R/f’ed concrete deep beam
It is a special structure in beams. In the deep beam, the depth of the beam is greater than eff. span. The assumptions of the normal beam are not applicable in a deep beam. As the depth is more, the distribution of the stress is not uniform. So we found some in finding the values of the lever arm & the other related things.
Defn of Deep Beam as per IS 456: 2000
The beam having greater depth in comparison to its eff. span is called as Deep Beam.
Uses of Deep Beam
- Foundation beams: The foundation beams transferring the conc column load to the supporting soil.
- Transfer girder or wall beams at an intermediate floor level of a bldg where some columns are read to be stopped for some particular reasons.
- Bunker or tanks where the walls itself act as a deep beam
IS Codes provisions of a Deep Beam
- As per IS 456:2000, a beam shall be deemed as deep beam,
- If 1 ≤ L/D ≤ 2 – For Simply supported
- If 1 ≤ L/D ≤ 2.5 – For Continuous beam
where, L = Eff span, D = Overall depth.
To calculate eff span = c/c dist betn supports or 1.15 x clear span. (Consider less value)
Lever Arm (Z): As the stress distn along the depth of the beam is non-linear. Due to this the lever arm bent the comp & tensile forces can’t be easily determined.
The values of Lever Arm as per IS 456:2000
- For simply supported beam:
Z = 0.2 (L+2D) If 1 ≤ L/D ≤ 2
Z = 0.6 L If L/D < 1
- For Continuous beam:
Z = 0.2 (L+1.5D) If 1 ≤ L/D ≤ 2.5
Z = 0.5 L If L/D < 1
R/F provision as per IS 456-2000
In the case of deep beam, tension is still to be extended over the whole length of the span & should be well anchored in the support.
- +ve moment r/f:
- It should resist the +ve bending moment & extended w/t curtailment betn the supports.
- This r/f should be embedded beyond the face of each support.
Embedded length = 0.8 x Development length
= 0.8 x (0.87 fy φ / 4 τbd)
Zone of depth = 0.25D – 0.05l
- -ve moment r/f:
Provided to resist the -ve B.M
Termination of r/f – Half of the r/f may be terminated at a dist of 0.5D from the face of the support & remaining should be extended over the whole span.
When 1 ≤ L/D ≤ 2.5, wholly have to distribute the total tensile r/f in 2 zones
Zone – 1 = 0.2D having Ast1 Ast1 = 0.5 (l/D – 0.5)Ast
Zone – 2 = 0.3D (on either side of mid depth) having Ast2 = Ast – Ast1
If l/D ≤ 1 = zone → 0.89 (evenly distributed)
- Vertical r/f: Hanging action → Bars or suspension stirrups to be provided.
- Side face r/f:
To resist the effect of shrinkage & temp
It also can be provided in vertical direction & horizontal direction.
% of total c/s’al area is to be provided given in IS 456:2000 cl: 29.3.4 → cl 32.4
Min req’ts of r/s in wall as per IS 456:2000 cl 32.4
|Type of r/f steel|
% of side face r/f of gross area of cone
|Deformed bar of dia ≤ 16mm & fy ≥ 415 N/mm2||0.12 0.2|
|Bars of other types||0.15 0.45|
|Welded wire fabric not larger than 16mm in dia||0.12 0.20|
Spacing of side face r/f ≤ 3 x thkness or 450mm