Abstract
Issues of load combinations of earthquakes and heavy trucks are important contents in multihazards bridge design. Current load resistance factor design (LRFD) specifications usually treat extreme hazards alone and have no probabilistic basis in extreme load combinations. Earthquake load and heavy truck load are considered as random processes with respective characteristics, and the maximum combined load is not the simple superimposition of their maximum loads. Traditional Ferry Borges-Castaneda model that considers load lasting duration and occurrence probability well describes random process converting to random variables and load combinations, but this model has strict constraint in time interval selection to obtain precise results. Turkstra's rule considers one load reaching its maximum value in bridge's service life combined with another load with its instantaneous value (or mean value), which looks more rational, but the results are generally unconservative. Therefore, a modified model is presented here considering both advantages of Ferry Borges-Castaneda's model and Turkstra's rule. The modified model is based on conditional probability, which can convert random process to random variables relatively easily and consider the nonmaximum factor in load combinations. Earthquake load and heavy truck load combinations are employed to illustrate the model. Finally, the results of a numerical simulation are used to verify the feasibility and rationality of the model.
Highlights
The current American bridge design specifications are mainly based on AASHTO load and resistance factor design methodology, which are fully calibrated against gravity load and live load
Ferry BorgesCastaneda model that considers load lasting duration and occurring probability well describes random process converting to random variables and load combinations of earthquake and heavy truck load
“Truck and earthquake new” is the cumulative probability curve of maximum value calculated with the model in this paper, “Truck and earthquake original” is the probability curve calculated with the traditional model, and “Truck and earthquakeT-Direct” is the probability curve of direct load combination of the earthquake and heavy truck loads in their 75 years
Summary
The current American bridge design specifications are mainly based on AASHTO load and resistance factor design methodology, which are fully calibrated against gravity load and live load. Among many load combination models, Ferry Borges-Castaneda model [15], Turkstra’s rule [16], and Wen’s method [17, 18] are the most frequently used. They have their own advantages and disadvantages. Turkstra’s rule considers that a load reaches the maximum value during design life of the bridge combined with the instantaneous value (or mean value) of another load. This rule is more practical and reasonable, but the results are generally unconservative [19]. Earthquake load and heavy truck load combinations are employed to illustrate the model
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