For Engineers

Passive fire protection is a life safety system and as such should be designed in a compliant manner. It is important for structural and fire engineers to understand the compliance requirements of each fire protection system so that appropriate measures can be enforced into their design. For instance, structural engineers are required to specify the limiting steel temperatures required for steel elements to ensure structural adequacy can be maintained. Fire engineers may also be required to ensure the adequate FRL has been achieved with the fire safety systems specified in a project. Discussing the issues of compliance ahead of time with Permax can assist throughout any project life cycle.

Passive fire protection is a crucial life safety system that must be designed and installed in compliance with relevant standards and regulations. Structural and fire engineers must have a thorough understanding of the compliance requirements for each fire protection system, so they can incorporate appropriate measures into their designs. For example, structural engineers must specify the limiting steel temperatures required for steel elements to ensure that structural adequacy can be maintained during a fire. Fire engineers may also need to ensure that the fire resistance level (FRL) has been achieved with the fire safety systems specified in a project.

Here is how we can help you throughout the time line of your project:

Detailed Design:

  • Obtain limiting steel temperatures and ensure all specified steel can achieve required FRL
  • Review design details of intumescent coated members to ensure appropriate expansion and heat transfer principles are designed for
  • Provide advice on where more than one passive fire protection system is used on a design

Tender Phase:

  • Steel schedule is developed by the Permax technical team with intumescent DFT loadings across the project
  • One of the Permax approved applicators can provide a tender quote based on the product and application cost.

Construction Phase:

  • Ensure passive fire protection installers are aware of the compliance and specific fire rating requirements, for instance coat back and direct fixings.


What is Hp/A and how it is relevant to passive fire protection?

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.

Why steel limiting/critical temperatures of 550ᵒC and 620ᵒC are often used?

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.

What is limiting steel temperature?

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.

What is the structural loading case used in a fire case?

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.

How are the rf (load ratios) calculated, and what limiting steel temperature should I use?

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.

What is Core filling?

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.

How are connections fire rated?

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.

Can plasterboards be positioned hard up against the protected steel? What are the limitations?

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

What is coat-back and where it is used?

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?


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You are about to download a file from the Permax site. Please note All technical advisory notes generated by Permax are based on research papers, indicative fire tests and any other existing evidence. These documents should not be used as an official evidence as design engineers should review the information and determine the reliability of the documents.

Permax constantly update the documentations based on the new fire testing outcomes and change of standards and regulations. To ensure the documents you read are up-to-date, please contact the Permax technical team.