The tragedy in Turkey has caused the entire region to look at the way buildings are built. Stoyan Georgiev’s special report today is dedicated to this topic.
We compare satellite images of the city of Gaziantep from before the earthquake and see that almost all the collapsed buildings have commercial areas and large storefronts on the ground floor.
Two structural engineers examine the destruction in Turkey and see a common feature in the way most of the completely destroyed buildings collapse, resulting in the deaths of over 20,000 people. Dr. Dimitar Stefanov is a seismic engineer from the Bulgarian Academy of Sciences.
“The first thing that makes an impression, which I would realize as a systemic defect in the construction, is that in the first floors, most of the buildings are exposed from the walls, these are commercial areas, restaurants and other spaces are open and the so-called flexible first floor is formed. What can be seen in all the clips is how the buildings first “sink” because the first floor is compromised, then in some of them there is a domino effect, the slabs collapse one after another. The absence of earthquake washers means that the structure has no structural elements capable of bearing the horizontal seismic load. And this is the main reason why these buildings collapsed,” explained engineer Stefanov.
Engineers note that the collapse occurs where there is a lack of brickwork between the reinforced concrete columns to act as an earthquake washer due to open commercial areas. Although in theory the brick walls are non-load-bearing and the reinforced concrete frame is what carries the building, the brickwork turns out to be decisive in whether the building will collapse when the block is subjected to horizontal earthquake loading.
The reason is that the brick wall does not allow the reinforced concrete columns to deform during shaking and, accordingly, break and collapse. Sometimes, even in buildings that have earthquake washers, the wall is the deciding factor in whether it will be strong enough to survive.
Video footage from a laboratory shows a simulation of a 20-second earthquake on a three-story monolithic building. It shows the brickwork on the first floor breaking to take some of the load, helping the column withstand the earthquake.
“Sometimes this is done at the design stage to provide access or storefronts. Other times it is due to renovations. In most cases, if we are talking about the Bulgarian practice, such reconstructions are totally illegal,” noted engineer Stefanov.
Globally, studies show that 50% of totally destroyed buildings are the result of a soft floor.
Engineer Todorov is the author of a study of the detrimental impact of the so-called soft first floor on the stability of buildings during an earthquake. His research is part of a scientific work on the condition of the buildings along Sofia’s main shopping street – Vitosha Blvd.
38 out of 64 investigated buildings in the very center of Sofia have the so-called soft first floor. But this is not part of the design of the buildings. Simply put, brick walls have been removed to open up commercial space. Totally illegal.
In 58% of the buildings on the central Sofia street, they have removed walls and rest only on columns. Thousands of square meters of soft floor, only on the pedestrian zone of “Vitoshka”.
Eng. Todorov prefers not to publicly announce exactly which buildings have undergone reconstruction, but emphasizes that the authorities are aware of the conclusions.
“The people who are in these buildings, I think, will not feel good because the conclusions are quite accurate. Especially the buildings that are the continuation of “Vitoshka”, the commercial ones, were not designed like that,” explained Engineer Todorov.
– Is there such a brick wall, even when we are talking about a monolithic reinforced concrete frame, the wall does not have any importance for its strength?
– The only option for a wall to be non-load-bearing is when it is built after the skeleton of the building is built. The column alone cannot take the earthquake. Analyzes from previous earthquakes show that a building that has walls with a good connection between the beams and the reinforced concrete slabs absorb 2-3 times more energy,” said engineer Stefanov.
– Is this the reason because we see pictures of buildings where the ground floor is literally flattened and the other floors are standing, even tilted.
– Yes, that’s exactly the reason.
– These myths that this wall is not load-bearing.
– We’ve been saying this for years, but it’s good to keep paying attention. The walls should not be removed, added engineer Stefanov.
– In your professional language, is there such a concept as a non-bearing wall?
– From the point of view of static loads, these walls do not carry, or at least they are not designed to carry, but when the earthquake comes, these walls help, and they help significantly. In certain cases, it is decisive whether the building collapses or survives. And for the first floor it is simply fatal.
There is no way a building will collapse, even with a magnitude 7.8 earthquake, if it was considered by the structural engineer during the design and if the project was followed by the contractors. The norms allow for damage, but the collapse of a building means the presence of criminal negligence, experts are categorical.
