Shear Buckling in Steel Webs

Steel beams are structural elements that resist bending and shear forces. The web of a beam is primarily responsible for resisting shear stresses. Under significant shear stress, the web, especially if it is thin compared to its depth, can become unstable and buckle. This phenomenon is known as shear buckling.

Unstiffened Web Buckling Check

IS 800 is the Indian standard code of practice for general construction in steel. It provides guidelines and criteria for the design and verification of steel structures to ensure safety and serviceability. For steel members with unstiffened webs (webs without vertical or horizontal stiffeners), the code specifies conditions under which the resistance to shear buckling must be explicitly verified.

The susceptibility of a web to shear buckling depends significantly on its slenderness ratio. A key parameter in this context is the web slenderness, often represented by the ratio of the web depth to its thickness, denoted as $\frac{d}{t_w}$. Here, $d$ is the depth of the web and $t_w$ is its thickness.

Another parameter involved is $\varepsilon$ (epsilon), which accounts for the yield strength of the steel material. It is defined as:

\( \varepsilon = \sqrt{\frac{250}{f_y}} \)

where \(f_y\) is the yield strength of the steel in MPa.

IS 800 Verification Condition

According to IS 800, for an unstiffened web, the resistance to shear buckling must be verified if the web slenderness ratio $\frac{d}{t_w}$ exceeds a certain limit related to the material property $\varepsilon$. If the web is stocky enough (meaning $\frac{d}{t_w}$ is below a certain limit), its shear strength is typically governed by shear yielding rather than buckling.

The code specifies that shear buckling verification is required when:

• The ratio of web depth to thickness, $\frac{d}{t_w}$, is greater than $67\varepsilon$.

This means that if $\frac{d}{t_w} \le 67\varepsilon$, the web is considered sufficiently stocky to resist shear buckling without needing an explicit buckling check; its capacity is limited by shear yielding. When $\frac{d}{t_w} > 67\varepsilon$, the web is slender enough that shear buckling becomes a critical limit state, and its resistance must be calculated based on buckling considerations.

Therefore, as per IS 800 for an unstiffened web, the resistance to shear buckling should be verified when $\frac{d}{t_w} > 67\varepsilon$.