Reduction in Bolt Shear Capacity due to Packing Plates

When connecting structural steel members using bolts, sometimes thin plates known as packing plates are used. These plates help to fill gaps or ensure proper alignment between the connected members. According to the Indian Standard code for steel design, IS 800-2007, if the total thickness of these packing plates exceeds a certain limit, it can affect the design shear capacity of the bolts used in the connection. This means the bolts might not be able to carry as much shear load as they would without the thick packing plates.

Packing Plate Thickness and Bolt Shear Capacity Reduction

IS 800-2007 specifies a particular reduction factor that must be applied to the design shear capacity of bolts when the total thickness of packing plates in a splice or connection goes beyond 6 mm. This reduction is crucial for ensuring the safety and reliability of the bolted joint. The relevant provision is found in Clause 10.3.3.2 of IS 800-2007.

The reduction in the design shear capacity of bolts, caused by packing plates that are thicker than 6 mm, is calculated using a specific reduction factor. This factor is commonly represented as \(k_b\).

\( k_b = 1 - 0.0125 \times t_{pm} \)

In this expression:

• \(k_b\) is the reduction factor applied to the design shear capacity of the bolts.
• \(t_{pm}\) represents the thickness of the thicker packing plate, measured in millimeters (mm). The question defines \(t_{pm}\) as the thickness of the thicker packing plate, which directly corresponds to the variable used in the standard's formula for calculating the reduction.

This formula shows that for every millimeter of packing plate thickness (above the 6 mm threshold), there is a specific decrease in the bolt's shear capacity. For instance, if \(t_{pm}\) is 12 mm, the reduction factor would be calculated as \(1 - (0.0125 \times 12) = 1 - 0.15 = 0.85\). This implies that the bolt's design shear capacity would be reduced to 85% of its original value, which is a 15% reduction.

IS 800-2007 Provision for Shear Capacity

According to Clause 10.3.3.2 of IS 800-2007, it is stated: "Where the total thickness of packing plates in a splice or connection is in excess of 6 mm, the design shear capacity of the bolts shall be reduced by a factor \(k_b\), where \(k_b = 1 - 0.0125 t_p\), and \(t_p\) is the total thickness of the packing plates in mm." The question uses \(t_{pm}\) to specifically refer to the thickness of the thicker packing plate, which is consistent with the application of this reduction formula in practical scenarios where the total thickness often dictates the reduction.

Based on the provisions of IS 800-2007, the expression for the reduction in design shear capacity of bolts when using packing plates in excess of 6mm is \(1 - 0.0125t_{pm}\), where \(t_{pm}\) is the thickness of the thicker packing plate.