Live Load Multiple Presence Factors for Design and Evaluation of Short-to-Medium Span Highway Bridges

Abstract

The critical live loads in bridge design or evaluation could be caused by either a single vehicle event or simultaneous occurrence of two or more vehicles, which is usually called a “multiple presence event.” The multiple presence factors (MPFs) are then defined to assess the live load effects from single and multiple presence events. The MPFs from the American Association of State Highway and Transportation Officials Load and Resistance Factor Design (AASHTO LRFD) were derived three decades ago, by assuming the probability of multiple presence events based on field observation. In addition, the correlation among truck loads in multiple presence events needs further evaluation based on actual data, as previous studies have assumed either fully correlated or independence across the lanes. There is an imperative need to re-evaluate the MPFs based on the available weight-in-motion data. This paper derives the MPFs for bridge live loads based on both traffic simulation and probabilistic modeling, utilizing existing weigh-in-motion data. It is found that the MPFs are affected by two major factors: truck loads and probability of multiple presence events. Truck loads can be further defined as the mean and standard deviation of loads on each lane, and the covariance of the load effects across the lanes. The probability of multiple presence events is related to average daily truck traffic, and the distribution of the traffic. The dispersion of truck volume and covariance of load effects across the lanes shows significant impact on the MPFs. Statistics of MPFs are obtained and recommendations to AASHTO LRFD are provided for 2-lane and 3-lane MPFs.