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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