How to calculate the thickness and select a fire protection system for steelwork

Approved Document B of the Building Regulations require certain elements of a structure to have a fire resistance for a specified minimum period. The main objective being to protect the structure so that the occupants can escape safely from the building and so that fire fighters can enter the building to save the lives of occupants that may have become trapped. The more difficult for the occupants to escape, the longer the fire protection period required.  This will be dependent on the type and use of a building and the height and number of storeys.

The function of fire protection is to reduce the speed at which the steelwork will heat up in a fire and individual members lose their strength characteristics. A steel frame is typically designed in a cold state to have a plus 50% safety factor. This level of redundancy ensures that the building will remain safe during the life of a building. During a fire the steel will lose the strength as the temperature begins to rise. Typically, at a temperature of 550 deg C a steel member will have lost 50% of the strength and the built-in safety factor has been removed and there becomes a risk that the steel will start to buckle, lose strength and building collapse. This is called the critical temperature. The critical temperature is not always 550 deg C and a structural/fire engineer may either decrease or increase the critical temperature by calculation depending on the load being applied on the structural members. Therefore, it is important when first pricing the installation of fire protection materials that the critical temperature is confirmed.

A steel member will also have a built-in fire resistance, the bigger the size and thickness the longer the steel will take to heat up and strength reduce. This built-in fire resistance is taken into account when calculating the level of fire protection by either calculating or looking up in tables the section factor (A/V) for that steel section.

The thickness of the fire protection material required to achieve the minimum fire resistance will depend on the following;

  1. Period of fire resistance specified
  2. The section factor of the steel(A/V). This is based on the size of the steel and exposure to the fire. The A/V ratios for steel sizes can be manually calculated or looked up in tables. Manufacturers literature or the ASFP Yellow Book, which can be downloaded via their website, contain tables listing the A/V ratios of most standard steel sizes.
  3. The thickness based on the steel limiting temperature for the steel as advised by the structural/fire engineer. The data will be included in tables within manufactures literature or the ASFP Yellow Book.

There are many options which could be specified to provide protection to steel work. These will depend on location with the building, aesthetics and installation cost. Many of these methods will have limitations because the fire protection board may need to be suitable for use in an area where the steel is in a semi exposed location or in areas where there is potential for moisture in the atmosphere.

Ken Francis, Technical Support Manager for Promat, gives an example:  A reactive or intumescent paint requires a void around the steel of 50mm for the product to intumesce.  This makes this solution impractical when protecting steel within, or behind the external building envelope.

Promat  boards have a long track record of use in semi exposed areas, will not degrade, have a high compressive strength and will provide fire protection to the steel work on all four sides for the design of the building.  It is possible to design solutions where the external building envelope can be fixed through the fire protection board to the steel work. The boards can also be left exposed to the weather during the construction phase before the envelope is fixed ensuring there are no delays to the build programme.

Promat Supalux®, Promat TD®, or Promatect® L500 can provide you between 30 minutes and 240 minutes fire resistance and the peace of mind that your specification will fully protect the external steel work.

For more information on CCF’s range of products and suppliers, view our product range here.

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