Solar arrays don’t live on spreadsheets. They live on hillsides that heave with frost, in deserts where sand scours metal, on rooftops that were never intended to carry more than a few layers of asphalt and a snow drift. As a structural engineer, I spend my days translating photovoltaic intent into bolted, welded, and anchored reality. That translation almost always comes down to a careful dance between steel and aluminum. Both materials can serve, both can fail when misused, and neither is universally “better.” The art lies in knowing when to lean on stiffness, when to chase weight savings, and how to keep the whole assembly working for 25 to 35 years of wind, temperature, and maintenance cycles.
Where the loads really come from
The static weight of panels and racks rarely governs on open ground. Wind does. Wind pushes, suctions, and vibrates. It pries at edges, accelerates over gaps, and uses the lever arms we give it. I have seen 10 to 15 psf uplift in moderate regions and well over 40 psf in hurricane zones once gust factors and edge effects are accounted for. On a 2 by 4 foot module, that can mean a quick 300 to 500 pounds of uplift along a leading edge during a rare event. If the rack deflects, load redistributes and clamps see amplified forces. If the foundation is soft, the structure rocks and the bolts loosen over time.
Snow is a different story. In the Rockies or northern Japan, we design for 30 to 70 psf ground snow, sometimes higher. Tilt angles can help shed, but drifting and sliding loads can exceed a naïve uniform snow assumption. For trackers, torsional wind coupled with snow accretion produces oddball combinations that wipe out simplistic load cases. Thermal expansion is the quiet companion to both. A 100 foot aluminum rail can grow 1 inch or more across a 100 Fahrenheit temperature swing, and if it is hard-fixed at both ends, the axial stress will find the weakest link.
These realities favor stiffness, good load paths, and connection details that do not punish materials for being what they are. That is where the steel versus aluminum question starts to make sense.
What steel does well
Steel brings modulus and yield strength at a low cost per pound. Mild structural steel has a modulus around 29,000 ksi. Aluminum sits near 10,000 ksi. For equal geometry, steel is almost three times stiffer. In wind-dominant designs, stiffness protects. It limits deflection, reduces flutter risk, and holds module clamps in their sweet spot so glass is not pinched one day and loose the next.
Steel also takes abuse well. Galvanized steel pipe racks survive pebble strikes and tools dropped by crews. Threads remain viable after a few cycles of wrong washer, right washer. Welds can be inspected easily and repaired in the field. If the site expects impact from maintenance vehicles or wildlife, steel is more forgiving. For long spans, rectangular hollow sections or open channels built from steel keep twist in check and mitigate torsional galloping, one of the more insidious failure modes on open plains.
Cost and availability matter. Structural steel sections and hot-dip galvanizing are globally standardized. In many markets, I can source ASTM A500 pipe or A36 plate faster than extruded aluminum of a precise shape. That shortens lead times for utility-scale plants, where construction sequencing can be the difference between delivering before a policy credit expires or missing it.
The trade-offs are not trivial. Steel is heavy, which means more crane time when lifting pre-assembled truss tables, and more effort at roof edges if a commercial array sits on a ballast frame. Steel also corrodes if protective systems fail. In saline coastal air, even good galvanizing develops tea stains and, over decades, measurable section loss, especially at cut edges and holes unless touched up.
What aluminum does well
Aluminum is light, corrosion-resistant, and friendly to extrusion. Those traits pair nicely with solar modules and rooftops. On a tight roof, shaving 30 percent off rack weight can keep you below a diaphragm upgrade threshold. Crews move aluminum rails by hand, reducing lift equipment and the hours spent waiting on it. In corrosive environments or near chemical plumes, the oxide layer on aluminum provides durable protection, especially when fasteners are chosen to avoid galvanic pitfalls.
Extrusion unlocks shapes that are difficult or expensive in steel. I can integrate cable trays, wire clips, and sliding slots directly into a rail geometry. I can thicken walls at bolt holes with local ribs, or use T-slots to simplify module clamp positioning. The part count drops, and assembly speeds go up. On trackers, aluminum torque tubes are less common than steel, but aluminum purlins under modules reduce rotating mass and help motors ride out gusts without over-torque trips.
The drawback is stiffness. With a third of steel’s modulus, aluminum deflects under the same load unless the section grows. To achieve equal stiffness, the geometry must change, usually by spreading material away from the neutral axis. That means taller rails or deeper ribs, both of which can catch wind or interfere with module backside clearances. Aluminum also creeps under constant stress. Clamp preloads and slip-critical connections need conservative assumptions and sometimes periodic torque checks. And while aluminum resists red rust, it is not immune to pitting, especially in alkaline soils or on rooftops with incompatible runoff.
Matching material to function within a single array
The most reliable systems use materials in the roles they serve best rather than forcing a single material across the board. On ground-mount fixed tilt, steel makes sense for foundations, posts, and primary beams. It holds alignment during installation, tolerates backhoe nicks, and gives enough torsional stiffness to keep module rows quiet in gusts. Aluminum shines in secondary rails and module clamps. It accepts self-tapping screws, provides built-in slots for wire management, and saves crews time.
On single-axis trackers, the torque tube and drive columns almost always benefit from steel. Torsional stiffness is essential to suppress coupled modes that can amplify with synchronized vortex shedding along a row. In more than one project, a shift from a thinner-wall tube to a slightly heavier steel section unlocked an entire wind certification pathway without resorting to complex dampers. Aluminum purlins then keep rotating mass down. Fewer Newton-meters to accelerate means smaller motors and lower power draw, which helps during storm stow operations when every amp matters.
On commercial rooftops, I default to aluminum where corrosion and weight dominate, then reintroduce steel where discrete compression points or ballast trays need robustness. TPO roofs bruise easily. Larger aluminum footpads distribute load and pair with isolating mats. Where parapet or equipment screen edges require cantilevered rack arms, a compact steel arm solves vibration problems without adding much mass.
Foundations and the soil beneath
No rack optimizes well if the foundation is mismatched to the soil. Driven steel piles work when refusal depths are predictable and rock is friendly. They are fast, repeatable, and cost-effective. In highly expansive clays or shallow bedrock, helical anchors often win, but they demand torque-based QA and corrosion checks for the shafts. Steel piles must be evaluated for stray current corrosion if the site neighbors rail lines or buried utilities that induce long-term DC.
For rooftop ballast, concrete mirrors the stiffness of steel, but it is not neutral to aluminum. Direct contact between aluminum rails and wet concrete invites alkali attack unless separated by non-absorptive pads or coatings rated for the service. Where roof deflection limits control, trading some ballast for mechanical anchorage can reduce total weight and movement, but it drags in roofing penetrations and warranty coordination. The optimal mix is rarely purely structural. It is construction, warranty, and operations all at once.
Galvanic couples, coatings, and what real sites teach
Mixing metals is not optional in solar. You will have stainless fasteners on aluminum rails, zinc-coated hardware on steel posts, copper ground wires terminating to both. The trick is to control the galvanic series and the environment. Stainless on aluminum is generally acceptable when isolated by a polymer washer or when the contact area is small and dry. Zinc-coated steel touching aluminum can be benign in arid climates, but in coastal areas the zinc sacrifices to protect the steel, leaving the aluminum to pit after the zinc is consumed.
I insist on three habits. First, keep like with like where possible. Aluminum-to-aluminum clamps, zinc-coated steel on galvanized steel bases. Second, isolate dissimilar pairs with nylon or EPDM washers, paint systems, or polymer gaskets that do not cold-flow away under bolt tension. Third, seal the entry points for water. Galvanic corrosion accelerates in thin electrolyte films that remain trapped. Closed-cell gaskets and proper roof pitch help here.
Real sites reveal what drawings miss. A coastal tracker project taught us that wind-blown salt settled in crevices at clamp interfaces more than on open faces. Five years in, bolts still shone, but crevice lines showed white corrosion on aluminum. Switching to slightly thicker anodizing and adding a tiny drainage slot relieved the trapped brine. A desert fixed-tilt farm taught us that sand equals sandpaper. Powder coat on steel looked pristine for the first year, then dulled where wind-driven grit scoured windward faces. Upgrading to hot-dip galvanizing with sufficient zinc thickness, despite a cost bump, halted measurable loss.
Stiffness, frequency, and the wind tunnel problem
Many solar failures are not ultimate strength failures. They are serviceability failures that snowball into damage: clamp slip, seal tears, fatigue cracks at bolt holes. Tracking the first natural frequency of a module table is as important as checking the beam’s bending stress. As a crude target, I try to keep the primary mode above 2 or 3 Hz for large tables, higher for small spans, unless specific wind studies justify lower. Steel gets me there more readily. Aluminum can as well, but the geometry must be right and the connections tight.
Full-scale wind tunnel testing changed the industry after several galloping incidents on single-axis trackers. In that setting, steel torque tubes with sufficient ovalization control and stiff post bases damped edge-row instabilities better than flexible alternatives. For fixed tilt, wind tunnel studies exposed local suctions at module corners that were not obvious in code-based simplified models. Reinforcing clamp areas with thicker aluminum extrusions or switching to steel at specific hotspots gave a good return on weight.
Thermal growth and connections that forgive movement
The expansion mismatch between steel and aluminum is notable. Aluminum grows about 12 to 13 microstrain per degree Celsius, steel about 11 to 12. Over 100 feet, aluminum’s extra movement is noticeable relative to steel bases. Rigidly fixing aluminum rails at both ends to steel posts guarantees stress on hot afternoons and chilly mornings. Slot holes, sliding clamps, and expansion joints solve it if they are placed deliberately. The slots need orientation that matches the dominant movement vector, and the clamping friction must be low enough to allow slip at safe loads but high enough to resist wind service loads. I have seen rails buckle slightly on long rooftops because every connection was locked tight out of caution. Teaching crews which slots remain fixed and which are sliding is part of the engineering.
Bolted connections live at the mercy of installation torque. Stainless hardware in aluminum galls if dry. A small tube of anti-seize in each installer’s pouch pays back in saved rework and consistent clamping force. Over-torqued bolts can crush aluminum under washers. Hardened washers with known diameter help spread load and keep clamp faces intact. For steel, protecting the galvanizing at field cuts matters. Cold galvanizing spray is a patch, not a panacea. For critical cuts, post-assembly zinc-rich paint with a verified DFT and proper cure makes a difference.
Cost, carbon, and the procurement puzzle
Unit price per pound misleads. A pound of aluminum does more work when it reduces labor steps on a roof. A pound of steel does more when it eliminates a brace and the time to install it. In utility-scale bids I often run two options with the same performance: one heavier with more steel and fewer bespoke parts, the other lighter with aluminum extrusions and more precise factory work. The field context chooses the winner. If the subcontractor has seasoned pile crews and rough-terrain cranes but limited fine assembly experience, the steel-heavy option typically beats schedule. If the site has transport limits over narrow roads and strict weight caps on equipment, the aluminum-heavy approach shines.
Carbon accounting adds another dimension. Primary aluminum has a high embodied energy unless sourced from low-carbon smelters, often tied to hydroelectric grids. Recycled content can reduce the footprint. Steel’s recycled content is typically high, especially from electric arc furnaces. Hot-dip galvanizing contributes its own footprint. Clients increasingly ask for cradle-to-gate numbers. When the AHJ processing time performance difference is marginal, I let these metrics tip the choice. In snowy regions where steel carries a clearer stiffness advantage, I focus carbon reduction on foundations and logistics instead.
Designing for installation, not just analysis
Racks arrive as bundles and crates, not as free-body diagrams. The fastest install I watched in the last decade succeeded because the structural design team sat with the field superintendent three months prior and walked through the sequence. We staggered post heights to match a subtle swale, swapped a batch of aluminum mid-rails to steel to tolerate a short-term cantilever during panel staging, and adjusted hole patterns to accept the sub’s preferred bolt guns. The calculations were unchanged in result, but the details saved roughly 15 percent labor.
Tolerances deserve respect. Aluminum extrusions can be precise, but field reality is not. Oversized slots, within reason, speed fit-up when posts wander within geotechnical tolerances. Shimming strategies should be designed in, not improvised. If shims are needed, steel shims on steel, aluminum on aluminum, with isolators if a cross-material shim is unavoidable. If load paths rely on friction, pray less and specify more. Use serrated flanges or knurled interfaces where slip is not an option.
Maintenance and the long tail of performance
Solar structures are low-touch compared to rotating machinery, but they are not maintenance-free. Bolts near vibrating elements, like tracker drives or wind-exposed edges, need planned torque checks in year one and after major wind events. Aluminum components should be inspected for creep at clamp interfaces, especially where thermal cycling is severe. Drainage paths should be kept clear. Leaves and debris that sit wet create localized corrosion cells on both steel and aluminum.
From a structural engineering company’s perspective, the best maintenance program begins at design. Consolidate fastener types so spares are manageable. Choose surface finishes common to the region’s trades. If a remote site in a humid region will need occasional coating touch-ups, specify systems that the local paint contractor already uses on agricultural equipment rather than boutique coatings that require special training and import lead times.
A few practical rules of thumb
- Use steel for primary load paths where wind stiffness governs or where torsional modes threaten, and reserve aluminum for secondary framing and clamps that benefit from extrusion features and weight savings. Treat every aluminum-to-steel interface as a design detail with isolation, drainage, and galvanic thinking. Dry and isolated is safe. Wet and trapped is not. Let the site drive material choice. Coastal and rooftop environments push you to aluminum where corrosion and weight dominate. Cold and high-wind plains push you to steel where stiffness and torsion rule.
Edge cases that change the calculus
Cold regions with rime ice and hoarfrost challenge both materials. Ice load is often ignored until a subzero fog event glazes every surface, increasing drag and weight. Steel’s stiffness helps when modules cannot stow below a certain angle. Aluminum’s low mass helps when de-icing sprays are limited and you rely on sun thaw. In cyclone regions with directional wind patterns, mixed-material arrays have performed best for us when we designed asymmetric stiffness intentionally, stiffening leading rows with steel and allowing interior tables to remain lighter in aluminum.
On brownfields with contaminated soils, driven steel piles may face environmental pushback. Helical piles with protective sleeves or shallow concrete footings with vapor barriers become attractive, which then interacts with rack weight. Aluminum’s lighter frame reduces overturning demands on shallow footings, which can lower concrete volumes and reduce handling of contaminated spoils. There, aluminum may win even if wind suggests otherwise.
What “optimized” really looks like
Optimization is not pure minimization of weight or cost. It is matching the structural behavior to environmental loads, installation capability, and long-term durability, then sizing components so no part works harder than it must. I aim for a system where the governing checks share utilization. If posts are at 80 percent, purlins at 80 percent, clamps at 80 percent, and bolt slip at 70 percent, the material is likely well placed. If one component governs at 98 percent while others idle at 30 percent, something is off. Aluminum’s extrusion flexibility can fix such imbalances by shifting material where it is needed. Steel’s broad catalog of standard sections can do the same cheaply if available sizes are used intelligently.
For a structural engineer, pride comes when an array rides out a design storm silently, the owner’s operations team forgets our names because nothing breaks, and the asset produces as planned. Mixing steel and aluminum smartly is part of that outcome. It respects stiffness where it matters, resists corrosion where it is relentless, and makes the most of crews’ time on rough ground or fragile roofs. Solar structural engineering is less about picking a champion material and more about making materials collaborate.
If you are choosing today
Start with climate, wind exposure, and geotechnical report in hand. Translate those into stiffness targets and pull-out capacities. Sketch a load path, then assign materials by role rather than habit. Use steel in foundations, posts, torque tubes, and any member where torsion or major bending makes serviceability critical. Use aluminum in rails, clamps, and other parts where weight, corrosion resistance, and extruded features add speed and reliability. Detail the interfaces like they are mini projects, because they are.
Then sit with the builders. Ask what they lift by hand, what they align with lasers, and what they prefer to fine-tune at height. Adjust hole sizes, slot directions, and the mix of pre-assembly versus field assembly. Verify that your galvanizing, anodizing, and paint specs have local supply and repair paths. Document torque values and anti-seize use explicitly. A structural engineer that listens early saves everyone late.
That is the view from a lot of sites and a lot of storms. Steel and aluminum, used with judgment, do not compete. They hold modules steady, share loads well, and keep a promise that solar developers make to owners, lenders, and communities: the array will stand, quietly and productively, for decades.