Building regulations require certain elements of a structure to have fire resistance. Whether or not an element requires fire resistance depends on the size of the building, what it will be used for and what the function of the element is.
When exposed to fire, all commonly used structural materials lose some of their mechanical strength. Heavily loaded steel will lose its designed safety margin at temperature around 550°C – regardless of the grade of steel.
The bigger volume of steel in the exposed area, the better fire resistance it has. How quickly the steel structure heats up in a fire can simply be described as the relation between the surface exposed to the fire and the steel volume of the profile. This relation is called the section factor A/V. A high section factor gives a quick temperature increase in the steel. In practice, this means that thin steel structures demand thicker protection.
To protect the structural steel in your building, use PAROC fire protection slabs with the following instructions:
Determine the needed fire resistance period
The fire resistance requirement for a building is defined in terms of fire resistance period and stated in terms of minutes (15, 30, 45, 60, 75, 90 or 120 minutes). This information is usually given in local building regulations and it depends on the height, occupants and type of the building. In practice it means that building frame has to maintain its load bearing capacity during the fire until everybody has left the burning building. It is the responsibility of the design engineer, using design codes such as ENV1993-1-2, to specify the appropriate limiting or failure temperature for a given section.
Different load bearing materials have different fire resistance periods. Those materials are usually tested by using the standard fire curve which demonstrates development of real fire. The temperature in a standard fire rises rather quickly and then increases indefinitely.
Fire resistance test results are expressed in terms of time of failure against one or more of three criteria:
- Load bearing capacity (R)
- Integrity (passage of hot gases/flames) (E)
- Insulation (temperature raise on the cold side of the structural element, usually max.140 °C) (I)
In some building constructions all of these are needed but for the steel frames only load bearing capacity is required.
Determine critical temperature and the section factor of the steel
All materials lose their strength as they get hot.
- Fully loaded steel beam exposed on four sides, fails at 550°C, regardless of steel grade.
- Fully loaded beam exposed on three sides, fails at 620°C. Temperature 450C – 500°C is commonly used as a rather safe limit value.
The design of fire protection is therefore based on this limiting temperature for elements exposed to fire on four sides. The aim is to keep the steel temperature below its critical temperature.
The rate of increase in temperature of a steel cross-section is determined by the ratio of the heated surface area (Am) to the volume (V). This ratio, (Am /V), has units of m-1 and is known as the section factor. Members with low section factors will heat up more slowly. The section factor is thus a measure of the rate at which a section will heat up in a fire and the higher its value, the greater will be the protection thickness required.
Determine protection method
The most practical way to limit the rise in steel temperature is to insulate it from the fire. Board materials would normally be used to form a box around the section or with higher profiles following the profile.
In case of ”box” protection, the surface area is taken as the sum of the inside dimensions of the smallest possible rectangular or square encasement
PAROC Fire Steel Protect
Design tools for prediction of stone wool fire protection board thickness have been made for 30–240 minutes endurance time in a Standard Fire Exposure (R30–R240) for open and closed (I/H and RHS sections) steel sections.
(RHS = retangular hollow section)
Based on graphical or tabulated design values PAROC Fire Steel Protect boards thickness can be chosen from the dimensioning tables as a function of
- fire endurance time,
- section factor A/V for the member and
- Critical steel temperature ranging from 300–700°C
For more information, go to www.paroc.co.uk/solutions/fire/steel-structures
Top image: Mechanically fixed and glue free PAROC Fire Steel Protect gives excellent fire protection for steel profiles up to 4 hours.
