Seeking Structural Advice: 25m Diameter Geodesic Dome, 8V Frequency, Coastal Wind Loads (Egypt)

[Hesham Elfeshawy]

Hello 

Project Overview:

• Diameter: 25 Meters (82 feet).
• Frequency: Considering 8V (to minimize strut length).
• Geometry: 3V 5/9 Kruschke or 8V (flexible based on engineering needs).
• Location: Coastal Egypt (Hurghada/North Coast).
• Design Wind Speed: 33 m/s to 42 m/s (approx. 75–95 mph) per Egyptian Code ECP-201.
• Cover: PVC-coated polyester (\(850\text{--}950\text{g/m}^2\)).

Material Availability:

• Pipe: Seamless Steel Pipe.
• Available Sections: 90mm OD with 2mm wall thickness (this is my primary concern).
• Connection Method: I have previously used the "Flattened-End" (No-Hub) method, but I am open to fabricating DIY steel hubs if required for this scale.

Specific Questions for the Group:

1. Buckling Risk: Is a 90mm OD pipe with only a 2mm wall thickness sufficient for a 25m span under high coastal wind loads, specifically at the "neck" where the pipe is flattened for bolt connections?
2. Frequency Choice: For a 25m diameter, is 8V the most stable option to prevent buckling in thin-walled pipe, or would a 6V with thicker steel be safer?
3. Hub Requirement: At this 25m scale, is the "Flattened-End" method fundamentally unsafe? Should I prioritize a sleeve-style hub that keeps the pipe profile round to maintain the Moment of Inertia?
4. Uplift & Anchoring: Given the "wing effect" of a 25m PVC cover, what is the best practice for anchoring the base ring in sandy or rocky coastal soil to prevent total uplift?

I appreciate any engineering insights or "lessons learned" from builders who have worked at the 20m+ scale.

Regards

Hesham El Feshawy

Sent from my iPhone

[Robert Clark]

Answer and image generated by Google Gemini for what it's worth.

💡 Key Takeaway: At 25 meters, the "Flattened-End" DIY method ceases to be viable. To prevent a collapse under Egypt's coastal wind loads, prioritize thicker steel (3.5mm+) and mechanical sleeve hubs.

Building a 25-meter (82ft) geodesic dome is a major engineering undertaking. At this scale, the structure moves from "temporary tent" territory into "permanent building" physics.

Based on the parameters provided, here is an analytical breakdown of your concerns.

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1. Buckling Risk & Pipe Dimensions

The 90mm OD pipe with a 2mm wall thickness is highly concerning for a 25m span.

• D/t Ratio: Your Diameter-to-thickness ratio is 45. While this is efficient for weight, it makes the pipe susceptible to local buckling (crinkling) before it reaches its global Euler buckling limit.
• The "Neck" Weakness: Flattening the pipe is the "Achilles' heel" of this design. By flattening the end, you destroy the pipe's Moment of Inertia (\(I\)) at the most critical point. Under the compression loads of a 42 m/s wind, the pipe is likely to fold at the transition point where the round profile meets the flat section.
• Conclusion: For a 25m dome, 2mm is too thin. You should aim for at least 3.5mm to 4mm wall thickness to provide a safety margin against the extreme axial loads generated by coastal winds.

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2. Frequency Choice: 8V vs. 6V

The choice between 8V and 6V at this scale involves a trade-off between member length and complexity.

• 8V Pros: Shorter struts reduce the risk of individual member buckling (\(P_{cr} \propto 1/L^2\)). It also creates a smoother surface for the PVC skin, reducing "pocketing" or flapping in the wind.
• 8V Cons: You are looking at over 2,000 struts. The cumulative "slop" or tolerance errors in 2,000+ connections can lead to structural instability or difficulty in closing the dome during assembly.
• Recommendation: Move to 6V with thicker-walled steel. A 6V frequency is a "sweet spot" for 20m–30m domes. The struts will be longer, but if you increase the pipe thickness/diameter, the assembly becomes significantly faster and the connections more robust.

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3. Hub Requirements (The "No-Hub" Question)

At a 25m scale, the "Flattened-End" method is fundamentally unsafe for several reasons:

• Eccentric Loading: In a flattened-end connection, the center of the bolt is offset from the center axis of the pipe. This introduces a bending moment that the pipe was not designed to handle.
• Bolt Bearing Stress: With only 2mm of steel, the bolt can "egg out" the hole under the rhythmic vibration of coastal winds, eventually leading to catastrophic tearing.
• The Solution: You must use a hub system that preserves the pipe's round profile.
  • Sleeve Hubs: Steel cylinders welded to a central plate where the pipe slides in and is pinned.
  • Star Hubs: Thick laser-cut plates that bolt to the interior or exterior of the pipe without flattening it.

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4. Uplift & Anchoring

A 25m dome with a PVC cover acts as a massive airfoil. In a 42 m/s wind, the lift generated can be several tons.

• The "Ring Beam": Do not rely on individual stakes. For sandy/rocky coastal soil, a reinforced concrete ring beam is the gold standard. It provides the necessary "dead weight" to counteract uplift.
• Soil Specifics:
  • Sandy: Use helical anchors (ground screws) tied into the concrete beam.
  • Rocky: Use chemical anchors (epoxy) with threaded rods drilled at least 500mm into the rock.
• Base Plates: Ensure your base hubs are heavy-duty steel plates with multiple anchor points, allowing for both vertical and shear (horizontal) force resistance.

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[Hesham Elfeshawy]

"Many thanks, Robert. Your advice on moving to 6V with thicker steel makes a lot of sense to avoid the complexity of 2,000+ struts. Since I am in a region where custom 3D hubs are hard to find, do you have any recommendations for a 'Flat-Plate' hub design that keeps the pipes round? I want to move away from the flattened-end method as you suggested. Thanks again for the help!"

[Ashok Mathur]

Dear Robert

During summers in Egypt, there are huge swings in day and night temperatures. During day the pipes will expend in length and as they form continuous rings of tension , where will they expend? 

Same thing in reverse direction happens near dawn .

So will fixed hubs last?

Regards 

Ashok

Regards

Ashok

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[Robert Clark]

Ashok and Hesham,

I'm not a structural engineer. I only have real world experience building an 8m backyard dome with wood struts and aluminum laser-cut hubs.

The temperature swings a 25m steel pipe dome in Egypt will experience are quite large. 15°C (60°F) at night to over 65°C (150°F) during the day. From what I researched, for a 25m span, a 50°C temperature change can cause the steel structure to expand by approximately 15mm to 20mm in total diameter. If your anchors are rigid and your hubs have zero "play," this expansion creates massive internal stress. Use oversized bolt holes in your hubs to allow for movement.

The only similar dome I can think of like this is the 36m Reunion Tower Dome in Dallas, Texas.

[Ashok Mathur]

Dear Robert

The hubs shown are suited for the planned dome.

In both 6v or 8v the number of six strut hubs that are difference from each other will be over 10 or thereabouts in number.

Should they be individually fabricated with different Dihedral angles or just a single hub with a common Dihedral angle?

I am suggesting a common angle as the variation in the dihedrals Is not going to be very large.

Regards

Ashok

Regards

Ashok

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