Young Researcher: Jamie Rickard email: rickarjm@aston.ac.uk
Establishment: Aston University
Supervisor: Long Yuan Li email: l.y.li@aston.ac.uk
Aims of research:
1) To develop a computer-based analysis and design package for purlin and rail systems in which the analysis is performed using finite strip methods; 2) to establish a full model which incorporates the purlin, sheeting and antisag bars as an integrated system and considers local, distorsional and lateral-torsion buckling and material yielding and thus it is able to predict the ultimate load of the purlin system; 3) to investigate the influence of antisag bars on the lateral-torsion buckling behavour of the purlin-sheeting system, the result of which can be used to design the necessary number of sag bars; 4) to improve existing design procedures and provide not only safe but also economical design rules.

Description of method:
Cold-formed steel members are widely used as purlins or rails, the intermediate members between the main structural frame and the corrugates roof or wall sheeting, in buildings for farming and industrial use. The shape of these members is mostly that of zed, channel, or sigma. These types of cross-section are inherently sensitive to local, distorsional, and lateral-torsion buckling. Historically, the load-bearing capacity of purlins and sheeting rails has generally been determined by carrying out full scale testing. However, the proliferation of new cladding systems, including those which offer little restriction to the supporting purlins requires that analytical design procedures should be developed. For the local buckling one can use the buckling theory of plates to predict the buckling load, which has been addressed in BS5950. For the lateral-torsion buckling, however, there is no rule that can be applied in the current British Codes of Practice, particularly when antisag-bars are present. With the recent publication of Eurocode, a method is now available which enables the designer to evaluate the ultimate load capacity of a purlin system. The method is largely based on the research work carried out in The Netherlands, Germany and France. The method requires only a limited amount of small-scale testing to evaluate the lateral and torsional restraints offered by the roof cladding system to the purlin itself. Different end conditions and the effects of antisag-bars are dealt by means of the effective length concept. The load tables calculated from the EC3, however, have been found to be on average 20% to 25% less than those obtained based on the full scale tests. This implies that the method suggested in EC3 is rather conservative. Although any design method should be a conservative estimate of the true behaviour of a member, it is clear that, with an inherent conservatism in the design method of up to 30%, the method still needs considerable refinement before it becomes to be an acceptable and economical replacement for full-scale testing. The proposed project is to develop the methodology and corresponding analysis method for the lateral-torsion buckling of the purling system with considering both restraints of roof sheeting and antisag bars. The analysis will be performed using the finite strip method (FSM) developed recently by Dr. Li at Aston University. In addition, tests will also be conducted in order to demonstrate the results obtained from the computer model.

Benefits to structural Engineering:
The research is intended to provide a better understanding of the mechanism of lateral-distorsion buckling of purlin system and the influence of the antisag bars on the buckling behaviour of the purlins. The immediate beneficiaries are likely to be academic researchers and industries who manufacture or use the cold-formed steel sections although the most beneficiaries will be the manufacturers since they can use the research results to produce load tables instead of carrying out the full scale testing.