Wood and Plastic Structures

2 Structural and chemical measures to protect against decay and combustion

The principle of structural protection of wooden structures against rotting is the creation of a temperature-moisture regime for wood, at which it is ensured that its humidity is maintained below 20% for the entire duration of operation. 

To do this, you need to carry out the following constructive activities. Bearing wooden structures shall be open, well ventilated and accessible for periodic inspection. It is necessary to ensure reliable waterproofing of wooden structures and their parts in contact with soil, foundations, concrete, masonry and massive metal parts. Since condensation is possible in the thickness of the enclosing elements located in the zone of temperature change, the bearing wooden structures should be located either entirely within the heated room or outside it. Paving panels and walls of empty structure shall not have wooden elements in the low temperature zone. Hollow enclosing structures shall have drying ventilation products ensuring fast drying of wood. At the same time, cold dry air is introduced under the cornice, and raw and warm is released at the skate. 

Wooden coverings should be made by external discharge of atmospheric water. Wooden walls are protected from oblique rain and snow by a wide crowning cornice or wide overhang. The ends of the bars or logs are protected from moisture penetration by planks.

Wooden coverings are not recommended to be arranged with top lights.

Structural measures to protect wood from rotting are not sufficient when using wooden structures under conditions of constant or periodic humidification. For such wooden structures, antisepting is the main protection against rotting, designed for the entire life of the wood. 

Antiseptic treatment of elements of wooden structures and products should be carried out in production conditions with specialized equipment. If it is impossible to centrally supply construction with elements of wooden structures chemically protected from rotting, it is allowed to carry out antiseptic treatment of wood at the construction site by mechanized, and in some cases by manual, methods. Before antiseptic treatment, wood must be cleaned from bark and lube. All mechanical processing of timber (sawing, hole drilling, etc.) is carried out before antisepting. The type of antiseptic treatment of wood is chosen depending on the operating conditions of wooden structures. When choosing an antiseptic, as well as determining the amount and method of its introduction into wood, it is recommended to be guided by the following brief information on the most effective types of antiseptic treatment of wood.  

Antiseptics are divided into three groups : oily , organo-soluble and water-soluble.

Wood burning occurs as a result of its heating to a temperature at which its thermal decomposition begins to form combustible gases containing carbon. Thus, wood as an organic material is combusted. However, due to the low thermal conductivity, the combustion of large elements is long limited by the outer layers, and they have a sufficient fire resistance limit - a very important indicator for successfully extinguishing a fire. It is determined by the time at which the loaded element retains bearing capacity at the fire temperature. Wood elements of large sections have higher fire resistance limits than others. For example, a bar beam with a cross section of 17X17 cm, loaded to a voltage of 10 MPa, has a fire resistance limit of 40 minutes, during which measures can be taken to extinguish the fire.

Wood fire and the spread of fire are impossible without certain favorable conditions. Prolonged heating at a temperature of 150 ° C or fast at a higher temperature can lead to ignition of wood. Ambient air enriches the oxygen combustion process and promotes flame propagation. Elements of structures consisting of separate boards with gaps between them are heated faster to a dangerous limit than monolithic ones, have large surfaces of contact with air and adjacent surfaces mutually heated by radiant heating. As a result, their fire resistance limit is significantly lower than that of monolithic elements.

Fire protection. The purpose of fire protection is to increase the fire resistance of wooden structures so that they resist fire longer and do not create or spread open flame during combustion. This is achieved by measures of structural and chemical protection of wooden structures from fire.

Structural protection of wood against fire consists in elimination of conditions favorable for fire occurrence and expansion. In structures of industrial buildings with hot processes, the use of wood is unacceptable. Wooden structures shall be separated from furnaces and heating devices by sufficient distances or fire resistant materials. To prevent the spread of fire, wooden structures should be divided into parts by fire barriers and fire-resistant areas. Wooden enclosing structures shall not have communicating cavities with the air rod, through which flame can spread, which is inaccessible for extinguishing. Elements of wooden structures must be massive glued or paved, having greater fire resistance than plank. Ordinary plaster significantly increases the resistance of wooden walls and ceilings to fire.

Chemical protection against fire is carried out in cases where an increased degree of fire resistance is required from enclosing wooden structures, for example, in premises where crowding is possible. It consists in fire-fighting impregnations and painting. For fire-retardant impregnation of wood, substances called flame retardants are used. These substances introduced into the wood melt or decompose under dangerous heating, covering it with fire-retardant films or gas shells preventing access of oxygen to the wood, which at the same time can only slowly decompose and melt, without creating an open flame and without spreading fire. Impregnation of wood with flame retardants is carried out under pressure in autoclaves, usually with simultaneous impregnation with antiseptics. Protective paints based on liquid glass, superphosphate and other substances are applied to the surface of the wood. When heated during a fire, the films swell from the emitted gases and create an air layer that temporarily prevents fire.

Calculation of the protective structure of the coating

We accept the ribbed structure of the plate with 4 longitudinal ribs of the frame. The upper plywood skin is adopted with a 7-layer thickness of 10 mm, the lower 5-layer thickness of 8 mm. To keep the insulation in the design position, we accept a grid of 25 x 25 mm bars, which is attached by nails to the ribs.

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