Project: An Experimental and Analytical Study of the Earthquake Response of R/C Frames with Yielding Columns►
About the Group
The structural framing systems permitted by existing design codes are in part a product of scientific scrutiny as well as a matter of subjective opinion. The use of reinforced concrete frame structures with girders flexurally stronger than the columns is avoided in seismic areas because of fear of inadequate response. The professional community, through building codes, prohibits the use of such a framing system. It is feared that when all the columns at a story yield, an elastoplastic mechanism will be formed leading to imminent. Codes also aim to prevent column strength decay under load reversals, joint failures, and shear failures of the columns. However, reinforced concrete structures exhibit post-yield stiffness under dynamic loading, rendering an elastoplastic mechanism unrealistic. This reserve stiffness may be used to limit drift to acceptable levels.
Architectural considerations often requiring ´open space´ lead to long spans and strong girders. Thus, this framing system is functionally desirable and is used, against the tenets of the building codes. Testing of ´strong beam´-´weak column´ structures or components has been very limited. Yet testing is required, if for nothing else, at least to evaluate the safety of existing structures. The objective of this investigation was to study the response of ´strong beam´-´weak column´ frames to strong ground motions by means of a combined experimental and analytical program.
To achieve the objective, two small-scale, nine-story, three-bay test structures were constructed and tested on an earthquake simulator. The structures were subjected to a single component of and earthquake base acceleration record in one horizontal direction. The test structures comprised two parallel frames and were coupled by rigid diaphragms which also acted as added story weights. The frames were designed so that the sum of the column flexural strengths at each joint was less than the sum of the girder flexural strengths. The principal variable of the test specimens was the strength of the stories. Column reinforcement for the first specimen was provided to satisfy design force requirements. For the second specimen column reinforcement quantities were increased. Test structures response to the base motions was interpreted and evaluated from displacements and accelerations measured at each level.
An analytical model was developed to reproduce ´strong beam´-´weak column´ frame displacement response to strong ground motions. A horizontal degree of freedom was defined at each level of the frames. All mass at a story was lumped at each degree of freedom, and all columns at a story were replaced by a single nonlinear spring. The force-displacement response of the nonlinear spring was defined by a story force-displacement primary curve and a set of hysteresis rules. Variations in column axial load due to overturning were neglected.
Various levels of complexity in the representation of loading and reloading behavior of reinforced concrete members were considered and evaluated in light of measured test structure response. Several representations of story force-displacement primary curves were considered.
After sufficient confidence had been developed in the analytical model, it was used to investigate the response of ´strong beam´-´weak column´ structures under a variety of conditions. Variables of the study included ground motion, frame strength, frame profile, and hysteretic representation. Frame performance in resisting the base motions was judged in terms of displacement response. Drift response was considered in relation to frame distortion, linear response, and inelastic action.