Most distortion problems in hot-dip galvanized solar structures are not galvanizing failures. They are design failures. The distortion was built in long before the steel reached the zinc bath.
This article covers the mechanisms behind HDG distortion, why solar carports are especially vulnerable, and the specific design and process decisions that determine whether your structure comes out straight.
Why HDG Distortion Happens
The galvanizing process immerses steel in molten zinc at approximately 450°C. The coating itself is not what causes distortion. The cause is thermal stress combined with residual stress that was already locked in the steel from rolling, cold-bending, and welding.
When a structural member enters the zinc bath, the surface heats faster than the core. Different wall thicknesses heat at different rates. This temperature gradient creates differential thermal expansion across the section. At the same time, the residual stresses from fabrication — which the steel had been holding in equilibrium at room temperature — are released by the heat. The result is warping, twisting, or bending.
Thin-walled members are more vulnerable for a straightforward reason: flexural rigidity is proportional to the cube of wall thickness. Halve the wall thickness and you get one-eighth the rigidity. Under the same thermal load, a thinner section deflects far more. Cold-bent C-sections, Z-sections, and square hollow sections (SHS) used in mounting structures accumulate concentrated residual stress at their bends — all of which releases at once during galvanizing.
Four Types of Distortion to Know
Longitudinal bow — the member curves along its length. This is the most common type and the most visible. Long purlins, beams, and columns are the usual victims.
Twist (torsional) — the cross-section rotates about the member's axis. Open profiles such as C-sections and Z-sections are most susceptible because they have low torsional stiffness.
Out-of-plane warp — a welded frame distorts diagonally into a saddle shape. This is the mode most likely to affect full carport bays welded as an assembly before galvanizing.
Local buckling — thin wall faces ripple or buckle under thermal stress. This can occur in box sections where internal pressure from trapped air adds to the thermal load.
Figure 1: Four typical galvanizing distortions — longitudinal bow, twist, out-of-plane warp, and local buckling.
Why Carports Are More Prone Than Other Structures
Solar carports concentrate every unfavorable condition for galvanizing into a single structure.
Long members at high temperature. Columns, rafters, and purlins are typically several meters long. Steel yield strength drops significantly at 450°C. A long member supported at one or two points will sag under its own weight while hot.
Single-end or limited hanging points. Batch dipping (吊镀) for large structural members typically applies lifting force at one end or at a limited number of points. As the member enters and exits the zinc bath, buoyancy and viscous drag from the molten zinc create asymmetric forces that promote twisting.
Progressive entry into the bath. A long member enters the zinc bath gradually, not all at once. The leading end expands while the trailing end is still cold. This length-wise temperature gradient directly causes longitudinal bowing. The same effect occurs in reverse during extraction and cooling.
Welded frames with high residual stress. Carport bays are often fabricated as complete welded frames before galvanizing. Weld zones carry the highest residual stress. When that stress releases simultaneously across an entire frame at 450°C, diagonal twist and out-of-plane warp are the typical result.
Thin walls over large spans. To keep weight and cost down, carport structures use relatively thin-walled profiles across large spans. This combines three unfavorable factors: low rigidity (thin), large moment arm (long), and high residual stress (welded). All three work against you in the zinc bath.
Six Design and Process Measures That Prevent Distortion
These measures should be resolved at the drawing stage, not discovered after galvanizing.
1. Wall Thickness and Section Symmetry
For carport main structural members, a minimum wall thickness of 2.0–2.5 mm significantly reduces distortion risk. Avoid sections where one side is substantially thicker than the opposite side. Asymmetric sections create built-in differential thermal expansion.
2. Stress Relief Before Galvanizing
For cold-bent or heavily welded critical members, schedule a stress-relief anneal before galvanizing. This releases the residual stress thermally before the steel reaches the zinc bath, rather than letting the zinc bath do it unpredictably.
3. Hanging Point Design and Dipping Angle
Discuss the hanging arrangement with the galvanizer before issuing drawings. Long members should use multiple hanging points distributed along the length. Specify angled (inclined) dipping rather than horizontal entry — this allows the member to enter the zinc bath progressively from one end, creating a gentle temperature gradient instead of a full-length thermal shock.
Reserve this discussion for the drawing stage. Adding hanging holes or redesigning attachment points after fabrication adds cost and delay.
Figure 2: Single-point hanging and flat dipping (incorrect) versus multi-point hanging and angled dipping (recommended).
4. Vent Holes and Drain Holes in Closed Sections
Closed hollow sections — square tubes, rectangular hollow sections (RHS), box-section beams — must have vent holes and drain holes. Without them, trapped air expands when heated, causing zinc splash, missed coating, or pressure-related deformation. In the worst case it creates a genuine safety hazard.
Position holes at the corners of the section, arranged diagonally at opposite ends. This layout gives the shortest path for air to escape and zinc to drain. Minimum diameter: 25% of the section diagonal, or as specified by the galvanizer.
Mark hole positions and sizes on the drawings. If you leave this to the galvanizer's discretion, they will drill wherever is convenient, which may affect structural capacity or finished appearance.
Figure 3: Vent holes at top corners and drain holes at bottom corners, arranged diagonally to ensure complete air exhaust and zinc drainage.
5. Straightening Allowance in Drawings
For long linear members, design in allowance for post-galvanizing straightening. Note on the drawings the permitted straightening range and method. This is normal practice, not a sign of a process failure — even well-designed members may require minor correction.
6. Segment Galvanizing with Site Bolted Assembly
For oversized frames where a full welded assembly cannot be galvanized without distortion risk, design the structure for segment galvanizing and site bolted connection. Each segment is galvanized independently at manageable dimensions, then assembled on site with bolted connections. This approach eliminates the risk of whole-frame warp and is compatible with high-tolerance bolted connections using non-standard splice hardware.
Coating Thickness: Which Standard Applies
Structural members and fasteners are covered by different standards, and the two systems — ISO and ASTM — are not directly interchangeable.
Structural Members
ISO 1461:2022 applies to profiles, steel plates, tubes, and welded structures (non-centrifuged):
| Steel thickness | Local minimum (µm) | Mean minimum (µm) |
|---|---|---|
| > 6 mm | 70 | 85 |
| > 3 to ≤ 6 mm | 55 | 70 |
| ≥ 1.5 to ≤ 3 mm | 45 | 55 |
| < 1.5 mm | 35 | 45 |
ASTM A123 uses coating grade designations. For structural steel profiles ≥ 6.4 mm thick, the minimum is Grade 100 (100 µm mean). For thinner profiles (1.6–3.2 mm), minimum is Grade 65. Individual specimens may be one grade below the mean requirement.
Fasteners
ISO 10684 covers M8–M64 hot-dip galvanized bolts: local minimum 40 µm, batch mean 50 µm.
ASTM F2329 (replacing A153 for fasteners): larger bolts approximately 50 µm mean; smaller bolts approximately 43 µm mean. Thread dimensions are not measured for coating thickness — zinc on the bolt threads protects the nut during assembly.
European and international project specifications typically reference ISO 1461 + ISO 10684. North American EPC clients will specify ASTM A123 + F2329 / A153. Note both standards separately on drawings — one for structural members, one for fasteners.
Pre-Galvanizing Design Checklist
Before issuing drawings or placing a galvanizing order, confirm the following:
Structural design
- Main structural member wall thickness ≥ 2.0–2.5 mm
- Section is symmetric; no single-sided thick/thin
- Large welded frames split into segments for site bolted assembly
Vent and drain holes
- All closed sections have vent holes at top corners and drain holes at bottom corners
- Hole diameter ≥ 25% of section diagonal
- Holes positioned on drawings with size callout
Welding and stress
- Welds continuous; weld spatter, slag, and burrs removed before galvanizing
- High-stress cold-bent or welded members scheduled for stress relief
- No dense weld concentration at a single cross-section
Hanging and process
- Hanging point positions and count confirmed with galvanizer
- Long members have hanging holes or provisions for multi-point support
- Angled dipping agreed with galvanizer
- Post-galvanizing straightening allowance noted on drawings
Standards and thickness
- Structural member standard noted (ISO 1461 or ASTM A123)
- Minimum coating thickness confirmed per steel thickness grade
- Fastener standard listed separately (ISO 10684 or F2329 / A153)
- Thread class after galvanizing noted (6AZ/6AX or tapped oversize)
- Touch-up acceptance criteria noted (A780 / ISO 1461 §6.3)
Order communication
- Steel grade and silicon content provided to galvanizer (affects coating thickness and appearance)
- Critical surfaces (significant surfaces) marked on drawings
- Wet storage white rust prevention packaging agreed
The Core Principle
Galvanizability must be considered at the structural design stage, not after production.
Distortion problems discovered after galvanizing mean rework, delay, and dispute. The root causes — thin walls, poorly placed weld lines, sealed hollow sections, no hanging provision — all could have been resolved with ten minutes of attention at the drawing stage.
At Solaracks, our structural design process treats galvanizability as a design constraint alongside wind load and deflection limits. We specify wall thicknesses, vent hole positions, hanging provisions, and galvanizing standards on every drawing set. If you are sourcing carport structures or large-span mounting systems and the supplier's drawings do not address these details, ask why.
We offer structural quotes within 24 hours. Contact us to discuss your project.
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