This study focused on the structural behaviour of

This study focused on the
structural behaviour of Cold-formed Steel (CFS) Built-up I beams composed of
two sigma sections placed face to face connected by using spot welding. It is
aimed to establish accurate finite element models for CFS built-up I beam
subjected to transverse load. The numerical model was developed by using Finite
Element (FE) software ABAQUS 6.13. The numerical model is validated by means of
comparison with the experimental results reported by Wang and Young (2015) in
terms of moment capacities, moment versus deflection curve and failure mode of
specimens. For different cross section geometries and different thickness of
built-up I beam under four-point bending, the numerical study has been carried
out by using the verified FE model and theoretical study using Direct Strength
Method (DSM) (AISI-S100:2012). The appropriateness of the DSM
method was checked and based on the comparison of results the effect of key
parameters on the moment capacities of CFS built-up I beams are discussed. The
comparison shows that the design strengths predicted by the current DSM are conservative for both local buckling and
distortional buckling in this study.  The advantages of using
CFS sections are high strength-to-weight ratio, flexibility in fabricating different
cross-section shapes, easy for construction and so on. Local buckling and
distortional buckling are usually the governing failure modes for cold-formed
steel sections, such as thin-walled plain channel and lipped channel sections.
However, when sections were stiffened by edge and intermediate stiffeners for
optimized section shapes, the computation of effective width for each plate
element could be quite tedious that involves iteration processes and the Effective
Width Method becomes much more complicated compared to the DSM. Hence, the DSM
was recommended for design of CFS members with complex stiffeners (Schafer et
al., 2006). On the other hand, the DSM in current specifications is a
semi-empirical approach (Schafer, 2008), which was calibrated to cover only the
pre-qualified sections specified in NAS (2012). The use of hot-rolled steel
sections become uneconomical for the steel structures subjected to light and
moderate loads and for the structural members of short span lengths. So the
study on behaviour of CFS members is unavoidable to reduce the cost of a
building made up of steel structures. CFS sections such as C-sections with or
without lips, I-sections, Z-sections, angles, T-sections, hat sections and
tubular are normally used as flexural members. When single sections are not
sufficient for design loads, built-up sections made of back-to-back C-sections
or nested C-sections are normally used as flexural members. In the design of
CFS ?exural member consideration should ?rst be given to the moment-resisting
capacity and the stiffness of the member.   Built-up
sections, with the advantage of flexibility in forming different sectional
shapes to meet design requirements have been used in a wide range of
constructional Steel applications. The built-up sections also have huge
potential in terms of improve strength and stiffness when applied in CFS
members, especially compared to single profiles. Doubly symmetric built-up
sections, has the advantage of larger torsional rigidity compared to the
traditional cold-formed singly symmetric open sections.

Schafer B.W. (2007) reviewed on the DSM of CFS member design and concluded
that reliability of the DSM equals or betters the traditional Effective Width
Method for a large database of tested beams and column. Wang L et al. (2014)
investigated the structural behaviour and evaluate the appropriateness of the
current DSM on the design of CFS stiffened cross-sections subjected to bending
and concluded that FE model well predicted the moment capacities and failure
modes of the beams and also they found that the design strengths by the current
DSM are conservative and suggested modified DSM to cover the new stiffened
channel sections. Wang et
al. (2015) found that
the local and distortional buckling behaviour of the built-up section beam
specimens were found to be different from the single profiles. They considered
Young et al (2008) recommendation that local buckling stress could be enhanced
by employing intermediate stiffeners to the slender plate elements of the
sections. The local and distortional buckling behaviour of the built-up section
beam specimens were found to be different from the single profiles. They
concluded, the numerical and
experimental results are in very good agreement in terms of ultimate moments,
failure modes and the moment-curvature behavior and aimed to develop
suitable design rules for cold-formed steel doubly symmetric built-up open and
closed sections with intermediate stiffeners under bending .They concluded that design predictions by
the current DSM equations are conservative for the built-up open sections but
current DSM equations are unconservative for the built-up closed sections
investigated in their study.

studies showed that predicting the Strength of a built-up cold-formed Steel
member as the sum of the strength of the individual profiles is not rational on
the actual structural behaviour of such members. In current scenario, only little
work has been carried out on the structural behaviour of built-up sections as
flexural members. So the proposed work aims to systematically
study the structural behaviour and moment capacity of the cold-formed steel
built-up I beam were subjected to local buckling and distortional buckling.