Detailing of Beam sections

Cover

Clear cover is the distance measured from the exposed concrete surface to the nearest surface of reinforcing bars. The requirement as per the codes are:

Mild exposure - 20 mm

Moderate exposure - 30 mm

Severe exposure - 45 mm

Very severe exposure - 50 mm

Extreme exposure - 75 mm

Minimum tension reinforcement

Minimum percentage of tensile reinforcement is provided in beams to prevent the sudden failure under applied load. For this reason the minimum reinforcement provided is such that it can produce moment of resistance equivalent to cracking moment of plain concrete section.

Minimum percentage of tensile reinforcement for different grades of steel is:

Fe 250 - 0.304

Fe 415 - 0.205

Fe 500 - 0.170

Maximum tension reinforcement

Providing excessive reinforcement in beams can result in congestion. This could affect the proper placement and compaction of concrete. For this reason IS 456: 2000 restricts the area of tension reinforcement to be not more than 4 percent of the cross-sectional area of beam.

Maximum spacing of bars

It is specified for the purpose of controlling crack-width and shall not exceed 300 mm.

For a given area of tension reinforcement providing several small diameter bars is more effective in controlling cracks and improving bond than providing fewer bars of larger diameter.

Minimum spacing of bars

Minimum center to center spacing is specified for the purpose of proper compaction of concrete so that the aggregate present in concrete should not get stuck in the mesh. It shall not be less than 75 mm ( Preferably 100 mm).

The clear spacing should not be less than 50 mm.