In the case of panel wall subjected to horizontal loads at right angles to the plane of the wall, with the mortar not leaner than M1 type, tensile stress in bending in the vertical direction may be allowed to the extent of:
🔬 Understanding Bending in Masonry Walls
This question deals with the structural design of unreinforced masonry walls according to building codes. A "panel wall" is an exterior non-load-bearing wall that is supported by a structural frame. It must be able to resist horizontal loads, primarily from wind pressure.
Bending in the Vertical Direction: When wind pushes against a wall that is supported by the floor and roof slabs, the wall bends like a vertical beam or slab. This bending creates compressive stress on the leeward (downwind) face and tensile stress on the windward (upwind) face. This tensile stress acts across the horizontal mortar bed joints, trying to pull them apart.
⚖️ Detailed Analysis of IS 1905 Code Provisions
The allowable stresses for unreinforced masonry are specified in IS 1905: Code of Practice for Structural Use of Unreinforced Masonry. Masonry is very weak in tension, so the code allows for only very small tensile stresses to prevent cracking.
Table 8 of IS 1905 specifies the "Permissible Tensile Stresses for Masonry." For bending tensile stress, it gives values for two conditions:
- Tension acting parallel to the bed joint (when the wall bends horizontally).
- Tension acting normal (perpendicular) to the bed joint (when the wall bends vertically).
The question asks for stress in the vertical direction, which corresponds to tension acting normal to the bed joints. The code provides the following values based on the mortar type:
- For M1 type mortar: 0.1 N/mm²
- For M2 type mortar: 0.07 N/mm²
Unit Conversion and Final Answer
The question requires the answer in kg/cm². We need to convert the code value for M1 mortar.
1 N/mm² ≈ 10.2 kg/cm²
Therefore, 0.1 N/mm² = 0.1 × 10.2 kg/cm² = 1.02 kg/cm²
This value is closest to option (c) 1.0 kg/cm².
💡 Study Tips for Masonry Design Codes
- Know Your Mortar Types: M1 is a high-strength mortar (e.g., 1:3 cement-sand), while M2 is a medium-strength mortar (e.g., 1:5 cement-sand). Higher strength mortar allows for higher permissible stress.
- Memorize Key Stress Values: The permissible tensile stress values from IS 1905 are frequently asked. For vertical bending, remember 0.1 N/mm² for M1 and 0.07 N/mm² for M2.
- Master the Unit Conversion: Be comfortable converting between N/mm² (which is the same as MPa) and kg/cm². The approximate conversion factor of 10 is often sufficient for multiple-choice questions (1 N/mm² ≈ 10 kg/cm²).