Bridge engineers often need to determine the maximum safe load that may be allowed to pass over a prestressed concrete bridge. frequently the need for the load to pass is urgent, requiring a swift butaccurate evaluation of safety. many times the maximum load is governed by flexural cracking, and the bridge must be analyzed to find the cracking moment capacity of the girders. a conventional bending analysis uses deterministic strength properties even though propertiessuch as concrete strength and prestress force are random variables.because of this, a conventional load capacity analysis cannot reveal the probability of failure (i.e., by flexural cracking) associatedwith the passage of an extreme load. therefore, bridge engineers have been forced to try to account for the probability of failure by acombination of experience and engineering judgement. this approach is sometimes disquieting for those engineers required to set load limits, issue overload permits, or approve large temporary construction loads. a computer program was written to use a very accurate analytical model and probabilistic methods to calculate a population of cracking moment capacities, rather than a single, deterministic value. using this data and information about the extreme load moment, theengineer can calculate an associated probability of cracking failure. this result along with other information can be used to help the engineer in making a final decision about the safety of the passage of an extreme load. this paper appears in transportation research record no. 1290, third bridge engineering conference, march 10-13, 1991, denver, colorado, volume 1.
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