Crashing into new railings to protect the strength of old bridges

More than 4000 aging bridges in Norway need to be secured with new railings. Researchers from the Norwegian University of Science and Technology (NTNU) are now crash testing new railings to see if it is safe to install them directly onto the old bridges. If the tests show that it is safe, the new railings can be attached directly to the concrete beams running along the bridge edges.

Easier, cheaper and more environmentally friendly

This makes the job both easier and more affordable. There is also an environmental benefit here, because much less new concrete will be needed.

"We need to preserve what we have, improve it where we can, and build new where we must,"

said project manager Vegard Aune, an associate professor at NTNU's Department of Structural Engineering.

A survey from 2018 shows that more than 4000 bridges on the Norwegian road network were designed according to the old load regulations, which were probably too strict. As a result, a large proportion of these do not meet current requirements.

Currently too strict

The researchers are testing how much edge beams built according to standard drawings from the 1947 and 1958 load regulations can withstand. The reason for this is that the current regulations, Vegnormal N101, are probably too strict and conservative. The load-bearing capacity of the edge beams has been calculated on the basis of slow, persistent, static load. By contrast, the load from a real collision is short and intense and lasts only 0.1–0.3 seconds.

This means that in many cases, modern railings cannot be installed on bridges built in accordance with the regulations from 70–80 years ago. At least not without extensive improvements being made. If the assumption that the regulations are too strict is confirmed, the project could provide a basis for changing the rules.

Unknown costs

Thousands of bridges in Norway require new railings, and the cost per bridge will vary. The Norwegian Public Roads Administration does not have exact estimates of what the total cost will be.

"Of course, it will cost a lot of money. Currently, we have to chisel away the existing edge beams, cast new ones, and then attach the new railings. If the tests at NTNU go well, we will simply be able to attach new bolts to the existing edge beams on the bridges," explained Fredrik Nyberg.

He is a senior engineer at the Norwegian Public Roads Administration and is responsible for the inspection and approval of road safety equipment in Norway. This responsibility covers everything that is installed on all bridges, not just those administered by the Norwegian Public Roads Administration.

Heavy vehicle versus railing

The red rig, known as the Pendulum Impactor, towers in the test hall, which is one of NTNU's largest laboratories. It is used to test how materials like aluminum, steel and concrete hold up against extreme loads—such as high-speed collisions.

On Thursday, January 22, everything is ready for testing, with the project's participants as excited spectators. When the red warning light flashes and the alarm sounds, the Pendulum Impactor's rig arm smashes the load into the railing. What the researchers are now investigating is an area that has not previously been addressed in the regulations. They are now going to calculate what actually happens when a heavy vehicle crashes into a bridge railing at high speed.

Important for road safety

Nyberg is following the tests closely.

"This is very useful for us. The project can also help us achieve two of the five top transport policy goals in Norway," said Nyberg.

The goals he is referring to are "More value for money" and "Contribute to achieving Norway's climate and environmental goals." According to Nyberg, the project is also important for Vision Zero.

"Currently, bridge managers refrain from replacing railings because it is too expensive. Now, more railings can be replaced, which will improve road safety," added the senior engineer.

30,000 frames per second

In addition to the dynamic tests involving force and speed in the Pendulum Impactor, the railings also undergo slow and controlled loading—right up to the point of failure (see image). Everything is filmed with a high-speed camera that takes 30,000 images per second.

"This helps us observe cracks developing in the concrete, pull-outs, and the strain caused exactly where the posts are attached to the edge beams," explained project manager Aune.

Requires full-scale testing

Tests were carried out on Mønster Bridge in summer 2022, which was constructed in 1955. The bridge was probably designed according to the load regulations from 1947. These tests showed no signs of damage. This supports the assumption that old edge beams can accommodate new, modern railings.

The NTNU project builds on a number of master's theses in which students used simulation tools to study the load-bearing capacity of edge beams from old bridges. The results suggested that the old edge beams meet the current requirements. However, it was concluded that full-scale tests were needed to confirm the accuracy of the simulations.

The results will be summarized in a report, which may propose regulatory changes to the client, the Norwegian Public Roads Administration.

Provided by Norwegian University of Science and Technology