The notion of limit state is fundamental in the AASHTO LRFD Bridge Design Specifications (AASHTO LRFD) (AASHTO 2012). A limit state is defined as the boundary between acceptable and unacceptable performance of the structure or its component.
The strength, or ultimate, limit states (ULS) of the AASHTO LRFD are calibrated through structural-reliability theory to achieve a certain level of safety. Exceeding the strength limit state results in a collapse or failure, an event that should not occur any time during the lifetime of the structure. Therefore, there is a need for an adequate safety margin expressed in the form of a target reliability index, âT. For bridge girders, the target reliability is taken as, âT = 3.5 (Nowak 1999; Kulicki et al., 2007). The strength limit states do not consider the integration of the daily, seasonal, and long-term service stresses that directly affect long-term bridge performance and subsequent service life.
The current service limit states (SLS) of the AASHTO LRFD are intended to ensure a serviceable bridge for the design life; assumed to be 75 years in AASHTO LRFD. When the SLS is exceeded, repair or replacement of components may be needed, repeatedly exceeding SLS can lead to deterioration and eventually collapse or failure (ULS). In general, SLS can be exceeded but the frequency and magnitude have to be within acceptable limits.
The current service limit states are based upon the traditional serviceability provisions of the Standard Specifications for Highway Bridges (AASHTO 2002). They are formulated to achieve component proportions similar to those of the Standard Specifications. However, these service limit states were not calibrated using reliability theory to truly achieve uniform probability of exceedence as the tools and data necessary to accomplish this calibration were not available to the code writers when AASHTO LRFD was developed. Currently, the development of calibrated service limit states remains a difficult task due to the lack of clear consequences of exceeding the SLS. The NCHRP Web-Only Document 201 presents the work performed on calibrating the service limit states related to concrete bridges in AASHTO LRFD.
