For masonry built in 1 : 1 : 6 cement-lime-sand mix mortar or equivalent, the horizontal shear stress permissible on the area of a mortar bed joint is:
🔬 Understanding Horizontal Shear Stress
Horizontal shear stress occurs in a masonry wall when it is subjected to lateral loads, such as wind or earthquake forces. This stress acts parallel to the horizontal mortar bed joints, trying to slide one course of bricks over the one below it. The bond between the mortar and the bricks provides the resistance to this shearing action.
Permissible Stress: To ensure the safety of the structure, design codes like IS 1905 specify the maximum (permissible) shear stress that a mortar joint can safely withstand. This value depends on the strength of the mortar used.
⚖️ Detailed Analysis of IS 1905 Provisions
The first step is to identify the grade of the mortar specified in the question. A 1 : 1 : 6 (Cement : Lime : Sand) mix is a standard composite or "gauged" mortar. According to IS 2250 and IS 1905, this mix corresponds to an M1 grade mortar, which has a minimum compressive strength of 5 N/mm² (MPa).
Next, we refer to Table 11 of IS 1905, which specifies the permissible shear stress for masonry.
| Mortar Grade | Permissible Shear Stress (N/mm² or MPa) |
|---|---|
| H1 | 0.15 |
| H2 | 0.15 |
| M1 | 0.15 |
| M2 | 0.1 |
| L1 | 0.1 |
| L2 | 0.1 |
(a) 0.15 MPa
Why it's correct: As shown in the table from IS 1905, for an M1 grade mortar (which is equivalent to a 1:1:6 mix), the permissible horizontal shear stress is 0.15 N/mm², which is the same as 0.15 MPa.
💡 Study Tips for Masonry Design
- Link Mix Ratio to Mortar Grade: This is a crucial first step. Memorize that a 1:1:6 composite mix is equivalent to an M1 grade mortar.
- Memorize Key Stress Values: For exams, knowing the permissible stresses for common mortar grades is essential. For shear, the key values are 0.15 MPa for stronger mortars (H1, H2, M1) and 0.1 MPa for weaker ones (M2, L1, L2).
- Distinguish Stress Types: Be careful to distinguish between permissible compressive, tensile, and shear stresses. They all have different values in the code.