Flanged concrete beams
Flanged beam properties can be specified under the Design Control heading in beam properties, by selecting Consider Flanges.
Typically, flanged beams can be either "T" shaped with a slab on both sides of the beam or "Γ" shaped with a slab on only one side of the beam.
The characteristic behaviour of flanged beams which can be made use of in design is the fact that the major axis bending resistance of the member is enhanced by the presence of adjoining concrete slabs which serve to increase the area of the compression zone activated during major axis bending.
This effectively raises the position of the neutral axis thereby increasing the lever arm of the longitudinal tension reinforcement and reducing the quantity of reinforcement required.
Validation of slabs for use in the flange effective width calculations
If a slab is present (and provided that a user defined flange has not been specified), the program automatically validates the slab as a potential candidate for being a beam flange using a number of criteria, the main ones being;
- the slab can be on one or both sides of the beam but
- it must extend for a distance ≥ the slab depth from the vertical face of the beam and
- it must extend for the full span length of the beam
- the slab must be a reinforced concrete slab
- if there are slabs on both sides of the beam, they may be of different depths and these depths may vary along the length of the beam
- the slab(s) must have a top surface coincident with the top surface of the beam or there must be a slab overlap ≥ 50 mm (2 inches) as shown in the following illustrations.
The effective width of any valid slab on each side of the beam, b eff,i, is calculated and the results that are appropriate at the mid-span length point are displayed along with the flange depth, under Design control in the Beam Property dialog.
When automatically calculated, the flange width and depth are only displayed in the Beam Property dialog and not in the Beam Properties window, (because the width and depth could vary if multiple beams were to be selected).
Include flanges in analysis
Selecting this option allows the flanged beam section properties to be considered in the analysis, stiffening the beam and reducing the deflection.
Consider as isolated (ACI/AISC)
ACI 318 clause 8.12.4 states:
"Isolated beams, in which the T-shape is used to provide a flange for additional compression area shall have a flange thickness not less than one-half the width of web and an effective flange width no more than four times the width of web."
When the Consider flanges checkbox is selected, an Isolated Beam checkbox is displayed to control whether or not the above code limit is applied. When the check is performed, if the flange geometry does not meet the above requirements the flanges are ignored.
Our understanding is that while this limit usually applies to precast beams, it is not usually applied to in-situ construction. Therefore, by the default the Isolated Beam checkbox is cleared, which means that the above check is not performed.
Effective Width of flanges (ACI/AISC)
For ACI 318-08 and ACI 318-11
For "T" shaped flanged beams the effective flange width, b eff, is given by :
beff = MIN(L/4, 16*hf + bw, b1 + b2 + bw) - Owi
For beams with slab one side only, the effective flange width, b eff is given by :
beff = MIN(L/12+ bw, 6*hf + bw, bi + bw) - Owi
L = span length
IF construction is continuous:
= distance of center-to-center of supports
= MAX(clear span + h, distance between centers of supports)
bi = 0.5 * the clear distance between the vertical faces of the supports for the valid concrete slab on side i of the beam or from the vertical face of the beam to the centerline of any supporting steel beam
Owi = the user specified allowance for an opening
For ACI 318-14
All limits on the flange width apply to the overhangs on each side of the beam. It is also clarified in this version that the clear span should be used in these calculations.
beff,i = MIN(ln/8, 8*hf , bi) - Owi
Effective Width of flanges (Eurocode)
The effective width of the compression flange is based on L0, the distance between points of zero bending moment.
For flanged beams the following values of L0 are to be used;
For a simply supported beam L0 = L
For a continuous beam, the value of L0 may be obtained using the following simplified rules;
End span of a continuous beam with a pinned end support L0 = 0.85*L
End span of a continuous beam with a fixed end support L0 = 0.70*L
Internal span of a continuous beam L0 = 0.70*L
L = the clear length of the span under consideration
The effective flange width, beff, is given by;
beff = bw + ∑beff,i
beff,i = the effective width of the flange on side i of the beam
= MIN[0.2*L0, bi,(0.2*bi + 0.1*L0)] - Ow i
L0 = the distance between points of zero moment as defined above
bi = 0.5 * the clear distance between the vertical faces of the supports for the valid concrete slab on side i of the beam or from the vertical face of the beam to the centreline of any supporting steel beam
bw = the width of the beam
Owi = the user specified allowance for an opening
If the slab thickness varies on each side of the beam, the thinner value is used in calculating the beam properties.
The above calculation for beff is also used for "Γ" beams with a slab on only one side although in this case, b1 or b2 as appropriate is = 0.