Steel Section High Shear Force Definition (IS 800-2007)

When designing steel sections, it is crucial to consider the combined effects of different forces, such as shear and bending. A steel section, like a beam, is often subjected to both an applied shear force (\(V\)) and bending moments. The Indian Standard Code for general construction in steel, IS: 800 - 2007, provides specific guidelines for the design of such sections, particularly regarding the interaction of these forces.

Shear Force and Capacity in Steel Sections

The question refers to two key terms:

• Applied Shear Force (\(V\)): This is the actual shear force acting on the steel section due to external loads.
• Shear Capacity of the Section (\(V_s\)): This represents the maximum shear force that the steel section can resist safely, calculated based on its material properties and geometric dimensions as per IS: 800 - 2007. In design, this is typically the design shear resistance (\(V_d\)) which is derived from the nominal shear capacity.

Defining High Shear Force per IS 800 - 2007

According to IS: 800 - 2007, the interaction between shear and bending becomes significant when the applied shear force is high. The code defines "high shear force" for a steel section when the applied shear force (\(V\)) exceeds a specific percentage of the section's shear capacity (\(V_s\)).

The exact condition, as specified in Clause 8.1.1 of IS: 800 - 2007 (for beams subjected to combined shear and bending), states that if the factored applied shear force (\(V\)) exceeds 0.6 times the design shear capacity of the section (\(V_d\), which can be represented as \(V_s\) for simplicity in this context), then the bending strength of the section should be reduced.

Therefore, a high shear force condition is defined as:

Implications of High Shear Force

Understanding this threshold is critical for safe and economical design of steel sections:

• When \(V \le 0.6 V_s\): In this case, the shear force is considered "low". The interaction between shear and bending moment is generally considered negligible, and the design bending capacity of the section can be taken as its full plastic moment capacity without reduction due to shear.
• When \(V > 0.6 V_s\): This is the "high shear force" condition. When the applied shear force exceeds 60% of the shear capacity, the shear stresses in the web become significant enough to affect the bending capacity of the section. In such scenarios, IS: 800 - 2007 mandates that the design bending strength of the section must be appropriately reduced to account for the combined effects, typically using interaction equations provided in the code. This prevents premature yielding or buckling failure modes that might occur due to the combination of high shear and bending.

This provision ensures that steel sections are designed to safely withstand the combined stresses they experience in real-world applications, preventing structural failures.