scope:

PURPOSE AND PHILOSOPHY

These Specifications establish design and construction provisions for bridges to minimize their susceptibility to damage from earthquakes.

The design earthquake motions and forces specified in these provisions are based on a low probability of their being exceeded during the normal life expectancy of a bridge.' Bridges and their components that are designed to resist these forces and that are constructed in accordance with the design details contained in the provisions may suffer damage, but should have low probability of collapse due to seismically induced ground shaking.

The principles used for the development of the provisions are:

1. Small to moderate earthquakes should be resisted within the elastic range of the structural components without significant damage.

2. Realistic seismic ground motion intensities and forces are used in the design procedures.

3. Exposure to shaking from large earthquakes should not cause collapse of all or part of the bridge. Where possible, damage that does occur should be readily detectable and accessible for inspection and repair.

A basic premise in developing these seismic design guidelines was that they are applicable to all parts of the United States. The seismic hazard varies from very small to high across the country. Therefore, for purposes of design, four Seismic Performance Categories (SPC) are defined on the basis of an Acceleration Coefficient (A) for the site, determined from the map provided, and the Importance classification (IC). Different degrees of complexity and sophistication of seismic analysis and design are specified for each of the four Seismic Performance Categories.

An essential bridge must be designed to function during and after an earthquake. In areas with an Acceleration 'The probability of the elastic design force levels not being exceeded in 50 years is the range of 80 to 95%. However, the design earthquake force level by itself does not determine risk; the risk is also affected by the design rules and analysis procedures used in connection with the design ground motion. Coefficient greater than 0.29 essential bridges must meet additional requirements. A bridge is designated essential on the basis of Social/Survival and SecurityDefense classifications presented in the Commentary.