Report on Analysis and Design of Seismic-Resistant Concrete Bridge Systems (ACI 341.2R-14)

Description

The stated objectives of seismic design provisions in major codes have evolved considerably over the last 20 years. The initial focus of preventing structural collapse under the design earthquake to prevent loss of life has shifted to broader design objectives, such as achieving a level of serviceability following a major earthquake that allows for emergency response and ensures that transportation life-lines remain operational. These newer design objectives focus on the need for structures to remain operational after an earthquake, particularly for structures important to emer-gency response and those housing emergency and high-risk facilities. Critical structures include bridges on key response routes, hospitals, public safety headquarters, communication centers, and nuclear power stations.

Bridge seismic design philosophies may use a traditional single seismic design level (AASHTO 2012; AASHTO LRFDSEIS-2-M) or a two-level approach (MCEER-ATC-49) where both functional-level and safety-level hazards are considered. Performance objectives for each level are composed of a performance level or functional requirement at a seismic hazard level. The functional-level event consid-ered in this two-level approach is typically a lower-level event with relatively high probability of exceedance (PE), and the safety-level event is typically a major seismic event with a very low PE.

The typical performance objectives for the two-level approach tolerate only slight damage to ensure uninter-rupted service of the bridge under the lower-level event, and allow only easily repairable damage under the higher-level event to ensure minimal or no disruption of lifelines.In setting minimum performance standards, design codes recognize that it is not practical to design a structure to resist a large earthquake elastically; therefore, some degree of damage is typically permitted under the higher-level event (Fig. 1.1). For critical structures, however, depending on expectations of how quickly the particular structure can be put back in service and repaired, the damage can be further restricted by tighter requirements defined by the owner