In a cavity wall, both leaves of which are load bearing, the effective thickness is taken as:
🔬 Understanding Effective Thickness of Cavity Walls
The effective thickness is a calculated value used to determine the slenderness ratio of a wall, which in turn determines its load-carrying capacity. For a simple solid wall, the effective thickness is just its actual thickness. However, for a cavity wall, the calculation is more complex because the two leaves (or wythes) and the air gap do not act as a single solid unit.
The Challenge: The two leaves of a cavity wall are connected by flexible metal ties, so they don't share loads as efficiently as a solid wall. The design codes, like IS 1905, provide a specific method to calculate an equivalent "effective" thickness that accounts for this composite action.
⚖️ Detailed Analysis of the IS 1905 Provision
The question asks how to determine the effective thickness when both leaves of the cavity wall are load-bearing. According to IS 1905, the effective thickness (tₑꜰ) is calculated in two ways, and the final value used for design is the greater of the two.
Calculation 1: (Option B)
The first method considers the combined stiffness of the two leaves acting together. The effective thickness is taken as two-thirds (⅔) of the sum of the actual thicknesses of the two leaves.
Formula: tₑꜰ₁ = ⅔ × (thickness of leaf 1 + thickness of leaf 2)
Calculation 2: (Option C)
The second method considers the strength of the stronger of the two leaves. The effective thickness is taken as the actual thickness of the stronger leaf. If both leaves are of the same material and thickness, this is simply the thickness of one leaf.
Formula: tₑꜰ₂ = thickness of the stronger leaf
(d) Larger of (B) and (C)
Why it's correct: The code mandates that the designer must calculate the effective thickness using both methods described above and then use the larger of the two values for the slenderness ratio calculation. This ensures a conservative and safe design. Therefore, this option is the complete and correct answer.
💡 Study Tips for Cavity Walls
- Two Rules for Two Leaves: Remember that for a load-bearing cavity wall, there are two rules to calculate effective thickness.
- Rule 1: ⅔ of the total thickness.
- Rule 2: Thickness of the stronger leaf.
- Always Take the Larger Value: The final step is to compare the results of the two rules and choose the greater one. This is a key detail often tested in exams.