The masonry structures built in the late nineteenth and early twentieth centuries were designed with empirical equations derived from construction practice (Heyman, 1995). Many of these structures are still standing today, which attests to the soundness of the design with respect to gravity loads. However, many of these buildings are unreinforced and have not yet been subjected to intense lateral wind or earthquake loads (Epperson and Abrams, 1989). Throughout eastern and mid-America there exists a large inventory of unreinforced masonry (URM) buildings that pose a great hazard in the case of an infrequent but large seismic event. The earthquakes of 1999 in Turkey and Taiwan, and 2000 in India, served as a reminder to the vulnerability of URM structures and the need for sound evaluation and rehabilitation techniques (Hays, et al., 1999). Seismic performance of these structures is largely dependent on the strength and behavior of in-plane shear walls. The lateral load capacity of URM shear walls is often limited by modern design codes (MSJC, 1999) to the onset of flexural cracking. However recent research has indicated that URM walls can resist considerable lateral force after the formation of the first crack and still behave in an elastic manner while resisting loads larger than those that caused initial cracking (Abrams, 1992 and Magenes, 1997). URM walls have been proven to be ductile elements capable of dissipating energy through rocking or bed-joint sliding, and thus design codes may be overly conservative in evaluating the capacity of these components. The leading document available today to guide an engineer on the seismic rehabilitation of URM walls is FEMA 356. This document titled “Prestandard and Commentary for the Seismic Rehabilitation of Buildings” was released in December of 2000 by the Federal Emergency Management Agency (FEMA), as part of the National Earthquake Hazards Reduction Program (NEHRP). This document provides methods, using the concepts of performance based design, for the analysis and rehabilitation of buildings made of masonry, concrete, steel, timber or any combination of these materials.Performance based design allows the engineer, and building owner, to jointly specify the desired building performance level for a particular earthquake magnitude, evaluate the capacity of the structural components to determine if they meet performance objectives, and design rehabilitations for the components if they do not meet the objectives. Performance levels are based largely on the amount of damage that would be incurred. Damage to a structural component is often considered to be a function of lateral displacements. Unlike design codes, where the acceptability of a component is determined by its strength, in FEMA 356 component acceptability is determined by the amount of damage which is expected to occur at a particular lateral deflection or drift. Numerical coefficients relating drifts to damage were deduced from laboratory tests available at the time. Further research is needed to validate and reduce the uncertainty in this data, particularly for the case of rehabilitated components such as URM shear walls.
|Name||Mid-America Earthquake Center CD Release|
- Mid-America Earthquake Center