The advantages of cold-formed thin-walled steel studs are many and their applications in building constructions continue to grow. They are used as load-bearing members. An example is lightweight wall panel assemblies which consist of channel steel studs with gypsum plasterboard layers attached to the two flanges, often with interior insulation. At present, expensive fire tests or advanced numerical modelling methods are necessary in order to discover the fire resistance of such wall assemblies. For common practice this is not effective and a simplified method, suitable for use in daily design, is necessary. The aim of this research is to develop such simplified methods.The first main objective of this study is to develop a simple approach to calculate the temperature distributions in the steel section, in particular the temperatures on both the exposed and unexposed sides when the panel is exposed to fire exposure from one side. These two temperatures are the most influential factors in the fire resistance of this type of wall assembly. The proposed method calculates the average temperatures in the flanges of the steel section and assumes that the temperature in the web is linear. The proposed method is based on a simple heat balance analysis for a few nodes representing the key components of the wall panel. The thermal resistance of these nodes are obtained by the weighted average of thermal resistances in an effective width of the panel within which heat transfer in the panel width direction is assumed to occur. The proposed method has been extensively validated by comparison with numerical parametric studies.In order to calculate the ultimate capacity of steel studs, the traditional method is by using effective width. However, this method is now being questioned as it considers elements of section in isolation and does not consider interaction between the elements. In addition, this method is not appropriate to be extended to steel studs under fire conditions. The cross-section under fire conditions has non-uniform temperature distribution which results in the non-uniform distribution of mechanical properties. Using an effective width method to deal with this problem will require many assumptions whose accuracy is uncertain. Recently, the direct strength method (DSM) has been developed and its accuracy for ambient applications has been comprehensively validated. This method calculates cross-sectional plastic resistance and elastic critical loads for local, distortional and global buckling modes with the aid of simple computer programs. The elastic and plastic resistances are then combined to give the ultimate resistance of the structure using interaction equations. This method is suited to steel studs with non-uniform temperature distribution in the cross-section.The second main objective of this study is to extend the direct strength method for application to thin-walled steel studs having non-uniform elevated temperature distributions in the cross-section. It has been found that the DSM concept is applicable, but the interaction equations should be modified to allow for the effects of elevated temperature (non-uniform temperature distribution and changes in stress-strain relationships). Also the effects of thermal bowing should be included when calculating the plastic resistance and the elastic buckling loads of the cross-section. This research has proposed new interaction equations and has developed design tools. By comparing the results of the proposed method with validated Finite Element simulations over a very large range of parametric studies, the proposed method has been demonstrated to be valid. The validation studies include both standard and parametric fire exposures and are generally applicable.