If the eccentricity ratio is more than 1/24, then increase in the permissible stress in the design of wall subjected to eccentric loading as per code is:
🔬 Understanding Eccentric Loading in Masonry
A load on a wall is "eccentric" if its line of action does not pass through the centroid (the geometric center) of the wall's cross-section. This is very common in practice, for example, when a floor slab rests only on the inner part of a wall.
Effect of Eccentricity: An eccentric load creates a combination of direct compressive stress and bending stress. This results in a non-uniform, trapezoidal stress distribution across the wall's thickness. The maximum stress occurs at one face, and the minimum stress at the opposite face.
Eccentricity Ratio (e/t): This is the ratio of the eccentricity (e) to the thickness of the wall (t). It's a measure of how off-center the load is.
⚖️ Detailed Analysis of the IS 1905 Provision
The design of masonry is governed by IS 1905: Code of Practice for Structural Use of Unreinforced Masonry. The code recognizes that when a wall is subjected to bending, the maximum compressive stress only occurs at the extreme fiber. To account for this non-uniform stress distribution, the code allows for an increase in the basic permissible compressive stress under certain conditions.
(b) 25%
Why it's correct: According to Clause 5.4.2.1 of IS 1905, for members subjected to eccentric loading (axial load + bending), the permissible compressive stress can be increased. The code states that if the eccentricity ratio (e/t) is greater than 1/24 but not more than 1/6, the permissible compressive stress may be increased by 25 percent. This increase is allowed because the peak stress is localized, and the overall section has reserve capacity.
Understanding the Limits:
- If e/t ≤ 1/24: The eccentricity is considered negligible. No increase in stress is permitted.
- If 1/24 < e/t ≤ 1/6: The load is moderately eccentric. A 25% increase in permissible stress is allowed. This is the condition specified in the question.
- If e/t > 1/6: The eccentricity is large, and tensile stresses will develop on one face of the wall. The design must then also check for permissible tensile stress, which is very low for masonry.
💡 Study Tips for Masonry Design
- Eccentricity can be "Good": Remember this counter-intuitive rule. A small amount of eccentricity allows for an *increase* in the permissible *compressive* stress because the stress is not uniform.
- Memorize the Key Ratios: The eccentricity ratios of 1/24 and 1/6 are critical boundaries in masonry design.
- The 25% Rule: The most common scenario tested is the one where the eccentricity falls between these two limits, which corresponds to the 25% increase.