sources such as physical
testing, hand calculations from design codes and FE analyses.
purlins and rails spanning between the main hot-rolled frames to support the
roofing/cladding. These purlins are typically light-gauge cold-formed steel members of
complex shape for which the thinness of the material means that local instabilities will
significantly influence their structural behaviour. In this thesis, the finite element (FE)
method (ABAQUS) has been used to develop numerical analyses to study the buckling
behaviour and degree of moment redistribution in continuous and sleeved cold-formed
steel 2-span purlin systems. Five types of nonlinear FE analyses have been validated
against reported physical tests: (i) continuous 2-span beams subjected to uniformly
distributed load (UDL), (ii) single span beams subjected to a moment gradient, (iii) single
span beams subjected to pure bending (iv) sleeved 2-span beams subjected to a UDL and
(v) single span sleeved sections subjected to a moment gradient. The FE analyses were
used to generate a large portfolio of new results for gravity and uplift loading for
continuous and sleeved 2-span arrangements covering a wide range of cross-sections by
varying the flange and web dimensions and material thickness. The effects of local and
distortional buckling and limited rotational capacities for single span FE models were
investigated. The 2-span FE results formed the basis for a simple modification to
conventional plastic design that recognises the possibility of a reduction in moment with
increasing rotation in the interior support region. The assumption of full moment
redistribution for gravity loading was found to be only valid for stocky sections but not
for slender sections. For uplift loading in addition to the potential reductions in moment at
the interior support, limitations in the span moment due to lateral torsional buckling
(LTB) for slender members were also accounted for. Based on the FE results, an α-
reduction framework was established to predict the collapse load for continuous and
sleeved 2-span systems. It was assumed that the cross-section or LTB resistance was achieved in the span while a reduced cross-section resistance allowing for the post-peak
fall in capacity was achieved at the interior support. The accuracy of the proposed design
method was compared against elastic and full plastic design cases by considering their
ultimate load carrying capacities. Whereas the elastic design method provides overly-conservative
results and plastic design overestimates the capacity of slender sections, the
proposed design method gave accurate predictions of the failure load with minimal scatter
for all cases. The developed α-reduction framework provides a foundation for allowing
the use of other purlin sections and interior support connections by inserting alternative
cross-sectional moment capacity inputs obtained from several sources such as physical
testing, hand calculations from design codes and FE analyses.
Version
Open Access
Date Issued
2014-03
Date Awarded
2014-08
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Gardner, Leroy
Nethercot, David
Sponsor
Ayrshire Metal Products (UK)
Publisher Department
Civil and Environmental Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)