Federal Highway Research Institute

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Calculation and upgrading of road bridges

Photo of the Kylltal Bridge on the A 60 federal motorway The Kylltal Bridge on the A 60 federal motorway

An efficient infrastructure is a key requirement for sustainable mobility and economic growth. The demands made of road infrastructure are high. Forecasts on the development of haulage transport indicate that a further increase in transport services may be taken for granted in the future. The existing bridges of the Federal trunk road network have now to a great extent already reached the limit of their capability. It may be assumed that it will not be possible to readily absorb the long-distance haulage forecast for the future without targeted upgrading measures being undertaken in combination with target-oriented maintenance planning.

Age distribution

A glance at the age distribution of bridges clearly reveals that some 60% of the existing bridges (in terms of area) were built before 1986. As a consequence of the increase in the age of bridges, particularly in the old West German states, deterioration in condition is observable. It is no longer sufficient to exclusively concentrate on the repairing of damage identified. There exists an urgent need for action in respect of implementing upgrading measures. Taking cost efficiency into account, these can include not only reinforcing measures but also replacement building measures – or also special considerations regarding the use of bridges.

Prioritisation procedure

Based on results of traffic load simulations, a prioritisation procedure has been developed for the purpose of estimating the extent of the upgrading measures required. This has helped in identifying the subgroups of existing bridges possibly affected by an upgrading. These investigations have enabled a network-wide qualitative risk analysis of existing bridges relating to the requirement for object-specific examinations.

Guideline

The policies applicable today are designed for the calculation of new structures and are only of extremely limited suitability as a means of sufficiently accurately recording the bearing capacity of structures, which have already been in use for a major part of their scheduled useful life. For this reason, a "Guideline for the calculation of existing bridges" was drawn up. With the introduction of this guideline, an approach to examine the identified structures is now available for the whole of Germany. The current status of this calculation guideline represents an initial version, which by means of refined identification techniques provides the experienced planner with the possibility of arriving at a load capacity assessment, which is as realistic as possible.

Methods of determination

Based on the available experience of already conducted calculations, more precise determination methods for assessing load capacity are being structurally developed by the exploitation of currently unused reserves. These should enable the previous regulations to be refined and supplemented in the course of the planned further development of the guideline. At the present time, various determination methods for realistically ascertaining the shear force bearing capacity and torsion load capacity of older existing prestressed concrete bridges are undergoing validation and verification, in order to be made available at short notice. In this respect, this is one of the most urgent identification problems when calculating these structures. The calculation ensues based on a tiered approach. Due to the particular expense involved when applying the higher calculations steps (3 and 4), these are only applied in special cases and only with the approval of the highest road construction and maintenance authorities in the Federal States. At the same time, these steps represent a major development potential for the Guideline. Current research projects are therefore particularly focussed on the application of scientific methods of assessing the existing structures within the framework of verification level 4.

Graphic shows a stepped procedures to the recalculation of existing bridge structures A tiered approach as part of calculating existing bridge structures

Deficits

Photo shows a view of a box-girder bridge A box-girder bridge with external post-tensioning to reinforce the system along its length and with CFRP laminates to upgrade the deck in the transverse direction.

In many cases when calculating road bridges, load bearing and usability deficits as well as deficits pertaining to durability and material fatigue are demonstrated. Tried and tested technologies are available to remedy these deficits. There exists a development potential with regard to a structured use of information on the structure, the relevant impacts and possible measures. BASt is realising decision-making aids to enable the more effective planning and execution of the upgrading of bridges in the future both from a technological and economic perspective. The user is envisaged to be able to work out the promising variant for a structure to be reinforced based on a transparent and standardised approach.

Monitoring concepts

Recent experience suggests new monitoring concepts for load-bearing-relevant reinforcement of bridge structures. Against the background of possible reinforcement measures, concepts for sensor monitoring are undergoing development and evaluation in relation to their function and reliability. Compensatory monitoring concepts are also being developed for the purpose of ensuring a continued traffic use of a bridge associated with a given load capacity. To ensure a sufficient level of reliability, it is possible to make use of compensation for additional safety in order to justify a lowering of the partial safety factors.


Replacement construction

In many cases, however, the issue of replacement construction is pushed to the forefront of considerations. In implementation, a particular challenge is to minimise the impact on the road network performance.