UC Berkeley

Most industrial and nuclear facilities rely on reinforced concrete structural walls as their primary seismic lateral-force-resisting components. These walls commonly have an aspect ratio smaller than 0.5 and have a very high stiffness and strength. There is a significant uncertainty regarding the behavior of these walls under earthquake loading, their failure modes, and their expected strengths and deformation capacities. Hybrid simulation is an effective experimental method to examine these issues: it enables simulation of the seismic response of squat and thick shear walls without the need to recreate the, often very large, mass associated with the rest of the prototype structure. A new method for hybrid simulation of the earthquake response of stiff specimens using a high-precision displacement encoder was developed and verified in this study. This method was implemented for hybrid simulation of seismic response of two large-scale squat reinforced concrete shear walls.

In order to examine the response of squat reinforced concrete walls to earthquake ground motion and to investigate the effect of ground motion sequence, two nominally identically 8 in thick models of a prototype 36 in thick structural wall, typically found in nuclear facility structures, were tested. Each wall experienced a different ground motion level loading sequence. After an initial combined shear and flexural response, a sliding shear failure occurred at the base of the walls. This response was quasi-brittle: the walls rapidly lost strength with small increments of post-peak strength deformation. A nominally identical specimen was tested at the State University of New York at Buffalo. Though the quasi-static cyclic test method has been shown to accurately predict the seismic failure modes of ductile, often flexure-dominated, specimens, there is considerable uncertainty associated with the predictive ability of the quasi-static cyclic test method when the tested specimens have brittle or quasi-brittle failure modes. In these cases, the effect of load magnitude history is so significant that it alters the deformation demand and the sequence of seismic failure modes. The quasi-static cyclic test at Buffalo was compared to the hybrid seismic response simulation experiments at Berkeley to evaluate its effectiveness with capturing the wall response to ground motion sequences.

The findings from the hybrid simulation tests were that displacement control hybrid simulation using a high-precision encoder for displacement feedback is an effective way to perform large-scale hybrid tests of stiff specimens. This new method is useful to address the shortcomings with understanding the dynamic behavior of these types of specimens. The results of the two wall hybrid simulation tests indicate that different earthquake magnitude sequences do not have a significant effect on the force-deformation response and the failure mode sequence of squat walls. After comparing the hybrid simulation test results to the quasi-static cyclic test at Buffalo, the quasi-static cyclic test was determined to be adequate for testing the quasi-brittle wall specimens. It effectively captured the global response of the squat shear walls in earthquake ground motion sequences.

Comparison of wall response to code based predictive equations showed that the code equations overpredict the peak shear strength of these squat rectangular walls by factors as large as 2. Modifications to code recommendations for the initial stiffness and peak shear strength of these walls are offered, and a definition for the "essentially elastic" region, used in nuclear facility design, is also suggested.