CHAR 21 dimensioning calculations A brief guideline for calculation of the coating amount needed for the application of CHAR 21 on steel members
1. Determine the section factor
a.In the most general case the section factor is the ratio of surface of the coated object to its volume. When exposed to fire, heat will flow into the object proportionally to its surface. For a given heat flow the temperature rise will be inversely proportional to the mass of the object, i.e. to its volume. Therefore the section factor determines how fast the object is heated up in a standard fire (low section factor: => slow temperature increase; high section factor => fast temperature increase).
When dealing with construction steel members, in most cases they are profiles with constant section. Therefore the section factor can be simply calculated by the ratio perimeter/area of the section. As above it is expressed in units of [m-1]. It is also named sometimes mass factor. It is sometimes referred to as A/V in symbols.
Most steel profiles have standard sizes. Tables are available to directly get the section factor. These tables give different values for the same profile depending on exposure: e.g. if a H profile is standing as a column and surrounded by fire on all perimeter it is said to be 4-sided exposed; a I profile used as a beam with the upper flange in contact with the concrete slab above is said to be 3-sided exposed. When the protection material is not applied over the surface but made in panels, “boxed” values are considered.
The attached table refers to standard European profiles, which are named e.g. HEA 300 = H section type A, 300 mm wide.
2. Determine the critical temperature
a.When a loadbearing steel member is heated up in a fire its mechanical strength will decrease with increasing temperature. At a certain temperature its loabearing capacity will no longer be enough for the applied load and it will collapse. This is the “critical temperature” (Tcr). It depends on load, or better on a “load factor”. In the general case it should be calculated by the structural engineer.
In many countries standard values of the critical temperature for typical constructions are given by building codes or by standard practice. These values generally range from 500 °C to 620 °C. Different values for beams and columns are often considered.
In several European countries we have obtained specific dimension tables for CHAR 21 approved by local regulatory bodies. These are generally simpler because one or few selected Tcr are considered. When the general tables from CHAR 21 assessment report are used, the critical temperature has to be selected by choosing the appropriate column.
3. Determine the resistance time
a.This is the time when the structure is expected to collapse for a given set of parameters, i.e. the maximum safe time allowed. In Europe it is generally expressed in minutes e.g. R60 = 60 minutes resistance. In the US it is generally expressed in hours, e.g. 1/2h or 1h.
The R value is determined by fire engineers or the fire brigade depending on type of building, its use, the estimated fire load, the estimated number of people to evacuate etc. At the moment of designing the protective material it is normally a given specification.
CHAR 21 tables are given for resistance time up to 2 hours (R120). However intumescent paints are particularly suited and prove more competitive for relatively short R values, generally up to R60 – R90.
4. Determine the coating thickness (DFT = dry film thickness)
a.Check if the profile is “open” e.g. H, I, U or “closed”. In the latter case whether it is square or circular. There are 3 separate sections in CHAR 21 dimension tables for each according to the European standards. Closed profiles require more protective material than open profiles of the same mass factor.
Check the resistance time R requested, than choose the corresponding page.
Select the appropriate table: one gives the “active layer thickness” i.e. the actual thickness of the intumescent coating, this is normally used for design. The other gives “total thickness” i.e. the dry thickness of the whole system including primer and topcoat.
Enter the table with appropriate mass factor (rows) and critical temperature (columns). A Tcr of550°C is generally a good first-guess estimate when this parameter is not known.
Read the dry film thickness value, in microns [µm].
5. Determine the amount of coating needed
a.The specification for the protection material is its dry thickness. This is the value that may be checked by independent auditing and must be guaranteed by the applicator taking into account the overspray or any other loss of material.
To have an estimate of the amount of wet paint needed per m2, just divide the DFT (in mm) by the standard yield rate. For CHAR 21 this is 0,55. You get the amount of paint in g/m2. This is a nominal value based on application in the lab in optimal operating conditions. It may vary to some extent depending on application conditions and must be corrected for losses. Experienced applicators know loss factors for different applications.
Multiply by surface per unit length (m2/m) of profile. For standard profiles tables are generally available. You get the amount per meter of each profile.
Multiply by overall length of all profiles.
EXAMPLE For R 60, with critical temperature = 550 °C
mass factor (m-1)
profile area (m2/m)
SHS * 100x100x5
* SHS = Square Hollow Section, the steel thickness determines the mass factor: when exposed on all perimeter A/V = 1/t (t in [m]) : A/V = 1/0,005 = 200 m-1.
** Value has been interpolated between rows. Approximate value can be taken for A/V immediately higher in table. The table is linear.
*** Value has been extrapolated. This procedure may not be allowed or be restricted to a maximum % depending on regulations.
**** Rounded. Should be increased by an appropriate “loss factor”.
CHAR 21 is certified according to the standard ENV 13381-4, test report n° pr01-02.093, assessment report n° PKO-02-1.027, PAVUS Institute, Prague. Values of thickness were obtained by multiple linear regression. The estimate of the amount needed of wet paint is based on a standard yield of 0,55 mm dry/kg wet. It is responsibility of the user to take overspray and waste into account to make sure that the final dry film thickness is as requested.
Though based on the results of long term tests and experience the present information is given as a guide only as utilization and operating conditions are extremely variable. We cannot accept any liability for use of the product that must be verified by the user.Accurate preliminary testing is highly advisable to obtain the full advantage of this product. Call in our technical department for any further information you may need.