SEISMIC DESIGN OF BRIDGE COLUMNS BASED ON CONTROL AND REPAIRABILITY OF DAMAGE

This report describes the development of a new seismic design paradigm referred to as Control and Repairability of Damage (CARD). Replaceable/renewable sacrificial plastic hinge zones that use fuse-bar details form the basis of the approach. Hinge zones are deliberately weakened with respect to their adjoining elements; all regions outside the hinge zones are detailed to be stronger than the sacrificial hinge (fuse) zone and remain elastic during seismic loading. The special detailing of the sacrificial hinge zone permits repair of damage inflicted on that zone after an earthquake. The CARD design methodology also lends itself to the use of precast and prestressed concrete construction. This is considered to be a particularly attractive proposition for bridge design and construction, since historically bridge piers have always been constructed of cast-in-situ concrete, inevitably requiring a longer on-site construction time-frame. A fatigue theory for structural concrete columns is proposed which is based on well-known strain-life fatigue concepts for metals. Using this theory, the fuse-bars in the potential plastic hinge zones are designed in such a way that their fatigue capacity exceeds the fatigue demand expected from a maximum capable earthquake ground motion. The remaining portions of the structure are then designed in accordance with well-known capacity design principles. To validate the proposed new design philosophy, an experimental investigation was conducted. Experimental results show that each of the repaired column hinges performed as well as their undamaged original counterparts. Damage to the specimens was controlled and constrained within the hinge zone. The experiments verified that this new approach to construction enables rapid restoration to full service immediately following an earthquake. The low cycle fatigue theory is shown to predict well the plastic rotation-life of bridge columns - no special calibration of the theory was needed for this purpose. Moreover, force-displacement hysteretic behavior and fatigue life of columns with fuse-bar detailing can be accurately predicted computationally using fiber element analysis.