Difference of Force-Based and Displacement-Based Seismic Design of RC Frames

Considering the performance of a structure in design procedure as an initial explicit and primary assumption is a valuable achievement, which enable the client to decide about acceptable cost and risk. In traditional Force Based Design (FBD) method after limitation of internal forces to the strength limits of members, performance of a structure will be checked as a secondary goal of design procedure. While actually it is the damage level that is defined by desired performance, it is the main concern of design as well. Performance based design methods are categorized into direct and indirect approach. In direct approach, in spite of indirect one, structure features and characteristics are predefined such that the desired mechanism, rotation of hinges, target story drift and material strain limits correspond to the desired performance level, have been included from the beginning of the analysis and design process. Direct Displacement Based Design (DDBD) is a direct performance method in which desired mechanism of inelastic deformation and optimal performance for frames are considered as initial and essential assumptions of the design procedure. According to predefined acceptable total drift of the structure as an index of performance, base shear will be determined by using displacement spectra considering nonlinearities and damping effects. After that, the calculated base shear will be distributed throughout the stories of structure according to predominant mode shapes. Internal forces of structural members will be calculated based on a plastic analysis considering assumed mechanism. In this paper, four concrete special moment frames with two 5 m bays and 4, 8, 12 and 16 stories with height of 3 m were designed by direct displacement based method and were compared with force based design method results. According to base shear calculation relation of DDBD, it is required to predefine the total drift as a performance indicator. The total drift for life safety performance level is suggested to be 2% in FEMA 356 for concrete moment resistant frame. It was observed that DDBD base shear was greater than FBD one. Besides, by increasing the height of the structure, the difference of base shear in DDBD and FBD become greater. It was concluded that columns dimensions and longitudinal reinforcement of DDBD are equal or greater than FBD. As a general conclusion, it has been understood that the issue of weak ductile beam against strong column that is the main design topic in special moment frame design is more feasible in DDBD approach. In fact, in direct displacement based design procedure, plastic hinges are assumed to be formed in beams ends, explicitly. This consideration leads DDBD columns to have more concrete volume and longitudinal reinforcement weight compared to FBD columns. Moreover, beams of DDBD have longitudinal reinforcement weight more than FBD beams but they have less concrete weight. In addition, it was observed that in DDBD method, obtained reinforcement arrangement in columns and beams and section dimensions are such that the formation of hinges in columns or beams mid span is prevented. Finally, it is concluded that DDBD leads to a more efficient design.