On the development of a robust element for second-order `non-linear integrated design and analysis (nida)'

This paper describes and applies a theory for practical second-order analysis and design of steel frames. The practicality, convenience and high precision of the method of `Non-linear Integrated Analysis and Design (NIDA)' that allows for the second-order effects for design of steel frames and trusses is demonstrated. The need for using two or more elements per member to accurately simulate the axial load effect on the element stiffness and to locate the maximum moment along an element is eliminated. Unlike previous work for `Advanced Analysis' which is difficult to use in practice as it ignores many important practical features such as element imperfection, the proposed method meets the current design code requirements and is based on the first-plastic-hinge concept which is currently adopted by engineers. Furthermore, the element by itself can predict the beam-column load carrying capacity using a single element per member to model the P-δ effect and, therefore, it can be used for simultaneous analysis and design. The use of several elements per member still possesses an error of idealising a physically smoothly curved member by a series of segments of straight elements. Conceptually, the suggested method designs a structure by modelling accurately its true behaviour, instead of making use of empirical formulae for individual member checks. It is readily available for design application of realistic structures and it is envisaged that the NIDA will initiate a revolution in the practical design of steel structures.