Basics of the metalwork fire protection

Basics of the metalwork fire protection

Load-bearing metalwork including light-gauge steel structures (LGSS), being an indispensable attribute of modern construction, lose their bearing capacity during fire.

Under effect of high temperature, steel elements become ductile during the certain period, linear thermal deformation occurs that causes changes in the dimensions of structures, their curling and fracture.

The study of the steel strength change at high temperature shows that at a temperature of 500÷550°C steel is able to withstand only 60% of the load applied thereto during normal operation. The temperature value 500°C is set as the critical steel temperature, at which structural elements are able to support the load. At the same time, calculations and studies show that the temperature, at which a steel construction loses its bearing capacity a fixed value; it varies depending on two factors – steelwork heating temperature and applied load.

To ensure the necessary requirements for fire resistance of metalwork set by applicable building codes, steel structures undergo the fire-retardant treatment, which is a quite important phase of steel construction. For the owner of the building, effective fire protection of steelwork allows to provide security during the evacuation of people from buildings, to maintain assets and to comply of the construction facility with all requirements of applicable law. To solve these issues, the various algorithms and methods are used for fire protection of metalwork; and the right solution permits to minimize costs and maximize the flame retardant performance during project implementation.

Classification of fire protection
Fire protection approaches are divided in two basic groups – active and passive.

Active methods of fire protection are analogue addressable systems of fire-fighting bodies - fire alarm systems, automatic extinguishing systems - water sprinkler systems and automatic smoke exhaust systems.

Passive methods involve application of cladding and heat-insulation coatings, which fire-resistant effect comprises thermo-physical properties of applied protective material, as well as reactive-type coatings, which bulge during thermal impact forming the heat-insulating coke layer on the surface to be protected.

The most common materials used in passive fire protection are structural fireproofing materials (plates, segments, shell, ceramic stoneware and blocks), fire-resistant plasters and thin-layer reactive intumescent coatings.

The way and particular mean for fire protection of steelwork are defined during designing of the particular facility taking into account the following conditions:
  • Required class of the metalwork fire resistance according to the degree of the building fire resistance
  • Steelwork type and arrangement in space
  • Limitations on the load of the fire-retardant coating over a structure
  • Conditions of carrying out of construction, assembly and fire protection works
  • Required timing of the fire retardant treatment
  • Aesthetic and architectural appeal
  • Environmental characteristics of the fire retardant coating
  • Operating conditions of the fire-retardant coating
  • The cost of fire-retardant treatment, which includes the price of the fire retardant material and the cost of works on fire protection
  • Comparing the methods of passive and active protection, it should be noted that they have fundamentally different purposes and differ significantly in terms of economic parameters.
The most widely used passive fire protection can be divided into the following types:

Reactive method of fire protection means application of thin-layer coatings, which form a dense insulating layer under effect of fire and protect the structure from thermal impact. Thermal transformation processes in these coatings are accompanied with chemical reactions, in which the substances are formed that retard the combustion process. These flame retardants called intumescent (swelling, heat-expanding) thin-layer materials. Reactive coatings are presented by two main groups: intumescent polyphosphate compositions and ones based on thermal-expansion graphite.
Fire-resistant dry construction mixtures (plaster, sprays) are (as a rule) cement(gypsum)-vermiculite compounds with a number of special additives, which form a coating with high adhesive power to metal surfaces and relatively low density. These compounds are delivered as dry construction mixtures and applied on metal surfaces by mechanical means after mixing with water.
Fire-resistant plates and fiber sheet materials relate to the constructive methods, which fire resistance consists mainly in thermo-physical properties of the material used. This method of fire protection gains more and more positions in the fire protection practice due to its decorative, environmental and operational characteristics.
Combined methods are applied in fire protection to solve complex and non-traditional engineering problems. They are a combination of flame retardant materials of different kinds, for example: heat-resistant fiber plates with coatings based on mineral binders or intumescent coatings; fiber insulation materials with gypsum wall boards; insulating materials with fire-resistant cement-vermiculite plates, etc.

Cladding by concrete or by bricks and insulation boards is performed using traditional construction materials; it may provide sufficiently high requirements for fire resistance. This method of fire protection is virtually not used in newly constructed buildings and is applied in renovation and restoration works to strengthen structures that have lost their strength properties due to prolonged operation.

Concrete, bricks, gypsum plasterboards and other tiled and sheet products as well as sand-cement plasters are used lining materials for the metalwork fire protection.

Application of concrete as the metalwork fire retardant often used, especially with simultaneous reinforcing of collar beams, pillars and struts. Embedding in concrete, as usual, is performed after attachment of reinforcing net to the product.

The advantages of concrete and brick cladding used as the fire protection consist in increased moisture resistance of the fire-proof materials, which significantly expands their operating conditions: this retardation method can be used at almost any temperature and humidity fluctuations, while exposing by corrosive environment, rainfall and dynamic loads.
Fire resistant cladding made of gypsum plasterboards has become increasingly popular in the buildings with steel support frame, with intermediate floors made from precast concrete slabs or monolithic concrete. These structures are significantly lightweight as compared with brick or concrete cladding; they are more efficient from the viewpoint of fire resistance. When using plasterboards, it is permitted to dismantle the fire resistant lining and to perform various works on reinforcing supporting structures as well as repeated application of corrosive-protecting coating on load-bearing structures of the building. The internal cavity between fire-protection elements and the supporting structure can be used for mounting a variety of utilities.

Below are the main characteristics and application fields for fire protection methods based on their characteristics

Fire resistant plasters Fire-resistant slabs Intumescent-type paints
Fire resistance class up to R240 до R300 до R90
Operating conditions by ETAG 018а) Y, Z1, Z2 X, Y, Z1, Z2 Z1 (with protective coating), Z2
Smoke removal factor, m2/g 0,5 - 1 0,5 - 1 500 - 700
Advantages The high limit of fire resistance, low material cost, environmental compatibility during operation and the absence of toxic combustion products, the possibility of outdoor application High level of fire resistance and long service life, high vibration resistance due to mechanical fastening to the structure, repair ability, no corrosive effect on metal, good decorative properties, environmental compatibility and the absence of toxic combustion products, accurate control of the thickness of the fire-protection layer, the dry assembly method Minimum thickness and low weight load on the structure; adaptability of operations on fire protection, repair capability, vibration resistance, good decorative properties
Disadvantages Shortages significant working time during application; complexity during recovery and repair, low decorative features, weak adhesion to the surface, decreased resistance to vibration The need to arrange fastening systems and elements, limited use for fire protection of structures with complex configuration  Limited operating conditions and fire resistant performance, high toxicity of combustion products
Field of application For structures with simple configuration (pillars, beams) For structures with simple configuration (pillars, beams) For structures with any configuration (pillars, beams, strings, cross-beams, lattice girders, bracing)
а) Type of operating conditions of fire protective coatings:
type X - in any environment (both indoors and outdoors, in ambient conditions); type Y - indoors or in semi-enclosed rooms with partial effect of the environment (temperature below 0°C, limited influence of ultraviolet radiation), but without effect of rain; type Z1 – inside rooms with high humidity, except the rooms intended for operation at a temperature below 0°C; type Z2 - indoors without the impact of high humidity, except the rooms, which are designed for operation at a temperature below 0°C.