On the structure of a PV array, the governing load is usually wind, which lifts and pushes the modules and calls for anchors, driven piles or ballast able to resist pull-out. The local building code gives the basic wind speed and the calculation procedure; ASCE 7 is used as a reference for solar panel pressure coefficients. On the ground, geotechnics and pull-out tests decide the foundation; on a roof, the capacity of the building and the anchoring.
Loads acting on the structure
A PV structure is a light, tilted and very exposed canopy. Its design loads are grouped as follows:
| Load | Origin | What it produces |
|---|---|---|
| Self-weight (dead load) | Modules, rails and accessories | Compression toward the ground or the roof |
| Wind | Pressure and suction on the modules | Lateral push and, above all, uplift |
| Seismic | Ground and structure acceleration | Horizontal and vertical forces on anchors |
| Installation and maintenance loads | People, tools, cleaning | Point load on modules and rails |
| Rain, hail, and ash or snow where applicable | Site climate | Additional vertical load |
| Temperature | Expansion of long rails | Stresses at joints |
In climates without snow, wind is the case that most governs the anchors, because on a tilted module the suction can exceed its weight by a wide margin and lift it. That is why the calculation of a solar structure starts with the wind pressure, and the seismic check and the other loads are done with the combinations set by the code.
Reference standards
Local building code. Each jurisdiction’s building code sets the basic wind speed for the site (usually through a wind map or table), the exposure categories and an analytical procedure to obtain the pressure on the structure, as well as the seismic loads. The design and settings are adapted to the local code and to the requirements of the client and the authority having jurisdiction.
ASCE 7. The US standard Minimum Design Loads and Associated Criteria for Buildings and Other Structures gives, in its wind chapter, the pressure coefficients for solar panels on roofs. For ground-mounted arrays, the 2022 edition introduces its own provisions for fixed-tilt systems, according to the sources consulted; confirm the clause and the edition with the structural engineer. Eurocode 1 (EN 1991-1-4) and wind tunnel studies are other references used by the designer, particularly for trackers.
Module. The module is tested separately: IEC 61215-2 includes a static mechanical load test, with test pressures set by the manufacturer datasheet (2400 Pa or more), and many manufacturers certify higher values. The load the module withstands is not the load the structure withstands, and the mounting method (clips, number and position of the points) must be the one the manufacturer authorizes for that value.
From speed to pressure
The wind velocity pressure in the formulation used by ASCE 7-type procedures is:
q = 0.613 × Kz × Kzt × Kd × V² × I (q in N/m², V in m/s)
with Kz the exposure coefficient according to height and terrain roughness, Kzt the topography factor, Kd the directionality factor and I the importance factor according to the use group. On an open panel, the net pressure is p = q × GCrn. GCrn is the net pressure coefficient. It depends on the tilt, on the position of the module in the array (edge or interior), on the height above ground and on the row spacing. The values of each coefficient come from the standard, not from a generic table.
Illustrative example (teaching assumptions, not design values). V = 100 km/h (27.8 m/s), Kz = 0.85, Kzt = 1, Kd = 0.85, I = 1.0 and GCrn = 1.5 are assumed, with a 2.58 m² module.
| Basic speed | q | p = q × 1.5 | Force per module (2.58 m²) | Table of 14 modules | Per post, if the table rests on 2 posts |
|---|---|---|---|---|---|
| 100 km/h | 342 Pa | 513 Pa | 1.32 kN | 18.5 kN | 9.3 kN |
| 120 km/h | 492 Pa | 738 Pa | 1.90 kN | 26.7 kN | 13.3 kN |
Pressure rises with the square of the speed: from 100 to 120 km/h, the force grows about 44%. That force is a resultant uplift; the engineering distributes it over each post, subtracts the self-weight and applies the combination and safety factors set by the code. The weight of the modules and the structure, which helps resist uplift, is on the order of a few tens of kilograms per module and is far below this force.
Ground-mounted structures
Types. Fixed structures on driven posts or rails are the most common in solar parks; single-axis trackers change the module angle during the day and are covered in solar trackers. There are also foundations with footings, screws or rock anchors, and ballast on a slab where drilling is not possible.
Pile driving. The post is driven with mechanical equipment to a depth that resists wind tension, compression and lateral thrust. The depth is not set by habit but with geotechnics: soil type, water table, presence of rock, corrosivity and expansiveness.
Pull-out tests. Before defining the driving depth, load tests are run on trial piles: tension (the one wind generates) and also compression and lateral load. The tension method uses ASTM D3689 as its reference. The results are compared with the design load and with the soil study and lead, if needed, to a different type of foundation.
Corrosion and drainage. The steel is protected with galvanizing or another coating according to the aggressiveness of the soil and the environment; the thickness and the test are set by the engineering and the applicable coating standard. Drainage and erosion around the posts are part of the site design.
Rooftop structures
On a roof, the limit is not the ground but the building: the roof must withstand the additional weight (modules, structure and, if any, ballast) and the uplift, and the anchors must reach an element able to receive them.
- Flat roof with ballast: the weight of blocks or filled trays counteracts the wind without drilling. It raises the dead load and calls for a check of the building structure.
- Roof with anchoring: anchors to purlins, beams or slab transmit the tension; sealing and waterproofing must be checked.
- Metal roof: the module is fixed to the rib of the sheet or with special clamps; the roof manufacturer must endorse the loads.
- Review of the existing building: consult the code in force and the structural engineer; the criterion depends on the age of the building and on whether the added load requires it.
This review is done by a structural engineer. Installing without it is the origin of most problems on roofs.
Roof versus ground
| Aspect | Roof | Ground |
|---|---|---|
| Main limit | Capacity of the building and its roof | Geotechnics and area |
| Foundation | Anchors or ballast | Driven posts, screws or footings |
| Prior tests | Structural review, anchor test | Soil study and pull-out tests |
| Wind | Roof coefficients, with edge and corner zones | Open-array coefficients over the ground |
| Maintenance | Access and work-at-height safety | Vegetation, erosion and drainage |
Frequent mistakes
- Taking the wind speed from another project or a generic table instead of the site zone.
- Applying to a ground-mounted array coefficients of another configuration (roof, building) without justification.
- Setting the driving depth without pull-out tests or a soil study.
- Forgetting the edge and corner zones, where pressures are higher.
- Relying on the load the module withstands without respecting the approved mounting method.
- Installing on a roof without checking the existing structure or the waterproofing.
- Not defining the tracker protection procedure for high wind.
- Not considering steel corrosion according to the aggressiveness of the soil.
Checklist
- Basic wind speed read from the code in force, for the site zone.
- Exposure category, use group and pressure coefficients justified.
- Calculation report prepared by structural engineering.
- Geotechnical study and pull-out tests (ground) or building review (roof).
- Module mounting as the manufacturer authorizes.
- Corrosion protection and drainage plan.
- Stow position and wind sequence defined for the trackers.
- Structure grounding coordinated with the electrical design.
Where electrical engineering comes in
The structure is work for structural and geotechnical engineering, but the electrical side depends on it. The layout of the tables defines the string lengths and the cable routes. The structure is also part of the plant grounding and of the PV array design. At Tiza Group we do the electrical engineering design and studies for solar plants, remotely, and coordinate them with the structural design of your project.
Related guides
References
- Local building code in force at the project site (wind and seismic loads).
- ASCE/SEI 7-16 and 7-22, minimum design loads, wind chapters (solar panels).
- Eurocode 1, EN 1991-1-4, wind actions.
- IEC 61215-2: qualification of PV modules, mechanical load test.
- ASTM D3689/D3689M: testing of deep foundation elements under static axial tensile load.
Frequently asked questions
Which standard applies to the wind load of a solar structure?
The building code of the site defines the basic wind speed and the calculation procedure. For the pressure coefficients of solar panels, designers commonly rely on the solar panel provisions of ASCE 7, and Eurocode 1 (EN 1991-1-4) and wind tunnel studies are other references. Structural engineering must justify the criteria used and adapt them to the local code and the requirements of the project.
What is the design wind speed?
It is set by the wind map or table of the local code for the site. It must be read from the code in force and for the exact location, not taken from a table of another project.
What is the pull-out test of the posts for?
It measures the real capacity of a post driven into the site soil against a tension force, which is the one generated by wind uplift. The method reference is ASTM D3689 (deep foundations under static axial tensile load). The results are used to adjust the driving depth and to decide whether the soil needs a different type of foundation.
Does a solar tracker withstand the same wind as a fixed structure?
Not in the same way. A tracker changes its angle, and manufacturers define a stow position (for example, flat) for high winds. The loads and the protection sequence are certified by the manufacturer with its own wind tunnel study; the plant must follow that procedure.