Tens of millions of us spend much of our lives in the buildings and structures where we work, reside, worship, and go for entertainment, relaxation, or medical care. Local and state government elected officials and administrators adopt and enforce the codes and standards governing the design and construction of these buildings. Insofar as building safety is concerned, these codes are the "law of the land." The seismic design provisions of the codes are especially important to the performance of buildings in areas subject to earthquakes. We have a right to know how the buildings we occupy will perform in earthquakes.
The Earthquake Engineering Research Institute, a national professional organization dedicated to improved earthquake resistant design, prepared this document. Its purpose is to help policymakers, code administrators, and others involved in the design, construction, and building maintenance processes understand how the seismic design provisions of the codes, knowledge and practices of our architects and engineers, and quality of construction affect the thousands of buildings of various types, sizes, and designs that we use daily. This paper attempts to establish expected levels of damage for buildings built to the 1991 Uniform Building Code (UBC 9 I), under various earthquake conditions.
First, we must dispel a myth: There is no "earthquake-proof' building. Although we are continuously improving our understanding of earthquakes and how buildings perform, there are limitations to building codes. Many older buildings were not built for earthquake resistance, and codes do not apply to many aspects of construction and use. As a result, we must expect losses from future earthquakes. These losses may take many forms: total or partial collapse due to shaking and ground failures, interior damage to nonstructural systems and elements, and damage to contents and equipment. While failures receive great media attention, we are heartened by the greatly improved performance of newer buildings constructed to recent building codes. But even new buildings are not immune to damage. Given the wide range of building types, site conditions, and earthquake characteristics, the performance of all building, even new ones, will not be the same. Many new buildings may suffer damage in a major earthquake, and a few should be expected to suffer serious damage.
The following sections cover the most important aspects that influence building safety. They include a discussion on earthquake causes and the accompanying shaking, fault rupture, and other ground failures. A brief summary is provided of common strategies for reducing earthquake hazards through planning, locating structures, and regulating construction. Building codes will be described in detail and the expected earthquake performance of new buildings built to the UBC 91 or older unreinforced masonry buildings retrofit to the 1991 Uniform Code for Building Conservation (UCBC) will be discussed. Initially, damage estimates have been limited to buildings in UBC Zone 4, because of the high probability of seismic events and the corresponding interest in this kind of information in this zone.