Understanding Longitudinal Skin Reinforcement in Beams

Longitudinal skin reinforcement plays a crucial role in controlling crack width in relatively deep beams. These reinforcements, typically in the form of deformed bars, wires, or bonded prestressed reinforcement, are placed uniformly on both sides of the vertical face of the tension zone within the beam. By doing so, they help in minimizing crack widths, thus enhancing the structural integrity of the beam.

Significance of Skin Reinforcement

In deep beams, without adequate skin reinforcement, cracks within the web of the beam can widen significantly. This can compromise the structural integrity and performance of the beam, leading to potential safety concerns.

Regulatory Standards

Various regulatory standards provide guidelines for the provision of skin reinforcement in beams. For instance, according to ACI 318-19, when the depth of a non-prestressed or class C prestressed beam exceeds 900 mm, skin reinforcement must be provided for a certain distance from the tension side of the beam. Similarly, BS8110 and Eurocode 2 offer their own specifications regarding the provision of skin reinforcement based on beam depth.

Purpose of Skin Reinforcement

The primary purposes of skin reinforcement in beams are:

1. Crack Width Control: By distributing the stress more evenly across the beam’s cross-section, skin reinforcement helps in minimizing crack widths, thus preserving the structural integrity.
2. Enhanced Load-Carrying Capacity: Skin reinforcement also contributes to increasing the ultimate load-carrying capacity of the beam, making it more resilient to applied loads.

Detailing of Skin Reinforcement

When it comes to detailing skin reinforcement in beams, certain factors such as bar size and spacing need to be considered:

Bar Size: The diameter of skin reinforcement typically ranges from No. 10 to No. 16, or alternatively, welded wire reinforcement with specific minimum area requirements per meter of depth.

Spacing: The spacing between steel bars is crucial in controlling cracks effectively. Maximum spacing for skin reinforcement is determined based on regulatory standards and calculations, as outlined in Table.

 Maximum Spacing for 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/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, typically taken as 2/3 fy or computed based on unfactored moment for deformed bars, measured in MPa.
• Cc: Concrete cover, measured in millimeters.
• Δfps: Change in stress in bonded prestressed reinforcement at service loads, equal to calculated stress based on cracked section analysis minus the decompression stress (fdc). The decompression stress is equal to the effective stress in prestressed reinforcement (fse). The value of Δfps should not exceed 280 MPa, and if it does not exceed 140 MPa, the spacing limit in Table-1 is not required to be satisfied.

In conclusion, longitudinal skin reinforcement plays a vital role in enhancing the performance and safety of beams by controlling crack widths and increasing load-carrying capacity. Adherence to regulatory standards and proper detailing are essential in ensuring the effectiveness of these reinforcements in concrete structures.