The basic stress in masonry units having height to width ratio of 1.5 may be increased by a factor of:
🔬 Understanding the Shape Modification Factor
The compressive strength of a masonry unit (like a brick or block) is not an absolute value. It is influenced by the unit's geometry. The standard crushing strength test is performed on a specimen with a specific shape. If the actual units used in construction have a different shape, their effective strength will be different. To account for this, design codes like IS 1905 provide a "Shape Modification Factor."
The Principle: The strength is related to the height-to-width ratio (or height-to-thickness ratio) of the unit. Squatter, more stable units (with a low ratio) are less prone to buckling and exhibit higher effective strength. Taller, more slender units (with a high ratio) are more likely to buckle and have a lower effective strength. The code allows for an increase in the basic permissible stress for squatter units.
⚖️ Detailed Analysis of IS 1905 Table 9
The question requires us to find the specific increase factor for a masonry unit with a height-to-width ratio of 1.5. This is given in Table 9 of IS 1905:1987.
| Height to Width Ratio of Unit | Shape Modification Factor |
|---|---|
| 0.6 or less | 1.0 |
| 0.75 | 1.1 |
| 1.0 | 1.2 |
| 1.5 | 1.6 |
| 2.0 | 1.8 |
| 3.0 | 2.0 |
| 4.0 or more | 2.0 |
(c) 1.6
Why it's correct: As shown in the table from IS 1905, for a masonry unit with a height-to-width ratio of 1.5, the basic permissible compressive stress can be multiplied by a factor of 1.6.
💡 Study Tips for Masonry Design Codes
- Squatter is Stronger: Remember the general trend: as the height-to-width ratio decreases (the brick gets squatter), the shape modification factor increases, meaning you can allow for higher stress.
- Memorize Key Ratios: For exams, it's highly beneficial to memorize a few key pairs from this table, especially the common ones:
- Ratio 1.0 → Factor 1.2
- Ratio 1.5 → Factor 1.6
- Don't Confuse with Slenderness Ratio: The shape modification factor applies to the individual masonry *unit*. The slenderness ratio applies to the entire *wall* or column.