Testing the load bearing capacity of bridges

With approximately 20,000 road bridges throughout Denmark, reliable knowledge is needed about the load that a road bridge can withstand before renovation or replacement is required. With such knowledge, it can be avoided that money is spent on reinforcing a bridge that turns out to be strong enough, and road-users also avoid delays due to unnecessary renovation works on bridges.

Together with the Danish Road Directorate and the consulting company COWI, DTU Civil Engineering examined a large number of concrete road bridges in 2016 and 2017 to determine the maximum load bearing capacity of the bridges. The actual load bearing capacity may, in fact, be much higher that currently expected.

One of the objectives of the project is to create some efficient tools which can provide bridge administrators such as the Danish Road Directorate, Rail Net Denmark, and the municipalities with a reliable picture of the strength of a given bridge type—without having to test all bridges in Denmark.

So far, one specific road bridge type has been examined in the project: concrete bridges with inverted T beams (OT beams) as the roadway base.

The measurements from this bridge type cannot simply be transferred to other types of bridges. To arrive at evaluation methods which do not only function for a single bridge type, but can be used more generally, DTU uses a combination of in-situ tests—i.e. tests on the actual bridges— laboratory tests, numerical calculations, and probability-based modelling.

During the load test of the bridge, deformations are measured in the concrete beams of the bridge. Photo: Jacob Wittrup Schmidt.

In situ tests

In connection with the in situ tests, the project partners have developed a special loading rig which can expose the bridge to a very precise load in a short period of time, informs one of the researchers behind the project, Associate Professor Jacob Wittrup Schmidt.

He is a structural engineering researcher at DTU Civil Engineering and has—for several years—worked with the assessment of the load bearing capacity of bridges, and monitoring, testing, and reinforcement of existing concrete bridges. He explains that, with the loading rig, they have been able to reduce the testing time in situ:

“Our loading rig makes it possible for us to perform load tests on a bridge in just one day. This is quick compared with other methods. The time aspect is crucial because a shorter testing time means shorter bridge traffic closure times and thus less traffic disruption in the area.”

One of the important factors in such tests is to make sure that the bridge is not subjected to overloading. It is therefore crucial to find the so-called stop criterion, which determines when the test is to be stopped.

“The stop criterion determines when the load test is to be stopped to ensure that there is a necessary safety margin in relation to a collapse. It also ensures that the bridge does not suffer permanent damage. Using advanced monitoring and calculations will bring us closer to a method for assessing the damage and fractures that occur in the bridges,” explains Jacob Wittrup Schmidt.

The load tests on the bridges are supplemented by calculations in the laboratory at DTU. Photo: Jacob Wittrup Schmidt.

Full-scale tests and laboratories go hand in hand

It is necessary to measure both inside and outside on a bridge to examine how it reacts to the load. It is a very cost-intensive job—in terms of both time and finances—because the work has to be done on the individual bridges.

With the reduced testing time of a single day, there is a large potential for in-situ tests. But even though the testing time has been significantly reduced with the application of the new methods, testing in situ is still costly and subject to time pressure.

The researchers are therefore using laboratory facilities to verify the measurements they have so far made in situ.

This enables comparing the measurements from the bridge tests with the theories developed on a continuous basis.

The laboratory tests will be performed in the Villum Center for Advanced Structural and Material Testing, which was inaugurated at DTU in November 2017. The Villum Center specializes in advanced testing of structures and materials and is an ideal place in which to repeat the full-scale measurements from the road bridges on a smaller scale.

“In the course of our full-scale testing, we discovered that the bridges on which we performed load testing could withstand much more than we expected. Now, our task will be to analyze and verify our measurements and theories in the laboratory to bring us a step closer to finding an efficient method for assessing the load bearing capacity of concrete bridges,” says Jacob Wittrup Schmidt.

If the project findings result in a method which can assess the load bearing capacity of bridges with inverted T beams, the next step will be to test the new methodology on other types of bridges.