ATTENTION! DANGEROUS BUILDINGS WITH "WEAK" FLOOR

The terms soft, cramped or weak floor are used in multi-storey buildings when one or more floors have a significantly smaller capacity to absorb earthquake impacts. A floor can be classified as "weak" when, figuratively speaking, its carrying capacity is below 70% of the carrying capacity of the floor above it or 80% of the average carrying capacity of the upper three floors. The presence of a weak floor is an extremely dangerous and common phenomenon in Bulgaria. Observed in the functional need for large openings for doors, car entrances or large windows, in place of antiseismic washers, especially along the facades of buildings.

Consequences of having a weak floor

Such openings may have been provided during the design of the building or may have been identified later. Modern scientific studies unequivocally prove that killing a door or window opening in brick buildings, without a reinforced concrete skeleton, reduces the load-bearing capacity of the wall to 70 ÷ 80%. In those with well-performed columns, beams and slabs, the load-bearing capacity varies from 50 to 70%.

The extreme seriousness of the problem is also proved by the results after the processing of information breaking the causes of severe damage in earthquakes. The main share of the severely damaged or completely destroyed buildings is due to the presence of a weak floor in the affected structures. An example is the Loma Prieta earthquake with a magnitude of 6.9, which in 1989 caused the destruction and severe damage to more than 160,000 homes. The subsequent report shows that nearly half of the buildings were damaged precisely because of the presence of a weak floor. The real danger and the great damage caused in the past have become the cause of government policies aimed at securing buildings suffering from this structural defect.

ATTENTION! The video contains disturbing footage!

Unfortunately, the weakening of load-bearing walls in the ground floors of buildings is an extremely dangerous but common practice in our country. The condominiums along the central city streets have been literally uprooted on their ground floors, in order to separate commercial areas with large entrances and windows. Even more dangerous are the reconstructions removing walls of two floors, as well as internal walls of the premises.

Mass reconstructions are justified only by a constructive opinion from a design engineer. He categorizes the affected wall as non-bearing, which avoids the need for mandatory antiseismic reinforcement of the hole. Apparently, the state services often neglect and blindly trust the "expert" opinion of the designer, which allows the implementation of real threats to the security of the building.

What are the reasons for not carrying out the mandatory reinforcement of the openings?
As in many other areas, in the first place, the neglect of a critical problem like this is dictated by the relatively high costs of strengthening the openings. Owners and tenants are willing to sacrifice the security of the entire building and its occupants, hoping for weaker earthquakes in the coming years. The prices for framing of openings start from 1200 ÷ 1500 BGN, as the quality execution and the dimensions of the new opening can increase the price several times to about 5000 ÷ 6000 BGN.

Another reason is the relatively slow procedure for legalization of reconstruction affecting the load-bearing elements of the building. For the new buildings it is necessary to obtain a constructive opinion, design visa, projects and permits, supervision and a licensed contractor. For older buildings, the procedure is longer and includes inspection, certification of the building and other activities. All these stages are mandatory and aim at a reliable end result ensuring the security of the building and its occupants.

The technological nature of the reinforcement is also an important factor, as sometimes the initial conditions significantly complicate the project. The presence of joists, thin slabs or weak masonry and foundations require the implementation of more complex and general fortifications providing the entire building.

Some design solutions include elements occupying part of the dimensions of the opening. Such obstacles may also impede the proper implementation of the project. Retail owners are looking for maximum light space and are often willing to risk the security of the building to get it.

Which buildings are most at risk?

Research indicates that the most affected by the phenomenon of "soft floor" buildings with brick construction, without the presence of reinforced concrete columns and beams. Buildings with a reinforced concrete skeleton but without reinforced concrete washers are also severely affected by the construction of new openings.

In general, constructions in which such alterations are made on the facade walls are more dangerous than those in which the removal took place inside the building. Prefabricated buildings also often fall victim to this type of renovation. In them, almost all interior walls perform a load-bearing function, and the facade mostly assists. Last but not least, higher storeys are associated with higher loads. As the height increases, the structures become significantly more vulnerable to earthquakes.

Why is it mandatory?

Recent decades have seen great progress in understanding the behavior of old structures in earthquakes. The global survey of buildings destroyed by earthquakes has clarified many issues and radically changed perceptions of anti-seismic construction of buildings. It turned out that the design models used before the end of the 20th century were generally misconceptions. The presence of a weak floor in the structures has been severely neglected, and in our practice this significant problem continues to be not considered with sufficient attention. Modern knowledge defines the buildings built before the 90s of the last century as not very reliable in an earthquake. Their additional weight loss by up to 80% in highly seismic areas such as the Sofia plain is an extremely risky and irresponsible process, which unfortunately continues to this day.

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Author:Dr. Eng. Teodor Todorov