State-of-the-Art Report on Seismic Performance of Unreinforced Masonry Buildings

Abstract

The author is commended for the summary of the progress in the reduction of earthquake hazard in unreinforced masonry buildings. However, the author repeats a common misunder­ standing of the Agbabian, Barnes, and Kariotis (ABK) methodology (Methodology 1984). The statement made in the ABK methodology is over a realistic range of building and soil characteristics, the ground motion is transmitted through the end walls with little amplification. This statement is not equivalent to saying these walls are infinitely rigid in plane. The ABK research studied the behavior of unreinforced masonry (URM) walls supported on flexible soils to support this assumption. The data obtained by the sensors placed in the Old Gilroy Fire Station (Plots 1990) support this assumption. The amplification of the ground displacements at the roof level was about 13 mm and 11 mm on a single pulse at the beginning of the record, and an average of 4 mm during the remaining period of ground shaking. This is a maximum amplification of the peak displace­ ment of 11 %. The amplification at the second-floor diaphragm was probably less than one-half of at the roof as its height is 3.76 m above the base, and the height of the sensor at the roof above the base is 8.1 m. The studies by Tena-Colunga (1992) identified that the dynamic response of the structure is controlled by the diaphragm action. And the diaphragm response is controlled by the input motion at its end. This is the key concept of the ABK methodology (Methodology 1984). The loading of the end shear wall results from its own mass and the diaphragm's dynamic response. There is no, and there need not be, assumption of rigid in-plane wall response. The inertial forces at any level in the end wall, applied to the end wall, are equal to the dynamic response forces of the diaphragm instead of to an arbitrarily assumed distribution of inertial forces. The discusser also disagrees with the assumption made by the author the objective of the U.S. codes and standards used to reduce earthquake hazard is to mitigate the risk of life loss and The ABK methodology was not intended solely to mitigate the risk of life loss or injuries. Mitigation of this risk is obtained by the reduction of property damage. The design-level ground motion used in the ABK research for the highest risk seismic zone had a spectral velocity of 0.76 mls (Seismic 1981). The design-level ground motion recommended in this highest-risk seismic zone by the Handbook for the Seismic Evaluation of Existing Buildings (FEMA 1992) is 0.61 mls for S2 type soils. The use of these codes and standards does meet historical pres­ ervation goals. It should be recognized the predicted earthquake damage resulting from design-level earthquake has a small probability of occurrence. The discusser has seen many instances in which features of historic buildings were lost in an attempt to save them.