Effective width of flanges [EC2 A§5.3.2.1]
[EC2 A§5.3.2.1] Τhe effective flange width b_{eff} depends on the distance l_{o} between consecutive zero moment points and on half the spans b_{1}, b_{2} at both sides of slabs.
Figure 3.1.21: Continuous beam: l_{o} distance between consecutive points of zero moments Figure 3.1.21: Continuous beam: l_{o} distance between consecutive points of zero moments
Figure 3.1.22: Continuous beam of frames: l_{o} distance between consecutive points of zero moments Figure 3.1.22: Continuous beam of frames: l_{o} distance between consecutive points of zero moments
Earthquake resistant structures require strong columns and fixed columnbeam connections. This requirement demands the creation of a frame set of beams, forming a continuous structure with respect to geometry, but autonomous with respect to the adjacent beams. This fact leads to the conclusion that, in general, the supports of a beam are rarely hinged. Therefore, l_{o}=0.70·l can be chosen for all the earthquake resistant beams.
Figure 3.1.23 Figure 3.1.23
b _{eff} =b _{w} +b _{eff,1} +b _{eff,2} ≤b _{lim}
where b_{eff,1}=0.20·b_{1}+0.10·l_{o}≤0.20·l_{o}andb_{eff,2}=0.20·b_{2}+0.10 ·l_{o}≤0.20·l_{o}.

Τhe effective widths at supports have practical meaning mainly for the dimensioning of inverted concrete beams under bending.

When an adjacent slab is cantilever, of a span l_{n}, the corresponding b_{1}ή b_{2} is equal to l_{n}.
Figure 3.1.24: Woodwork plan and section 11 Figure 3.1.24: Woodwork plan and section 11
The end beam b5 has crosssection 300/5 and span l=5.00 m. The clear span on the left of slab s1 is l_{n1}=4.00 m. The calculation of the effective flange width is requested.
l_{o}=0.70·l=0.70x5.00=3.50 m
b_{1}=l_{n1}/2=4.00/2=2.00 m, b_{lim}=b_{w}+b_{1}=0.30+2.00=2.30 m
Τhe effective width is equal to:
b_{eff}=min(b_{w}+b_{eff,1}, b_{lim})=min(0.30+0.70, 2.30)=1.00 m
b_{eff,1}=min(0.20·b_{1}+0.10·l_{o},0.20·l_{o})=min(0.20x2.00+0.10x3.50, 0.20x3.50)=min(0.75, 0.70)=0.70 m.
The middle beam b6 has the same dimensions as b5. The span of the slab on the left of b6 is l_{n1}=5.00 m. The span of the slab on the right of b6 is l_{n2}=4.00 m. The calculation of the effective flange width is requested.
This flange is of type T.
b_{1}=l_{n1}/2=5.00/2=2.50 m, b_{2}=l_{n2}/2=4.00/2=2.00 m, b_{lim}=b_{w}+b_{1}+b_{2} =0.30+2.50+2.00=4.80 m
Τhe effective width is equal to:
b_{eff}=min(b_{w}+b_{eff,1}+b_{eff,2}, b_{lim})=min(0.30+0.70+0.70, 4.80)=1.70 m
b_{eff}_{,2}=0.70 m (same with b_{eff}_{,1} of beam b5)
b_{eff,1}=min(0.20·b_{1}+0.10·l_{o},0.20·l_{o} )=min(0.20x2.50+0.10x3.5,0.20x3.50)=min(0.85, 0.70)=0.70 m.