The code behind every tower, mast and mounting structure we build. The formula, the four factors, and the number that applies where we are.
IS 875 Part 3 gives the wind loads used to design buildings and structures in India. It starts from a basic wind speed read off the national map, then modifies it for return period, terrain and height, topography and importance.
Kolkata and Howrah sit at a basic wind speed of 50 m/s — Zone V, the highest band on the map, shared with Bhubaneswar, Chennai, Guwahati, Vijayawada and Visakhapatnam. For comparison, Delhi is 47 m/s, Mumbai and Nagpur 44 m/s, and Bangalore 33 m/s. Steel fabricated here is designed for the harder end of the Indian wind envelope.
Vz = Vb × k1 × k2 × k3 × k4
| Factor | What it accounts for | Typical value |
|---|---|---|
| Vb | Basic wind speed from the zone map | 33–50 m/s depending on location |
| k1 | Risk coefficient — the return period the structure is designed for | 1.0 for 50-year return; 1.07 for 100-year |
| k2 | Terrain category and structure height | roughly 0.67 to 1.35 |
| k3 | Topography — hills, ridges and escarpments | 1.0 on flat ground, up to about 1.36 |
| k4 | Importance factor for cyclone-prone coastal regions | 1.0 standard; 1.15 in cyclonic coastal areas |
Design wind pressure follows as pz = 0.6 × Vz² in N/m². The two factors most often misapplied are k2, because terrain category is a judgement and height is measured to the top of the structure, and k4, which many designers omit on coastal sites where it belongs.
| City | Basic wind speed Vb |
|---|---|
| Kolkata / Howrah | 50 m/s (Zone V) |
| Bhubaneswar, Chennai, Guwahati, Vijayawada, Visakhapatnam | 50 m/s |
| Delhi | 47 m/s |
| Mumbai, Nagpur | 44 m/s |
| Bangalore | 33 m/s |
Always read the value for the actual site off the map in the current edition of the code rather than from a list. City names are a guide; the zone boundary is what governs.
Because wind is the load case that sizes almost everything we make. A telecom tower, an OHE mast, a solar module mounting structure and a substation gantry are all wind-governed structures, and the difference between a 44 m/s site and a 50 m/s site is roughly a 29% increase in design pressure — pressure goes with the square of the speed.
We fabricate to the designer's approved drawing and do not carry out the wind analysis. What we can tell you is that structures leaving our works are built for a 50 m/s environment, because that is the one they stand in.
Answered from our own line, not from the standard's cover page.
50 m/s — Zone V, the highest band in IS 875 Part 3. Howrah, where our works is, sits in the same zone.
Vz = Vb × k1 × k2 × k3 × k4, where Vb is the basic wind speed from the map, k1 is the risk coefficient, k2 covers terrain and height, k3 covers topography and k4 is the importance factor for cyclone-prone coastal regions. Design pressure is pz = 0.6 × Vz² in N/m².
It combines terrain category with the height of the structure, and it typically ranges from about 0.67 to 1.35. It is the factor most often applied loosely, because terrain category involves judgement about the surroundings and height is measured to the top of the structure, not the eaves.
No. We fabricate to the approved drawing supplied by the line designer or consultant. What we do is build and galvanize accurately against it, and certify the material and the coating.
Effectively all of them — telecom towers and monopoles, transmission and substation structures, railway OHE masts and portals, and solar module mounting structures are all sized by wind rather than by dead load.
Tell us the standard, the coating requirement and the component, and we will confirm in writing what we can hold and how we will certify it. Galvanizing is on our own floor, so the answer comes from us and not from a sub-contractor.
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