For cast-in-situ beams supporting slab (monolithic casting), a beam considered as a flanged section than a rectangular one, and this is because the slab part just right adjacent to the beam bends in flexure together with the beam integrally.

As a result of deformation variation [quantitatively] between rectangular and flanged beams, many international codes recommend the use of a T or inverted L section beam for modeling [analysis] and design as a result of flexural stiffness variation. For the use of a flanged section both on a model for analysis and design for dimensioning first, the effective width needs to be computed.

Flanged section, effective beam width  according to EN 1992: Clause 5.3.2.1

   Effective flange width, beff ,for a T and inverted L beams defined as ; 

  beff  = ∑ beff,i + bw  ≤ b and    beff,i  = 0.2 bi + 0.1 lo ≤  0.2 lo

 beff,i ≤  bi

Typical flanged section:

sample example

The effective flange width for a beam marked below, and b/h = 25 cm / 50 cm .

Solution:

Location of the beam: Exterior beam

         

There is no transverse beam to the left (L – beam, l2 = 0 ), whereas a transverse beam to the right is, l1 = 6.00 m. b1 is the distance between the support face (column) and the mid-span of the transverse beam.

           i.e.  lo = 0.85 * 6.00 m = 5.10 m and      b1 = (l1 -bw) / 2 = (6.00m – 0.25 m) /2 = 2.875m

               beff,1 = 0.2 b1 +  0.1 lo  < 0.2 lo  

               beff,1 = 0.2 * ( 2.875m )  +  0.1 * ( 5.10 m)   < 0.2 * ( 5.10 m)

                beff,1 = 1.085 m < 1.02 m

                beff = beff,1 + bw = 1.02 m + 0.25 m

                    ∴  beff = 1.27 m = 1270 mm

Note: the consideration of a flanged section beam on a finite element model would be a necessity for a moment-resisting ductile frame while analyzing of a seismic load.

Try: For the same beam cross-section above [b/h], put your answer on the comment section below for the effective flange width of the marked beam.

 

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