Exploiting design to extend the safe working lives of bridges

Author: Eugene O'Brien, director of Roughan O’Donovan Innovative Solutions and professor of Civil Engineering, University College Dublin Eugene O'Brien will present at the upcoming CPD seminar entitled, 'Practical Steps to Extend the Lives of Bridges', which will take place on Friday, 31 January at 22 Clyde Road (early-bird rate available until 17 January). This one-day seminar will span the gap between research and practical steps, informing delegates of the latest investigations and methods to extend or improve the safe working lives of bridges and usage of bridge infrastructure.

Bridges, like many civil engineering structures, are designed quite conservatively – the probability of dying due to the collapse of a new bridge is about one in 10 million. Part of the reason for this conservatism is that adding extra strength to a bridge when it is being built is not expensive. For older bridges, however, the situation is quite different – there is a huge difference between the cost of strengthening an existing bridge and not doing anything. Fortunately, it is often possible to prove that a bridge is perfectly safe, despite having deteriorated since it was first built. The Long Life Bridges (LLB) project is a €890,000 European Seventh Framework research project under the ‘Industry and Academia Partnerships and Pathways’ programme. The LLB consortium has been working for the past two years on ways to exploit the conservatism in bridge design so as to extend their safe working lives. The group consists of two small/medium enterprises – Roughan O’Donovan Innovative Solutions and French company, Phimeca – plus two universities, Aalborg University in Denmark and the Royal Institute of Technology (KTH) in Sweden. The budget has been used to fund secondments between the partners and is intended to deepen the relationship between the industry and university partners. Across Europe, there are many bridges that are vital to the economies they serve and which, if taken out of commission, would have a devastating impact on local, national and international economies. Take for example, the Forth Road Bridge in Scotland, which connects Edinburgh to Fife (Figure 1). [caption id="attachment_10884" align="alignright" width="1024"] Fig 1: Forth Road Bridge, Scotland[/caption] This bridge, which opened in 1964, is a vital element of Scotland’s transportation infrastructure and is of prime importance for the local and national economies. Due to inadequate maintenance strategies, the bridge, which was designed to be serviceable up to 2084, was found to be undergoing significant deterioration, with the result that by 2017 it is predicted that it will no longer be able to safely carry vehicular loading. A new bridge is currently under design. Traditionally, this replacement of infrastructure, usually at the end of its design life, has been the adopted strategy. However, advanced analysis techniques now exist to accurately assess and identify, on an ongoing basis, maintenance and repair strategies that would prevent a recurrence of the Forth Road bridge scenario. Due to conservatism in their initial design, many bridges have sufficient reserve capacity to remain in service for an extended period of time beyond their initial design life. The goal of the LLB project is to develop techniques to extend the lives of these bridges. It will allow identification of old bridges that are safe to remain in service and those that need maintenance plans to optimise their remaining life. In times of economic recession this is particularly important, ensuring the maximum return possible from the existing bridge infrastructure as opposed to undertaking expensive new design and build projects. LLB aims to deliver: • More road and rail bridges being proven to be in a safe state; • Higher speeds on our (non-high-speed) railway lines; • Less demand for non-renewable and carbon intensive resources; • Less cost associated with achieving these goals. The partnership brings together experts in the fields of structural assessment, probabilistic analysis and risk quantification from both academic and industrial backgrounds. Three main research threads are studied in the project: (i) bridge loading and dynamics, (ii) life cycle evaluation and (iii) fatigue.

BRIDGE LOADING AND DYNAMICS

A bridge is safe if the stresses due to the applied loads are less than its capacity to resist those stresses (load < resistance). Many assessments of bridge safety – with or without dynamics – come back to this basic point and calculate some indicator of the probability that load is less than resistance. It can be argued that both sides of this equation – load and resistance – are equally important. Site-specific measurement of traffic load and quantification of the associated uncertainty has great potential, as there are many bridges where the actual load is much less than what the bridge was designed for. Recent improvements in weigh-in-motion (WIM) technology have made this possible, by providing road authorities with large databases of vehicle weights, axle configurations and inter-vehicle gaps. For long-span bridges, site-specific bridge loading is a real challenge as convoys of trucks can form in the traffic stream that are critical for the bridge. In LLB, micro-simulation is being used to explore the sensitivity of convoy formation to driver behaviour parameters and to find the traffic scenarios and truck weights that the bridge needs to resist. Long Life Bridges is also looking at probabilistic approaches to the dynamic interaction between traffic and bridges. There is great potential in this as the Eurocode factors for road bridges range from 20% to 70%, whereas the true dynamic amplification is generally less than 10%. By finding the true dynamic allowance, bridges can be safely retained in service that would otherwise be replaced or strengthened. Equally, for railway bridges, trains can be allowed to travel at higher speeds than would otherwise be allowed. The usual factors to allow for dynamics make the assumption that the critical loading event is also the critical event when dynamics is ignored. This is a hugely conservative assumption. Critical dynamic factors are commonly associated with very light trucks and dynamic amplification tends to reduce greatly when the truck is very heavy or when there are several trucks on the bridge. In LLB, a greater understanding of how to quantify the uncertainty associated with bridge dynamics is being developed and will be applied to railway as well as road bridges. A semi-active damping system has also been developed to reduce the risk of dynamic excitation in railway bridge hangers.

LIFE-CYCLE EVALUATION

Life-cycle analysis (LCA) has been developed on the basis of statistical decision theory and applied to infrastructure systems, especially bridges, in recent years. All uncertainties, and all costs and benefits, in the life cycle are accounted for. The main objective is to minimise the total expected costs by optimising the maintenance actions taken during the design lifetime of the structure. Figure 2a shows the life cycle performance of a structure with no intervention measures, with Figure 2b illustrating the effect of preventative and corrective actions. As part of Long Life Bridges, LCA techniques will be applied to calculate the probability of a bridge failure. Generally, this is a calculation at one specific point in time indicating whether or not the bridge, in its current condition, is safe to remain in service. More recently, consideration of the complete remaining life of the bridge, allowing for its deterioration through time, is the state of the art approach. LLB will investigate the use of probabilistic measures in the assessment of bridges and implement them in a case study. Focus will be given to the development of a probabilistic framework for the whole life assessment of new cable-stay bridges, such as the Boyne Bridge shown in Figure 3.

FATIGUE EVALUATION

The evaluation of fatigue in civil engineering structures (including bridges) often involves significant uncertainties. These uncertainties should be taken into account in the design process by using a probabilistic approach from which the reliability of the structure can be estimated. The University of Aalborg divides the uncertainties related to fatigue design into physical, model and statistical uncertainty, which can be estimated from tests using classical statistical methods. During the structure’s service life, information from monitoring can be used to update the reliability using Bayesian methods. [caption id="attachment_10563" align="alignright" width="251"] Fig 3: Boyne Bridge, M1[/caption] This branch of the project will develop a probabilistic framework for fatigue design of steel bridges, building on the fatigue model described by Paris’ Law, which relates the crack growth to the number of stress cycles. Further to the development of a fracture mechanics model, the work will include a probabilistic description of both the fracture parameters and the time-variant loading. The structural reliability assessment will be incorporated into a maintenance planning approach, whereby measurements of crack length are monitored in time. This will lead to an optimal maintenance planning framework (such as that shown in Figure 2) that combines fracture models and monitoring data. The goal of Long Life Bridges is to extend the lives of the existing bridge stock in Europe. ‘Europe 2020, A European Strategy for Smart, Sustainable and Inclusive growth’ is the EU’s policy that sets out an economic growth strategy for the coming decade. Long Life Bridges will make a significant contribution to the goals set out in this plan by reducing the cost of transportation and reducing demand for non-renewable construction materials.