ENERGY-BASED ANALYSIS AND DESIGN OF BUCKLING-RESTRAINED BRACED FRAMES FOR DUAL EARTHQUAKE-WIND HAZARDS

Current design codes for buildings are based either on the force-controlled approach or the displacement-controlled approach. Compared to these conventional design methods in which sufficient strength and allowable deformation are used as primary design parameters, the energy-based approach to design is more preferable since it can account for progressive damage due to inelastic cyclic effect from strong ground motions and/or high winds. In an energy-based design, a structure is considered satisfactory if its energy dissipation capacity is greater than the energy demand on the structure. To achieve an economical design, structures are rarely designed to behave elastically under strong earthquake and/or wind loads. They are expected to undergo yielding to absorb energy. To determine the energy demand, spectral or time history analysis is commonly used to carry out the dynamic analysis. While current design guidelines treat strong winds and earthquakes independently, these two commonly encountered natural hazards could occur simultaneously. As a result, this research introduces the use of a dual earthquake-wind hazard spectrum for analysis. This dual hazard spectrum is obtained by combining the power spectral densities of earthquake and wind using the square root of the sum of the squares (SRSS) combination rule. An equivalent time excitation function is then computed using inverse fast Fourier transform (IFFT) and used as input for the dynamic analysis. With the dual hazard spectrum, engineers can design buildings without having to determine whether the design is governed primarily by a single hazard or both hazards. Using time history analysis on the OpenSees platform, the dynamic responses expressed in terms of peak and residual inter-story and roof drifts of three steel frames (a portal frame, a 3-story frame and a 9-story frame) in four different geographic locations in the United States (Los Angeles, Miami, Charleston, and Boston) designed to perform at the immediate occupancy (IO) and life safety (LS) performance levels are used to verify the validity of the proposed approach. In addition, to reduce the amount of inelasticity induced in the frame members, buckling-restrained braces (BRBs) are used as energy dissipation devices. These BRBs are not only capable of dissipating energy, they also provide additional strength and stiffness to the frames and therefore allow them to withstand the combined impact of the dual hazards. A simplified energy-based method is proposed to design the steel core of these BRBs. The method is based on the equivalent lateral force (ELF) procedure. The energy in excess of what the bare frame is capable of dissipating without exceeding the (FEMA-356, 2000) recommended drift limits for each performance level is taken as the energy demand on the BRBs. These BRBs are then designed to have an energy capacity that can overcome this energy demand. The energy capacity of the BRBs is derived from their strain and hysteretic energy when the steel core undergoes yielding under cyclic load due to ground excitations. The proposed design procedure is then validated by nonlinear time history analysis using OpenSees. The present study has shown that the use of the dual hazard spectrum for energy-based design of steel frames susceptible to dual earthquake-wind hazards and the procedure proposed for determining the steel core size of BRBs to allow the frames to satisfy the drift limits for the performance levels of immediate occupancy (IO) and life safety (LS) are viable means to design structures against dual earthquake-wind hazards