View PDF

ABSTRACT

The strongback braced frame (SBF) is a hybrid of a conventional inelastic system and an essentially elastic steel truss. Input seismic energy is absorbed by supplemental inelastic elements typical of a traditional system; e.g., through primary yielding and/or buckling in the bracing elements and secondary flexural yielding in the beam links. The opposite strongback truss is then proportioned to remain essentially elastic, resulting in a relatively stiff and strong vertical spine. Axial forces and bending moments developed in these inelastic elements are transferred vertically to adjacent stories through the strongback.
The strongback is not intended to provide supplemental lateral strength. Rather, the spine pivots about its base to maintain an imposed first mode shape. Inelastic demands are not eliminated but averaged, resulting in smaller peak and residual drifts distributed across the frame height, increased behavior predictability, and reduced probability of yellow or red tagging following an earthquake. Notably, the strongback system represents an in-between stepping stone between basic and enhanced performance objectives.