Overview and Objectives: The goal of the hybrid frame research is to develop a new type of seismic resistant system that utilizes a variety of strategies to provide improved performance over traditional systems. Key to the hybrid concept is the development of some minor yielding at ground motion levels well below the design basis earthquake (DBE), and to provide a minimum residual stiffness at the maximum considered earthquake (MCE). The “early yielding” is intended to provide sufficient energy dissipation to control resonant response when the system is still quasi-elastic, and the residual stiffness is intended to limit the development of residual deformations and associated tendencies towards dynamic instability.

The following basic strategies are being considered to develop hybrid behavior:

1. Use of very low strength steels (F_y < 20 ksi) and high strength steels in the same system
2. Use of gravity preloading
3. Use of gravity system as an intended backup to the main system
4. Use of high damping rubber as an auxiliary energy dissipation system

Work Description: The hybrid frame concept will be developed in detail. At least two hybrid frame systems will be developed, one of which will include the hybrid moment resisting frame. The second system will be based on a mixture of braced frame systems, including buckling restrained braces. Each hybrid frame system will be formally evaluated using an ATC 63-like process.

Deliverables: Design rules for hybrid frames will be developed in concert with a formal analysis/evaluation procedure that is modeled after that developed for the ATC 63 project. The result of the work will be code-language design rules for hybrid frames, together with appropriate design values (R, C_d, W_o).

Benefit: Hybrid frames may perform better than traditionally designed systems subjected to seismic loads.

Milestones: Extensive investigations on Strategies 1 and 3 (see above) have been completed, resulting in intermediate work products consisting of white papers and reports.

(image courtesy of Dr. Charney)