For Engineers

Hp/A, is a method of calculating the steel section factor. It is the heated perimeter to cross sectional area ratio of a steel member. Hp/A gives an indication of the steel heating rate and governs selection of steel sections that can achieve the required FRL. Generally, a larger, thicker steel member will have a lower Hp/A and thus have a lower heating rate than a smaller, thinner steel member. This is important to consider because members with high Hp/A will require more passive fire protection than members with low Hp/A.

When limiting steel temperatures are not provided for structural steel work, the standard limiting steel temperatures of 550ᵒC or 620ᵒC are used as default. It is important to ensure structural steel limiting steel temperatures are confirmed with the project structural engineer.

Limiting steel temperature, also known as the critical steel temperature is the temperature which a structural member must be prevented from reaching, in order to retain its required strength capacity. Limiting steel temperatures can be calculated as per AS4100 cl 12.5.

The structural loading case used in a fire case can be determined in AS1170.0. It is different from the ultimate load case structural loading. For example, the ultimate limit state loading for beams is 1.25G + 1.5Q, where G is the designed dead load, and Q is the designed live load. Under a fire load case the design load which structural engineers determine is G + Ψc Q. Where Ψc is the combination factor determined by the use of the building.

Load ratio, rf, is calculated based on the design load under a fire load case over the design member capacity at room temperature. The load ratio determines the limiting steel temperature as per clause 12.5 in AS4100.1998 – Section 12 Tl= 905 – 690rf where Tl is the limiting steel temperature.

For example, a 460UB67.1 beam with 399kNm design flexural strength capacity at room temperature and 130kNm design bending moment in a fire environment will have a load ratio of approximately Rf = 0.33.

Core-filling is the process of adding a grout or concrete mixture of 40MPa or less to a hollow structural steel section to create a composite column. The purpose of core-filling is to reduce the section factor of the structural steel, allowing it to achieve a higher FRL rating with a thinner coat of intumescent.

Connections are usually fire-rated to the same dry film thickness of the primary member directly attached. Connections are usually masked during off-site spraying, and are sprayed on-site after the steel is erected. Permax have pre-formed intumescent bolt caps that can also be used to fire rate steel bolts.

There are many possible configurations of plasterboard sheeting used in conjunction with Nullifire SC902 coated steelwork. Please contact Permax for detail specific advice.

Coat-back is a process of coating an area beyond the specified protected structural steel member. It can be required when fire protected steel is connected to steel which is not fire protected. In this case it is recommended 500mm coat-back is provided to the non- fire rated element to prevent heat transfer to the member which requires fire protection. Follow this link to read more about coat-back in an advisory note. Link?