When is a Structure Required to be Retrofitted

The question often arises, when must a bridge or a building be retrofitted to resist seismic forces? Structure retrofitting is a process whereby an existing structure is enhanced to increase the probability that the structure will survive an earthquake. This can be accomplished through the addition of new structural elements, the strengthening of existing structural elements, and/or the addition of base isolators. The requirements for seismic retrofit vary depending on the location and type of structure.

The basic code document that governs the design of bridges in the United States is the AASHTO Standard Specifications for Highway Bridges. Most states have adopted this document with modifications or guidelines as required to account for their particular special requirements. The Federal Highway Administration uses this document as a standard for the design of bridges that receive federal funding.

There are no federal or other jurisdictional requirements providing guidelines to determine when a bridge has to be retrofitted. Bridge retrofits are prioritized by governments along with other transportation funding needs. After the Loma Prieta Earthquake, the California Legislature placed more importance on retrofitting bridges and significantly increased the level of funding for seismic retrofitting of bridges. Prior to that event, less than $10,000,000 per year had been allocated for seismic retrofits.

The federal government is now allowing bridge replacement funds to be used for seismic retrofit projects. This will make it possible for local agencies to supplement their funding to retrofit seismically deficient bridges in their area.

Bridges must be retrofitted to the same criteria as new bridges. The methods required to achieve these results differ, but the level of earthquake design is the same as for a new bridge. These techniques include: adding base isolators, wrapping columns, strengthening footings, adding hinge restrainers, and increasing the width of supports at abutments so that the superstructure will not fall off the support.

Buildings on the west coast are typically designed to the requirements of the Uniform Building Code (UBC). This code is generally adopted by cities, counties, and states with only minor changes. The seismic portion of the code is written and maintained by members of the Structural Engineers Association of California.

The UBC does not specify when a structure must be retrofitted. Chapter 34 of the UBC requires all alterations, additions, or repairs comply with the requirements for new construction. When a building is retrofitted, the entire structure does not have to comply with the requirements for new construction. Sometimes, the building may need to be retrofitted if the use or occupancy changes. Many cities and counties look at each project that is submitted and use judgment to determine when the structure has to be retrofitted for seismic forces. Some jurisdictions, such as the City of Los Angeles, actually have a formal program requiring structures to be retrofitted within a certain time period. However, the majority of jurisdictions do not have any requirements regarding seismic retrofit. Therefore, it is up to the building owner, architect, structural engineer, and the building official to determine if the building should be retrofitted.

Government buildings are funded as priorities change, much like bridges. However, government funding sources are not available to assist the private owner when they decide to retrofit their building.

Unlike the retrofit of bridges, there are many standards that cover the retrofit of buildings. The UBC is sometimes used as the standard for designing seismic retrofits. But, the UBC states that only the elements and connections being added need comply with the requirements of the UBC. Other existing elements do not have to be strengthened as long as more force is not being added to existing elements.

It is recognized in the building community that some times it is not economical to design seismic retrofits to the requirements of the UBC. The appearance of a historic structure may be altered by a retrofit designed per the requirements of the UBC. Many older buildings are constructed of archaic materials that would not be incorporated into modern buildings. Because of this, the Uniform Code for Building Conservation (UCBC) was written by the same building code writers as the UBC. The UCBC does not have as stringent of requirements for seismic design. The UCBC allows some construction materials (such as unreinforced masonry) to be used in the lateral force system if it already exists in the building whereas, the UBC does not. The UCBC also has lower force levels than the UBC. However, the use of the UCBC for a seismic retrofit must be approved by the local Building Official. A similar approach is taken by other government agency standards.

In these publications, the forces applied to the structure are only 2/3 of the forces applied to a new building. Therefore, many retrofitted buildings do not have the same seismic safety as a new building.

The methods for retrofitting a building are similar to the retrofit of bridges. They include base isolation, element strengthening, and element addition.