Ivar Björnsson

Determining Appropriate Design Impact Loads to Roadside Structures Using Stochastic Modeling

The design and verification of built structures requires structural engineers to consider accidental loading situations. The accidental loading situation investigated in this paper is heavy-goods vehicle (HGV) collisions with roadside structures; focus is on the design of bridge-supporting structures. The impact loads were determined from Monte Carlo simulations of a probabilistic model in which highway traffic measurements and accident statistics in Sweden are input. These loads were calculated for structures adjacent to straight roads as well as roads with curvature, and include considerations of the directional load components. Comparisons were made between the simulation results and approaches given in design codes, with focus on the Eurocode. The simplified approaches provided in the code were inadequate in their treatment of these design situations. Alternative equations for calculating impact forces and energies are presented. These equations can be used for determining design values for impact forces or for conducting probability/risk-based assessments of bridge supports subjected to HGV impacts. In this way, a more consistent treatment of HGV impacts in the design of bridge structures is achieved.

From Code Compliance to Holistic Approaches in Structural Design of Bridges

In the process of designing structures such as bridges, engineers have to make design choices in light of uncertainty with the expectation that the structures they design, once built, perform as intended and do so with a sufficient degree of safety against failure and nonperformance. Consequences as a result of structural failures can be very high indeed, and it is vital that events such as these rarely occur. To achieve this end, the risks of failure should be addressed, understood, and controlled in such a way that they are deemed within acceptable limits for all stakeholders involved. According to Elms (1992), two primary strategies are used for controlling risks in engineering design: the first “is to be more conservative in design to allow for uncertainties : : : [and the second] : : : is to put more effort into careful risk assessment, to maintain safety levels and reduce risk while refining designs and reducing costs.” The prior strategy is a form of engineering heuristics (Koen 2003), which by definition cannot be absolutely proven but is, nevertheless, considered legitimate because it has been successfully implemented by generations of engineers in the past without any disproportionate rates of failure; examples include factors of safety applied in allowable stress design (ASD). The second approach to control risk, although arguably also a form of heuristic (e.g., Koen 2003), has its basis in the rational treatment of uncertainty as a calculable entity— something that can theoretically be reduced and altered such that risk-optimal design solutions can be obtained that are both safe as well as being cost-effective. The scientific rationalization and formalization of risk assessments in engineering design has led to theories, such as structural reliability theory (Freudenthal 1956), which are necessarily confined in their application compared with previous tried-and-tested methods of risk control based on design conservatism and engineering judgment. It is the virtue of a competent engineer to understand the limitations of such theories and give equal thought to the importance of heuristics in engineering design and the role of engineering judgment in yielding good (quality) designs (Addis 1990; Vick 2002; Davis 2012; Bulleit et al. 2014). The aforementioned approaches focus on what engineers themselves can do to help control risks in the process of designing structures. Viewed from a wider societal standpoint, the management of risks in engineering design requires addressing the broader issue of providing assurances that engineering professionals achieve this goal satisfactorily. To help address these issues (and to help legitimize the engineer as a professional), design codes and other formal standards of practice have been established (Shapiro 1997). Some of the primary purposes of these documents, hereafter simply referred to as design codes, include codifying current best engineering knowledge and practices as well as helping to ensure (and assure) that design risks are adequately treated by engineers, i.e., the design codes act as instruments for controlling risk. This development is important because it indicates a transfer of responsibility for controlling risk away from the individual engineers carrying out the design to the code-writing committees.

Reliability of RC Bridge Supports Designed to Resist Heavy Goods Vehicle Collisions

Abstract The reliability of bridge-supporting structures with regard to resistance to impacts from heavy goods vehicles (HGVs) is investigated. Probabilistic simulations are carried out to calculate the reliability index of a circular reinforced concrete column that has been designed using historical values for equivalent static impact loads provided in the Eurocode. Considerations are made for the uncertainties related to the dynamic response and resistance of reinforced concrete bridge supports subjected to vehicular impact. A general procedure is outlined for determining the dynamic resistance of the structure. As input for the impact force, results from previous probabilistic simulations of HGV impacts to roadside structures, have been used. It is found that the design based on the codified approach does not provide adequate safety levels in the case of the structure studied. An alternative formulation for determining more appropriate values for the impact load is suggested and some proposals presented pertaining to other possible design strategies for the treatment of these types of loading situations.