Session I - Practical Assessment, Requirements and Examples¶
Moderator: Peter Tanner
Aim of the Session¶
Provide an overview, based on a series of case studies, of the methods and procedures currently used in different countries to assess the performance of existing structures and identify research and development needs for future codes from the perspective of practicing engineers.
Lessons learnt assessing structures in the last 30 years¶
by Aurelio Muttoni, Muttoni et Fernández Ingénieurs Conseils and Ecole Polytechnique Fédérale de Lausanne, Switzerland
The activity of assessing existing structures can be nourished by experiences gathered in designing new structures, observing the construction process from design to execution, investigating accidents and collapses as well as conducting experimental research (failure studies in a laboratory are just collapses observed under controlled conditions). Likewise, experiences gained in assessing bridges and buildings can be very useful to enhance the design of similar structures. In this regard, the similarities between design and assessment are far more significant than the differences between them. In both activities, an overview and an overall assessment of the critical elements, of the governing risk situations, of the associated uncertainties and of the governing failure modes are of paramount significance. The verification of an existing structure should start from a global assessment, where the site investigation should play a major role. The verification should follow a “Levels of Approximation Approach” (LoAA) combined with a “Levels of Knowledge Approach” (LoAKA), where the upper level should be selected depending on the significance of the uncertainties and on the ratio between costs of interventions and investigations. In this context, the choice of the parameters to be refined by site investigations or analysis should also be based on an estimate of the relationship between the uncertainties and the sensitivity of these parameters to the final result. This process is illustrated by a practical assessment and retrofitting example that has some similarities with collapses in comparable structures. It is shown that the Partial Safety Factors Method enables a simple refinement of the result according to the LoAKA. The discussion of the results makes it possible to finally identify some research needs.
Monitoring bridges using bridge weigh-in-motion technology¶
by Aleš Žnidarič, Slovenian National Building and Civil Engineering Institute, Slovenia
Bridge management requires accurate information on the bridge condition to achieve an optimal balancing of maintenance costs, potential risks and overall bridge performance. One of the key performance indicators is structural safety, the quantification of which on the one hand requires knowledge about the condition of a bridge and its effect on resistance, and on the other hand, as accurate as possible data on heavy goods traffic. Structural safety assessment is especially needed for ageing deteriorated bridges that are close to the end of their life cycle. Their safety is often difficult to prove with the traditional analytical methods. To avoid unnecessary remedial actions, such as strengthening or even replacement of a bridge, it is therefore beneficial to support the assessment with structural and material testing.
The presentation addresses the use of bridge weigh-in-motion (B-WIM) technology to measure parameters that are critical for optimal bridge assessment. On the loading side, B-WIM systems provide information about the actual axle loads and spacings of all freight vehicles that cross a bridge. At the same time, these systems can measure bridge performance by calculating the actual influence lines, girder distribution factors (GDF) and dynamic amplification factors (DAF). The true, not modelled values of these parameters significantly reduce uncertainties associated with the load effects used in the analyses. Applying measured not assumed data even allows reducing the target safety levels, partial safety factors or risks of failure, used in the calculations. All presented procedures will be illustrated with practical examples from several bridges, with consequences for the decisions adopted as a result of the assessment.
Some experiences with proof loading of bridges¶
by Svend Engelund, COWI, Denmark When planning the proof loading of bridges there are a number of questions that need to be answered. These questions concern the magnitude of the load(s) that must be applied and the location(s) where it (they) should be applied in order to demonstrate that the bridge has an acceptable reliability with respect to a given failure mechanism. The magnitude of the load(s) that must be applied may be determined on the basis of a probabilistic model of the traffic load, relevant acceptance criteria and a model by which the structural response of the bridge may be predicted. Unfortunately, the tests performed so far indicate that the actual structural response of a given bridge subject to proof loading is very different from the predicted response. This implies that we are faced with the problem of evaluating the test results without a sufficiently accurate model of the structural response of the bridge. This also makes it difficult to perform an update of the structural reliability. These problems will be addressed in the presentation to find simple solutions to these challenges.
Rehabilitation and seismic upgrading of the Villafranca bridge¶
by Pietro Croce, University of Pisa, Italy
The assessment of historical structures is a challenging topic that encompasses multidisciplinary aspects. The process, which includes the reconstruction of the most significant events over the years, is an iterative cyclical process that allows for an increasingly refined appraisal of the structure itself. This process, which was basically outlined in the ISO 13822 (2001) standard, has recently been refined so that an updated ad-hoc flowchart is now available. Indeed, the actual situation of a structure is the result of a usually complex history that leads from erection to its current state. Therefore, it is important not only to recognize the original aspect at the end of the erection phase, resulting from the building materials, workmanship, codes or cannons in force at the time of construction, but also to know how it was changed over the course of the years due to the modifications or alterations related with human interference, degradation caused by climatic actions, damage from earthquake or accidental actions and so on. In this field, modern useful tools such as in-situ survey, remote sensing, real-time monitoring and even the use of H-BIM or applications for artificial intelligence are available. Despite the general view that an existing structure can be known much better than a new one, thus reducing epistemic uncertainty, it must be taken into account that often hidden aspects have to be addressed. The topic is dealt with on the basis of the rehabilitation and seismic upgrade of a historic masonry arch bridge, which partly collapsed in 2011 as a result of the flooding by the Magra river. Special emphasis is placed not only on the assessment of the existing structure, but also on the design of strengthening interventions involving complex interactions between structural and geotechnical aspects.