METALWORK FIREPROOFING

When using metalworks to build a frame for a building, it is important to pay special attention to fireproofing the load-bearing frame.

The fire-resistance rating describes the structural element’s capability to retain its load-bearing qualities during a fire. Normally, metal is not flammable, but it is a perfect conductor of heat. During a fire unprotected metalworks can quickly reach the critical temperature of 480 degrees Celsius (their fire-resistance rating is around a quarter of an hour) and lose their rigidity. Increasing the fire-resistance rating assists the evacuation of people out of the burning building.

According to a study, the collapse of the World Trade Center towers during the terrorist attack in New York was caused by a fire that started after the planes crashed into the buildings, since the metal frame of the towers was not protected against high temperatures.

Fireproofing of metalworks is done by creating non-flammable insulating layers on the surface that slow down the heating of metal.

One of the most prominent methods consists of coating the metal surface with a layer of inflatable fire-proof mixture, that increases the fire-resistance rating to 3 hours. There is a broad variety of such mixtures based on water and organic solvents. A layer of 2 mm is applied on a prepared stretch of metal surface. When temperature rises, the paint inflates and its volume increases several dozen times over.

Different types of wall plaster are also used to fireproof metal surfaces. This is an effective method that increases the fire-resistance rating up to 3 hours. Its drawbacks are the relative complexity of the works, significant weight of the plaster layer and the need for a frame of wire mesh to support sand-cement plaster. Metal covered with plaster must also be protected against corrosion. A series of lighter plasters based on asbestos, perlite and vermiculite has been developed recently. When necessary due to aggressive environment or aesthetic concerns, it is possible to cover the fireproof layer with finishing materials, a list of which should be provided by the manufacturer in the manual.

And effective but labor-intensive method is to cover metal structures in fireproof slabs and brick walls, which increases fire-resistance rating to 3 hours and more. Slabs are usually made out of slag wool and are 5 to 10 centimeters thick or more. Fireproofing may be done in several layers and with combined methods. When using several types of materials, inner layers are made softer, while outer layers are harder.

The choice of the fireproofing material depends on the fire-resistance rating required, the complexity of the structure and aesthetic considerations and usually is determined by a thorough techno-economical study.

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