Why Was The Earthquake So Devastating In Turkey The Engineering Explanation Of The Damage
The recent earthquake in Turkey has shown its clear destructive potential, an event so catastrophic that it caused massive damage and killed thousands. But why was there so much damage? We analyze the situation of the buildings before and after the earthquake from an engineering point of view to understand the extent of the damage.
The violent earthquake measuring 7.9 that struck southern Turkey and Syria on Monday caused the collapse of thousands of buildings, causing many deaths and injuries. At the moment, the recovery operations from the rubble continue but, unfortunately, the balance is increasingly dramatic. In this article we want to focus on the analysis of the structural effects and the consequent damages observed, unfortunately, in terms of victims and economic losses. From an initial analysis of the data collected, in fact, the USGS (American Geological Service) estimates the possible final toll at 10,000 victims and about10 billion dollars in economic losses. To have an objective view of the consequences of the event, it is worth making some observations of a technical nature on the three main causes that have contributed to the creation of such an apocalyptic scenario.
The intensity of the tremors and their effect on buildings
The reasoning to be done is simple: an earthquake generates actions on the structures (mainly horizontal forces). These actions can damage a building to a greater or lesser extent depending on the relative relationship between the intensity of the tremors and the capacity that the structural part has to counteract them . Let's focus for now on the intensity of the earthquakes, observing some data processed starting from the seismic signals recorded during the event.
From a physical point of view, we can focus on the first graph at the top left, which shows three curves of different colors, corresponding to the three different seismic components of the recorded signal (EW=East-West, NS=North-South, UD= UP-Down). On the vertical axis of the graph we find one of the fundamental data in seismic design, called spectral acceleration . It represents a measure of the entity of the forces that solicit the structure during the seismic event. It can be seen that the highest values recorded are above 2 g, i.e. twice the acceleration of gravity: it is as if a building had to bear twice its weight horizontally. Roughly, imagine rotating the building 90 degrees and placing another building of the same weight on top of it.
Under these extreme conditions, the system should oppose the applied actions. This very crude comparison confirms that major structural damage and collapse could still be expected after an event of this magnitude, no matter how well designed the structures. Simply put, the effect this earthquake has had on buildings is far from modest . Wanting to compare the data with the Italian seismicity, it could be said that actions of this magnitude are expected in L'Aquila (area of high Italian seismicity) on average about every 10,000 years .
The structural capacity of buildings before the earthquake
A second point is that relating to the structural capacity of the building present in the areas affected by the event. From the images and videos that are currently circulating on the net, we can deduce a substantial number of multi-storey buildings in reinforced concrete affected by significant structural damage and, in many cases, collapsed. They are residential buildings that are apparently similar (in terms of conformation and construction technology) to those present in Italy. The presence of various collapses mainly triggered by structural crises starting from the lower floors also appears evident – these are symptoms of the lack of adequate anti-seismic design criteria .
The consequences of replicas on buildings
During a given seismic event we always have to deal with several earthquakes, usually concentrated in space and time. In engineering jargon they are said to occur in clusters . For this reason, seismic swarms (where there is not an event with a prevailing magnitude) and seismic sequences (where there is one or even several earthquakes with a relatively high magnitude compared to the remaining ones) are usually distinguished . In the case of recent news we are therefore dealing with a seismic sequence! We also speak of mainshock, to indicate the main shock, and aftershockto indicate all aftershocks that occur, also called aftershocks. This phenomenon can lead to two types of problems:
- A generic aftershock could have intensities close to those of the main event, ie the mainshock, and for this reason be equally destructive;
- Buildings, which in principle may have already been damaged by the mainshock, could suffer further damage as a result of the aftershocks, increasing the observed damage.
This phenomenon is greatly emphasized in the case of Turkey, as some aftershocks after the main one have had magnitude values so high as to be comparable to that of the main shocks. It therefore occurs that a building may have been severely damaged by the mainshock but, for a series of reasons related to its structural capacity, has maintained its load-bearing capacity against gravity, i.e. its weight. However, subsequent replicas, acting on a sensibly proven structurefrom the immediately preceding events, can compromise the stability of the system and lead it to collapse. There are several home videos that report collapses of this type, i.e. buildings collapsed as a result of aftershocks rather than the effect of the actual main earthquake. The extent of these aftershocks which eventually leads to the collapse depends on the previous state of damage of the structure . In principle, therefore, even a gust of wind could be enough to cause the collapse due to an aftershock.
