Many times openings (cutouts) are required to be provided in reinforced concrete slab in buildings to provide way for lifts, or cables, ducts or other instrument to pass through one floor to other floors, mainly in the case of industrial buildings. But in that case, special care need to be taken while detailing of reinforcement for such openings in slabs.
Detailing of opening in slabs is based on factors like size of opening, loads on slabs, floor vibrations etc. In the case of large openings it is desirable to place beams under the opening area so that no additional detailing of slab reinforcement is needed and load is directly transferred to main beams through secondary beams.
Small opening is slabs:
Where openings are small and are less than 150 mm in larger dimension, with no special loading or vibration condition of slabs, following type of detailing should be done:
Detailing of small slab openings
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| Detail of Small Slab Openings - Cutouts |
For Holes less than 150mm, reinforcement bars may be displaced, additional reinforcement is not required.
Slab openings of 150 mm to 450 mm
For slabs with openings more than 150 mm and less 450 mm, at least one half the quantity of main steel reinforcement intersected by the opening is provided parallel to the main reinforcement bars on each side of the opening extending a length of development length Ld beyond edges of the opening as shown in figure below. This reinforcement is provided on both top and bottom face of the slab.
Detailing of medium slab openings
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| Detail of Medium Slab Openings - Cutouts |
Slab openings of 450 mm to 900 mm
For slabs with openings more than 450 mm and less 900 mm, at least one half the quantity of main steel reinforcement intersected by the opening is provided parallel to the main reinforcement bars on each side of the opening extending a length of development length Ld beyond edges of the opening on both top and bottom slab face as shown in figure below. In addition to this, diagonal bars are provided on both top and bottom face of the slab as shown.Detailing of large slab openings
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| Detail of Large Slab Openings - Cutouts |
Larger Slab Openings:
For larger slab opening larger than 900 mm, it should be specifically designed and detailed. It is recommended to use beams below larger slab openings.
