In BS8110, it states that secondary reinforcement should be provided for beams exceeding 750mm deep at a distance measured 2/3 depth from the tension face....What is the purpose of skin reinforcement for deep beams?
Skin reinforcement in deep beam is provided for the following reasons:
1. For confinement (composite action)
2. To prevent lateral bucking of beams
3. To enhance torsional resistance
For deep beams some additional reinforcement should be placed near the vertical faces in the tension zone to control cracking in the web. Without such auxiliary steel, the width of the cracks in the web may greatly exceed the cracks width at the level of the flexure tension reinforcement. The width of the cracks arising from flexure is about 2-3 times larger than the width of the same crack at the level of the surface where the crack originally forms.
The presence of cracks poses potential corrosion problems to reinforcement of deep beams
The principal function of skin reinforcement is to control crack width. It is also employed for providing bending resistance of the section.
In case of deep beam, shear cracks penetrate towards surface. To prevent catastrophic failure, it is essential to provide ductility to the deep beam. Hence side face reinforcement is provided in addition to tension steel needed for bending and dowel action.
A web can be considered formed by the stirrups of the beam and small diameter longitudinal bars added to prevent cracking of the vertical surfaces that lie between the center of compression and that of the tension bars.
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