Peter Gergely

Implications of Experiments on the Seismic Behavior of Gravity Load Designed RC Beam‐to‐Column Connections

This paper summarizes recent experimental research at Cornell University conducted on the behavior of gravity load designed reinforced concrete building frame components subjected to reversing cyclic loads (simulated seismic effects). Reinforced concrete framing systems, designed primarily for gravity loads, with little or no attention given to lateral load effects, are typically characterized by non-ductile reinforcing details in the joint regions and in the members. The seismic response of connection regions for gravity load design (GLD) frames has received relatively little attention in earlier studies, thus making it difficult to reliably evaluate GLD frames and to properly plan repair or retrofit strategies. Thirty-four full scale bare interior and exterior beam-to-column joints have been tested under reversed cyclic bending to identify the different damage mechanisms and to study the effect of critical details on strength and deformations. The discussion of test results focuses on the definition of joint shear strength factors for GLD frames to complement those provided by ACI-ASCE Committee 352 for frames designed with better details.

Behavior of Gravity Load Design Reinforced Concrete Buildings Subjected to Earthquakes

Two small-scale reinforced concrete building models were tested on the Cornell University shake table. The models were a 1/6 scale two-story office building and a 1/8 scale three-story one-bay by three-bays office building. Both structures were designed to resist purely gravity loads without regard to lateral loads (wind or earthquake forces). The reinforcement details were based on typical reinforced concrete frame structures constructed in the central and eastern United States over the past 50 to 60 years, as characterized by (a) low reinforcement ratio in teh columns, (b) discontinuous positive moment reinforcement in the beams at the column locations, (c) little or no confining reinforcement in the joint regions, and (d) lap splices located immediately above the floor level. Both models were tested using the time-compressed Taft 1952 S69E ground motion scaled to increasingly large peak ground accelerations. Test results indicated that gravity load design (GLD) reinforced concrete buildings without walls will experience very large deformations associated with a considerable stiffness degradation during a moderate earthquake. The high flexibility produced significant P-Δ effects in the three-story building model. Although the nonseismic details associated with the gravity load design philosophy forms a source of damage, the experiments indicate that these details will not necessarily lead to collapse or to a complete failure mechanism. Comparison with analytical results indicated that inclusion of the slab contribution to beam flexural strength is a vital step in the assessment of the performance of GLD reinforced concrete structures since it has the potential of altering the relatively ductile strong column-weak beam mechanism to a more brittle soft-story mechanism.

Strength and stiffness of reinforced concrete containments subjected to seismic loading: Research results and needs☆

Abstract The primary purpose of this paper is to present results of an experimental investigation on the strength and stiffness of reinforced concrete subjected to combined biaxial tension and simulated seismic forces. The test specimens represent a section of a wall of a containment structure carrying combined pressurization and seismic loading. Shear stiffness and strength, and their degradation with shear cycling, are given, along with simple expressions for predicting strength and extensional stiffness. The secondary purpose of the paper is to discuss research needs for improved prediction of the response of containment structures to seismic effects.

Design considerations for concrete nuclear containment structures subjected to simultaneous pressure and seismic shear

Abstract Improvements in design code provisions for tangential shear in secondary concrete nuclear containment vessels are needed. This paper presents a brief summary of an experimental research program conducted at Cornell University on tangential shear. Six inch thick reinforced concrete panels were subjected to combined in-plane tension and shear as a behavioral model of a section of the wall of the containment under the combined loading of internal pressurization and seismic shear. Approximately 50 panels were tested. Parameters studied included: tension level and direction (biaxial or uniaxial), shear level and type (monotonic, cyclic, or a combined mode), sequence of applied loading, and reinforcing ratio and orientation. The results of the research indicate that current code provisions are overly conservative with regard to the amount of tangential shear to be carried by the orthogonally reinforced concrete. By increasing the allowable stress, the required amount of diagonal reinforcing would be reduced. This would result in savings in fabrication costs and construction time, and improved structural reliability through improved concrete placement. The research also indicates a need for a more exact consideration of containment displacements. Shear stiffnesses for the panels were extremely low, indicating that containment displacements may be larger than anticipated. The code provisions in this area are limited and unsubstantiated.

Seismic fragility of reinforced concrete structures in nuclear facilities

Abstract The failure and fragility analyses of reinforced concrete structures and elements in nuclear reactor facilities within the Seismic Safety Margins Research Program (SSMRP) at the Lawrence Livermore National Laboratory are evaluated. Receiving special attention are uncertainties in material modeling, behavior of low shear walls, and seismic risk assessment for nonlinear response. Problems with ductility-based spectral deamplification and prediction of the stiffness of reinforced concrete walls at low stress levels are examined. It is recommended that relatively low damping values be used in connection with ductility-based response reductions and that static nonlinear force-deflection curves be studied for better nonlinear dynamic response predictions.

Peripheral shear strength of biaxially tensioned reinforced concrete wall elements

Abstract The results of a series of tests on biaxially tensioned, orthogonally reinforced concrete panels subjected to punching shear are presented and discussed. Contrary to existing U.S. code provisions, the punching shear capacity is not reduced significantly as the biaxial tension level is increased to as much as 0.8 f y in the reinforcement. A design equation is proposed that gives 4√ f ′ c shear stress for zero biaxial tension and a linear decrease to 3.1√ f ′ c as the tension is increased to 0.9 f y . The size of the loading pad under the punching force and the shear span have little effect on the strength but the pattern of the failure crack does change with these geometric variables. The splitting crack tends to connect the edge of the loading pad and the supports. More testing is recommended to evaluate a few additional variables, such as the use of inserts which receive the punching force.

Seismic effects in secondary containments: Research needs

Abstract The purpose of this paper is to discuss the status of current and projected research on the behavior of nonprestressed secondary containment structures carrying combined pressurization and seismic shear. Ongoing experimental research at Cornell University on specimens carrying combined biaxial tension and static cyclic shear is described. The remainder of the paper treats research needed to better predict the response of containments to seismic effects and to serve as the basis for improved design methods for reinforced concrete containments.

Nonlinear dynamic response of cracked reinforced concrete nuclear containment vessels

Abstract In the design of reinforced concrete nuclear vessels horizontal cracks are assumed to exist as a result of pressurization. Seismic shear forces must be transmitted across these cracks. The nonlinear dynamic response of cracked vessels is studied. The force-displacement relationship across the cracks are taken from the experimental investigation that included the shear transferred by the concrete but not by dowel action of the vertical steel. The stiffness is highly nonlinear, hysteretic, and degrading. A modal analysis technique, based on an eigenvalue reanalysis procedure, is developed and it is compared with a direct numerical integration solution. Only typical response values are given for particular values of the variables and for one particular earthquake input.

Research needs for design of concrete containment structures

Abstract Potential failure modes of reinforced concrete containment shells are outlined, especially those associated with pressure-induced cracking and seismic forces. A summary is given of experimental and analytical research needed to evaluate tangential shear capacity and stiffness, the interaction between liner and cracked concrete, peripheral (punching) shear capacity, radial shear behavior, and nonlinear dynamic analysis approaches.

Seismic rating of existing buildings

As part of the US‐Japan co‐operative research programme, this report presented at the 1985 UJNR Panel Conference reviews two workshops, the first held at Tsukuba in 1983 and the second at Berkeley, California in 1984. The highlights of those workshops are discussed here, and some conclusions drawn.