Temperature development
Following temperature curves are used for the description of the gas temperature according to EN 1991-1-2.
Standard temperature curve
The fundamental nominal curve, the temperature of gas in the fire compartment is given by the expression
Where is: | θg |
|
t |
|
External fire curve
The external fire curve is defined by the expression
Where is" | θg |
|
t |
|
The gas temperature is limited by the value 680°C. Therefore, the critical temperature won't be achieved in certain cases.
Hydrocarbon fire curve
The hydrocarbon curve is defined by the expression
Where is: | θg |
|
t |
|
Also this curve is limited, the limiting value is 1100°C.
Parametric temperature curve
This curve is valid for fire compartments up to 500m2, without openings in the roof and for a maximum compartment height of 4m. The curve consists of two phases: heating phase and cooling phase. Following parameters describe the geometry of the curve:
- The time tlim for maximum gas temperature in case of fuel controlled fire
- The thermal absorptivity for the total enclosure b given by the expression
Where is: | ρ |
|
c |
| |
λ |
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- The opening factor O given by the expression
Where is: | Av |
|
heq |
| |
At |
|
- The design value of the fire load density qt,d related to the total surface area At of the enclosure.
Temperature development
The temperature development differs for protected and unprotected fire details.
Following expression is used for unprotected members:
Where is: | Δθa,t |
|
Am/V |
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ca |
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ρa |
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hnet |
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Δt |
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The specific heat of steel differs according to the temperature in accordance with the chapter 3.4.1.2 of EN 1993-1-2. The net heat flux hnet is given according to the chapter 3.1 of EN 1991-1-2. The temperature of steel member is checked with the help of temperature increments, the time period specified for the temperature increment Δt is 5 seconds.
The temperature development for protected members is defined by following expression:
where
Where is: | Δθa,t |
|
λp |
| |
Ap/V |
| |
dp |
| |
ca |
| |
ρa |
| |
θg,t |
| |
θa,t |
| |
Δt |
| |
Δθg,t |
| |
cp |
| |
ρp |
|
The specific heat of steel differs according to the temperature in accordance with the chapter 3.4.1.2 of EN 1993-1-2. The temperature of steel member is checked with the help of temperature increments, the time period specified for the temperature increment Δt is 30 seconds.
The critical time is given as a sum of time increments Δt, that reach the critical temperature. The critical temperature is calculated in advance with the help of utilization of the member.