FREQUENT QUESTIONS REGARDING PASSIVE FIRE PROTECTION
Fire definition:
Fire represents the complex process of burning, with uncontrolled evolution in space and time due to the presence of combustibles and ignititon sources, whose occurence and development have negative effects, which consist in loss of human lifes, material damages and which requires organised interventions for extinction.
This is the reason why passive fire protection is an important element in construction for all civil and industrial buildings and it must be treated very seriously.
How is the passive fire protection calculated?
In case of steel structures , the calculation for passive fire protection depends on three analytic factors, indifferent of the used products (resistant mortar, thermofoaming paint or fire resistant boars)
- The fire ressistance time, as it is determined in project
- Section factor of the element which must be fire protected
- Critical temperature of steel element, as it is determnined by the structural engineer
How is the fire resistance time determined?
The fire resistance time of steel structures is determined according to the project surface, destination of the project, resistance level. All these factors can be found in Fire Safety Norm P118 ( Romanian regulation )
How am I calculating the section factor (massivity factor)?
We define section or massivity factor: the relation between the exterior section perimeter exposed (to fire) and straight area section of the profile (m2)
For an easier calculation, it can be used the following formula:
Massivity =P/A(m-1)
where:
P = perimeter of straight section wich will be fire protected (m)
A = area of straight section of the profile (m2)
EXAMPLES FOR CALCULATION THE MASSIVITY FACTOR FOR AN HEB - 180 PROFILE
Dimensions of HEB profile - 180
h = 180 mm / b = 180 mm / t = 8,5 mm
Example of calculation for massivity factor on 4 sides
1. Calculation of fire exposed perimeter:
P = 4 x b + 2 x h - 2 x t = 4 x 180 + 2 x 180 - 2 x 8,5 = 1063 mm = 1,063 m
2. Calculation of the profile section:
A = 65,3 cm2 = 0,00653 m2
3. Calculation of massivity factor:
1,063/0,00653 = 162,8 (m-1)
Example of calculation for massivity factor on 2 sides:
1. Calculation of fire exposed perimeter:
P = 2b + h - t = 2 x 180 + 180 - 8,5 = 531,5 mm = 0,5315 m
2. Calculation of profile section:
A = 65,3 cm2 = 0,00653 m2
3. Massivity factor:
0,5315/0,00653 = 162,8 (m-1)
Once it is known the massivity factor of the profile, this can be found in the table with thickness determination for the layer of mortar PARAFOC F and than it will be applied the corresponding massivity value, to acomplish the requested fire resistance.
Why does a Technical Agreement contains more than one thickness tabel?
It is absoltelly normal that a Technical Agreement which reffers to passive fire protection of metallic structures to contain more thinckness tabels, for different fire resistance times .
It exists two analytical factors that are making the difference between these tabels:
1.CRITICAL TEMPERATURE, and we will find tabels with thickness for critical temperature, starting with 350 Celsius degrees up to 750 Celsius degrees
2.THE SHAPE OF THE PROFILES, and we will find different tabels for the profiles with opened section (profile I,H) and for profiles with closed section (circular and rectangular profiles)
Regarding the tables with differential thickness for critical temperature, is neccessary that the Agreement to contain the entire range of Critical temperatures and the designer of the resistance structure to have the possibillity to choose the neccessary critical temperature for the project. Different projects may have different critical temperatures, in conclusion, different fire protections.
The critical temperature is determinated using an calculation, according with the EUROCOD 3. If this this is not intended, the critical temperature wich can be used without calculation for passive fire protection of steel structures is between 500 and 550 Celsius degrees, and will be applied thickness indicated in tabels with critical temperature 500 or 550 Celsius degrees.
Also it is very important to apply the thickness from tables to corresponding shape of the profile wich follows to be fire protected.
What does critical temperature mean?
Critical temperature means temperature which corresponds with the loss of bearing capacity for a structural element subjected to external tests, and also to temperature. Is for note that fire protection of an structural steel element must be chosen so that during the imposed period of fire resistance, the temperature in section do not reach the suitable temperature for the element to collapse.
The fire protection is determinated not only according with the massivity of transversal section (using section factor), but also using the loading factor of the structural element. The parameter which contributes practically for the loading factor of the element in fire situation is CRITICAL TEMPERATURE. As higher is the loading factor (or as higher is element's load in case of fire, based on the specific loadings), as lower is the critical temperature at element's subside.
Because the critical temperature depends on forceing degree of the structural element in question, it results that only the structural engineer, who administrates all efforts in system, can calculate the critical temperature. In case that the structural engineer doesn't calculate critical temperature, the beneficiary may require an technical expertise to determine this parameter.
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