Danger of Collapse Averted

Whether they are subjected to earthquakes, terror attacks or explosions, buildings need to be able to withstand a whole range of calamities. Structural engineer Dr. Akanshu Sharma is conducting research into where the weakest points of buildings are found and how these can be retroactively reinforced to withstand shocks, heavy impacts and fire. In January 2017, he took up a junior professorship in “Innovative Reinforcement Methods with Fixings” at the University of Stuttgart. The chair was donated by the Fischer Group, the global leader in fixing systems.

In 2016, a series of earthquakes in central Italy killed over 300 people, destroyed numerous buildings and left tens of thousands of people homeless. Germany too experiences several thousand earthquakes per year – primarily in the Rhine Valley Rift and Swabian Jura regions as well as in and around Gera. Most of these are weak shocks that barely register on the Richter scale. But, stronger earthquakes also occur here every 30 years on average. Whilst they never reach catastrophic dimensions, they do cause visible damage. The worst such quake since 1900 occurred in 1992. The epicentre was close to Roermond in the Netherlands and the quake measured 5.9 on the Richter scale. Houses shook, chimneys and roof tiles fell down, and trees toppled over, causing millions of euro of damage.

Modern buildings usually comprise a reinforced concrete frame into which the slabs and walls are integrated. The weakest points in older buildings are usually the nodal points in these reinforced concrete frames, i.e., the points at which the vertical columns and horizontal beams intersect. The greatest risk is that one of the load-bearing columns of a building buckles without warning, causing the ceiling slab to collapse, potentially burying people below it. “Our goal is to ensure that people can get out in time and survive the quake”, Dr. Sharma explains. Smaller cracks in a beam are acceptable, he goes on to say; they can even be allowed to bend. But, the main thing is that there is suffi cient warning before a structural failure and that the building doesn’t suddenly collapse like a house of cards. “By contrast with normal buildings”, says Dr. Sharma, who previously worked as a building safety engineer at the Bhabha Atomic Research Centre (BARC) in his native India, “we do not accept any cracking whatsoever in nuclear reactors through which nuclear radiation could escape.”

The main cause of building collapse is concrete, which has outstanding compression resistance properties but fails to withstand tractive forces. In the event that a frame node is insuffi ciently reinforced by steel reinforcing rods, then it will collapse. By comparison with concrete, steel has a high tensile strength and will, therefore, bend without simply breaking. Engineers can still improve the earth-alquake resistance of older buildings by retroactively reinforcing the sensitive frame nodes. One option that Dr. Sharma discovered in the course of his doctoral studies at the University of Stuttgart involves steel diagonal members (gusset plates) attached to the inner corners of the frame node. This deflects some of the forces around the frame nodes so that, instead of appearing in the nodes themselves, any cracks will appear in the connecting beams. However, according to Dr. Sharma’s findings, that only works if the steel diagonals are firmly fixed to the concrete.

Befestigte er einen Stahlwinkel mit diagonal aufgeschweißter Platte auf einen maßstabsgetreu nachgebildeten Rahmenknoten, so hing das Verhalten im Wesentlichen vom Verhalten des Befestigunsmittels ab. Als Befestigungsmittel verwendete Sharma beispielsweise Betonschrauben, Metallspreizdübel oder in das Bohrloch des Betons geklebte Verbunddübel. Sind diese für gerissenen Beton und seismische Belastung ungeignet, so ergeben sich unerwünschte Versagensarten. „Es kann beispielsweise zu starken Verschiebungen des Befestigungsmittels kommen, die wiederum eine Schwächung der Stahldiagonalen bewirken“, hat Sharma beobachtet. Ob sich ein Befestigungsmittel für diese Anwendung eigne, lasse sich anhand der Daten aus der allgemeinen bauaufsichtlichen Zulassung abschätzen, so der Experte.

Whenever he attached a steel angle member with a diagonally welded gusset plate to a scale model of a frame node, he discovered that the resulting behaviour essentially depended on the fi xing materials. For example, he tried concrete screws, metal expansion dowels and bonded anchors glued into a hole drilled into the concrete. If these are unsuitable for cracked concrete and seismic loads, the result can be unwanted failure profi les. As Dr. Sharma observed: “the defl ection of the fi xing mechanism can be too great, which in turn weakens the steel diagonals.” Whether a given fastening element is suitable for this application can be estimated using data from the general type approval documentation, as the expert explains.

Originally, researchers in New Zealand thought of fastening the steel diagonals to the reinforced concrete frame using threaded rods, which they would insert through both the beam and the supporting column. The benefi t of Dr. Sharma’s solution is that the frame nodes only need to be accessed from the inside and that the slab does not need to be drilled through. He has also developed numeric models with the aid of which it is possible to study the failure of specifi c structural components in detail on the computer, as well as engineering models that can be integrated into traditional engineering software packages. Engineers can use the latter to calculate the likelihood of a given building being able to withstand a future earthquake or some other heavy stress load. The same scenario can then be repeated on the computer but with reinforced frame nodes. This enables construction experts to calculate such things as the ideal dimensions of the steel diagonals or which fastening system would be best suited to secure the building against future earthquakes. “Many of these models are already mentioned in the relevant literature, but they are much too complicated for daily use and are relatively costly”, As Dr. Sharma found: “I was always fascinated by the question of whether we could find a way to ensure that building structures behave exactly as we want them to under specific conditions rather than in a completely unpredictable manner”, says Dr. Sharma, who, even as an adolescent, visited high-rise building sites with his father, who was also a structural engineer. Ultimately, the son followed in his father’s footsteps and studied structural engineering. He loved his later work at the BARC in India. Nevertheless, Dr. Sharma began to realise that he was being forced to spend more and more of his time on administrative tasks, which limited the amount of research he was able to do. Finally, nine years ago, he took the risk of leaving India for the first time – and arrived at the University of Stuttgart. He first arrived as a visiting researcher as part of an Indo-German collaboration at the Institute of Construction Materials. A short time later he returned for his doctoral studies, which he completed at this institute and in India.

“One of the benefits we have in the Indian public sector”, says Dr. Sharma, “is that we can conduct our experiments cost-effectively and that the research work is primarily funded through equity capital. On the other hand, Germany offers an excellent research environment with fewer restrictions. However, that also means that one is responsible for finding the necessary funding.” One thing in particular strikes the structural engineer about Germany. Dr. Sharma comes from a country where buildings are shooting up everywhere, thanks to the current economic boom, and where new bridges and subway tunnels are being built in many places. In Germany, by contrast, he is confronted with a completely different set of challenges: “Most bridges and buildings are over 30 years old and are showing the initial signs of decay. They need to be reinforced and gradually renovated.”

Four years have now passed since Dr. Sharma made the final break with India and relocated with his family to Fellbach to pursue his research career in Germany. “Although the decision was not easy for us”, he says, “I had the full support of my wife and my daughter, who is now eleven years old.” As Group Leader for Reinforcements and Fixing Systems, Dr. Sharma is now also looking into the behaviour of reinforced concrete structures exposed to fire or impact loads, such as a heavy collision. The overriding question for all of these aspects is how structural weak points can be retroactively reinforced. The primary focus of the new Endowment Junior Professorship is on fixing and fastening systems for reinforcement measures carried out, for example, on dilapidated buildings – Dr. Sharma’s area of specialism.

Someone who was already thinking about fixing and fastening systems some 50 years ago was the inventor Artur Fischer, who achieved world renown as the inventor of the wall plug (German: Fischer-Dübel). The company founded by Fischer will be funding the Junior Professorship at the University of Stuttgart with 1.6 million euro over the coming six years. In retrospect, relocating from India to Germany was a huge step for Dr. Sharma, but one that he in no way regrets. The initial language problems are now largely a thing of the past, even though, as an ardent cinema goer, he still prefers to watch films in English. “Once one is properly set up and is able to speak the language”, says Sharma, “life here is pretty easy.” The only thing he has not yet succeed in getting used to is the fact that the shops are closed on Sundays! Helmine Braitmaier