Distortional buckling behaviour of cold-formed steel compression members at elevated temperatures
Ranawaka, Thanuja (2006) Distortional buckling behaviour of cold-formed steel compression members at elevated temperatures. PhD thesis, Queensland University of Technology.
In recent times, light gauge cold-formed steel sections have been used extensively in
residential, industrial and commercial buildings as primary load bearing structural
components. This is because cold-formed steel sections have a very high strength to
weight ratio compared with thicker hot-rolled steel sections, and their manufacturing
process is simple and cost-effective. However, these members are susceptible to
various buckling modes including local and distortional buckling and their ultimate
strength behaviour is governed by these buckling modes. Fire safety design of
building structures has received greater attention in recent times due to continuing
loss of properties and lives during fires. Hence, there is a need to fully evaluate the
performance of light gauge cold-formed steel structures under fire conditions. Past
fire research has focused heavily on heavier, hot-rolled steel members. The buckling
behaviour of light gauge cold-formed steel members under fire conditions is not well
understood. The buckling effects associated with thin steels are significant and have
to be taken into account in fire safety design. Therefore, a research project based on
extensive experimental and numerical studies was undertaken at the Queensland
University of Technology to investigate the distortional buckling behaviour of light
gauge cold-formed steel compression members under simulated fire conditions.
As the first phase of this research program more than 115 tensile coupon tests of
light gauge cold-formed steels including two steel grades and five thicknesses were
conducted at elevated temperatures. Accurate mechanical properties including the
yield strength, elasticity modulus and stress-strain curves were all determined at
elevated temperatures since the deterioration of the mechanical properties is one of
the major parameters in the structural design under fire conditions. An appropriate
stress-strain model was also developed by considering the inelastic characteristics.
The results obtained from the tensile coupon tests were then used to predict the
ultimate strength of cold-formed steel compression members.
In the second phase of this research more than 170 laboratory experiments were
undertaken to investigate the distortional buckling behaviour of light gauge coldformed steel compression members at ambient and elevated temperatures. Two types of cross sections were selected with various thicknesses (nominal thicknesses are
0.6, 0.8, and 0.95 mm) and both low and high strength steels (G250 and G550 steels
with minimum yield strengths of 250 and 550 MPa). The experiments were
conducted at six different temperatures in the range of 20 to 800°C. A finite element
model of the tested compression members was then developed and validated with the
help of experimental results. The degradation of mechanical properties with
increasing temperatures was included in finite element analyses.
An extensive series of parametric analyses was undertaken using the validated finite
element model to investigate the effect of all the influential parameters such as
section geometry, steel thickness and grade, mechanical properties and temperature.
The resulting large data base of ultimate loads of compression members subject to
distortional buckling was then used to review the adequacy of the current design
rules at ambient temperature. The current design rules were reasonably accurate in
general, but in order to improve the accuracy further, this research has developed
new design equations to determine the ultimate loads of compression members at
ambient temperature. The developed equation was then simply modified by
including the relevant mechanical properties at elevated temperatures. It was found
that this simple modification based on reduced mechanical properties gave
reasonable results, but not at higher temperatures. Therefore, they were further
modified to obtain a more accurate design equation at elevated temperatures. The
accuracy of new design rules was then verified by comparing their predictions with
the results obtained from the parametric study.
This thesis presents a description of the experimental and numerical studies
undertaken in this research and the results including comparison with simply
modified current design rules. It describes the laboratory experiments at ambient and
elevated temperatures. It also describes the finite element models of cold-formed
steel compression members developed in this research that included the appropriate
mechanical properties, initial geometric imperfections and residual stresses. Finally,
it presents the details of the new design equations proposed for the light gauge coldformed
steel compression members subjected to distortional buckling effects at
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|Item Type:||QUT Thesis (PhD)|
|Supervisor:||Mahendran, Mahadeva & Mahaarachchi, Dhammika|
|Keywords:||light gauge cold-formed steel, distortional buckling, compression members, elevated temperatures, axial compression load, reduced yield strength, reduced elasticity modulus, stress-strain model, fire safety design, fire test, finite element analysis|
|Divisions:||Past > QUT Faculties & Divisions > Faculty of Built Environment and Engineering
Past > Schools > School of Urban Development
|Department:||Faculty of Built Environment and Engineering|
|Institution:||Queensland University of Technology|
|Copyright Owner:||Copyright Thanuja Ranawaka|
|Deposited On:||03 Dec 2008 04:03|
|Last Modified:||28 Oct 2011 19:47|
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