The prefabricated houses, irrelevant of the type of building, is a relatively new technique for the Bulgarian construction practices and therefore the type of construction is not yet fully regulated by the law in the areas of tax institutions, cadastral maps, the banking sector or the insurance sphere. There is distinction in the different types of buildings according to their bearing structure and the relation to this distinction, the individual institutions determine for themselves whether the building is reliable or not. The categories are as follows (Annex 2 to the LTCT – Tax Valuation Rules for Real Estate):
Codes used for types of constructions
Code Building
1 2
FB frame-built
SM semi-massive
M1 solid without reinforced concrete elements or from
prefabricated ethereal and other boards (bungalows),
massive with partial reinforced concrete elements
M2 massive – one large panel
M3 massive with load-bearing brick walls and fully mono-
prefabricated or prefabricated reinforced concrete flooring
constructions, massive – prefabricated scaffolding and
Frame reinforced concrete structures raised slabs-
large-scale and creeping form-work,
сskeleton-beam-less constructions, special
construction (steel, etc.)
From a modern engineering standpoint these concepts are clear, but only to a certain extent, because these types of constructions were up to date when the mass construction was mainly made of reinforced concrete and virtually no wood was used for new construction. That is why there are so many divisions of constructions types. There are currently many subsets that don’t fall into any of these categories. This is exactly the case with prefabricated houses (with a wooden or steel supporting skeleton). Anyone who is uninitiated on the subject would immediately classify them as flimsy buildings (which would be wrong).

Frame built buildings were constructed up to the beginning of the last century. They are a basement (or a basement of stone walls) on which a one or two storey wooden skeleton structure is built, with diagonal elements between the columns of frame .They serve for the structural reinforcement and stability of the skeleton. It is precisely this clamping element, that comes from the name of this type of construction. The empty space between the wooden columns and beams is filled with plastered hedges, tiles or bricks. The floors are made out of wooden beams and the roofs consist of capitals, beams and pliers. These buildings are not sized and calculated, but are made by craftsmen (carpenters) who have chosen the right beams sections based on their experience over the years. In fact, razor structures are a prototype of the skeletal structures we know in modern construction. Each production hall whether it be made of reinforced concrete, wood or steel, consists of columns, beams, and stiffeners. These reinforcements are not called clutches but are spatial reinforcement (e.g. X links). From a engineering stand of view, the way in which the forces are assumed is the same in the old ‘fraythan’ structures, as well as in the modern skeletons of reinforced concrete, steel or wood.

Real frame built buildings, are old technology which is unheard of in modern times.The latter were built a century ago, without any legal regulation (on construction and execution).Not calculated and measured. The wood has random quality and strength. They do not have the characteristics of modern dwellings, such as thermal insulating properties, convenient distribution, energy efficiency. Each and every repair of these type of buildings is costly because the technology is from a century ago, and so are the materials that are used. That’s why they are not attractive to customers. There is no market (who would want to live in a house where the doors are 160cm high, the windows are just like a prison, the winter is horribly cold and there is a single room for absolutely everything. Most are two bedrooms and the toilet is in the back yard).

Technically speaking, these buildings are strong enough and stable enough, proof of which is the test of time. They have resisted the most extreme conditions in the past 100 years.

It should be clear that ABSOLUTELY ALL NEW BUILDINGS are calculated, measured and legalized according to the current legislation in Bulgaria – Eurocode. This is a set of documents obliging engineers to design buildings according to strictly defined requirements. BDS EN 1990 provides general requirements for sizing them up. The concept of project lifetimes (BDS EN 1990 – 2.3), which is a  50 year old  ‘category 4’ – constructing  buildings and other simple constructions. Measures to ensure durability are described. BDS EN 1991 – identifies the impacts that buildings have to take – payload, snow, wind, earthquake and accidental loads. BDS EN 1998 – shows what the earthquake loads are. These loads are the same regardless of the type of construction and the material from which it is constructed. The other parts of Eurocode (BDS EN 1992-1999) refers to the measuring of structures of different materials. There are guidelines that take into account the specificity of each material.

This means that regardless of whether a house or facility is made of reinforced concrete, steel, wood or masonry, it is secured against the same loads and has the same load bearing capacity. In addition, all buildings have the same design lifetime which is 50 years. Assessing whether the house is healthy or risky can not be given by the type of building or the material from which it is executed. This is the engineering point of view.

At the moment, for each new building, a building permit is required, which requires calculations and measuring of the construction by an engineer who complies with the current legislation (Eurocode). Each new building must withstand a specific load. It should use wood with proven ‘physico-mechanical’ properties and have appropriate certificates. Every project is approved, and only after the construction has started. Protocols are then issued for each stage to ensure the quality of the construction. The prefabricated houses rise on a pre-built reinforced concrete foundation with or without a basement. The construction consists of a skeleton made of wood or steel. Wind and earthquake precautions are achieved through diagonals or planes enveloped on both sides of the structure. The empty space between the beams is filled with heat insulation, and an additional layer is placed on the outside.

Modern houses are comfortable, energy efficient, heat insulated and guaranteed to be of quality. They are built quickly and easily, no matter the season, because they are prefabricated, they are made under factory conditions under the constant control of an engineer.

On the other hand, they are modern, attractive to customers because of their qualities. They are much more energy efficient and green than traditional buildings with a reinforced concrete structure. We can safely say that their value will continue to increase at the expense of houses with a traditional construction. In this way, they will become a preferred way of construction as in the developed countries, where the advantages of this type of construction have long been appreciated. We can note the the only thing, that resembles the flimsy houses, is the timber used in the construction (if it is uses, that is).

None of the aforementioned qualities are fulfilled in the frame-built buildings.

Semi-solid buildings are also a pretty old method that doesn’t interest us.

Massive buildings are divided into many subcategories. From a engineering point of view, a building that is massive should consist of a reinforced concrete or masonry structure. Here we come across a rather big paradox in the naming of the categories.

In Category M1(above) it is stated:

massive without reinforced concrete elements or from
prefabricated ethereal and other boards (bungalows)

This automatically means that there are no massive buildings in category M1 because there is neither concrete nor brickwork.

In Category M2it is said:

massive – large panel

Here are all of the panel buildings, implying they are made out of reinforced concrete (which is not mentioned), although it is absolutely not a problem to make a panel building made out of wooden elements. It has been done repeatedly and is being done right now. There are houses that consist of 3-4 massive modules, and there are houses that are delivered whole. How much in a large-scale building.

In Category M3 there are also quite interesting cases:

massive with brick-bearing walls and fully monolithic
prefabricated or prefabricated reinforced concrete flooring
constructions, massive – prefabricated scaffolding and
Frame reinforced concrete structures,
raised slabs, large-scale and creeping form-work,
skeleton-beamless constructions, special
construction (steel, etc.).

In this category there are skeletal structures, which are made out of reinforced concrete, but notice also made out of steel. Steel also does not automatically belong to massive buildings because of the lack of concrete or bricks, but it is still included.

We can say that the concepts of jagged, semi-massive and massive have evolved over time and have lost their purity on the engineering side, and should be understood as follows:

– Frame-Built – A wooden-brick building from the past, that is extinct and presumably is considered unstable because of the unclear normative base through which it is built.

– Semi-massive – Building with load-bearing walls of concrete or brick and non-bearing wood.

-Massive – due to the variety of different types of constructions, we can summarize that it is a modern building, built according to the existing normative system with clear properties, which is not necessarily made out of reinforced concrete.

The tax assessment should not be determined by the type or material of the structure. This may have been a way of determining how much a structure was secured in the past, but nowadays it is completely irrelevant. The type of material of the structure is decided by the designers, who judge which type is most appropriate. The strength of the construction is guaranteed by the signature of the engineer.

We can summarize that prefabricated houses, with a wooden or metal structure, as a new type of construction in Bulgaria, has not yet found a place in the above categorization, but they can certainly be included in category M3, in special construction (steel, etc.)

The most appropriate thing would be for these categories to be renewed and updated.

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