Estimating axial force demand in columns of seismic-resistant steel structures

Current design procedures that are based on capacity design principles assume that all the stories of a steel building yield simultaneously, while calculating axial force demand in columns. This, evidently, results in large and economically inefficient sections in tall structures. Estimation of number of simultaneously yielding stories, NSYS, in a building is considered to be the key in proper estimation of axial force demand in columns. Based on the concepts of wave propagation, mathematical equations for estimating NSYS were first developed for shear type buildings subjected to full-sine velocity base excitation. The estimation procedure was then extended for buildings subjected to earthquake base excitation. This paper presents the application of the proposed procedure for estimating the NSYS values for buckling restrained braced frame (BRB) subjected to earthquake base excitation and utilizes the results to further develop a procedure for estimating axial force demand in columns of such structures. In the proposed procedure, axial force demand in column is calculated by summing the vertical force transferred from the NSYS simultaneously yielding stories and the square root of sum of squares combination of the vertical force transferred from the rest of the stories above the column under consideration, assuming their full yield capacity. Application of the estimation procedures on shear-type buckling-restrained braced frames subjected to 18 different earthquakes belonging to three categories, namely: earthquakes having i) a single dominant pulse, ii) more than one distinct pulse, and iii) no distinct pulse, in their velocity record, was found to provide good prediction of the NSYS value and the axial force demand, especially, for the first two categories of earthquakes.