The number in a TMT grade is the minimum yield strength in N/mm². The letter D means higher ductility: the bar can stretch more before breaking. That matters in earthquake zones, and West Bengal includes zones III, IV and V.
Mechanical Requirements (IS 1786:2008)
| Grade | Min. yield / 0.2% proof stress (N/mm²) | Tensile strength | Min. elongation (%) |
|---|---|---|---|
| Fe415 | 415 | ≥ 1.10 × yield, and ≥ 485 N/mm² | 14.5 |
| Fe415D | 415 | ≥ 1.12 × yield, and ≥ 500 N/mm² | 18.0 |
| Fe500 | 500 | ≥ 1.08 × yield, and ≥ 545 N/mm² | 12.0 |
| Fe500D | 500 | ≥ 1.10 × yield, and ≥ 565 N/mm² | 16.0 |
| Fe550 | 550 | ≥ 1.06 × yield, and ≥ 585 N/mm² | 10.0 |
| Fe550D | 550 | ≥ 1.08 × yield, and ≥ 600 N/mm² | 14.5 |
Fe500D is the usual choice for residential RCC in Kolkata today. It has good strength and the ductility margin structural engineers want for seismic design.
Checking Weight: the d²/162 Rule
The nominal mass of a bar in kg per metre is its diameter in mm squared, divided by 162.
| Diameter (mm) | 8 | 10 | 12 | 16 | 20 | 25 |
|---|---|---|---|---|---|---|
| kg per metre | 0.395 | 0.617 | 0.889 | 1.58 | 2.47 | 3.85 |
Weigh a few 1 m cut pieces from each lot. Consistently light bars are a red flag.
On Delivery
- BIS mark, manufacturer and grade embossed on the bar
- Mill test certificate matching the lot and heat number
- Diameter and weight spot checks
- Steel stored off the ground and covered, away from standing water
- For larger projects, samples sent to a lab for tensile and bend tests
Reference
- IS 1786:2008 High strength deformed steel bars and wires for concrete reinforcement
Summary for site use. Always work from the current edition of the standard and your project specification. Where they differ, the structural engineer’s specification governs.