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ABSTRACT 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, βT. For bridge girders, the target reliability is taken as, βT = 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. This report presents the work performed on calibrating the service limit states related to concrete bridges in AASHTO LRFD. xvi
EXECUTIVE SUMMARY 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 were not statistically calibrated; rather, they are based upon the traditional serviceability provisions of the Standard Specifications for Highway Bridges (AASHTO 2002). The lack of tools and data necessary to accomplish the statistical calibration precluded the statistical calibration when the AASHTO LRFD was originally developed. Currently, the development of calibrated service limit states remains a difficult task due to the lack of clear consequences of exceeding the SLS. Very little useful information exists in the literature. This report presents the work performed on calibrating the service limit states related to concrete bridges in AASHTO LRFD. To accomplish the SLS calibration, the main steps performed were: ⢠Available literature was reviewed to identify information on the existing service limit states ⢠Existing service limit states in AASHTO LRFD and in other design specifications were identified ⢠Current service limit-state design practices and the need for additional service limit states were investigated through a questionnaire to major bridge owners across the US ⢠Sources of information and databases needed for calibration were identified ⢠Limit states amenable to calibration were identified and the limit state function; i.e. the criteria that determines whether the limit state was exceeded, were determined ⢠A large set of Weigh in Motion (WIM) data was analyzed to determine the live-load model to be used for service limit-state calibration ⢠A calibration process for service limit states was developed ⢠The calibration process was applied to the selected limit states and the load and resistance factors that resulted in uniform reliability, and the associated reliability indices, were determined ⢠Revisions to existing design specifications were developed The limit states that were found amenable to statistical calibration using the information currently available are: ⢠Cracking of reinforced concrete components ⢠Tensile stresses in concrete in prestressed concrete components ⢠Fatigue of concrete and reinforcement 1
The results of the work indicated that the main problem in calibrating the service limit states is the lack of clear consequences to exceeding the limit state and the ability to define more than one limit state function to address the same phenomenon. In the absence of reasons to increase or decrease the reliability inherent in the designs performed using the current specifications, the goal of the calibration was to achieve uniform reliability with an average reliability similar to that inherent in current designs. The study of the Weigh-In-Motion (WIM) data indicated that the number of incidences of heavy vehicles existing in adjacent traffic lanes is very small. Therefore, for most limit states, the live loads used on the load side of the service limit state calibration were assumed to occupy a single traffic loads. The calibration of the limit state for Cracking of Reinforced Concrete Decks through the Distribution of Reinforcement indicated that the existing provisions produce uniform reliability and no revisions to the specifications were proposed. The calibration for the limit state for Tension in Prestressed Concrete Beams indicated that, for most cases, an increase of the load factor for live load from 0.8 to 1.0 is necessary to improve the uniformity of the reliability and to maintain the average level of reliability inherent in existing bridges. The calibration of the fatigue limit state indicated that an increase in the load factor for the Fatigue I limit state from 1.5 to 2.0 is needed along with revisions to the constant amplitude fatigue threshold for reinforcing steel. 2
