Structural adequacy, integrity and insulation are the three components of a Fire Resistance Level (FRL).
The National Construction Code Volume 1, otherwise known as the BCA, demonstrates the fire protection requirement for different buildings. Depending on the type of construction, in some cases where the structural elements function as compartment separating elements, integrity and insulation may be critical also. However, it is important to note that most structural steel fire protection can only achieve the structural adequacy component of an FRL (e.g. 60/--/-- or 120/--/--), as structural components are designed to withstand the building loads, rather than separate compartments.
Fundamentally structural adequacy of a building when exposed to fire, requires structural engineers to ensure the loads acting on a structure in a fire event can be resisted by the capacity of the structural members and their connections at elevated temperatures. This involves understanding the effects of thermal expansion on the structure, thereby ensuring the structure has enough capacity and robustness to allow occupants to evacuate, fire fighters to supress the fire and safeguard fire spread to surrounding buildings.
Load bearing structural elements can be made of many different building materials. Common material types include reinforced concrete, timber and the material most commonly requiring passive fire protection, structural steel. These materials all behave differently in a fire event, depending on their inherent thermophysical and material properties. Steel begins to lose its strength at 215˚C. Timber chars when the surface reaches 300˚C, resulting in a reduced effective cross-section. Concrete has a low thermal conductivity which is critical in providing insulation to steel reinforcement thereby limiting the loss of strength in the composite member.
Without fail, structural engineers document the required concrete cover to insulate reinforcement in a fire event or oversize timber members to allow for char loss within their design process.
Why is it not common practice in Australia, for structural engineers to document the same design detail when it comes to structural steel fire resistance?
The attached PDF outlines the methods structural engineers can adopt to assist in documenting the required structural details for structural steel members. Click the link below to download the full PDF now.
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