Strength and Stiffness-Based Methods for Reducing Hinging in Columns of Reinforced Concrete Frames Subjected to Strong Ground Motion
The focus of this research is the development of stiffness-based and strength-based rules for encouraging a structural mechanism to define the maximum deformed shape of reinforced concrete buildings subjected to earthquakes. The emphasis of the program is to improve the response of reinforced concrete buildings by eliminating yielding in the columns, thereby distributing the nonlinear deformations over the entire height of the building. The rules are to be implemented as a performance-based design approach, where by satisfying the criteria, localized story distortions are reduced.
Inelastic behavior is traditionally encouraged to occur in the girders and not in the columns of a frame by providing greater strength in the columns framing into a joint than the girders at that joint. This rule, however, does not guarantee the desired results. Initial research results have shown that the ratio of required column to girder flexural strengths increases with increasing number of stories without bound. For example, for a ten-story building, the ratio of strengths (SMc/SMg) must exceed 4.0 to have a structural mechanism become the controlling mechanism. With a ratio of 6/5, currently required by ACI318 (1999), the controlling mechanism would indicate that yielding (as well as the most concentrated story deformations) will occur mostly in the bottom two-thirds of the building (Fig. 1). The current grant explores a combination of stiffness and strength criteria that will improve this distribution and ideally lead to a structural mechanism defining the maximum deformed shape of the structure. The results of this study will improve methods for locating probable hinging locations in a frame for providing special detailing for inelastic response.
There are three phases to the project. In the first phase, equivalent static lateral load procedures are studied and compared with a series of nonlinear dynamic analysis results to determine the best method for reproducing the distribution of yielding in the members. The second phase consists of a parametric analysis of reinforced concrete frames to determine the optimal combination of stiffness and strength-based rules for redistributing inelasticity in the building to minimize story and member distortions. An undergraduate research assistant is completing the third phase of the project in which control methodologies are being investigated for improving the performance of reinforced concrete frames. The student is developing a windows-based structural analysis tool for combining control technology with reinforced concrete systems (Fig. 2).