An introduction to performance control for moment frames of uniform response under lateral loading

The paper offers an exact, closed form solution for performance-based elastic-plastic design of moment frames under lateral loading. It introduces the concept of uniform response, which in turn, enables the engineer to manually define, predict and to control structural response at pre-selected design stages, such as before and at first yield, any fraction of the failure load or specified drift ratio, up to and including incipient plastic collapse. It is assumed that the moment frames are composed of imaginary, symmetric, rectangular modules that are stacked on top of each other to form vertical subframes of uniform response where individual modules are designed to deform identically and to develop internal stresses of equal magnitude simultaneously throughout the subframe. These subframes respond as structures of uniform strength and stiffness in which members of the same group such as beams and columns, share the same demand-capacity ratios regardless of their location and number of similar elements within the framework. The subframes are eventually integrated to reconstruct the original system. The proposed solutions are unique since they satisfy the prescribed yield criteria, static equilibrium as well as the boundary support conditions. While moment frames of uniform response are ideally suited for preliminary performance control studies, the method is further enhanced by the introduction of moment control factors. These factors are used to control the propagation of plasticity within the structure and symbolize the formation or elimination of plastic hinges as needed. In practical terms, cover plates, reduced beam sections or similar technologies may be used to avoid and/or to induce formation of plastic hinges in selected locations. The proposed drift control and moment modification equations appear to be the only ones of their kind that can analytically estimate the lateral displacements and element moments of such frames, including the P-delta effects, throughout the history of loading of the structure.