When it comes to new building construction, there is a predominant focus on keeping construction costs as low as possible while still satisfying the performance requirements in the building codes. Regarding fire rating, we are often asked whether the structural steel section is sufficient in achieving the proposed Fire Resistance Level (FRL) without using any fire protection. Although on most occasions, the answer is no, several considerations still need to be made before jumping to the conclusion. In this piece, we dissect why for you.
To answer the question, we will need to look into two different areas: the Performance Requirements in the National Construction Code (NCC) and the fire performance of the member associated with the limiting steel temperature.
It is essential to satisfy the Performance Requirements in the NCC to make the building complies with the Australian construction codes. Depending on the project structure, the person/party in charge could vary. For example, suppose the building surveyor/certifier is responsible for informing the parties about the fire performance, and they are likely to go with the requirements in the NCC, that is, disregarding the inherent fire performance of the unprotected steel, the steel member should still have fire protection on even if it is only 30/-/-FRL. It is a more conservative approach as it ensures the building would satisfy the FRL requirement.
On the other hand, if the structural engineer is in charge of the structural steel fire protection, then, in terms of fire performance, there is a possibility that the steel’s inherent fire performance can remain below the limiting steel temperature over the fire period. To do this, we will need to look at the time-temperature profile of unprotected structural steel firstly.
Temperature Profile of Structural Steel
The below time-temperature graph for unprotected steel under the standard heating condition was developed based on the European Standard EN1993-1-2 due to the lack of information in the Australian Standard.
The section factor (HP/A) is the factor that represents the fire performance of a steel section; the higher the figure is, the worse fire performance it is. The typical steel sections used in the Australian Construction Industry are usually within the 200 – 350HP/A range. According to the graph, these members are extremely close to the standard heating curve displays in AS1530.4 when they are unprotected, making structural steel nearly impossible to achieve the required FRL without using any fire protection material. However, in some rare scenarios, it is achievable.
In every building, the loads or forces acting on the structural members varies depending on the building design. Some structural members would experience a significant amount of load, and some would experience less, which causes the difference in limiting steel temperature for structural steel (refer to Permax advisory note PTA005). In some infrequent cases, the limiting steel temperature of a member could be above 800°C. If we look back at the time-temperature graph for the unprotected steel, the section factor is 100HP/A with a limiting steel temperature above 800° (let’s say 850°C). It is likely the unprotected steel temperature would still be lower than the limiting steel temperature at 30 minutes, which means, theoretically, the unprotected steel will still be able to bear the loads acting on it for the first 30 minutes in a fire event. Under this scenario, if the structural engineer is satisfied with this theory, they might not need to fire these steel members. However, it is crucial to understand that such an approach will make it a Performance Solution instead of a Deem-to-Satisfy Solution.
Passive Fire Protection for Structural Steel
Disregarding the NCC requirements on fire protection, it is essential to understand the purpose of implementing passive fire protection to the structure to reduce damages on properties and save lives. With this, the building designer (primarily structural engineer) will need to understand how the selected product works to ensure it is suitable for their design structure and capable of providing the required performance. The better understand the engineer has, the more flexible design they could produce and potentially come up with a more cost-effective solution.
If you would like to have further discussion about structural steel fire protection or inquire about different products, please give us a call. We have a dedicated team with rich passive fire protection knowledge and experience with varying products of painting. We trust that our professionals will provide you great benefits.
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