About the Section factor

After a thermal analysis , the ‘Section factor’ can be requested in the data table . But this value does not correspond to section factors that you can find in (most) tables (such as this one).

This is because Diamonds shows the shadow factor k_sh multiplied by the section factor A_m/V.

Example: IPN 80 (b=42mm; h=80mm; t_w=3,9mm; t_f=5,9mm, r=3,9mm)

The perimeter V of the cross-section equals:
$V=2\cdot b\cdot t_{f} + (H-2\cdot t_{f})\cdot t_{w}+(4-\pi )\cdot r^{2}=775mm^2$
The area of the cross section per unit of length equals A_m
$A_m= 4\cdot b-2\cdot(t_{w}+2\cdot r)+2\cdot (h-2 \cdot r) + 2 \cdot \pi \cdot r=314mm$
The are of the bounding box of the cross-section pr unit of length equals A_m.box
$A_{m.box}= 2 \cdot (h+b)=244mm$
The shadow factor k_sh then equals (EN 1993-1-2 equ. 4.26a)
$k_{sh}= 0.9 \cdot \frac{A_{m.box}}{A_m}=0.7$
The section factor A_m/V(EN 1993-1-2 Table 4.2):
$A_{m/V}=\frac{A_{m}}{V}=404m^{-1}$
The result shown in the data table of Diamonds in the column ‘Section factor’ equals:
$k_{sh} \cdot A_{m/V} = 0.7 \cdot 404m^{-1}= 283m^{-1}$

The choice to show the product of the shadow factor k_shand the section factor A_m/Vhas several reasons:

On the one hand, Eurocode allows the use of the shadow factor when working with nominal fire curves (EN 1993-1-2 §4.2.5.1) (which is almost always the case in Diamonds). By already including this multiplication in the ‘section factor’ column, there is no discussion for the end user that the shadow factor will be charged.
On the other hand, there are several manufacturers of fire-resistant paints whose data is based on a k_shx A_m/V, which makes the interpolation of the results a lot easier for the calculation core.
Some tables also include the shadow factor like this one.

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