Intumescent paint coating frequently-asked questions for engineers

Steel section factor, also known as Hp/A or ksm, is the ratio of perimeter to cross sectional area of structural steel members.

Hp/A gives an indication of the heating rate and dictates selection of the steel sections that can achieve the required FRL.

Generally, a larger, thicker steel member will have a lower Hp/A and lower heating rate than a smaller, thinner steel member.

This is important to consider because structural steel members with high HP/A will require more passive fire protection than those with low a Hp/A.

See more on steel section factor here.

You can also click the link below to download a comprehensive guide from Promat.

The standard critical steel temperature reflects a conservative estimate of loads based on the most common loading scenarios.

For floor loads, we assume columns are the most heavily loaded elements, requiring a load ratio of 0.51, beams require a load ratio of 0.4 and members supporting a roof load requiring a load ratio of 0.3.

Critical steel temperature is calculated based on these varying load ratios so it varies - columns are 550˚C, beams are 620˚C and roof members are 720˚C.

The standard structural loading used in a fire case can be found in AS1170.1.

It is different from a standard structural loading.

For example, a standard limit state loading for beams is 1.25G + 1.5Q, where G is the designed dead load and Q is the designed live load.

In a fire case the standard loading for beams is G+0.4Q.

Rf load ratios are calculated based on the member capacity at room temperature and the designed member capacity in a fire environment.

The load ratio determines the limiting steel temperature based on equation 12.5 of 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.

Core-filling achieves this by increasing the thermal insulation property and structural compressive capacity of the composite section.

See more about core filling here.

Core-filling is possible even for small structural hollow section sizes as low as 75X75X3 SHS and 114.3X3 CHS.

Connections are usually fire rated to the same dry film thickness of the primary member directly attached. They are usually masked during off-site spraying and are sprayed on-site after the steel is erected.

We strongly recommend that primary steel should not be penetrated.

In some cases, however, it is possible to use low diameter studs to penetrate protected steelwork in order to support light metal framing, light panels, window fixings, etc.

It is important to consult the intumescent coating product manufacturer if you are planning on penetrating structural steel elements.

The minimum gap for intumescent expansion is 30mm.

This is the approximate distance for a thin layer of intumescent to activate before lightweight elements such as plasterboards and unprotected members begin to fail and fall off, allowing the rest of the intumescent to char and expand.

Metal stud framing and lightweight elements can be directly fixed to structural steel.

The main condition is the frame itself must be coated with intumescent using the same dry film thickness as the primary structural steel member.

Please refer to this linked file below from Nullifire for more details.

Indicative tests indicated that plasterboard encasement does not degrade fire performance of intumescent protected steel but rather, improved it.

We can use the standard product loading tables for Nullifire SC902 with encased sections for both columns and beams.

The only condition being that the plasterboard cannot be mechanically fixed directly to the protected steelwork.

Refer to Nullifire Australia technical advisory note linked below for more details.

Coat-back is a process of coating an area beyond the specified protected structural steel member.

It is mainly used when an intermediate zone between protected structural steel and unprotected steel, other substrates or other passive fire protection systems.