BEHAVIOR OF ECCENTRICALLY BRACED STEEL FRAME

Abstract

This report discusses the dynamic behavior of a practical and efficient earthquake resistant structural system, eccentrically braced frame, which possess a higher stiffness in moderate earthquake than moment resisting system and a better energy dissipation mechanism than concentrically braced frame in severe earthquakes.

In an eccentrically braced frame, the inclined braces are connected to the beams, instead of the intersections of beams and columns. The eccentric beam elements will be subjected to very high shear forces from the braces during an earthquake. By limiting the lengths of the eccentric beam elements, they can be yielded under the action of shear rather then bending to form plastic shear hinges. Thus the energy is dissipated in the form of shear strain energy. This kind of energy dissipation is more desirable than that of bending yield because it preserves more structural stability and the plastic deformation can be restricted in the beams which are much easier to repair than columns after a severe earthquake. In this report, the plastic behavior of beam element under shear action is studied using sandwich beam theory and the flexibility matrices are derived for cases with plastic shear hinge and bending moment hinges. These matrices are incorporated into the computer program DRAIN-20, and the overall structural behavior predicted by this program, under ground motion, is then checked experimentally by means of a relatively new and powerful experimental technique -- pseudo-dynamic method. The basis of pseudo-dynamic method is also discussed in this report and a complete experimental set-up is presented.