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

1. Foundation beams: The foundation beams transferring the conc column load to the supporting soil.
2. 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.
3. 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,

1. If 1 ≤ L/D ≤ 2 – For Simply supported
2. 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: 

1. It should resist the +ve bending moment & extended w/t curtailment betn the supports.
2. This r/f should be embedded beyond the face of each support.

Embedded length = 0.8 x Development length

= 0.8 x (0.87 f_y φ / 4 τ_bd)

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.

Distribution

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 Vertical                                        Horizontal
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