The roof of FC Twente’s stadium failed. A multi-storey car park in Eindhoven collapses. That’s when TNO’s forensic engineers come into action. Like true detectives, they reconstruct events and trace causes.
Happily, things don’t go wrong too often. But in the Netherlands, if buildings or other structures (or parts of them) collapse, this immediately makes the headlines. Especially when people are injured or killed. Something like this happened a few years ago. While a roof was being erected over FC Twente’s ‘Grolsch Veste’ stadium, part of this structure collapsed. Forensic engineer Huibert Borsje was asked to investigate the cause of this collapse. “A forensic investigation like this can have various goals, such as determining liability. However, the primary goal is always to identify the exact causes, to prevent any repetition of the event and to avoid any future problems.”
“The primary goal is always to identify the exact causes involved, to prevent any repetition of the event and avoid any future problems”
Detectives at the ‘crime scene’
TNO has been investigating damage to various structures – ranging from buildings to locks and bridges – for over fifty years. Over the past decade, however, there has been a real boom in forensic engineering, according to Mr Borsje. “Our investigation procedures have become more highly structured and more refined. As a result, when reconstructing the events involved in cases of damage, the approach we adopt is almost scientific in its precision. I can confidently say that TNO ranks among the best in the world in this field.” As an example, he cites TNO’s bottom-up approach. “While others often cite various possible causes, starting from the top, we switch that approach by 180 degrees. Working like detectives at a ‘crime scene’, we devote considerable effort to gathering information from the collapsed section of the structure. Based on these findings, we start searching for causes. We literally start at the bottom, down among the debris. In addition to helping us identify the cause of the event in question, this bottom-up approach has the advantage of eliminating all those factors that were not involved.”
In the case of FC Twente’s stadium, for example, the deformations of the collapsed roof sections showed how the collapse had occurred. This evidence revealed that the various stages of construction has been carried out in the incorrect sequence. The situation during the construction phase was unsafe, as certain parts of the support structure had not yet been installed. Huibert Borsje notes that “When investigating construction faults, those using a top-down approach often tend to focus primarily on the construction plan. But this could cause you to overlook instances in which the building plan was not followed correctly, or cases where mistakes were made during construction. Our approach, however, makes this very evident.”
Just like a real-life detective, this forensic engineer from TNO always works with a partner. That person is a ‘materials expert’, such as a materials scientist or a structural engineer. TNO’s forte is that it usually has materials experts available in-house, so it can put an investigative team together soon after a collapse.
“An example of this is our bottom-up approach. While others often cite various possible causes, starting from the top, we switch that approach by 180 degrees”
Taking a closer look at the construction method
Gerrie Dieteren, a structural engineer specializing in concrete structures, is often called in. A multi-storey car park was under construction in Eindhoven when a floor suddenly collapsed. Fortunately, no-one was hurt. During the investigation into the cause of this event, one specific detail was subjected to a structural analysis. Mr Dieteren discovered that, on-site, concrete had been poured onto prefab filigree slabs of ‘self-compacting’ concrete. The bond between these two types of concrete was not sufficiently strong. The critical nature of this factor had not been fully recognized and, as a result, it had not been designed for all possible failure mechanisms. This conclusion has had far-reaching consequences, as the same construction method has been applied elsewhere in the Netherlands. The affected buildings are currently being investigated. Buildings at several sites across the country were strengthened or temporarily closed.
Another shining example of TNO’s forensic work is the Rijkswaterstaat building in Utrecht. The building’s occupants suddenly started complaining about unexplained vibrations. The cause was a mystery. The local geology meant that the possibility of an earthquake could be excluded. Also, it was not known whether these vibrations posed a risk to the building’s structure. So the building was immediately evacuated, as a precautionary measure. “We were called in after several partial investigations had already been completed. Our job was to provide a second opinion about those investigations”, Mr Borsje says. “Naturally, we started by searching those reports for any clues. Statements by various witnesses led us to suspect that the window-cleaning installation on the roof might be the cause of the vibrations. And that, as it turned out, was indeed the case.”
“We literally start at the bottom, down among the debris. In addition to helping us identify the cause of the event in question, this approach has the advantage of eliminating all those factors that were not involved”
The service life of structures
Concrete has a long service life. But how long, exactly? That varies from one structure to another. Gerrie Dieteren points out that “Sometimes, older buildings no longer meet the requirements in current standards. In such cases, we investigate the actual strength of the structure in question. We may find that the building meets all of the safety requirements, even though it is no longer fully compliant with the standard design rules. TNO contributes its expertise in this area to various international (standardisation) bodies charged with drawing up the standards or codes for concrete structures. In this way, we contribute to the development of regulations and to the pool of knowledge about concrete. At the same time, this approach can prevent the unnecessary demolition of older structures. In many cases, these structures can continue to be used for years, perhaps with minor strengthening. This way, we can boost the sustainability of our built environment.”