Controlling deflection and cracking

Controlling deflection

• Span/depth ratio method (IS 456 cl. 23.2.1): keep $l/d$ within the basic value (7, 20 or 26 for a cantilever, simply supported or continuous member) multiplied by the modification factors for tension steel, compression steel and flanged section.
• Increase the depth of the member, or provide more tension steel (this reduces the service stress), and add compression steel to reduce creep deflection.
• Use better concrete (higher modulus), cure it properly to reduce shrinkage, and keep a low water-cement ratio.
• Limit the deflection to $\text{span}/250$ (total) and $\text{span}/350$ or 20 mm (after finishes).
• Camber (pre-camber) the member, and delay the fixing of partitions and finishes.

Controlling cracking

• Limit the bar spacing to the maximum allowed (IS 456 cl. 26.3.3, cl. 43 and Table 15), and use many small bars rather than a few large ones.
• Provide sufficient cover (not more than 50 mm to the tension bars), and provide side-face steel (0.1% of the web area) in beams whose web depth exceeds 750 mm.
• Limit the steel stress and the crack width to 0.3 mm (IS 456 cl. 35.3.2), checked by the calculation of Annex F when necessary.
• Control shrinkage and thermal cracks by proper curing, minimum steel and contraction joints.