Development of Improved Methods to Establish Toughness Requirements for North American Steel Highway Bridges

Abstract

Brittle fracture is a major concern to structural engineers as it can have significant consequences in terms of safety and cost. Although modern day occurrences 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 more significant concern due to the harsh climate present through much of the country, which, if the toughness properties are improperly specified, is sufficient to put many steels on the lower shelf of the toughness-temperature curve. The provisions for avoidance of brittle fracture in various bridge design codes vary in complexity. The existing Canadian CSA standards take a fairly simplistic approach for design against brittle fracture, using design tables that have two temperature zones. Depending on the minimum mean daily temperature of the location of interest, one can determine the Charpy V-Notch testing requirements for the grade of steel. However, it is known that temperature is not the only factor that plays a role in the fracture behaviour of steels. Other factors influencing fracture, such as plate thickness, crack size, demand-to-capacity ratio, and considerations related to traffic, are currently neglected. It is generally known, for example, that thin plates (e.g., less than 12.5-19.0 mm in bridge applications) are less susceptible to brittle fracture, due to the rolling reduction ratio at the mill. However, for the same steel grade (with a small distinction between base and weld metal), the same CVN requirements are applicable to a wide range of plate thicknesses (i.e., from the minimum allowed for corrosion considerations up to 100 mm). The existing CSA standards also assign responsibility for identifying fracture-critical members (FCMs) to the design engineer, though regulations on how to identify them are limited and vague, leaving much to engineering judgement. A comparison of brittle fracture design provisions around the world reveals that more sophisticated approaches have been developed in terms of modelling and understanding brittle fracture in existing and new bridges than the ones currently in use in North America. One of these more involved methods is the fracture mechanics method in the European EN 1993-1-10 standard, which allows factors such as plate thickness, crack size, and strain rate to be considered. This standard also gives designers the option of using a simplified method or a much more involved, fracture mechanics-based approach. While the current Canadian brittle fracture provisions generally appear to be meeting the needs of the code users, two issues are noteworthy. The first, which has already been alluded to, is that the North American provisions offer less flexibility and guidance for handling unusual situations than the Eurocode methods. The ‘one size fits all’ approach in the Canadian design standards may not be optimal and may result in structures being overdesigned or under-designed, leading to inefficiencies in safety and cost. This highlights the need for answering questions regarding the feasibility of allowing reduced toughness requirements for bridges fabricated with thinner plates or experiencing lower traffic volumes or demand-to-capacity ratios. The second issue is that few studies can be found in the literature around the world attempting to assess the level of reliability against brittle fracture provided by any of the existing design provisions. The lack of a probabilistic assessment of brittle fracture risk in Canada and the few studies globally highlights a gap in the current understanding and implementation of these design standards. This thesis includes a literature review on: 1) factors affecting material toughness, 2) common methods of evaluating toughness, 3) North American and European brittle fracture provisions, and 4) previous work on design code calibration and reliability analysis for steel structures subject to various failure modes, including brittle fracture. A comparison of the North American and European design provisions using the example of a typical steel-concrete composite highway bridge is then presented. For this case study, it was found that North American codes are typically more conservative than the Eurocode for bridge elements made with thinner plates and less conservative for elements made with thicker plates. Following this, the fracture mechanics-based European brittle fracture limit state is then evaluated in a probabilistic framework using Monte Carlo Simulation (MCS). In order to do this, statistical distributions are established for the various input parameters, and – in particular – statistical models for the live traffic load and temperature are established. Prior to application of the model, a calibration step is performed to establish a design crack depth. Sensitivity studies are then performed where key input parameters are varied to examine how the failure probability is affected by variations in each parameter. The work is then cast in a time-dependent reliability framework, using historical temperature and traffic data, to determine the failure probability with temperature and traffic loading fluctuating on a time scale throughout the year. This time-dependent model is then used to assess the reliability level provided by the current Canadian brittle fracture provisions. Given certain plate thickness, crack size, load levels and geographical temperature data, the annual probability of failure and annual reliability index, β, are obtained. The obtained reliability indices are compared with a target reliability index to assess the extent to which the Canadian provisions provide consistent and adequate levels of reliability against brittle fracture. On the basis of the results, the North American brittle fracture design provisions are critically assessed, and new design tools from these probabilistic studies are presented. Opportunities for improvement in the existing Canadian standards and areas warranting further study are lastly highlighted.