SEAU Paper

 

Seismic Strengthening

of

Existing Buildings

Seismic Committee Report

July 8, 1999

Executive Summary

The history of understanding the nature of earthquake dangers in Utah began shortly after the arrival of the Mormon pioneers in the mid 1850’s. As surrounding areas experienced the destructive forces of earthquakes, both public and private sectors in Utah took note and sought a better understanding of the risks for the state. The Uniform Building Code has recognized the seismicity of Utah since 1935 and continues with improved understanding today. As our appreciation of seismic risk increases, it becomes necessary to look back on our existing building inventory and address the risks which are inherent in older designs and construction materials.

The Structural Engineers Association of Utah (SEAU) is a member organization of Engineers involved in the design and construction of buildings and other structures. Formed in 1980, it’s goals include promoting legislation and codes relating to structural engineering and public safety.

The Utah Seismic Safety Commission (USSC) has requested that the Structural Engineers Association of Utah contribute to discussions regarding the need for evaluating and retrofitting existing buildings and other structures. SEAU welcomes this opportunity to lend its opinion regarding the technical issues of building rehabilitation and to comment on the need for attention to this class of buildings.

Building code regulations in the State of Utah are intended to provide a minimum acceptable level of building performance for both gravity (vertical) and lateral (earthquake) loading. The codes are continually updated to reflect new knowledge about behavior of materials, building systems and applied forces. Nowhere is this change more noticeable than in the design for seismic loading. The code has a long history of response to lessons learned from past earthquakes. Each edition refines the knowledge available for building design and provides regulation of acceptable design methodologies. With earthquake design, the code blends both theoretical and observed behaviors and attempts to give practical design parameters for economical designs.

We can readily acknowledge the impact of present building code requirements for seismic design in new buildings, however applying regulation to our existing buildings is an area less well defined. Presently, there is a diverse list of existing code references which could be interpreted to require seismic upgrades of existing structures. Unfortunately, these references do not provide a clear path toward addressing the hazards of our existing buildings. It is apparent that the need to focus on this area of building safety is a priority for the protection of both property and life.

The building code establishes a degree of protection by compromising acceptable risk and the economics of construction. This underlying premise is often little understood or appreciated. A definition of risk which refers to an interval of time for the reoccurrence of an event (earthquakes) is significantly different than understanding it as the probability of damage to a building or structure with known deficiencies. With this latter realization, risk of building damage is highly probable when structures which have shown to be poor performers are allowed to remain without some measure of improvement. It is this concern which should fuel the effort to require some form of review and rehabilitation of existing buildings.

Today, engineers have many tools to assist in the evaluation and upgrading of existing buildings. These resources offer several appropriate means to help reduce existing hazards in buildings and significantly improve their performance in an earthquake. Ranging from partial reduction of lateral force requirements to performance and component based approaches, each offers a means to focus some measure of attention on the need for rehabilitation. SEAU believes that the minimum level of safety for all existing buildings should be for collapse prevention. This threshold addresses the most basic reason for any regulation concerning existing buildings, protection of life.

Consideration of any technical regulations regarding rehabilitation of existing buildings is tempered by the extent of the program desired. While it would be presumptive to assume a level of compliance at this time, it is the opinion of the Structural Engineers Association of Utah that many existing buildings pose some measure of liability in view of the progression of knowledge regarding building performance and seismic design. In this light, we recommend that a Task Force be established to focus the various interests and proceed with recommendations for specific program(s) regarding seismic strengthening of existing buildings

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