GUIDELINES FOR SLAB DESIGN | REQUIREMENTS FOR SLAB DESIGN | LCETED

• Practically, it is a common practice that if 𝐿/𝐵 > 1.5, design it one way slab.

 

• If 𝐿_𝑥 > 3 𝑚: it is advisable to provide a two-way slab.

 

• Common depth of slabs = 5”, 6” (125 mm, 150 mm); max=200 mm (for load = 5 𝑘𝑁/𝑚𝑚2 ).

 

L/D ratios:

• One way S.S slab 𝐿/𝐷 = 20 × 𝑚. 𝑓 = 20 × 1.4 = 28 𝑓𝑜𝑟 𝑝𝑡 = 0.3%, 𝑚. 𝑓 = 1.4) . Adopt 𝐿 𝐷 = 25.

• One way continuous, above value (25) shall be multiplied by 26/20 = 1.3 = 25 × 1.3 = 32.5. Adopt 𝐿/𝐷 = 32.

• Two way S.S slab 𝐿/𝐷 = 28 (for 𝐿𝑥 ≤ 3.5𝑚 𝑎𝑛𝑑 𝐿. 𝐿 ≤ 3 𝑘𝑁 𝑚² )

• Two way continuous slab 𝐿/𝐷 = 32 (for two-way slab, shorter of the two spans be used to calculate 𝐿/𝐷 ratio

 

• Common diameter of bars = 8, 10, 12 mm. (dia. ≯ 𝐷/8)

 

• Approx. area of main steel in slabs, 𝐴_𝑠𝑡 = 𝑀𝑢/0.8𝑑.𝑓𝑦 (𝑀𝑢 = 0.138𝑓_𝑐𝑘𝑏𝑑² )

 

Theoretically, calculated 𝐴_𝑠𝑡 is provided in the middle strip and min. 𝐴_𝑠𝑡 is provided in edge strip. In practice, bars are uniformly provided spaced throughout spans in both directions.

• Assume 𝐴_𝑠𝑡 = 0.3 − 0.45% (one way slab)

• Assume 𝐴_𝑠𝑡 = 0.2 − 0.3% (two way slab)

• Assume 𝐴_𝑠𝑡 = 0.5% (one way continuous)

• Maximum area of main steel in slab = 4% 𝑜𝑓 𝑏𝐷 (As per ACI).

 

• All top reinforcement of slabs shall continue up to 𝐿/3 of span at both sides of C/L of wall or Beam.

 

• Corners free to lift: cast over walls and no parapet above

 

• Roof slab with corner held down is achieved by 230 mm parapet wall, 600 mm high and 2 m length on either side of the corner

 

• Area of distribution steel = 0.12% 𝑜𝑓 𝑏𝐷 (For floor slab)

• Area of distribution steel = 0.24% 𝑜𝑓 𝑏𝐷 (For roof slab)

 

• Short span bars are provided in the bottom layer.

 

• If two different dia. bars are used, provide larger dia. bars at the bottom layer.

 

• Due to openings in slabs, area of steel interrupted be replaced by an equivalent amount, half of which should be placed along each edge of the opening.

 

Practical considerations of one-way slab

• Spacing shall be between 150-200 mm (main bars)

• Spacing shall be between 150-300 mm (distribution bars)

• Max. Spacing of bars 3𝑑 or 300 mm, whichever is less.

 

 

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