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In many design codes for roadway bridges, it is required that a design truck be passed over influence lines for stress at various locations on the bridge to obtain nominal stress ranges for design. For the fatigue design of Canadian bridges in the infinite or very long-life domain, the nominal stress range is compared with a fatigue limit, after modification by an appropriate correction factor to account for the difference between the nominal stress range and some measure of the extreme stress range in the expected real traffic histogram, which governs infinite life design. The extent to which the influence of simultaneous truck crossings was considered in the establishment of the correction factors is believed to be limited. With this in mind, a simulation-based study, conducted to investigate the effects of simultaneous vehicle crossings on the fatigue limits for steel bridges under Ontario highway traffic loading, is presented in this paper.

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Brittle fracture is a major concern to structural engineers as it has significant consequences for safety and cost. Although modern day occurrences of brittle fracture are rare, it is well known that they can occur without warning and may lead to the sudden closure of a bridge, loss of service, expensive repairs, and/or loss of property or life. In Canada, steel bridge fracture is a particularly significant concern because of the harsh climate. If the toughness properties are improperly specified, many steels could be on the lower shelf of the toughness-temperature curve. A comparison of brittle fracture design provisions around the world reveals that more sophisticated approaches have been developed for modeling and understanding brittle fracture in existing and new bridges than those currently in use in North America, including Canada and the U.S.A. This paper describes the European brittle fracture provisions and presents a comparison of the North American and European design provisions using the example of a typical steel-concrete composite highway bridge. On the basis of this comparison, situations where one set of design rules may be more or less conservative are identified, and opportunities for improvement and areas warranting further study are highlighted.

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