In the central urban areas, there are still many buildings with wooden floors. Their attractive location dictates a constant trend of change in their purpose. Their transformation from residential buildings into offices and public places is associated with increased requirements for the buildings themselves. The increased load, the need for sound insulation, thermal insulation and fire protection require changes to the existing wooden joists. In this article, special attention will be paid to the constructive point of view in accordance with modern understandings of structural safety and measures for its provision.
Behavior of the joist in an earthquake
The main feature that a number of modern scientific studies focus on is the vulnerability of buildings with wooden beams to earthquakes. The old masonry structures, whose storey levels are solved with wooden joists, show extremely unfavorable behavior and are the predominant part of the severely affected or completely destroyed buildings in past earthquakes.
After a thorough structural analysis of existing historic buildings, the engineers came to the conclusion that the joist is the weakest link in this type of construction. The classic, well-executed and preserved joist also does not meet the basic structural requirements formed after the analysis of such damage. The main disadvantage of this type of elements is the inability to properly redistribute the horizontal earthquake forces to the vertical elements - columns, walls. Simply put, the "soft" joists do not allow the transmission of forces to the walls that are designed to absorb them. As a result, there is an overload of individual vertical elements until their destruction.
Another serious structural problem with wooden joists is that, unlike reinforced concrete slabs, it does not provide transverse space stability of the load-bearing masonry walls. Unwanted behaviors that occur, most often outside the plane of the walls, lead to their premature destruction.
Despite their significantly higher number of storeys and weight, modern buildings, filled with reinforced concrete skeleton and reinforced concrete slabs, prove their reliability in any major earthquake. Under the same influences, different constructive systems show radically different behavior. Some remain literally unaffected, while those with wooden beams suffer fatal damage.
Since the buildings in question with trimmers obviously have a deficit to absorb earthquake impacts, several logical questions arise.
Can this be compensated?
What are the main directions in which engineering thought should be focused in order to achieve acceptable levels of security?
At the same time, how to preserve the original vision and harmony, especially in cultural monuments?
The task is further complicated by the financial factor and the variety of masonry and joists. Undoubtedly, for each project an individual solution must be sought according to the specifics. Several possible variants are shown in the illustrations below, and their application must take into account the desired results and the initial parameters of the structure. Thus, for example, it is not reasonable to apply a concrete fill if the walls of the building are not able to absorb greater transverse forces due to the increased own weight of the slab.
The decision to select an amplification system must be motivated by the results of a thorough on-site analysis. There are many cases in practice in which the performed reinforcing measures have a rather negative overall effect on the structure.
Important moments when repairing the joists
One classic solution, widely used in our practice, is concrete filling combined with reinforcing mesh and anchoring elements.
Such a solution increases the qualities of the floor construction, but adds extra weight on the vertical elements. Detailing the connections between the old structure and the reinforced concrete fill is a key point that in most cases incompetent contractors miss.
During construction, the monolithic construction of the existing beams in the masonry was carried out only with masonry mortar. The created connection is fragile and transmits horizontal forces only in one direction. In order to increase the bearing capacity of the joist in all directions, its planar hardening is necessary. Such systems consist of additional fasteners transmitting forces between the individual elements of the joist and leading them securely to the walls intended for earthquake absorption. Designing such a system is a delicate task. The omission of some moments in the solution and leads to a significant reduction in the efficiency of the repair.
Thermal insulation, fire protection
In the planned repair measures it is good to consider the issue of reducing the weight of the floor structure. Replacing the filling between the beams of the joists with modern thermal insulation and sound insulation materials is a cost-effective and logical solution. Given the location of the new thermal insulation, it is desirable that it be made of non-combustible and non-toxic material such as glass or mineral wool.
Last but not least, during the constructive repair of the joist, it is important to take modern measures for its fire safety. The Bulgarian norms regulating the fire protection of wooden structures are Eurocode 5 - Part 1.2: Design of structures against fire. One successfully applied method for passive fire safety of wooden elements is cement or gypsum plaster with a thickness of 20-30 mm, laid on a netting.