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.
Contents:
Purpose of Skin Reinforcement
- Minimizes crack width in the web of a beam.
- Increases the ultimate load-carrying capacity of the beam.
Skin Reinforcement Detailing
Bar Size
- The diameter of skin reinforcement ranges from No. 10 to No. 16.
- Alternatively, welded wire reinforcement with a minimum area of 210 mm2 per meter of depth.
Spacing
The spacing between steel bars is limited to control cracks. The maximum spacing for skin reinforcement is determined based on Table-1.
Table-1: Maximum Spacing for Bars, Wires, and Bonded Prestressed Reinforcement Used as Skin Reinforcement
| Reinforcement type | Maximum spacing, mm |
| Bars and wires | Smallest of (300(280/fs)-2.5Cc), and (300(280/fs)) |
| Bonded prestressed reinforcement | Smallest of ((2/3)380(280/(Δfps))-2.5Cc), and (2/3)300(280/(Δfps)) |
| Combined deformed bars or wires and bonded prestressed reinforcement | Smallest of ((5/6)380(280/(Δfps))-2.5Cc), and (5/6)300(280/(Δfps)) |
Where:
fs: stress in reinforcement which is taken as 2/3 fy or computed based on unfactored moment for deformed bars, MPa
Cc: concrete cover, mm
Δfps: change in stress in bonded prestressed reinforcement at service loads which is equal to calculated stress based on cracked section analysis minus the decompression stress (fdc). The decompression stress is equal to effective stress in prestressed reinforcement (fse). The value of Δfps should not exceed 280 MPa. If Δfps does not exceed 140 MPa, the spacing limit in Table-1 is not required to be satisfied.
Placement of Skin Reinforcement
The placement of skin reinforcement for positive and negative moments is shown in Figure-1.
FAQs
Longitudinal skin reinforcement is deformed bars, wire, or bonded prestressed reinforcement placed uniformly on both sides of the vertical face of the tension zone in relatively deep beams to control cracks.
1. Minimize crack width in the web of a beam.
2. It may also be provided to increase the ultimate load-carrying capacity of the beam.
If 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.
Read More
Design of rectangular reinforced concrete beam
Thumb rules and specifications for the design of reinforced concrete beam