How safe are we and where?

January 1, 2021 – As the earthquakes continue in Croatia, look at earthquakes and building statistics – how safe are we and where?

After a short conversation with the architect, Boris Mardia, at his suggestion, I also spoke to statistician Ivika Girkinich, in order to get an expert opinion and answers to some questions. Many people are on the lookout these days, and perhaps the only thing we can control to some degree is the quality and amount of information that can help us look at the situation more realistically and assess the risks. Accordingly, we can then determine which options are available to us.

What regulations have been established in the last 50 years?

As early as 1964, Provisional Technical Regulations, that is, guidelines for building facilities in an earthquake zone, were adopted. Then I took a big step in increasing home and building security. A more recent version of these guidelines (for high-rise construction in seismic regions) was released in the early 1980s. The last regulations to be adopted were the European ones – Eurocode 8, and they have been in place since the first part of the first decade of the 21st century. Each country adapts these guidelines to its earthquake and climate risk assessments. So quality regulations have been around for decades, and the only question left is their strict enforcement.

What is the strength of the earthquake designed by the buildings (Mercali / Richter)?

In the Zagreb region, according to these guidelines, buildings are designed to withstand earthquakes of 8 and 9 degrees, respectively (depending on the rating area, Zagreb has two regions) of the Mercalli scale – that is, 6.5 to 7.0 on the Richter scale.

On the one hand, these are the magnitudes that we have reached for the boundary values ​​in these earthquakes, but projections indicate that earthquakes stronger than this cannot occur in this region (i.e. the probability is very small). Unofficially, there is information that earthquakes are occurring slightly stronger than before.

How many weaker earthquakes can a building withstand? From 1 to infinity? Does physical fatigue happen?

If there are no deformations or cracks in the body, then the organism can withstand a large number of these stresses. Unlimited in theory, but in practice, there is one earthquake after which the first deformation occurs, which would disturb the stability of the body. Therefore, all facilities must be carefully inspected in order to notice their actual condition in time. This is a broader issue related to building maintenance, because it is seldom required by anyone in our country and it appears that there are no companies specializing in that. Repetitive work on the structure is of course not good and can stress the material which will then begin to wear down at once, but it is difficult to determine when this will happen, after 10, 100 or 500 of these stresses. For these and other reasons, it is often written in documents that the building was constructed for 50 years.

Can it be examined and how, what strength of earthquake can the building withstand? What about things that are over 50 years old and how do we know what they can handle?

In theory, this can be verified. It is necessary to make an architectural picture of the building, analyze the materials and their quality (the quality of concrete, mortar, bricks and steel), record what has been installed (slabs and beams), and examine some samples of the materials. Sometimes it is possible to make an assumption about the quality of the material and proceed to the static analysis and based on it to strengthen the structure, walls and install additional beams.

In practice, this rarely happens, because it is very expensive and there are not many organizations that can implement it well. IGH and possibly the School of Civil Engineering have the laboratories needed for all analyzes (according to the information currently available).

All old buildings should do such an analysis, but this is unrealistic given neither the owners nor the state pay attention to the price. Mostly after such events, most of it is due to home painting and surface patching. A large number of old houses have long known the need to strengthen them, but no one will demolish them alone.

What is the situation with bridges? Who checks and tests its durability and how often?

As with tall buildings, the necessary analysis can be performed to check the quality of concrete and steel. The bridges appear to be holding together well at the moment but need to be checked more regularly. In their regulations, standards are set to protect against corrosion and iron corrosion, which is another problem in marine areas. The thickness of the protective layer plays a special role in it. Again, these analyzes can only be done by a few institutes in Croatia, but it seems that part of these processes are organized at the country level.

Is there an estimate of the number of contractors who did not operate according to all regulations but made some savings on the account of safety?

There is no rough data on this either. Until the war, most construction companies were large and fairly organized companies, and in the 1990s a large number of small companies appeared that were difficult to control. All of them met the minimum requirements, but it happened that one engineer registered in the company supervised 5 construction sites, which, of course, is very difficult to do in a quality way. In those years of automatic construction, there was a shortage of professional employees in private companies. Also, in the past, building buyers had demands to move some walls or add windows, so these were sometimes met. The question is whether they have done all the necessary analyzes for all of these changes from the start. Of course, there were companies that were as good at the time as they are today, and today there are also companies that operate with questionable quality. This is somewhat of a problem for the workforce as well, since a large number of people from the building have exited, and our companies are helping with employees from Far Eastern countries who work very small hours.

When the earthquake passes, if the building is not damaged, is it better to stay inside or outside and does that make sense?

During an earthquake, of course, it is not a good idea to leave the building immediately because we do not know what could fall on us. Once things calm down, the facilities that sustained some damage should be evacuated. Ladders can be critical points. When you go out, you need to go to a meadow or space far enough from the building (a large portion of people stayed near the building and in the parking lot in front of it). If the building remains intact (which is sometimes difficult to assess from the inside) then there is no need to go out. People do this because of potential new earthquakes, but unfortunately we cannot know if it will happen in the next half hour or hour, day or two, or in two months or not again. While in the building, we must adhere to the load-bearing walls, which are walls that are more than 20 cm thick. These are facade walls but there are also internal walls that go in both directions. It is constructed of reinforced concrete, takes a lot of strength on itself, but, unlike partition walls, it does not crack.

What is the safety of underground garages for cars and people?

Basement garages have hewn concrete walls and breathe with the ground. At the bottom, the offset is smaller than on the upper floors. In principle, it is relatively safe from breakage and damage. Of course, there is only a risk of closing their exits due to very strong earthquakes due to the collapse. The ground floor is often the most important.

For more information on the Petringa earthquake, follow our dedicated section.

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This article originally appeared on Medium, where you can find more Aco articles.

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