- Author
- El-Borgi, S. | Stone, W. C. | White, R. N. | Gergely, P.
- Title
- Analytical Study on Seismic Behavior of Lightly Reinforced Concrete Frame Buildings.
- Coporate
- Cornell Univ., Ithaca, NY
- Sponsor
- National Institute of Standards and Technology, Gaithersburg, MD
- Report
- Report 3, September 15, 1992, 186 p.
- Contract
- NIST-PROJECT-50SBNB1C6543
- Keywords
- reinforced concretes | structures | frames | analytical models | inelastic dynamic analysis | dynamic loading | parameters | frame structures | inelastic seismic analysis | beam column section | seismic design | critical parameters
- Abstract
- There are many thousands of existing multi-story Reinforced Concrete (RC) frame structures that were designed with litle or no account for lateral forces. Attention has been focused on frame structures designed and built during the period of 1940 to 1970. It is necessary to evaluate the lateral load resistance of these structures because the amount of reinforcing and the reinforcing details used are in sharp contrast to those now used in modern seismic design. Improved evaluation techniques and retrofit methodologies are needed for these "Lightly Reinforced Concrete" (LRC) structures. A joint research program between NIST and Cornell University was initiated in early Spring 1991 with the primary objective of developing practical engineering guidelines for improving the seismic performance of LRC frame structures. To this end, an analytical study was undertaken at Cornell University to gain a better understanding on performance of these structures in a seismic loading environment. This was done by improving existing analytical tools and then using the improved analytical approaches in analyzing a number of typical buildings. This report, which is the last of three to be provided to NIST under the current contract, summarizes the findings of this analytical investigation. The required improvements in analytical capabilities are based primarily on experimental results from a parallel research program at Cornell University on full-scale joint regions of lightly reinforced concrete structures. The research (under the sponsorship of the National Center for Earthquake Engineering Research) focuses on studying the effects of the critical parameters influencing (a) deterioration of load carrying capacity, (b) degradation of stiffness, (c) ductility, and (d) energy dissipation. The present study consisted mainly of developing a smooth hysteretic model that represents accurately the physically non-coservative behavior of RC elements under reversed loading. The model can simulate effectively stiffness degradation, strength deterioration, unsymmetric hysteresis, and pinching effects. A System Identification method was developed to calibrate the hysteretic model parameters based on available experimental data. The model was implemented in an existing dynamic analysis program. With an improved version of this program and with a rational selection of the hysteretic parameters, two typical LRC frame structures were evaluated: an East-Coast 3-story, 3-bay structure designed for gravity loads alone, and a West-Coast 10-story, 3-bay building designed primarily for gravity loads with little attention to lateral forces. Both structures were detailed according to the 1950's and 1960's U. S. practice. Nine earthquake records were used in studying the seismic performance of these buildings, corresponding to different Peak Ground Accelerations (PGA's) of Nahanni, Taft and El Centro earthquakes. The two LRC structures exhibited a somewhat different behavior under the same earthquake loadings. The 3-story building became relatively more flexible than the 10-story, resulting in a higher reduction in the seismic demand. This flexibility was caused mainly by two factors: (1) partial pullout mechanism of discontinuous positive beam reinforcement from joint panels; and (2) yielding of columns. Larger drift levels occurred in the 3-story structure. The flexural response of the 10-story building was generally less inelastic than the 3-story. Unlike the 3-story structure, shear failure of the 10-story interior columns was imminent under Taft and El centro with a PGA of 0.35g.