Assessing heavy-load bridge options: refurbish or replace?

PTV has developed a methodology for the optimization of bridge refurbishment and/or renewal measures (Photo: iStock).

Growing traffic volumes, including heavy-load transport, have a negative impact on roads and bridges, in particular on the 39,500 bridges of the federal motorway system in Germany. Most of them were built between 1965 and 1985. That is why some of them have reached their performance limit. What needs to be done? Older bridges must be retrofitted by increasing their load-bearing capacity, for example. Or they will even have to be replaced.  But how exactly? And in what order? On behalf of the Federal Highway Research Institute and in cooperation with the KHP Planungsgesellschaft mbH, PTV has developed a methodology that ensures optimized planning of refurbishment measures and/or the replacement of important bridges that are part of the German motorway network. Together with the engineering office Bornmann und Jauck GmbH, PTV now focuses on the further development of this concept. Annette Kindl and Andreas Stadler, both senior project managers at PTV Group and in charge of the project, explain how they will proceed.

Compass: Many bridges of the federal motorway network are over 40 years old and are therefore not designed to carry today’s and future traffic loads, in particular with regard to the significant increase in road freight transport. But how can you make sure that bridges are future-proof?

Annette Kindl: If the analysis reveals that a road bridge’s load capacity will no longer be sufficient, options for action need to be developed. The aim of the research and development project is to develop a prototypical process that helps optimize bridge refurbishment and renewal strategies. At the same time, this procedure is intended to minimize the macroeconomic impact arising from the fact that bridges are not available or can only be used to a limited extent. The focus is on highly critical network elements such as the Rhine crossings.

During the first project, we developed a methodology for a multi-tiered procedure: It includes approaches to the selection of structures, a methodology for the analysis of refurbishment options for individual structures and a methodology for establishing enhanced refurbishment programmes for a sub-network. All approaches encompass both structural and transport-related aspects.

Compass: How can you identify which bridges would most urgently need to be retrofitted?

Annette Kindl: The pre-selection procedure developed by us enables public authorities to identify highly critical building structures at an early stage and to integrate them into an action plan at sub-network level. The effects of the closure of the structure will be evaluated, including the load capacity of other critical structures, which would have to cope with higher traffic volumes due to the traffic flow shift.

“The aim is to develop a software prototype that calculates an optimized set of refurbishment measures,” says Andreas Stadler; Senior Project Manager, PTV Group.

Compass: How do you identify the best refurbishment option?

Andreas Stadler: There are numerous options ranging from the reinforcement of individual elements to the replacement of the bridge. We have developed standard refurbishment and renewal options and allocated standard values, for example in terms of construction time and construction site management. With relatively little effort, planners can thus assess possible actions at an early stage of planning.

The analysis of potential refurbishment options allows for the assessment of structural effects in terms of the achievable remaining useful life of the building. Another important aspect that has to be taken into consideration is the fact that all refurbishment options will hamper traffic flow – mainly due to the construction sites. This is why all macroeconomic and/or traffic-related effects, such as changes in travel times, emissions and accidents, will be part of the analysis.

Compass: How do you identify when bridges of a sub-network need to be refurbished?

Andreas Stadler: The procedure of ranking options for action regarding an individual structure contributes towards preparing the assessment of structures within a sub-network. Based on this method, we have developed approaches that allow us to set up refurbishment programmes for sub-networks. The aim is to generate maximum macroeconomic benefit for the selected sub-network. This means the negative impact of traffic restrictions on road users should be kept to a minimum.

Compass: Has the methodology been verified?

Andreas Stadler: Yes, it was verified as part of the Rhine Crossings project in North Rhine-Westphalia, between Bonn and Duisburg. The application included the method for the pre-selection of structures, the assessment and sequencing of options for action at object level and the calculation of an optimized set of refurbishment measures. We used PTV Validate the road network and traffic data model for Germany, as database, and the transport planning software PTV Visum to identify the traffic restrictions caused by load capacity issues and construction sites. The transport model allows planners to analyse the impact of closures on road users, i.e. those directly affected as the bridge is located along their route as well as those indirectly affected.

“The focus is on highly critical network elements such as the Rhine crossings,” explains Annette Kindl, Senior Project Manager, PTV Group.

Compass: What major challenges did you face?

Annette Kindl: A major challenge was to assess the impacts of measures carried out simultaneously at several structures. As the project progressed, this aspect played an increasingly important role, which is due to the solid financial resources of the states. As a result, these measures can be carried out at sub-network level. In terms of transport modelling, this means that the transport model takes all traffic-related interdependencies, including traffic restrictions on different bridges into account. In terms of sub-network optimization, the methodology must be suitable for the analysis of large areas. Within the scope of the first project, we succeeded in developing a prototypical method and using it in an exemplary manner. It was designed for sub-networks with comparatively few structures and a small number of simultaneous measures.

The follow-up project, which started in December, will focus on the further development of the methodology in terms of its application to larger sub-networks. The aim of the project is to develop a software prototype for the calculation of an optimized set of refurbishment measures.