IS 456 sets a beam's nominal cover by exposure: 20 mm mild, 30 mm moderate and 45 mm severe, and never less than the bar diameter. Many specifications use 25 mm for beams inside buildings. ACI 318 asks for 1½ in (40 mm) to the stirrups in interior beams.
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Minimum cover
20mm
In inches
0.79 in
Governing rule
IS 456:2000
Cover is to the outermost steel, including stirrups, ties and links. Fire rating or the drawings may require more.
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Table 16 note 2: the actual cover may be up to 10 mm more than the nominal cover, never less.
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IS 456 allows 20 mm in mild exposure, but older practice (IS 456:1978) used 25 mm for beams and many engineers still specify it. The drawing always governs.
Is beam cover measured to the stirrup or the main bar?
To the outermost steel, which in a beam is the stirrup. The main bars sit one stirrup diameter further in.
What cover does a beam in a car park need?
Car parks see wetting, drying and de-icing or coastal salts, so IS 456 severe (45 mm) or more is common. Use the exposure your engineer specifies.
References
IS 456:2000 Table 16 and cl. 26.4 (nominal cover); Table 3 (exposure conditions)
ACI 318-19 Table 20.5.1.3.1 (specified concrete cover); NSCP 2015 uses the same values
