A beam is a structural element that is capable of withstanding load primarily by resisting bending. We design beams by limit state method as per IS 456: 2000. We should consider different detailing specifications for a beam section for the proper design.
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: When exposure of concrete is moderate, clear cover should be 20 mm.
- Moderate exposure: When exposure of concrete is moderate, clear cover should be 30 mm
- Severe exposure: When exposure of concrete is severe, clear cover should be 45 mm
- Very severe exposure: When exposure of concrete is very severe, clear cover should be 50 mm
- Extreme exposure: When exposure of concrete is extreme, clear cover should be 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, i.e., for 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 the 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.
Conclusions:
This article provides information on the specifications for beams used in construction. The article covers aspects such as material selection, load capacity, and dimensional tolerances.
Some of the key learnings are as follows:
- Cover: Clear cover depends on different exposure condition.
- Min. tension reinforcement: for Fe 250 – 0.304, Fe 415 – 0.205, Fe 500 – 0.170
- Max. tension reinforcement: should be not more than 4 percent of the cross-sectional area of the beam.
- Min. spacing of bars: It shall not be less than 75 mm.
- Max. spacing of bars: It shall not exceed 300 mm.
Check this post to know about the different type of beam sections and its applicability.
Beam design
- RCC Beam Design is a free app for designing reinforced concrete beams as per Indian Standards.
- RCC Design and detailing could be performed by Limit State Method specified in IS456:2000
- Option to save the design projects in local storage.
- Detailed calculation steps presented for verification and validation.
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