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.

Abstract

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

elevated temperatures.

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ID Code: 16417
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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