Doubling the span between solar carport columns does not double the bending load on the primary beam. It quadruples it. That single relationship — bending moment scaling with the square of span length — is the most important structural fact in solar carport engineering, and the one most often ignored when buyers compare price per kilowatt-peak.
This article explains the mechanics, the cost implications, and how to choose span lengths that balance structural economy with parking function.
The Bending Moment Equation
For a simply supported beam under uniform load, the mid-span bending moment is:
M = wL² / 8
Where:
- M = bending moment (kN·m)
- w = distributed load per unit length (kN/m), from panel self-weight + wind + snow
- L = span length (m)
The load w stays roughly constant regardless of span — it depends on panel weight, wind pressure, and snow zone, not on how far apart the columns are. Span L is the variable that changes when you move columns further apart. Because L is squared, every incremental increase in span creates a disproportionate jump in the structural demand on the primary beam.
At 6m span: M = w × 36 / 8 = 4.5w
At 8m span: M = w × 64 / 8 = 8.0w
At 10m span: M = w × 100 / 8 = 12.5w
At 12m span: M = w × 144 / 8 = 18.0w
Going from 6m to 12m span — which might look like a modest doubling in column spacing — multiplies the bending load by four. The beam must be dramatically deeper and heavier to carry that load within acceptable deflection and stress limits.
How Span Affects Beam Depth and Weight
Structural steel sections are selected to keep the maximum bending stress below the yield limit of the material and to keep mid-span deflection within the service limit (typically L/250 to L/360 for solar mounting applications).
Both constraints — stress and deflection — are governed by the section modulus and moment of inertia of the beam, both of which scale roughly with the cube of beam depth. In practical terms:
- 6m span: A 200×100 H-beam or equivalent at 25–35 kg/m typically suffices
- 8m span: Requires approximately a 250×125 or deeper section, 40–55 kg/m
- 10m span: Moves into 300×150 or built-up box beam territory, 60–85 kg/m
- 12m span: Requires 350mm+ depth or a fabricated plate girder, 90–130+ kg/m
These are approximate order-of-magnitude figures — exact sizing depends on wind zone, panel weight, deflection limit, steel grade, and connection design. But the trend is unambiguous: beam weight roughly doubles between a 6m and 10m span, and again between 10m and 12m.
Relative Steel Weight Comparison
| Span (m) | Relative Bending Moment | Approx. Primary Beam Unit Weight | Relative Steel Cost Index |
|---|---|---|---|
| 6 | 1.0× | 25–35 kg/m | 1.0 |
| 8 | 1.8× | 40–55 kg/m | 1.6–1.8 |
| 10 | 2.8× | 60–85 kg/m | 2.4–2.8 |
| 12 | 4.0× | 90–130 kg/m | 3.5–4.5 |
Note: Indices are relative to a 6m baseline with equivalent uniform loading. Actual values vary by wind zone, steel grade, and deflection limit.
The Material Inflection Point: Why Aluminum Works at 6m but Not at 12m
Aluminum alloy (6005-T5) has a yield strength of around 240–260 MPa and a Young's modulus of approximately 70 GPa — roughly one-third that of structural steel (210 GPa). This lower stiffness means aluminum deflects three times as much as steel under identical loading, which rapidly becomes the governing constraint as span increases.
At 5–7m spans, aluminum extrusions can be sized to meet deflection limits while keeping weight competitive with steel. The lower material density (2.7 vs 7.85 t/m³) helps offset the lower stiffness for short spans.
Above approximately 8–9m, aluminum beams become impractically deep — or impractically heavy — to control deflection. At that point, structural steel (HDG or ZAM-coated S355) is the only viable primary beam material. This is the cost and material inflection point that separates small residential and light commercial carport designs from large commercial and industrial ones.
Optimum Span Ranges by Project Type
There is no single "correct" span. The right span depends on what the project is trying to achieve:
Residential / Small Commercial (1–20 spaces)
Optimal span: 5–6m. Aluminum extrusions viable. Low steel cost. One or two rows of standard 6m sections. Columns at 5–6m centres match standard bay dimensions and keep beam sizing straightforward.
Commercial Parking Lot (20–200 spaces)
Optimal span: 6–8m. The practical sweet spot. Standard H-beam or C-frame columns at 7.5m centres (standard parking bay multiple of 2.5m) cover three bays per span. Structural steel but relatively light sections. EV charging conduit fits within the column spacing without obstruction.
Large Commercial / Industrial (200+ spaces)
Optimal span: 8–10m for multi-span frames; 7–8m for cantilever configurations. At this scale, column placement must coordinate with parking row geometry and vehicle manoeuvrability. Span optimisation at this level should be done with site-specific structural analysis (SAP2000 or equivalent) — generic span tables stop being reliable.
Logistics / High-Clearance Sites (>4.5m clearance)
Any increase in clearance height adds bending moment from greater wind exposure and increases the column's effective buckling length. Effective design at these sites often reduces span to 6–7m to avoid a compounding cost penalty from both height and span simultaneously.
Multi-Span vs Cantilever: The Steel Economy Trade-Off
The structural efficiency of a carport frame depends not just on span length but on how the frame distributes load across columns.
Multi-span continuous frames (where beams continue over internal columns) benefit from moment redistribution at intermediate supports. The peak bending moment drops compared to a simply supported single span of the same length. A continuous beam over three 6m bays carries lower maximum moments than a single simply-supported 6m beam with the same total load. This makes multi-span configurations structurally efficient — but they require internal columns that pass between parking bays.
Cantilever configurations (7-shaped or C-frame designs) eliminate internal columns by projecting the roof from a single row of columns at one side. The bending moment at the column head is now M = wL²/2 — four times higher than the simply-supported equivalent for the same span. Cantilever arms of more than 5m become very heavy very quickly. Most practical cantilever designs stay at 4–5m projection with heavily reinforced column-to-beam connections.
Y-shaped bilateral frames — Solaracks' signature product — are essentially two cantilever arms back-to-back on a single central column. Each arm covers one parking lane. This is more efficient than a single cantilever because the two arms partially balance each other's overturning moment at the column head, reducing foundation uplift and improving overall frame economy at 6–8m total covered width.
The Column Trade-Off: Fewer Columns vs Less Steel
The central design tension in carport engineering is between these two things:
Wider column spacing (longer spans) means:
- Fewer columns to obstruct parking bays
- Better vehicle manoeuvrability
- Simpler cable routing for EV charging
- Dramatically heavier and more expensive primary beams
Closer column spacing (shorter spans) means:
- More columns that can block parking access
- More foundation piers (civils cost)
- Lighter, cheaper primary beams
- Easier structural compliance in high-wind zones
For most commercial projects, 7.5m column spacing (three 2.5m parking bays per span) hits the best structural economy. This is not coincidental — 2.5m is the standard parking bay width in most markets, and 7.5m is the span where beam weight and column frequency are both reasonable.
Pushing to 10m or 12m to eliminate one column row in a 30-bay car park rarely saves money. The additional beam steel usually costs more than the concrete pier and column steel it replaces. The exception is sites with underground services or rock conditions that make foundation drilling prohibitively expensive — in those cases, longer spans that reduce pier count can genuinely lower total project cost.
What This Means for Procurement
When comparing carport quotations, the span length is one of the most important parameters to verify. Two quotations with the same kWp capacity and similar panel specifications can differ by 20–40% in structural cost if one specifies 6m spans and the other specifies 10m spans.
Neither is automatically wrong — a wider span may be correct for the site. But buyers who focus only on price per kWp without understanding the span design can end up comparing structurally incompatible offerings.
Ask every supplier:
- What is the column spacing (span length)?
- What is the primary beam section size and unit weight?
- Is the design based on a site-specific wind speed, or a generic catalogue assumption?
- Has the structural design been verified against ASCE 7-16, JIS C 8955:2017, or the applicable local standard?
A supplier who can answer all four questions with specific numbers is doing engineering. One who cannot is selling a catalogue.
Solaracks Span Optimisation
Solaracks designs solar carport structures in SAP2000, using site-specific wind speeds under ASCE 7-16 and JIS C 8955:2017. Every project starts with a span optimisation review: which column spacing minimises total steel tonnage while meeting the site's parking bay geometry, clearance requirements, and structural code.
We can typically turn around a structural proposal — including primary beam sizes, column section, and estimated steel tonnage — within 24 hours of receiving site drawings and local wind zone data.
If you are evaluating carport designs and want an independent check on whether the proposed span makes structural sense for the cost being quoted, contact Solaracks. Be supported.