Cold-formed Steel Purlins Research Articles

Numerical Study of Buckling Behaviour of Cold-formed C-channel Steel Purlin with Perforation

Cold-formed C-channel steel purlins are widely used in construction industry. Since the cost of the steel is the main consideration in design, the concept of introducing web opening has been applied in the construction. A numerical study was undertaken to investigate the lateral-torsional buckling behaviour of cold-formed C-channel purlin with web opening. For this purpose a finite element model was developed using LUSAS software. It was then used in a detailed parametric study to simulate the lateral-torsional buckling and capacity of cold-formed steel purlins under varying perforated conditions. Five different shapes of opening were established to investigate the shape that gives the least reduction in buckling moment. Opening in the form of C-hexagon opening shape was used to develop models with different edge distance, different numbers of openings, different opening spacing and also different size of openings. For 3 m span, 0.5D opening size, where D is 175mm, 750mm edge distance with a total number of 13 openings with spacing 125mm is found to reduce buckling moment by 3.56% but at the same time reducing the volume of the section by 5.16%. For the same span, edge distance, number of openings and spacing with 0.75D opening size, buckling moment reduced significantly by 9.16% while volume reduction by 11.61%. For all different size of openings, spacing and edge distance, the deformation pattern are similar. In conclusion, perforation do affects the buckling moment capacity of the C-channel purlin.

Web-flange distortional buckling of partially restrained cold-formed steel purlins under uplift loading

It is well-known that cold-formed steel (CFS) members of open section can buckle locally, distortionally and/or lateral-torsionally. Since they are usually used as the secondary structural members in buildings to support roof and side cladding or sheeting, CFS beams are mostly treated as the restrained beams either fully or partially in its lateral and/or rotational directions. For a thin-walled channel- or zed-section beam subjected to uplift loading, if its upper flange is fully restrained in its lateral and rotational directions, the beam will not buckle lateral-torsionally, but may have a web-flange distortional buckling. In the literature there is limited information on the web-flange distortional buckling and currently the critical stress for the web-flange distortional buckling is calculated mainly by using numerical methods. In this paper an analytical model is presented to describe the web-flange distortional buckling behavior of the partially restrained CFS beams when subjected to uplift loading. Formula used to calculate the critical stress of web-flange distortional buckling is derived. Comparisons of the predicted critical stresses with those obtained using finite strip and finite element methods are provided to demonstrate the appropriateness of the model proposed.

Buckling Analysis of C-Section Cold-Formed Steel Purlins under Uplift Loadings

In this paper, a finite element analysis model for C-section cold-formed steel members partially restrained in its rotational direction by sheeting is established. Analyses are carried out using the model to examine the ultimate load carrying capability of the members when they are subjected to uplift wind loading. The analyses consider the material and geometric nonlinearities. The numerical analysis results show that the sheeting restraints have significant influence on the structural performance of the C-section purlin. It can increase the ultimate load carrying capacity evidently.

Numerical investigation of cold-formed lapped Z purlins under combined bending and shear

The plain Z-section is one of the most common cold-formed steel purlins in use for roof systems. Its lapping ability provides continuity, and double thickness material at the support regions results in greater performance and more economical designs. Design methods for this section are normally specified in the Australian/New Zealand Standard for Cold-Formed Steel Structures or the North American Specification for Cold-Formed Steel Structural Members. The newly developed Direct Strength Method (DSM) presented (Chapter 7 of AS/NZS 4600:2005, Appendix 1 of NAS S100-2007) is limited to pure compression and pure bending. Recently, design proposals for shear, and combined bending and shear for DSM have been added to the 2012 Edition of the North American Specification. The situation of combined bending and shear as occurs in a continuous purlin system is not considered in detail. Hence, this paper presents numerical simulations using the Finite Element Method (FEM) to determine the ultimate strength. The simulations are compared with and calibrated against tests at two lap lengths. For each lap length, tests were also conducted with and without straps screwed on top flanges. These straps provide torsion/distortion restraints which may enhance the capacity of the purlins. The accurate results from FEM allowed extension of test data by varying the lap lengths and section thicknesses. The results of both the experimental tests and FEM were used and plotted on the recently proposed DSM design interaction curves. Proposals for an extension to the DSM in combined bending and shear are given in the paper.

Load–deflection behaviour of sleeved joints in modified Z purlin system

The nonlinear rotational behaviour of bolted joints in the cold-formed steel purlin system is normally approximated to a linear theoretical model. Based on the experimental and numerical results, this paper employs a new method to obtain the nonlinear flexural rigidity of the sleeved connections for deflection analysis. Two equations are proposed to predict the moment resistance and the effective flexural rigidity ratio respectively, for the sleeved modified Z connections with different configurations. The nonlinear load-deflection behaviour of sleeved connections can then be predicted using these equations.

GBT-based assessment of the buckling behaviour of cold-formed steel purlins restrained by sheeting

This paper presents and discusses the results of a GBT-based numerical investigation concerning the local, distortional and global buckling behaviour of lipped channel and zed-section cold-formed steel purlins restrained by steel sheeting and subjected to an uplift loading. Strengthened (lapped) joints, commonly employed at internal supports to preclude the occurrence of local/distortional buckling phenomena, are also investigated and an illustrative application of the use of GBT to determine strengthening lengths is also presented. The sheeting restraint is modelled by means of elastic translational and rotational springs, located at the purlin upper flange, and the joint strengthening is modelled by doubling the cross-section wall thickness. For validation, the GBT-based results are compared with values yielded by Ansys shell finite element analyses.

Theoretical analysis of partially restrained zed-purlin beams subjected to up-lift loads

Cold-formed steel section beams are usually used as the secondary structural members in buildings to support roof and side cladding or sheeting. Thus these members are commonly treated as the restrained beams either fully or partially in its lateral and rotational directions. In this paper an analytical model is presented to describe the bending and twisting behaviour of partially restrained cold-formed steel purlins when subjected to uplift loading. Formulae used to calculate the bending stresses of the roof purlins are derived by using the classical bending theory of thin-walled beams. Detailed comparisons are made between the present model and the simplified model proposed in Eurocodes (EN1993-1-3). To validate the accuracy of the present model, finite element analysis is also conducted and the bending stress distribution along the lip, flange and web lines obtained from the finite element analysis is compared with those obtained from the present and EN1993-1-3 models.

Numerical simulation of high strength cold-formed purlins in combined bending and shear

The paper provides numerical nonlinear simulations, based on the finite element method (FEM) using the software package ABAQUS/Standard, of high strength C-section cold-formed steel purlins in shear and combined bending and shear. The simulations are compared with and calibrated against tests performed at the University of Sydney on a variety of section sizes and thicknesses. Studies of the effects of boundary condition, geometric imperfection, and element type as well as mesh size are included. Geometric imperfections are often taken as a scaled multiple of the eigenvalue modes. The selection of eigenmodes and their scaling is given in the paper. The accurate results of the numerical simulations show that finite element analysis can be used to predict the ultimate loads of thin-walled members including the post-buckling behavior of thin-walled sections in shear and combined bending and shear. It is demonstrated that finite element analysis can therefore be used to design and optimize thin-walled sections of high strength steel.

Design of Cold-Formed Steel Purlins Attached to Roof Sheeting Based on Eurocode 3: Effect of Gravity Load

In the design of cold-formed steel purlins based on Eurocode 3, the lateral bending moment in the free flange of the purlin, when in tension, is assumed to be zero due to flange curling and second order effects. To investigate the validity of this design assumption, non-linear analytical and finite element models for analyzing how the gravity load affects the maximum mid-span stresses in cold-formed steel purlins attached to the roof sheeting are presented in this paper. The second-order effect of tensile stresses induced by the gravity load in the free flange of purlins is investigated by emphasizing on Z and C sections. The results obtained from the two models are compared with Eurocode 3, and they are included in this article. It is observed that for shorter purlin span lengths the design assumption underestimates the maximum mid-span stresses.

Modelling of cold-formed steel sandwich purlin-sheeting systems to estimate the rotational restraint

Sandwich panels are attached to cold-formed steel purlins in roofs of industrial buildings to provide insulation. As the strength of the attached purlins is considerably increased due to the lateral and rotational restraints provided by the sandwich panels, estimating these restraints is important in the design of purlins. The rotational restraint is generally determined by experiments, as no design rules exist for sandwich purlin-sheeting systems. In this paper, a non-linear finite element model is presented to estimate the rotational restraint provided by the sandwich panels to the attached purlin. The model is validated with experimental tests and is in good agreement. In order to develop a design method for estimating the rotational restraint in sandwich purlin-sheeting systems, the model could be useful for parametric studies to investigate the influencing factors.

A theoretical numerical study of the rotational restraint in cold-formed steel single skin purlin-sheeting systems

Cold-formed steel purlins are commonly used in the construction of industrial buildings. The roof sheeting increases the strength of the attached purlin to a considerable extent by providing lateral and rotational restraints. Further, the rotational restraint plays an important role on the buckling behaviour of the attached purlin. There is a need for a design method that accurately predicts the rotational restraint without experimental tests. Addressing this problem, in this research work, a non-linear finite element model is developed to estimate the rotational restraint provided by the first and second generation trapezoidal sheeting to the attached purlin. This model is applicable to trough-fixed and crest-fixed single skin purlin-sheeting systems commonly employed in steel roofs. The performance of the finite element model is validated by conducting experimental tests and found to be in good agreement. The factors influencing the rotational restraint can be investigated by using this validated finite element model. It opens the way to the development of a design method for estimating the rotational restraint provided by the sheeting to cold-formed steel purlins.

Distortional Buckling Tests on Cold-Formed Steel Beams

Failure in cold-formed steel beams is generally initiated by one of three instabilities: local, distortional, or lateral-torsional buckling. For cold-formed steel joists, purlins, or girts, when the compression flange is not restrained by attachment to sheathing or paneling, distortional buckling may be the predominant failure mode. Experimental results on cold-formed steel beams with unrestrained compression flanges are scarce. Therefore a series of distortional buckling tests on cold-formed steel C and Z sections in bending was conducted to establish the capacity in distortional buckling failures. Test details were selected to allow distortional buckling to form, but restrict lateral-torsional buckling to the extent possible. These distortional buckling tests also provide a direct comparison against the local buckling tests previously performed by the writers. As expected, large strength reductions are observed in the tested specimens when distortional buckling initiated the failure instead of local buckling. U.S., Canadian, and joint North American standards for design, which are known to primarily focus on local buckling, provided unconservative predictions of the observed strength. The Australian/New Zealand Standard and the direct strength method, which provide explicit methods for calculating the capacity in the distortional buckling mode, provided reasonably accurate and reliable predictions.

Influence of lateral restraint on lateral-torsional buckling of cold-formed steel purlins

In this paper, the influence of the lateral restraint provided by cladding on the lateral-torsional buckling of zed-purlin beams is investigated in detail. The analysis is performed using the energy method. The influence of the lateral restraint on the critical load is discussed for beams with various different boundary conditions, loading positions, with and without interval anti-sag bars.