After calculating the Dead Load (DL) and Wind Load (WL), the next step in Solar Module Mounting Structure (MMS) design is to develop the design load combinations. Structural members should never be designed using only one load case. Instead, different load combinations are checked to identify the most critical condition for each structural member.
In this article, we will learn how to develop load combinations for a Ground Mounted Solar MMS using IS 875 and Limit State Design principles.
PROJECT DETAILS
- Project Type
- Ground Mounted Solar Power Plant
- Structure Type
- Fixed Tilt Ground Mounted MMS
- Module Configuration
- 2P × 28
- Total Modules
- 56 Nos
- Tilt Angle
- 11°
- Design Method
- Limit State Design (LSD)
Applicable Standards
IS 875 (Part 1) – Dead Load
IS 875 (Part 3) – Wind Load
IS 800:2007 – General Construction in Steel
WHAT IS A LOAD COMBINATION?
A load combination is a mathematical combination of two or more structural loads acting simultaneously on a structure.
The objective is to determine the worst possible loading condition that may occur during the service life of the structure.
For Solar MMS, the governing loads are generally:
- Dead Load (DL)
- Wind Load (WL)
Since live load on solar tables is generally insignificant during normal operation, the primary design is based on Dead Load and Wind Load.
LOADS CONSIDERED
Dead Load (DL)
Dead Load consists of
- Solar Modules
- Hat Purlins
- Rafters
- Columns
- Front Bracing
- Rear Bracing
- Wind Bracing
- Cleats
- Bolts
Wind Load (WL)
Wind Load consists of
- Downward Pressure
- Upward Uplift
- Horizontal Wind Force
- Overturning Force
WHY MULTIPLE LOAD COMBINATIONS?
Every structural member experiences different critical loading conditions.
Example
Hat Purlin
Usually governed by
DL + Downward Wind
Column
Usually governed by
DL + Wind Uplift
Foundation
Usually governed by
Wind Uplift + Overturning
Bracing
Usually governed by
Horizontal Wind Force
Therefore, different combinations must be checked before finalizing the member size.
LIMIT STATE LOAD FACTOR
According to IS 800:2007 (Limit State Design), factored loads are used for strength design.
Typical Ultimate Load Factor
1.5
This factor provides an additional margin of safety against uncertainties in loading and material strength.
LOAD COMBINATION 1
Dead Load Only
DL
Purpose
Used for calculating the self-weight effect on the structure.
Typical Check
- Deflection
- Initial Reactions
LOAD COMBINATION 2
Dead Load + Wind Pressure
DL + WL
Purpose
Used when wind acts downward on the module surface.
Critical Members
- Hat Purlins
- Rafters
LOAD COMBINATION 3
Dead Load + Wind Uplift
DL + WU
Purpose
Used when wind produces uplift on the solar table.
Critical Members
- Rafters
- Columns
- Foundations
LOAD COMBINATION 4
Dead Load + Horizontal Wind
DL + WH
Purpose
Used for checking lateral stability.
Critical Members
- Wind Bracing
- Front Bracing
- Rear Bracing
- Columns
LOAD COMBINATION 5
1.5 × (DL + WL)
Purpose
Ultimate Strength Check
Used to verify bending, shear and axial capacity under downward wind pressure.
LOAD COMBINATION 6
1.5 × (DL + WU)
Purpose
Ultimate Uplift Check
Used to verify
- Column Pull-Out
- Foundation Stability
- Bolt Tension
LOAD COMBINATION 7
1.5 × (DL + WH)
Purpose
Ultimate Horizontal Stability
Used for
- Wind Bracing
- Column Base
- Foundation
LOAD COMBINATION 8
0.9DL + 1.5WU
Purpose
Critical Uplift Condition
During high wind events, the dead load may not be sufficient to resist uplift.
This combination checks whether the structure remains stable when only 90% of the dead load is available to counteract uplift.
This is one of the most critical combinations for Solar MMS.
LOAD TRANSFER
Dead Load
↓
Solar Module
↓
Hat Purlin
↓
Rafter
↓
Column
↓
Foundation
Wind Load
↓
Solar Module
↓
Hat Purlin
↓
Rafter
↓
Column
↓
Foundation
WHICH MEMBERS ARE GOVERNED BY WHICH LOAD?
Hat Purlin
Mostly controlled by
Dead Load
+
Wind Pressure
Rafter
Controlled by
Dead Load
+
Wind Pressure
+
Wind Uplift
Column
Controlled by
Wind Uplift
+
Horizontal Wind
Front Bracing
Controlled by
Horizontal Wind
Rear Bracing
Controlled by
Horizontal Wind
Wind Bracing
Controlled by
Horizontal Wind
Foundation
Controlled by
Wind Uplift
+
Overturning Moment
DESIGN CHECKS
Every load combination should be checked for
- Bending Moment
- Shear Force
- Axial Force
- Deflection
- Buckling
- Bearing Stress
- Connection Capacity
COMMON MISTAKES
- Designing members using Dead Load only.
- Ignoring uplift load combinations.
- Ignoring horizontal wind effects.
- Using only one wind direction.
- Not checking the most critical combination for every member.
CONCLUSION
Load combinations are the bridge between load calculation and structural member design. Once the critical combination has been identified, the design of each structural member can begin. Every member should be checked against the governing combination before finalizing the section size.
In the next article, we will convert the wind pressure acting on the solar table into line loads acting on the Hat Purlins and calculate the reaction at every support. These line loads will form the basis for the manual design of the Hat Purlins without using STAAD Pro.