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.
Experimental works revealed that at or close to mid-depth of deep beams, the maximum width of cracks arising from flexure may be about two to three times larger than the width of the same crack at the level of surface where the crack originally forms.
The presence of crack is undesirable from aesthetic point of view.
Moreover, it poses potential corrosion problems to reinforcement of deep beams.
To safeguard against these crack formation, skin reinforcement is designed on the sides of deep beams to limit the formation of flexural crack widths.
Though the principal function of skin reinforcement is to control crack width, it may be employed for providing bending resistance of the section.
Side facing reinforcement also called skin reinforcement is provided in a beam when the depth of the beam exceeds 750mm, or when there exists torsion in the beam, these reinforcement is provided for beams having depths greater than 450mm.
Their main purpose is to prevent the lateral reinforcement a.k.a shear reinforcement a.k.a stirrups from buckling.
Think of a 12 inch scale and a 6 inch scale.
which one do u think is easier to buckle (Buckling is characterized by a sudden sideways failure of a structural member subjected to high compressive stress, where the compressive stress at the point of failure is less than the ultimate compressive stress that the material is capable of withstanding)??? The 12 inch one off course.
Similarly in case of stirrups, the longer the stirrups in height the easier for them to buckle and hence we provide Side facing reinforcement.
Other uses can be like to prevent crack width etc.