ABSTRACT
Tubular steel elements are suitable for structures comprised of continuous beams (purlins) and sway frames (industrial buildings) that may be designed according to elasto-plastic methods of analysis. In these cases the plastic deformation capacity may play a critical role. Different approaches, based on flange and web slenderness ratios, have been proposed in the structural codes to classify the sections for plastic behaviour. However, there is a need to know the actual plastic rotation that the systems may require. The purpose of this paper is to find out the actual inelastic rotation capacity, measured in degrees, required at the plastic hinges for the types of structures mentioned above.
Both, the Ultimate Limit State as well as the Serviceability Limit State are considered to determine the necessary rotations for rectangular hollow sections in the following type of structures: first, continuous beams of two and three bays (purlins); and second, portal sway frames of industrial buildings with clamped supports. Two types of methods, namely, elasto-plastic hinge and plastic zone method are used to determine the necessary plastic rotations measured in degrees. In the case of slender designs the deflection criteria is more restrictive, and therefore no plastic hinges are developed. In other cases the deflections reach the allowable values before the collapse plastic mechanism is reached. Only in the very stiff designs the plastic collapse mechanism is reached for which the largest rotations are required. All these results are shown to provide the designer with some means to compare the available rotation of the sections with the required ones.
