Wind load is the most critical load in the design of a Ground Mounted Solar Module Mounting Structure (MMS). In most utility-scale solar projects, the governing design condition is not the dead load but the uplift and lateral forces generated by wind. Therefore, every structural member such as the hat purlin, rafter, column, bracing and foundation should be designed to safely resist the design wind pressure.
In this article, we will learn the complete step-by-step procedure for calculating wind load on a fixed tilt Solar MMS as per IS 875 (Part 3).
PROJECT DETAILS
- Project Type
- Ground Mounted Solar Power Plant
- Structure Type
- Fixed Tilt MMS
- Module Configuration
- 2P × 28
- Total Modules
- 56 Nos
- Module Size
- 2382 × 1134 × 35 mm
- Tilt Angle
- 11°
- Design Standard
- IS 875 (Part 3)
WHAT IS WIND LOAD?
Wind load is the pressure exerted by moving air on the surface of a structure.
For a Solar MMS, wind produces
- Downward Pressure
- Upward Uplift
- Horizontal Force
- Overturning Moment
The direction and magnitude of these forces depend on wind speed, terrain, structure height and tilt angle.
INPUT DATA REQUIRED
Before starting the wind load calculation, collect the following project data.
- Basic Wind Speed (Vb)
- Terrain Category
- Structure Class
- Topography
- Importance Factor
- Ground Level
- Structure Height
- Tilt Angle
- Module Dimensions
- Table Width
- Table Height
STEP 1 – BASIC WIND SPEED
The Basic Wind Speed (Vb) is obtained from the Wind Speed Map of IS 875 (Part 3).
Example
Project Location
Note
Always use the value corresponding to the project location.
STEP 2 – RISK COEFFICIENT (K1)
The Risk Coefficient depends on the design life of the structure.
Obtain K1 from IS 875 Part 3.
STEP 3 – TERRAIN FACTOR (K2)
Terrain Factor depends upon
- Open Land
- Agricultural Land
- Industrial Area
- Urban Area
Determine the Terrain Category and Structure Class as per IS 875.
Obtain K2 from the code.
STEP 4 – TOPOGRAPHY FACTOR (K3)
Topography affects wind speed over hills and slopes.
For level ground,
If the site is located on a hill or ridge, calculate K3 as per IS 875.
STEP 5 – DESIGN WIND SPEED
Formula
Where
Vz = Design Wind Speed
Vb = Basic Wind Speed
K1 = Risk Coefficient
K2 = Terrain Factor
K3 = Topography Factor
Example
STEP 6 – DESIGN WIND PRESSURE
Formula
Example
This is the design wind pressure acting on the structure.
STEP 7 – MODULE PROJECTED AREA
Determine the exposed area of the Solar Module Table.
STEP 8 – PRESSURE COEFFICIENT
The pressure coefficient depends on
- Tilt Angle
- Wind Direction
- Ground Clearance
- Spacing Between Rows
Obtain the coefficient from IS 875.
STEP 9 – DESIGN WIND FORCE
Formula
Where
Pz = Design Wind Pressure
Cp = Pressure Coefficient
A = Projected Area
Example
kN
STEP 10 – WIND UPLIFT
Wind acting underneath the modules produces uplift.
Formula
Where
Obtain the coefficient from IS 875.
STEP 11 – HORIZONTAL WIND FORCE
Where
This force is resisted by
- Columns
- Bracings
- Foundations
STEP 12 – OVERTURNING MOMENT
Wind creates overturning action about the base of the structure.
Formula
This moment is resisted by
- Columns
- Foundations
- Piles
STEP 13 – LOAD TRANSFER
Wind Load
↓
Solar Modules
↓
Hat Purlins
↓
Rafters
↓
Columns
↓
Foundation
Every structural member should safely transfer the wind load to the foundation.
IMPORTANT DESIGN NOTES
1. Wind Load is usually the governing load in Solar MMS Design.
2. Wind acts in both positive and negative directions.
3. Uplift should always be checked.
4. Load combinations should include Dead Load and Wind Load.
5. Wind coefficients should always be selected from IS 875 (Part 3).
COMMON MISTAKES
- Using incorrect basic wind speed.
- Ignoring terrain category.
- Ignoring uplift.
- Using gross area instead of projected area where applicable.
- Ignoring pressure coefficients.
- Not checking wind from both directions.
CONCLUSION
The design wind pressure has now been determined. This pressure will be converted into line loads acting on the hat purlins and rafters. These loads will then be used to calculate bending moments, shear forces and reactions for the manual design of the complete Solar MMS.
In the next article, we will learn how to develop Load Combinations for a Ground Mounted Solar MMS using Dead Load and Wind Load as per IS 875.