Fire resistance of unprotected metal structures.
This concept is crucial when assessing the fire resistance of steel structures that require fire protection treatment to achieve a rating of R15.
According to SHNK 2.01.02-04 "Fire Safety of Buildings and Structures", roof structures in buildings classified as fire resistance level II must undergo fire protection treatment if the intrinsic fire resistance rating of each individual roof element is below R8.
The intrinsic fire resistance limit refers to the duration a metal structure without any fire protection can maintain its load-bearing capacity during a standard fire event.
It’s common during the design phase for roof load calculations that clients, aiming to reduce costs, instruct designers to use minimal metal profiles. Often, a single 50x50x6 mm angle bar, with its intrinsic fire resistance rating below R8, can compromise the entire structural integrity under fire conditions. This typically forces the client to incur additional expenses for fire-retardant treatment of the roof.
However, our experience and calculations demonstrate that opting for larger, more robust metal grades can be more cost-effective. This approach not only meets the requirements of SHNK 2.01.02-04 "Fire Safety of Buildings and Structures," but also eliminates the need for costly fire-retardant treatment of the roof.
The book "Fire Resistance of Building Structures" by I.L. Mosalkov, G.F. Plyusnina, and A.D. Frolov (edited by V.I. Kuznetsov, Moscow, Spetstekhnika, 2001) provides data on how the temperature of unprotected metal structures of varying steel section factors changes over time under standard fire conditions (standard test curve of temperature rise versus time).
The graphs below illustrate these dependencies.
Heating time’s effect on unprotected steel structure element temperatures.
The heating process depicted follows the "standard" fire conditions, represented by curve 1.
The numbers on the curves correspond to different section factors.
Using these curves, the actual fire resistance limit of unprotected steel structures can be determined based on the loss of load-bearing capacity upon reaching a temperature of 500 ºC. The intersection of the curves with the 500 ºC temperature line is plotted on the "Time" axis, and the fire resistance limit is determined.
The dependence of the time to reach the critical temperature on the steel section factor is plotted using the coordinates: section factor / heating time up to 500 ºC. This relationship can be approximated by a trend line (an averaged straight line between the data points), yielding the following formula:
Y = 0,64*X + 7,2
As an example, let's calculate the time required for a steel structure with a transformed cross-section of 17.85 mm to reach the critical temperature. Using this relationship, we obtain:
Y=0,64*17,85+7,2
Y=18,60
The fire resistance rating for this structure is 18.60 minutes.
The following graph can also be used.
WHY IS IT IMPORTANT TO CONSIDER THE FIRE RESISTANCE OF METAL ROOFING STRUCTURES
When calculating loads on metal roofing structures, it is important to consider not only the strength but also приведённую толщину металлаthe steel section factor, which directly impacts its fire resistance limit..
At the design stage, it is crucial to determine whether the fire resistance of trusses and roofing structures requires enhancement with fire protection compounds. This proactive approach prevents costly rework, expert revisions, and unnecessary expenses down the line.
WE'LL SUPPORT YOU AT THE DESIGN STAGE.
Our specialists provide professional fire protection calculationsfor metal structures, including roof trusses and roofing.
We engage you at the technical specification and design stages,consult on the need for fire protection treatments, and ensure compliance with the requirements o SHNK 2.01.02-04..
👉 Order a fire resistance and fire protection calculation during the design stage – save your time, reduce costs, and pass the assessment on the first try.
The first fire protection calculation is FREE for new clients.
