What are the wind speed zones in India under IS 875 Part 3?

Six zones of basic wind speed: 33, 39, 44, 47, 50 and 55 m/s. Bengaluru is at 33, Pune and Ahmedabad at 39, Hyderabad and Mumbai at 44, Delhi and Jaipur at 47, and Chennai, Kolkata and Visakhapatnam at 50 m/s. Appendix A of the code lists the value for each major town.

What is the design wind pressure for a solar structure in Hyderabad?

Hyderabad has a basic wind speed of 44 m/s. With k1, k2, k3 and k4 all equal to 1.0 at Category 2 terrain, the design speed is 44 m/s and the pressure pz = 0.6 × 44² = 1,162 N/m², before the Kd, Ka and Kc factors and the pressure coefficients for the table geometry.

What does terrain category mean for a solar plant?

Terrain category describes the roughness of the surroundings. Category 2, open terrain with scattered obstructions under 10 m, applies to most ground-mount sites and gives k2 = 1.00 at 10 m. Category 3, closely spaced obstructions up to 10 m, gives k2 = 0.91 and is only justified for sites inside built-up industrial estates. Using Category 3 for open farmland under-designs the structure by about 17 per cent in pressure.

Does the cyclone factor k4 apply to solar plants?

Yes, for sites inside the coastal cyclonic strip defined in IS 875 Part 3: 2015, about 60 km wide on the east coast and parts of the Gujarat coast. Solar plants are normally treated as industrial structures with k4 = 1.15, which raises design speed by 15 per cent and pressure by 32 per cent over the same basic speed inland.

How much more steel does a higher wind zone need?

For a fixed-tilt 2P ground mount, moving from 39 m/s to 44 m/s adds about [8 to 12 per cent] to structure weight, to 47 m/s about [15 to 22 per cent], and to 50 m/s inland about [25 to 35 per cent]. Adding the k4 cyclonic factor at 50 m/s takes the increase to [40 to 50 per cent]. Foundations and pile embedment rise in step.

Engineering

How Wind Zone under IS 875 Part 3 Changes Solar Mounting Structure Design

IS 875 Part 3 wind speeds from 33 to 55 m/s, terrain category, the k1 to k4 factors, and how design pressure drives member sizes, foundations and steel per MW.

Vasistron engineering team · Published 11 September 2026 · Last reviewed 11 September 2026

Wind is the governing load

A solar array is a light tilted plate a short distance above ground or roof. Self-weight is [15 to 25 kg per square metre] including modules and there is no snow load over most of India, so wind uplift and drag govern every member from clamp to pile. IS 875 (Part 3): 2015, Wind Loads, is the reference, read with IS 800: 2007 for steel design, IS 801 for cold-formed light-gauge sections, IS 811 for cold-formed section dimensions and IS 456 for RCC foundations.

Basic wind speed zones

IS 875 Part 3 divides India into six zones of basic wind speed Vb: 33, 39, 44, 47, 50 and 55 m/s. Vb is a 3-second gust speed at 10 m height in open terrain (Category 2) with a return period of 50 years. Appendix A of the code lists Vb for major towns, and Figure 1 maps the zones. Wind pressure is proportional to the square of speed, so the 55 m/s zone carries 2.8 times the pressure of the 33 m/s zone.

Vb, m/s	Pressure 0.6 Vb², N/m²	Example cities from Appendix A	Typical design implication for solar structures
33	653	Bengaluru	Lightest sections; 2P post spacing up to [4.0 to 4.5 m]
39	913	Pune, Ahmedabad, Bhopal, Coimbatore, Kochi	Base case for many standard rooftop clamp designs
44	1,162	Hyderabad, Mumbai, Nagpur, Surat, Vadodara	Deccan reference case; purlin thickness steps from [1.6 to 2.0 mm] on longer spans
47	1,325	Delhi, Jaipur, Jodhpur, Lucknow, Kanpur, Patna	Post spacing reduced or rafter depth increased; pile embedment [1.5 to 2.0 m]
50	1,500	Chennai, Kolkata, Visakhapatnam, Bhubaneswar, Guwahati, Bhuj	Cyclonic belt on the east coast adds k4; bracing in both directions, HDG posts common
55	1,815	Darbhanga; small areas on Figure 1 in north Bihar, the north-east and the Saurashtra coast	Project-specific design; steel per MW [40 to 60 per cent] above the 39 m/s case

From basic speed to design speed: k1, k2, k3, k4

The design wind speed at height z is Vz = Vb × k1 × k2 × k3 × k4. Each factor has a plain meaning.

k1, probability or risk factor

k1 adjusts the 50-year basic speed for design life. Table 1 gives k1 = 1.0 for general structures with a 50-year life in every zone, slightly lower values for 25 years and lower still for temporary structures. Some designers adopt the 25-year value of about [0.90 to 0.94] for solar plants; the more common practice is k1 = 1.0, and a buyer should ask which was used.

k2, terrain roughness and height factor

k2 covers ground roughness and height. The four terrain categories are: Category 1, open sea coast and flat treeless plains; Category 2, open terrain with scattered obstructions 1.5 to 10 m high, which describes most ground-mount sites; Category 3, numerous closely spaced obstructions up to 10 m, such as industrial estates and suburbs; Category 4, city centres with tall buildings. At 10 m height Table 2 gives k2 of 1.05, 1.00, 0.91 and 0.80 for Categories 1 to 4, and the 10 m value applies to a ground mount at 3 m. For a rooftop array on a [12 to 20 m] industrial shed in Category 2, pressure at roof level is [5 to 10 per cent] higher than at 10 m. Choosing Category 3 for open Category 2 farmland underestimates pressure by about 17 per cent and is a common error.

k3, topography factor

k3 is 1.0 on flat ground and rises to 1.36 on the crests of hills, ridges and cliffs where wind accelerates; sites on plateau edges and slopes should be checked against Appendix C.

k4, importance factor for cyclonic region

Added in the 2015 revision, k4 applies within the 60 km coastal strip on the east coast and parts of the Gujarat coast. It is 1.30 for structures of post-cyclone importance, 1.15 for industrial structures and 1.00 otherwise. A solar plant on the Andhra Pradesh or Odisha coast is normally treated as industrial, k4 = 1.15, which raises pressure by 32 per cent over the same Vb inland.

From design speed to design pressure

Wind pressure is pz = 0.6 Vz² in N/m². The 2015 code then gives design pressure pd = Kd × Ka × Kc × pz, where Kd is the directionality factor, 0.9 for most structures; Ka is the area averaging factor, 1.0 up to 10 m² of tributary area, 0.9 at 25 m² and 0.8 at 100 m² and above; and Kc is the combination factor for internal and external pressure. pd may not be less than 0.7 pz.

For a 2P table in Hyderabad, Category 2: Vb = 44, all k = 1.0, pz = 1,162 N/m²; with Kd = 0.9 and Ka about 0.9 for a rafter tributary area near 25 m², pd is about 940 N/m² before pressure coefficients. The same table at Visakhapatnam with Vb = 50 and k4 = 1.15 has Vz = 57.5 m/s and pz = 1,984 N/m², 71 per cent more.

Pressure coefficients and member sizing

Module pressure is pd multiplied by a net pressure coefficient. The code gives coefficients for free-standing monoslope roofs, the closest match to ground-mount tables and carports, and local coefficients at edges, ridges and corners of pitched roofs, which apply to rooftop arrays. Net uplift coefficients on an open tilted table are about [1.2 to 2.0], highest at end tables and windward edges.

Sizing follows the load path. Clamps and rails see the local coefficient over a small area. Purlins are checked to IS 801 for bending and lateral-torsional buckling under uplift, which puts the unrestrained flange in compression. Rafters and posts are checked to IS 800 for combined bending and axial load, with deflection limits of [span/150 to span/180] for purlins and [span/240] for rafters. Bracing takes the drag along and across rows.

Effect on foundations

Uplift governs foundations. The net upward force per post on a 2P table in a 44 m/s zone is of the order of [15 to 25 kN], and can exceed [35 kN] in a 50 m/s cyclonic zone. Driven piles resist by skin friction and need [1.2 to 2.0 m] embedment in typical soils, confirmed by pull-out tests on [1 to 2 per cent] of piles to [1.5 times] design uplift. RCC piers to IS 456 resist by self-weight and soil friction. On RCC roofs the uplift is matched by ballast of [60 to 120 kg per square metre of array] or tested anchors; on metal-sheet roofs it passes to the purlins, whose fixing to the rafters becomes the critical check.

Effect on steel weight per MW

Because some members are governed by minimum thickness or deflection rather than strength, steel weight rises more slowly than pressure. Relative to a 39 m/s Category 2 design, typical weight increases for a fixed-tilt 2P ground mount are about [8 to 12 per cent] at 44 m/s, [15 to 22 per cent] at 47 m/s, [25 to 35 per cent] at 50 m/s inland, [40 to 50 per cent] at 50 m/s with k4 = 1.15, and [40 to 60 per cent] at 55 m/s. Rooftop clamp systems see smaller changes because the existing purlins carry the load, but the number of fixings per module rises.

About Vasistron

Greentek Vasistron Private Limited, Dhulapally, Hyderabad, designs the SolBase Terra ground mount, SolBase Aero carport and canopy, and SolBase Tecta rooftop and shed structures to IS 875 Part 3 for the basic wind speed, terrain category and k-factors of each site. The structures use GI cold-rolled C, Z and U sections with SS304 fasteners and are assembled by bolting with no site welding.

FAQ

Questions on this topic

What are the wind speed zones in India under IS 875 Part 3?: Six zones of basic wind speed: 33, 39, 44, 47, 50 and 55 m/s. Bengaluru is at 33, Pune and Ahmedabad at 39, Hyderabad and Mumbai at 44, Delhi and Jaipur at 47, and Chennai, Kolkata and Visakhapatnam at 50 m/s. Appendix A of the code lists the value for each major town.
What is the design wind pressure for a solar structure in Hyderabad?: Hyderabad has a basic wind speed of 44 m/s. With k1, k2, k3 and k4 all equal to 1.0 at Category 2 terrain, the design speed is 44 m/s and the pressure pz = 0.6 × 44² = 1,162 N/m², before the Kd, Ka and Kc factors and the pressure coefficients for the table geometry.
What does terrain category mean for a solar plant?: Terrain category describes the roughness of the surroundings. Category 2, open terrain with scattered obstructions under 10 m, applies to most ground-mount sites and gives k2 = 1.00 at 10 m. Category 3, closely spaced obstructions up to 10 m, gives k2 = 0.91 and is only justified for sites inside built-up industrial estates. Using Category 3 for open farmland under-designs the structure by about 17 per cent in pressure.
Does the cyclone factor k4 apply to solar plants?: Yes, for sites inside the coastal cyclonic strip defined in IS 875 Part 3: 2015, about 60 km wide on the east coast and parts of the Gujarat coast. Solar plants are normally treated as industrial structures with k4 = 1.15, which raises design speed by 15 per cent and pressure by 32 per cent over the same basic speed inland.
How much more steel does a higher wind zone need?: For a fixed-tilt 2P ground mount, moving from 39 m/s to 44 m/s adds about [8 to 12 per cent] to structure weight, to 47 m/s about [15 to 22 per cent], and to 50 m/s inland about [25 to 35 per cent]. Adding the k4 cyclonic factor at 50 m/s takes the increase to [40 to 50 per cent]. Foundations and pile embedment rise in step.