LONGITUDINAL SKIN REINFORCEMENT-ALL YOU NEED TO KNOW

LONGITUDINAL SKIN REINFORCEMENT-ALL YOU NEED TO KNOW

Longitudinal skin reinforcement is deformed rebar, wire, or bonded prestressed reinforcing bars evenly placed on either side of the vertical surface of the tensile region in a relatively deep beam to control the width of the crack. If the deep beam is not provided with such reinforcement, the crack width in the web will be greater than the cracks developed at the level of longitudinal tensile reinforcement.

The principal function of skin reinforcement is to control crack width. It is also employed for providing bending resistance of the section.

Longitudinal skin reinforcement is deformed bars, wires, or bonded prestressed reinforcement placed uniformly on both sides of the vertical face of the tension zone in relatively deep beams to control crack width. If the deep beam is not equipped with such reinforcement, the width of cracks in its web will be larger than the cracks developed at the level of longitudinal tension reinforcement.

When the depth of a non-prestressed and class C prestressed beam is greater than 900 mm, skin reinforcement should be provided for a distance ½ depth from the tension side of the beam, as per ACI 318-19.

Nonetheless, the British Standard for the design and construction of reinforced and prestressed concrete structures (BS8110) states that skin reinforcement should be provided for a distance measured 2/3 depth from the tension face of the beam if the depth of the element is larger than 750 mm.

Additionally, Eurocode 2, part 1-1 specifies that such reinforcement should be provided for a beam with a height of 1000 mm or more. This article presents the purpose and detailing of skin reinforcement based on ACI 318-19.

Skin Reinforcement for deep beams:

The beam with the depth of web that will exceed 3 feet has the capacity to make extremely wide cracks in above parts of their zone of tension. Hence, in order to decrease these crack, it is required to add some extra longitudinal reinforcement in flexural zone of tension. This tension zone is adjacent to the vertical side face of their respective webs.

According to the code, the extra skin reinforcement should be distributed uniformly over the both side face of the member. Spacing along the height direction should be greater than 36 inch for the distance equals to half of the height, which is nearest to the flexural steel reinforcement.

The code provides the spacing between skin reinforcement. Hence, the additional bar might be used for calculating the strength in bending of member only, when the suitable strain for their respective position is taken relative to the neutral-axis. This section will be used for computing the stress in the bar.

The entire area of skin-reinforcement on both face sides should not go beyond the value of one half of required bending tension reinforcement in beam. The code does not provide the area of skin reinforcement and it states that the extra reinforcement must be located adjacent to the vertical face of zone of tension. This is done in order to protect the formation of crack in web region of beam.