What is one way slab? Design Considerations

sThe slab in which is supported to the two opposite sides RCC beams which are responsible to carry the slab load along one direction.

In one way sab the beams length difference is huge, that’s is why the load will not transfer toward the shorter RCC beams. In these slabs on in one direction, the main reinforcements were provided.




According to Indian standard (IS 456:2000), the slab must have a,

  1. The longer span divided by
  2. To shorter span must
  3. Equal or greater than 2.

Formula:

One way slab = Longer span /Shorter span ≥ 2

In those slabs, in a shorter span, the main reinforcement was provided and in the transverse direction, the distribution bars were provided.

one way slab detail

Design consideration of one-way slabs:

The design consideration is very important in the construction of this slab to get maximum strength and minimum deflection (In between limits). The area of steel and the thickness of the slab are to be determined according to the conditions given below.

 

(a) Effective span (cl.22.2 of IS 456)

According to the slab boundary condition, the effective span of the slabs was determined. In the given below table (Table 1.0) the supports condition and the effective span guidelines are given.

In this (Table 1.0), the Effective span of the slabs was to be determined according to (cl.22.2 of IS 456)




 

Slr.NoSupport conditionEffective span
1With its supports the simply supported will not build integrally.Lesser the (a) clear span of slab + the effective depth, and (b) the support of center to center.
2If the clear span 1/12th is greater than the width of the support than continuous ( Width of supports < 1/12th of clear span)same

 

3Continuous when If the support width is greater of the 1/12th of clear span or 600 mm.

(i) If one end continuous and the one end is for intermediate spans,

(ii) If one end is continuous and the one end is free,

(iii) Spans with rock bearings or may on the roller.

(i) Clear span of slab b/w the slab supports.

(ii)  The clear span of sab + half of the slab effective depth. or

(a) Slab clear span + half of the discontinuous support width.

(iii) The distance b/w the centers of bearings

 

4The cantilever slab having an end continuous.Up to the center of support length.
5Cantilever span onlyUp to the face of the support span length + half of the slab effective depth.
6FramesCentre to center distance

 

 

 (b)The slabs effective span to its effective depth ratio (cls.23.2.1a-e of IS 456)

The one way sab deflections will always in under limits if the ratios of the effective span of the slabs to its effective depth are to be chosen within the limits and provisions was mentioned in (cl.23.2.1a-e of IS 456). The beams stipulations are also applicable to this slabs and they are considering design them as the beam of unit width.

(c) Nominal cover (cl.26.4 of IS 456)

Mainly the nominal cover of the slabs is totally depended upon the fire resistance requirements and durability. In the IS 456, the Table 16 or 16A provide these values. According to the structure requirements, the proper nominal cover was provided.

(d) Minimum reinforcement (cl.26.5.2.1 of IS 456)

According to the IS 456, the minimum reinforcement shall not be less than 0.12 and 0.15 % of the total mild steel (Fe250) cross-sectional area and deformed steel bar having high strength (Fe 415 and Fe 500)/welded fabric wire, respectively.

(e) The maximum diameter of reinforcing bars (cl.26.5.2.2)

The reinforcement diameter use in one-way slabs will not exceed the 1/8 of the slab total depth. 

(f) Maximum bars distance (cl.26.3.3 of IS 456)

The horizontal spacing between the parallel main steel bars is always less than of the, Three times the slab effective depth, or maximum 300 mm.

In the distribution bars, the spacing shall be the less than, five times the slab effective depth, or 450 mm.

One Way Slab Reinforcement Details:





Detail of one way reinforcement

The diameter of the reinforcements and the spacing of bars is to be used according to the (cls.26.5.2.2 and 26.3.3 of IS 456).

The quantity of reinforcement along the large span of the slabs shall be as per (cl.26.5.2.1 of IS 456).

In the shorter span, the main reinforcements were provided because in shorter span the bending moment will be high.

In the longer span, the distribution bars will be provided for purpose of load distribution purpose. We also have discussed the difference between main and distribution bars in the separate topic you can also check.


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