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Last edited 14 Jul 2016
Many parts of the world continue to provide opportunities for major new bridge construction. These include the massive investment in infrastructure in China and the Far East; the urban metro schemes in India and the Middle East; and the high-speed rail lines in Europe and the USA.
However, many countries now have extensive stocks of mature bridge assets. A typical example is London Underground, which has a stock of approximately 8,000 bridges and other assets, of which nearly half were constructed before 1900.
Assessment is a key tool for the management of existing bridges. Assessments may be undertaken to check that bridges are safe under the loads they are already experiencing. In addition, increasing traffic capacities and loading often require the evaluation of the carrying capacity of existing structures. Changes to structures, such as modifications or external damage or deterioration, may also need to be assessed.
Bridge assessment, although a challenging subject, can also be a rich field that offers opportunities for research, innovation and the development of technical expertise. The rewards can also be significant: analytical approaches are generally less costly than structural strengthening, so there can be many benefits in progressing sophisticated investigation and analysis methods to justify the strength of structures.
In situ tests show that sometimes bridges have a reserve strength that is not accounted for in design codes of standard assessment methods, since the codes may conservatively neglect contributory mechanisms. Full-scale load testing can demonstrate these reserves of strength. However, proof load testing up to the design load may have risks of inducing cracking or damage in the structure if it is not properly performed and controlled, owing to the high level of load in the bridge.
Acoustic emission has been identified as a useful technique to stop the load increase before any damage can be inflicted on the bridge. In 169 BE2, of the ICE's Bridge Engineering journal, Olaszek et al. (2016) present tests on a three-span bridge at Barcza, Poland, and shows that monitoring with acoustic emission sensors made it possible to evaluate the cracking limits of the concrete members and stop the load increase prior to the onset of plastic behaviour.
Certainly structural health monitoring can produce valuable data-sets that can aid key decisions on current performance, margins of safety, actual loading, stress history, extent of deterioration and residual life. However, the collection of data is of little value unless it can be used to inform and influence decision-making.
Facilitating formal discussions between key stakeholders before any deployment will ensure that scarce resources are not wasted in the pursuit of data as opposed to information. Vardanega et al. (2016) propose an approach is based around a questionnaire to be completed in partnership between the asset manager, structural engineer and monitoring engineer.
The approach can be used to determine if there is a case for specifying monitoring on a project and assess the potential value of any information that may be obtained. It has been trialled against five historical monitoring case studies.
 Numerical modelling
Non-linear three-dimensional finite-element modelling also plays a major role in bridge assessment. For example, it can be used to consider progressive damage in masonry arch bridges. This allows for the investigation of damage and crack propagation at service level loads, in contrast to traditional methods that only consider the ultimate capacity state of masonry arch bridges (Gibbons and Fanning, 2016).
It can also be used to demonstrate the adequacy of web stiffeners and longitudinal stiffeners in box girder bridges. At Tame Valley Viaduct in the UK, post-buckling capacity and the Eurocodes effective-area approach were used to calculate the resistance of plated structures (West, 2016).
On this project the desire to optimise the strengthening works led to the creation of a structural information model and bespoke software tool to automate the assessment process rather than conservatively selecting critical panels. The project clearly demonstrates how investment at assessment stage can leverage major savings in construction by significantly reducing the quantity of strengthening required.
This article was published by ICE on 1 July 2016. It was written by Simon Fullalove. You can see the original article here.
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