Fire Resistance of Steel Framed Buildings – Site Applied Protection Materials

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Article updated on 19/02/20 by Brett Smith and Clare Kiernan

According to the International Building Code (IBC), steel structural frames should be able to withstand two hours of fire-level temperatures, typically around 1000 °F (538 °C). The IBC does lay out the various kinds of fire resistance measures that facilities should include.

While steel is more fire-resistant then building materials such as wood or vinyl, steel can still sustain significant damage from the temperatures associated with fire, and the results of such damage to a steel structure can be disastrous. Steel loses around one-half of its load-bearing capacity after it has been subjected to temperatures greater than 1100 °F (590 °C.). Once a steel structure can no longer hold the weight that it is supposed to, it will collapse, possibly resulting in the total collapse of a building. Therefore, it is extremely important to fireproof a steel structure as much as possible.

IBC fire protection requirements are based on four factors: intended use, type of construction, intended occupancy and location. Certain uses, such as the storage of toxic or combustible materials, call for much greater fire protection than others. The degree of occupancy is an obvious factor as fire protection relates directly to saving lives. Structures that are located close to other buildings also require higher degrees of protection than structures located far away from other buildings or properties. 

Passive fire protection materials insulate steel structures from the effects of high temperatures that are generated by fire. They can be divided into two types: non-reactive, of which the most common types are boards and sprays, and reactive, of which intumescent coatings are the best example.

Boards

Board systems are the most popular type of fire protection in the UK. They are widely used both where the protection system is in full view and where it is hidden.
Board systems are a type of external cladding that can be made of fire-resistant materials. As a fire resistance measure, board systems have been popular for years. However, significant safety issues have recently been associated with their use, and now only certain types of boarding have been deemed safe for new construction. 

The principal advantages are:

  • Appearance – rigid boards offer a clean, boxed appearance which may be pre-finished or suitable for further decoration. 
  • Installation – The application is dry and may not have significant effects on other trades.
  • Quality assured – boards are factory manufactured thus thicknesses can be guaranteed.
  • Surface preparation – boards can be applied to unpainted steelwork.

The principal disadvantages are:

  • Safety – as noted above, serious safety concerns have been raised with some types of boarding systems.
  • Cost – a non-decorative board system can be relatively cheap however a decorative system can significantly increase costs.
  • Application – fitting around complex details may be difficult.
  • Speed – board systems may be slower to apply than some other methods.

Sprays

Often containing minerals and ceramics mixed with building materials such as gypsum or concrete, spray-on coatings convey fire resistance to wherever they are applied. There is a wide range of fire-resistant coatings and sprays available. Building designers, contractors, and managers should only use sprays that are rated for their particular facility, with ratings being based on the amount of time it takes for protected steel to reach 1000 °F in fire conditions.

Spray protection systems have decreased in popularity in the past decade, despite being the cheapest in terms of application costs.

The principal advantages are:

  • Cost – spray protection can usually be applied for less than the cost of the cheapest board. Because the cost of sprayed material is low compared to that of getting labor and equipment on-site, costs do not increase in proportion to fire resistance times.
  • Application – it is easy to cover complex details
  • Durability – some materials may be used externally
  • Surface preparation – some materials may be applied to unpainted steelwork

The principal disadvantages are:

  • Appearance – sprays are not visually appealing and so are usually used only where they are not visible.
  • Over spraying – masking or shielding of the application area is usually required on-site.
  • Application – this is a wet trade, so it can have significant knock-on effects on the construction program resulting in the real, total cost of spray protection being significantly higher than just the application costs.

Thin Film Intumescent Coatings

Intumescent coatings are hydrate-containing, paint-like substances that are inert at low temperatures but provide insulation by swelling to provide a charred layer of low conductivity materials at temperatures of approximately 400-480 °F (200-250 °C). At these temperatures, the properties of steel will not be affected.

The principal advantages are:

  • Aesthetics – the thin coating allows the shape of the underlying steel to be expressed.
  • Finish – attractive, decorative finishes are possible. Also, coatings can be painted over to match a building’s interior or exterior.
  • Application – complex details are easily covered
  • Servicing – post-protection fixing is simplified

The principal disadvantages are:

  • Cost – typical application costs are higher than sprays and generally comparable with board systems.
  • Application – this is a wet trade that requires suitable atmosphere conditions during application and precautions against overspray.
  • Limited Fire Resistance Periods – most intumescent coatings can provide between 30 and 60 minutes of fire resistance. Some can be sustained for longer periods, i.e. between 90 and 120 minutes; however, the cost increases considerably for periods over 60 minutes.

Flexible/Blanket Systems

Flexible fire protection systems have been developed as a response to the need for a cheap alternative to sprays but without the adverse effects on the construction program often associated with wet application. Mineral insulation blankets are effective in temperatures up to 2000 degrees °F (1090 °C). 

The principal advantages are:

  • Low cost – blanket systems are comparable with cheap boards
  • Installation – The application is dry and may not have significant effects on other trades.

The principal disadvantage is:

  • Appearance – unlikely to be used where the steel is visible.

Concrete Encasement and Other Traditional Systems

Until the late 1970s, concrete was by far the most common form of fire protection for structural steelwork. However, the introduction of lightweight, proprietary systems such as boards, sprays, and intumescents has seen a dramatic reduction in concrete use. At present concrete encasement has only a small percentage of the fire protection market with other traditional methods such as blockwork encasement also used occasionally.

The principal advantage of concrete and blockwork is:

  • Durability – these robust encasement methods tend to be used where resistance to impact damage, abrasion, and weather exposure are important e.g. warehouses, underground car parks, and external structures.

The principal disadvantages are:

  • Cost – concrete encasement is normally one of the most expensive forms of fire protection.
  • Speed – time-consuming on-site
  • Space utilization – large protection thicknesses take up valuable space around columns.
  • Weight – building weight can increase considerably

Source: Corus

Additional References

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