Cold-formed steel trapezoidal profiles provide efficient solutions for roofing and often use the Gerber joint to effectively utilize capacities. The previous design of Gerber joint was sensitive to uneven distribution of loads and accidental loads, which imposed bending moments in the joint and lead to its failure. In this experimental program, the design of Gerber joint has been modified to work as a hinge under service loads and carry moments in accidental conditions. Also, the design of CFS is based on elastic methods that underestimate their capacity, especially for multi-span systems. Full-scale tests were conducted on highly stiffened double-span trapezoidal sheeting profiles with modified Gerber joint to investigate elastic capacity, inelastic behaviour, moment redistribution in the post-elastic phase, ultimate load capacity and feasibility of modified Gerber joint. Comparison of elastic load capacity with EWM and DSM predictions revealed that EWM design predictions were conservative by 30% while DSM predictions were accurate. For multi-span application, residual moment capacity ratios of 0.76 and 0.81 in the post-elastic phase allowed for moment redistribution and increased ultimate load capacity by 7.14% and 8.80% for 0.85 mm and 1 mm thick profiles respectively. Performance of modified Gerber joint to behave as a hinge under service loads and as continuous in the post-elastic phase was also found to be satisfactory. The study concluded that the economy in design and capacity utilization of multi-span CFS profiles can be improved by allowing for moment redistribution and using the modified Gerber joint.

The goals of structural design are fundamentally different when designing structures at normal temperature or when designing them in a fire situation. While structures are primarily designed for normal temperature situations considering the different design limit states, in the fire design situation, however, the already designed structure is assessed for its resistance in the fire design limit state. The assessment of the structure in the fire limit state may lead to either active or passive fire protection measures. The assessment of the structure in fire may be done in several different domains such as its structural resistance, integrity of structural components to prevent spread of fire and insulation properties of materials. The focus of the thesis presented here is on the structural resistance of steel structural members particularly steel beams and trusses in fire situations.The Eurocodes permit designers to use either a simple prescriptive design procedure or a more complex performance based procedure for design of structures in fire. The prescriptive design is a simple choice regarding design of steel structures in fire due to their use of simple analytical equations; but through several studies it has been established that this approach might be conservative and in some situations it might not reflect the complexity of interaction between the heated structural members and its surrounding colder parts of the structure. The performance based approach has therefore been increasingly adopted in structural fire design, which, although more complex than the prescriptive approach, is closer to the real structural behaviour.Through a performance based approach, this thesis aims to establish that steel structural members are able to offer structural resistance in fire situations that are much higher than would be expected from a prescriptive approach. Two different types of structural members such as steel beams in multi-storey buildings and trusses in single storey buildings were considered here. It has been shown through extensive finite element analysis in both cases that actual resistance of these structural members in fire situations can exceed their primary resistance mechanism through flexure. Alternative load transfer mechanism through catenary action offers the added resistance at much higher temperatures than the conventional critical temperatures from prescriptive design. The thesis also proposes simplified calculation procedures that can be used to reasonably predict the structural resistance at elevated temperatures considering the catenary action.

Structural fire design is exceedingly adopting the performance based approach. There are evidentadvantages of this approach compared to the prescriptive methods from codes. An analytical procedure, based on thereal performance, must accurately predict the beam behaviour in fire. The study presented here proposes one suchsimplified analytical procedure aim to predict the real behaviour of a restrained steel beam. The proposed analyticalprocedure is validated through FE Analysis using FE models validated through test results. The study also attempts toestablish the importance of using semi-rigid connection strength with respect to accurately predicting the behaviorof the restrained beam at catenary stage.

Structural fire design is exceedingly adopting the performance based approach. There are evident advantages of this approach compared to the prescriptive methods from codes. An analytical procedure, based on the real performance, must accurately predict the beam behaviour in fire. The study presented here proposes one such simplified analytical procedure aim to predict the real behaviour of a restrained steel beam. The proposed analytical procedure is validated through FE Analysis using FE models validated through test results. The study also attempts to establish the importance of using semi-rigid connection strength with respect to accurately predicting the behaviour of the restrained beam at catenary stage.

The methods proposed by the design codes for single member design in fire situation assume that these members are isolated in their response. The real response of structural members such as beams is, however, more complex due to thermal expansion and the presence of restraints against this expansion by the surrounding structure. It is therefore imperative to study the response of structure at high temperature in a way which includes its interaction with its surroundings such as in a full-scale fire test and in numerical analysis. This paper focuses on the numerical investigation of steel beams, with a concrete slab and connected to concrete filled tubular (CFT) columns through reverse channel connections. The finite element software ABAQUS has been used in this study. The aim of the investigation is to study the behaviour of the composite steel-concrete beam exposed to increasing temperature in fire.

The design methods currently proposed by the codes prescribe the strength assessment of structures to be based on their strength limit state. These design methods can be applied to isolated steel members to determine their design strengthin fire. The real response of a structural member is, however, more complex due to the thermal expansion and the presence of restraints against this expansion by the surrounding structure. It is therefore imperative to study the response of a structural member at high temperature in a way which includes its interaction with its surroundings. This paper focus on the numerical investigation of steel beams in structural frames connected to concrete filled tubular (CFT) columns through reverse channel connections and comparison to hand calculation procedures. Finite element models (FEM) of the sub-frames were validated against fire tests conducted on sub-frames and then their results were compared to the proposed simplified hand calculation procedures (HCM).

This paper presents the experimental results of the investigation on the coupled joint-structure behaviour of the composite sub-frame, using the reverse channel connections between an I-beam and the concrete filled tube (CFT) columns. This experimental programme includes seven full-scale tests: three tests at ambient temperature and four tests under heating-cooling curves. The parametric study was dedicated to: temperature-time curve and channel wall thickness (8, 10 and 12 mm). The main objective of these tests is to provide experimental information on the behaviour of the reverse channel joints and its influence on the structure under a heating-cooling fire. The restraining effects from the unaffected part of surrounding structure induce highly variable loading histories on the joints during fire; therefore the investigation on coupled joint-structure behaviour should lead to a realistic prediction of progressive collapse of the structure.