ABSTRACT
In the past decades European highways have experienced a steady increase in daily vehicle numbers and axle weights. In Germany the total heavy freight traffic on roads has approximately doubled in the last 20 years, with authorities anticipating a further increase in the future (Kaschner, R. et al. 2009). Concrete structures constitute more than 66% of the bridge area of the German highway network, more than half of them however have been constructed before the 1980s. Since then the traffic loads for bridges with several lanes (DIN EN 1991-2/NA 2012) have approximately doubled for both the concentrated and uniformly distributed loads.
In Germany the “Guideline for the calculation of existing bridges” (Nachrechnungsrichtlinie 2011) issued by the Federal Ministry of Transport, Building and Urban Development provides a framework for the re-assessment of these structures. The guideline features a multi-stage approach where, in case the requirements for a specific stage cannot be met, in the next stage considerations that are more elaborate may be carried out. In the past couple of years a systematic re-assessment of German highway bridges was propelled, Schömig-Plan Consulting Engineers have been involved in the analysis of more than a dozen structures.
In the present paper the results of the re-assessment are shown for a bridge consisting of precast girders with I-section connected by an in-situ concrete deck, a multi-span hollow box with longitudinal and transversal prestressing and a composite structure with pre-stress by imposed deformations. Materials tests broadened the scope of the analysis for the last two bridges. With the three case studies presented, the main provisions of the re-assessment guideline are demonstrated and common shortcomings of existing bridges are highlighted. In addition, the regulations of the different stages are compared based on selected examples.
If ultimately some deficits remain, compensating or upgrading measures are required. It can be seen from the results of the case studies and also based on statistical analysis that shear and fatigue are the most common causes for safety deficits of reinforced concrete highway bridges in Germany. An established method to increase the load bearing capacity is the installation of external tendons with deviation and anchorage blocks on the existing superstructure.
A study on the structural safety of the load transfer of 17 retrofitting measures by external tendons (Novák, B. et al. 2013) has concluded however, that such systems may be vulnerable to a number of factors, that need to be addressed in the design process. Therefore considering recent experience some pitfalls of the technique are pointed out and key requirements are emphasized. Based on these results improved proposals for anchor and deviation construction concepts have been developed. Three examples of such anchorage concepts are introduced and the feasibility of different concepts is discussed, important observations on the design and construction of anchorage blocks are provided.
