In the past, seismic factors were considered in highway bridge design only in areas such as California, where seismic risk was known to be high. In areas where risk was assumed to be low, seismic codes have been minimal. Even in California, codes were not revisited until the San Francisco earthquake of 1989 when the level of damage raised new concern in insurance companies, politicians, and code writers.

Outside California, most states have used the same set of uniform codes, based on the probability and likely magnitude of seismic events. Although earthquakes have been recorded in the past in many regions throughout the country, we have tended to dismiss those that occurred prior to recent memory and ignore the possibility of future events.

In the past 10 years, major earthquakes that were not previously anticipated have occurred in the U.S. and elsewhere. These events have caused extensive damage to highway bridges, including structures where AASHTO codes were applied, increasing awareness of the problem worldwide.

Because current design codes are based on criteria of no collapse, unexpected events can result in unforeseen damage so great that the structure is not usable and repair/replacement costs are prohibitive. These risks can now be substantially reduced by better initial design and through the seismic retrofit of bridges.

Regional criteria

In the past, all designers from Texas to Alaska, were simply tied to the same codes. Now, they have a more rational means of making choices and still conforming to those codes. With available information, probabilities and risks can be identified and bridge owners can choose an acceptable level of damage and determine the costs involved in limiting it. An owner can examine the photos in the PEER database to evaluate the damage they can expect if the bridge ever has to fully develop that particular level of performance.

In the United States, the new generation of seismic codes for bridges is moving towards performance-based design as a rational and cost-effective approach to seismic design. Current seismic codes are based on accelerations and forces, which result in stronger and more rigid structures, which in turn attract higher forces. Instead, the performance-based design approach uses displacements and deformations along with the proper detailing to allow the deformations to develop, permitting structures to be somewhat more flexible. In addition, the necessary details to meet given performance criteria usually cost less to construct than those required to meet the criteria of force-based approaches — the money is spent where it is needed.

Progress is being made in the development of seismic codes for the new millennium. The Multidisciplinary Center for Earthquake Engineering Research, an independent Buffalo-based group, is involved in Comprehensive Specifications for the Seismic Design of Bridges, a project sponsored by the National Cooperative Highway Research Program, whose goal is to write a newer, more seismic, design specification for bridges. Included in the scope of the project is the development of design and performance criteria.

A national approach to the issue is still needed, as well as the publication of maps to define the probability of occurrences and the magnitude of events within the United States. The most recent, developed by the U.S. Geological Survey and incorporated into the International Building Code 2000, provides maps and databases and some applications.

Retrofitting minimizes risks

Because of lack of proper reinforcement in the past, older facilities don’t meet newer standards, and this is where retrofit comes into the picture. To start a retrofit program, owners need to review their inventory and understand which facilities may be at risk. If there is risk, the owner must prioritize and determine which facilities are most important in order to schedule the order of retrofits and ensure that those structures will not suffer damage in the future.

Once this is determined, the owner must design appropriate measures to carry out the program. Among the standard measures, there are different levels of sophistication that may be required depending on the level of risk.

The most basic retrofit strategy is restrainers applied to bridge girders to keep them from becoming unseated and falling to the ground in an earthquake. Another standard measure is steel jackets placed around concrete columns to confine the concrete, a solution that is widespread in California. These are least-cost options that provide the best leverage of cost versus reliability.

More complex strategies include replacing conventional bearings with seismic isolation bearings, bigger shear keys on piers and abutments, and foundation strengthening methods. The most complex tactics involve measures such as replacement of all or significant parts of sub- or superstructures.

It is important to note that any step taken improves the reliability index. It is the owner’s choice how far to go, depending on the level of risk, the performance level selected, and the amount to be spent.