Do old bridges last longer than expected?

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The 163 meter-long bridge is characterized by a continuous prestressed concrete
The 163 meter-long bridge is characterized by a continuous prestressed concrete superstructure and seven spans. (Image: Chair of Concrete Structures / TUM)
Research news

More traffic, heavier loads: When bridges in Germany over the age of 50 are evaluated according to current standards, calculations show that many of them theoretically have substantial deficiencies. Nevertheless many bridges exhibit no damage that confirms the calculated structural shortfalls. Engineers at the Technical University of Munich (TUM) are now working to explain this contradiction, conducting experiments on the transverse load bearing capacities of a real pre-stressed concrete bridge.

Researchers had been looking for a bridge like this for a long time. The requirements were specific: It had to have been built before 1966, had to have as many spans as possible between the bridge piers, had to be easily accessible and of course had to already be out of service. The 60-year-old bridge over the Saale in Lower Franconia’s Hammelburg fits this profile perfectly. Over the years the bridge exhibited more and more damage; renovation would not have been economically feasible. As a result, road traffic has been routed over a new bridge since December 2016.

Engineers at the TUM want to use Hammelburg’s Saale bridge to investigate the load bearing characteristics of real bridges in order to resolve a contradiction between theory and practice. Compared to the standards in effect 50 years ago, current standards, formulated at the European level, require a significantly higher shear load bearing capacity. One reason for the change is the substantial increase in heavyweight traffic moving over bridges, since the major strain placed on bridges results from heavy-load vehicles.

Computations indicate deficiencies, but no damage is visible

Shear is acting vertically along the longitudinal axis of the bridge. "Bridges built before 1966 have virtually no vertical reinforcement (stirrups) to carry shear forces," explains Prof. Oliver Fischer of the TUM Chair of Concrete Structures. When these bridges are evaluated according to the new regulations, they exhibit massive shortcomings, in some cases at levels of more than 100 percent. As a result, these bridges will have to be strengthened, traffic loads will have to be reduced or, in extreme cases, entire structures will have to be demolished and rebuilt. However, there is a discrepancy between the theoretical and the actual load bearing capacity. "There are many bridges for which computations indicate a deficiency that is however not confirmed by visible damage," Fischer says.

Measurements near the bridge piers

Shear load bearing characteristics are highly complex, and as a result a variety of theoretical approaches to describing them exist. "One difficulty is that the corresponding experimental analyses have been conducted almost exclusively in the laboratory," Fischer explains. "Many load bearing systems react differently on a small scale than under realistic conditions." It is also impossible to simulate in the laboratory the impact natural wear due to weather and decades of aging have on the bridges. The planned experiments with the Saale bridge are expected to close this gap.

The 163 meter-long bridge is characterized by a continuous prestressed concrete superstructure and seven spans. "Shear forces are greatest in the vicinity of the bridge piers and supports," says Fischer. This makes measurements at these points particularly interesting. The experiments are being conducted in five spans and in the area of the respective supports.

Load equivalent to 400 small cars

The shear load will be applied in the individual experiments using a beam especially constructed for these major experiments, referred to as a loading girder ("Belastungsträger"). This girder is approximately 32 meters long, 1.80 meters high and weighs some 40 tons. The overall load applied by in total up to eight hydraulic jacks can be increased to as much as 400 tons, equivalent to the weight of ten 40-ton trucks or 400 small cars.

The measurement technologies are complicated: For example, the scientists can use optic glass fibers to determine how the concrete expands and where cracks develop. The TUM Chair of Geodesy is supporting the investigations with the application of high-resolution cameras to document crack formation. The resulting images are then analyzed with special software.

In addition to the open-field experiments, the researchers are conducting both extensive numerical simulations and laboratory investigations. They have developed an innovative experimental apparatus in which they can install a part of bridge and test it under realistic conditions. Fischer: "Our clear objective is the formulation of new approaches to dealing with older bridges and to make even better use of load-bearing reserves without compromising safety. This will make it possible to save resources and money on an individual case basis."