IS 875 Part 3 — Wind Load Calculations for Indian Solar Mounts

Wind is the primary structural load that governs solar mounting design in India. Module racking that survives a Rajasthan dust storm or a Tamil Nadu cyclone season is not accidental — it is the product of a rigorous wind load calculation under IS 875 Part 3, the Indian standard for wind loads on structures. Specifying the wrong wind zone or missing the pressure coefficient for an inclined surface can mean the difference between a 25-year structure and one that fails in Year 3.

Direct answer. IS 875 Part 3 (2015 revision) governs wind load calculations for solar mounting structures in India. The design wind pressure is: Pz = 0.6 × Vz², where Vz = Vb × k1 × k2 × k3 × k4 is the design wind speed at height z. For inclined solar surfaces, pressure and suction coefficients (Cp) for the panels are applied separately to uplift and downforce. The Six-Factor Calculation Chain framework systematizes this into a repeatable calculation that feeds directly into STAAD Pro structural analysis.

This guide is written for Indian EPC engineers, structural consultants, and developers who need to understand the IS 875 Part 3 calculation chain — from basic wind speed selection through to the STAAD Pro load combination that yields the final pile or foundation design. Every formula is explicit.

IS 875 Part 3 — The Regulatory Foundation

IS 875 Part 3:2015 (Wind Loads), published by the Bureau of Indian Standards, is the Indian counterpart to wind load standards like ASCE 7-22 in the USA or EN 1991-1-4 in Europe. It applies to all structures in India, including solar mounting systems, which are classified as “open structures” or “canopies” depending on configuration.

The 2015 revision of IS 875 Part 3 introduced several changes relevant to solar:

• Updated basic wind speed map: The zonal map was revised based on 50 years of meteorological data. Several districts in Rajasthan, Gujarat, and coastal Tamil Nadu moved to higher wind speed zones.
• Updated terrain categories: Four terrain categories (I–IV) now more precisely distinguish open terrain from urban environments.
• Revised pressure coefficient tables: Tables for flat and inclined surfaces were updated, affecting the design of ground-mount arrays at tilts from 0° to 35°.
• Topography factor k3: Better guidance for hillside installations, relevant for elevated ground-mount sites in hilly terrain.

India’s Basic Wind Speed Zones — Selecting Vb

India’s basic wind speed map in IS 875 Part 3 divides the country into zones with basic wind speeds (Vb) from 33 m/s to 55 m/s, corresponding to 3-second gust speeds at 10 m above ground in terrain category II (open country with scattered obstructions) with a 50-year return period.

Basic Wind Speed (Vb)	Regions Covered	Structural Implication
33 m/s	Parts of central India (Chhattisgarh, Jharkhand)	Lower wind zone — lighter structure possible
39 m/s	Most of the Indo-Gangetic plain (UP, Bihar, MP)	Moderate wind zone
44 m/s	Rajasthan, large parts of Gujarat, Delhi NCR	High wind zone — governs most ground-mount bids
47 m/s	Parts of Andhra Pradesh, Karnataka interior	High wind zone
50 m/s	Coastal Andhra, Odisha coast, parts of Tamil Nadu	Very high — cyclone exposure
55 m/s	Andaman & Nicobar Islands, cyclone-prone coastal areas	Maximum zone — requires detailed analysis

The Six-Factor Calculation Chain — The Named Framework

The Six-Factor Calculation Chain is the complete sequence from basic wind speed to design wind pressure, applied step by step with no approximations skipped.

Pressure Coefficients for Inclined Solar Surfaces

The design wind pressure Pz gives the base load. For a solar panel at a given tilt angle, the actual wind force depends on the pressure coefficient Cp, which accounts for the direction of wind relative to the inclined surface.

IS 875 Part 3 Section 6.3 provides Cp values for “open-topped” and “closed” canopy roofs — the appropriate analogy for ground-mount solar arrays. For solar panels:

Wind Direction	Tilt Angle	Cp (Pressure)	Cp (Suction)
Wind toward the lower edge	10°	+0.5	-1.3
Wind toward the lower edge	15°	+0.5	-1.5
Wind toward the lower edge	20°	+0.6	-1.5
Wind toward the higher edge	10°	-0.9	+0.4
Wind toward the higher edge	15°	-1.1	+0.5
Wind toward the higher edge	20°	-1.3	+0.5
Wind parallel to rows	10°–20°	±0.7	±0.7

Net force per unit area = Pz × Cp

For downforce (pressure coefficient positive toward the surface): F_down = Pz × Cp_pressure For uplift (suction coefficient negative, pulling surface upward): F_up = Pz × |Cp_suction|

The critical case for most ground-mount designs is uplift when wind strikes the high edge of the array (wind “going under” the panel) — this produces the highest suction force on the panel and the highest tension force in the anchor bolts.

STAAD Pro Integration — From IS 875 to Structural Analysis

The IS 875 Part 3 calculation produces wind pressures, not forces. To input into STAAD Pro, convert to nodal forces at the mounting structure support points:

Wind force per support = Pz × Cp × Tributary area per support

Tributary area = (Horizontal projected area of panels supported by one structure) / (Number of support points per structure)

For a two-column structure supporting 24 panels of 2.1 m × 1.1 m at 15° tilt:

• Panel area = 24 × 2.1 × 1.1 = 55.4 m²
• Horizontal projected area = 55.4 × cos(15°) = 53.5 m²
• Number of support points = 4 (2 per column, top and bottom)
• Tributary area per support = 53.5 / 4 = 13.4 m²

For Pz = 898 N/m² and Cp = -1.5 (maximum suction, wind toward high edge):

• Uplift force per support = 898 × 1.5 × 13.4 = 18,051 N = 18.05 kN (upward)

This nodal force is applied to the STAAD Pro model at the four support points with an upward direction vector. The pile or footing is then designed to resist this uplift plus the self-weight of the structure.

According to BIS technical standards for structural engineering, IS 875 Part 3 in combination with IS 800 (general structural steel design) constitutes the mandatory design code for all steel-framed solar mounting structures in India.

The IS 800 load combination that governs wind:

1.2(DL) + 1.2(WL) for wind upward case (most critical for uplift) 1.5(WL) for pure wind case

Where DL = Dead Load (self-weight of modules + racking), WL = Wind Load (calculated as above).

IS 875 vs. ASCE 7-22 — Key Differences for Solar

Many Indian EPCs work with US investors or lenders who reference ASCE 7-22. Understanding the key differences between the two standards is essential for projects with international financing:

Parameter	IS 875 Part 3 (India)	ASCE 7-22 (USA)
Wind speed basis	3-second gust, 50-year return period	3-second gust, varies by risk category (700-year for Risk Cat IV)
Terrain categories	4 categories (I–IV)	3 exposure categories (B, C, D)
Pressure coefficients for solar	Canopy roof analogy (Table 14)	Chapter 29, open building solar panels
Return period for structures	50 years (Class B)	700 years for high-risk; 25 years for pre-engineered
Partial load factors	IS 800 load combinations	ASCE 7 strength design combinations
Standard for structural steel	IS 800	AISC 360
Equivalence	Broadly similar for low-rise structures	More conservative at high wind speeds

For projects seeking financing from IREDA or PFC under Indian schemes, IS 875 is the governing standard. For projects seeking international DFI financing (AfDB, IFC), some lenders require dual compliance or acceptance of IS 875 with a third-party structural review confirming equivalence.

According to IRENA’s cost analysis for solar in India, structural failure due to wind — including inadequate wind load design — remains one of the top three causes of early-life insurance claims in Indian solar installations.

Foundation Design Under Wind Load

The wind uplift force calculated from IS 875 governs the foundation design. For driven pile foundations (most common in India):

Pile pull-out capacity = Skin friction along pile length × Safety factor

Skin friction depends on soil type (SPT N-value from geotechnical investigation). For a sandy loam at N = 15 blow/30 cm:

• Unit skin friction ≈ 12 kN/m²
• For a 65 mm OD, 2 m embedded pile: perimeter = π × 0.065 = 0.204 m; embedded area = 0.204 × 2.0 = 0.408 m²; capacity = 12 × 0.408 = 4.9 kN
• At 18.05 kN uplift per support (from earlier example), four piles provide 4 × 4.9 = 19.6 kN > 18.05 kN ✓ with a safety factor of 1.08

For soft soils or expansive black cotton soils (common in Maharashtra, Karnataka), the safety factor against pull-out must be increased to 2.5–3.0, requiring deeper piles or concrete footings.

The complete structural calculation — from IS 875 wind pressure through IS 800 load combinations to IS 456 foundation design — is what the STAAD Pro report delivers as a bankable structural calculation document accepted by CEIG offices and lenders.

Special Cases — Tracker Systems and Floating Solar

For single-axis tracker systems, the wind load calculation changes because the panel tilt varies continuously from 0° to 60°. The worst-case wind load is typically at maximum tilt angle (60°) with wind striking the high edge. IS 875 Part 3 does not have a dedicated section for tracking structures — the structural engineer must apply the canopy roof pressure coefficients at each relevant tilt angle and identify the governing case.

Most tracker manufacturers provide their own structural qualification reports to IS 875 or ASCE 7-22 through wind tunnel testing, which provides more accurate Cp values than the IS 875 table analogy. Always request the tracker manufacturer’s wind load report and verify it references the correct Indian wind zone for the project site.

For floating solar, wind load is applied to the floating structure rather than to ground-mounted supports. The floating body adds horizontal wind drag and wave-induced forces not covered by IS 875 Part 3. Floating solar structural design typically requires IS 875 for wind plus IS 456 for anchoring, with hydrodynamic loads analyzed separately per IS 4651 (for ports and harbors) or project-specific guidance.

According to IEA-PVPS Task 13 report on floating solar (2022), wind load is the dominant structural design parameter for floating systems in inland water bodies, where wave loading is lower than in coastal applications but wind exposure can be high due to unobstructed fetch.

How Heaven Designs Helps With IS 875 Structural Design

Structural calculations under IS 875 Part 3 require site-specific input data, correct pressure coefficient selection, and a STAAD Pro model that reflects the actual array geometry. Errors in any of these three areas produce a calculation that the CEIG office or the lender’s independent engineer will reject.

• Solar Civil and Structural Engineering — STAAD Pro and SAP2000 structural analysis for ground-mount and rooftop solar, covering IS 875 Part 3 wind loads, IS 456 foundation design, and IS 800 steel member sizing. Bankable calculation reports accepted by CEIG and IREDA.
• STAAD Pro Reports — standalone structural calculation reports for mounting structures, usable for CEIG submission and lender due diligence.
• Solar Ground Mount Design — full utility-scale layout including structural design that integrates IS 875 wind zones into the mounting design from the first draft.
• Site Survey and Land Feasibility — includes terrain category assessment and wind exposure check that feeds directly into the IS 875 calculation.
• Download design samples — see a sample IS 875 calculation package.

The structural calculation report Heaven Designs produces includes: Vb selection with source citation, k1–k4 factor justification, Cp selection with IS 875 table reference, STAAD Pro model screenshots with load diagrams, member stress results, and foundation pull-out calculations. Contact us for a project-specific quote.

FAQ

Which edition of IS 875 Part 3 applies to solar projects in India?

The current edition is IS 875 Part 3:2015, the third revision. This edition supersedes the 1987 version and includes the updated wind speed map, revised terrain categories, and new Cp values for canopy-type structures relevant to solar panels. All new structural calculations for solar mounting should reference the 2015 edition. If you have older STAAD Pro reports referencing IS 875:1987, they should be updated before submission to CEIG or any lender performing technical due diligence.

What wind speed should be used for a 25-year solar project in Rajasthan?

The basic wind speed Vb for most of Rajasthan is 44 m/s per IS 875 Part 3:2015. With k1 = 1.0 (Class B, 25-year design life), k2 = 0.88 (Category 2 terrain at 5 m height), k3 = 1.0 (flat terrain), and k4 = 1.0 (non-cyclonic), the design wind speed is Vz = 44 × 1.0 × 0.88 × 1.0 × 1.0 = 38.7 m/s, giving a design wind pressure of 898 N/m². Confirm the exact Vb for the specific district using IS 875 Part 3 Appendix A, as Jaisalmer and Barmer may have different values from Jodhpur.

Does IS 875 Part 3 cover tracker systems?

IS 875 Part 3 does not have a dedicated section for solar tracking structures. The standard practice is to apply the canopy roof pressure coefficients from IS 875 Part 3 at the maximum tilt angle and with wind attacking from the most critical direction. Most tracker manufacturers supplement this with wind tunnel test data at various tilt angles, which provides more accurate Cp values. The structural engineer must evaluate both approaches and apply the more conservative result.

What is the difference between basic wind speed and design wind speed?

Basic wind speed (Vb) is the 3-second gust wind speed at 10 m height above ground in Terrain Category 2 with a 50-year return period — a geographic constant from the IS 875 wind speed map. Design wind speed (Vz) is the site-specific value calculated by multiplying Vb by the four IS 875 factors (k1, k2, k3, k4) that account for design life, local terrain, height, and topography. The design wind pressure Pz = 0.6 × Vz² is then used for structural load calculations.

How do cyclone-prone coastal states affect IS 875 calculations?

In cyclone-prone coastal states — primarily Andhra Pradesh, Odisha, and Tamil Nadu (designated cyclone zones in IS 875) — the basic wind speed Vb is already elevated (47–55 m/s) compared to inland areas. The importance factor k4 = 1.15 applies to life-safety structures in these zones. For solar plants in cyclone-prone areas, the design wind pressure can be 40–60% higher than for equivalent-latitude inland sites, significantly affecting foundation design, pile depth, and mounting structure sizing.

Can I use ASCE 7-22 instead of IS 875 Part 3 for an Indian project?

No. For projects regulated by Indian authorities (CEIG, state electricity boards, MNRE-funded schemes), IS 875 Part 3 is the mandatory structural code. ASCE 7-22 is a US standard and is not accepted by Indian regulatory bodies. However, for internationally financed projects, some lenders ask for a peer review confirming that IS 875 compliance is equivalent to or more conservative than ASCE 7-22 for the specific site. Heaven Designs has provided such equivalence statements for projects financed by ADB and IFC.

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Related reading: For the full structural calculation chain from IS 875 to STAAD Pro output, see our guide on solar structural engineering in India. For how wind zone selection affects PVsyst yield modeling inputs (soiling, tracker performance), see our bankable PVsyst reports guide.