Geodesic dome retrofit?

[Bernardo Guarderas]

Hello,

I wanted to tap into the vast knowledge of geodesic domes in this group.

We have a geodesic dome project that is going to turn 8 years old this summer. Built in 2018, it was meant to be an outdoor awning, a cover for a round pen we had for horses.

It is a straightforward 2V flat-bottom geodesic dome made from bamboo (dendrocalamus asper) with a radius of 6 meters. It appears to me that, although it is a 2V configuration and therefore a half-sphere, the height of the dome is lower than 6 meters (just a small observation). We used aluminum 4-inch pipes at either end of the bamboo canes with an inner steel flange, and flattened them so we could make overlaid joints as seen in this picture. https://imgur.com/a/Nz3iGj1

The tarp panels we had installed (as seen in pictures) have degraded away, and we have vast access to concrete. So we have decided to use a mortar spray application instead of this tarp to make the structure weather-resistant.

In terms of structural reinforcement, we have inserted calibre 5 steel rebar in all the bamboo canes, welded the joints, and filled both the canes and joints with concrete. We have also welded steel rebar mesh over the exterior of the dome, where we plan to spray this concrete mortar.

All we need to do now is schedule a concrete mixer and mortar sprayer and hope our dome structure will not collapse. We've had a structural engineer look at our structure, and his calculations are not the most certain, since he suggests a classic catenary arc dome-structure reinforcement and did not make a geodesic calculation.

If you've reached this far, thanks for reading this.

Any advice will be welcomed!

Please help us succeed with our dome retrofit.

Gracias!

https://maps.app.goo.gl/HMDN5uLrz6Y28Mqy8

https://maps.app.goo.gl/i7GzhPdYBsmtZsYR6

https://maps.app.goo.gl/XHPUcndoH3fR2dqF8

[Ashok Mathur]

Dear Gracias,

How thick a layer of spray will you apply and what will it weigh?

It might surprise you, but when you have finished the spraying and it has dried, you have created a thin shell dome structure in addition to the 2V geodesic dome that existed before.

This is going to be very strong.

I expect the spray will take between 3 to 4 week to dry and attain its full strength.

You might consider supporting each vertex with a bamboo pole under it till the spray dries. 

Then the poles can be safely removed.

Beautiful work done before hand

Regards

Ashok

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[Dx G]

Bernardo,

  Very interesting project.  Here are a few items you might find of interest.  If you check the internet, you will find past and current information about concrete and stucco dome with various kinds of information.

https://monolithicdome.com/
https://www.monolithic.com/dome

https://www.masonryarches.com/domeblock

There was also a company called American Ingenuity Domes (in Florida USA), or AIdomes, that build stucco-on-frame domes, but they closed several years ago.

Many thanks for posting your pictures and background about your dome work.   I'm working on a universal strut connector (USconn) that would make it easier for people to make domes and space frames out of pipe, tubing, rods, lumber, and yes, bamboo, without going to the time and expense of making their own connector.  However, I admire all the work you put into this.   Let us hear from you as you make further progress.

Dx G

[Bernardo Guarderas]

Thanks Ashok and Dx G for the responses and reassurance.

After looking at monolithic.com this particular article about Chain Shell Coating provided some crucial information.

Therefore, to answer Ashok, following a Chain Shell Coating technique, we can aim to make the mortar 2 inches thick. As far as the weight, what would be the best way to calculate that?

Instead of chain-link fencing, we will be using electrowelded rebar mesh. I assume this won't make a big difference?
Finally, is there any particular suggestion for fixing this rebar mesh to the base of the dome? We recently removed our brick&mortar base and made a thick concrete one.

Bernardo 

[Ashok Mathur]

Dear Bernardo

A Two inch mortar shell is both an overkill and likely to destroy the structure.

A one inch mortar shell with a rich cement mixture with chicken wire mesh underlay will work very well .I will try find a ppt I made on Ferrocement work some years ago .

You can search the archives of this group and find old photos of Ferrocement 

Latur domes that I have previously posted. Those domes were constructed about 30 years ago and are in a fine conditions.

Monolithic dome current owner has put out her PhD on the subject of monolithic domes structures that may have more authentic data on thickness that you need.

A simple way to approximately calculate the weight of the layer is to first calculate it’s volume and then multiply that by it specific density. A simple way to get the surface area of the dome is to treat it as 2*3.14*r*r where r is the radius of the dome.

Regards

Ashok

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[Ashok Mathur]

So generated answer. Treat with caution!

Ferrocement layers for domes, commonly used in cost-effective structures, typically have a total thickness between 

10 mm to 25 mm (roughly 3/8 to 1 inch). This thin shell construction involves applying rich cement mortar (1:2 or 1:3 ratio) over layers of small-diameter wire mesh, ensuring a minimum cover of 3 mm, creating a lightweight yet strong structural element. 

ignited.in +4

Key Details of Ferrocement Dome Construction:

• Thickness: Generally ranges from 10–25 mm, though some experimental structures can reach 50–60 mm depending on design requirements.
• Components: Comprises skeletal steel (6–8 mm bars) supporting fine wire mesh (0.5–1.5 mm diameter).
• Construction: Wire mesh is applied in layers, often applied by two people (one inside, one outside) to ensure full penetration and avoid voids.
• Curing: A crucial 28-day curing period is necessary to achieve full strength.
• Advantages: High crack resistance, no formwork required for some applications, and high durability. 
  www.challengejournal.com +5

Regards

Ashok

[Paul Kranz]

Bernardo: I would try one face and see if it holds up. If not, you can break down the triangles and keep the overall shape of the dome. See what I had to do with a 1v dome breakdown.

Paul sends...

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Very high regards,

 

Paul C. Kranz, LMFT

Kranz & Associates, LLC

ReviveTheFlame.com

[Dx G]

Bernardo,

I'm not a structual engineer, but will share a few thoughts to pass along on your shell structure.

Firstly, you already know you need to determine if the finished structure will be self supporting, so it won't collapse under its own weight, the dead load.  Then one needs to add live load to the requirements,  from wind, rain, snow, and anything the structure needs to support other than just its own weight.

Even if the finished structure is deemed adequate, you need to consider what support it may need during construction, before it is completed.

As an example, when a lot of people look at a stone arch made of massive blocks, they don't ask how one would erect such a thing. Sure, once the stones are all in place, they can stand for centuries and bear enormous loads. But how did they hold it up until all the stones were placed?  The most common method (of many) is to build a frame underneath the arch to hold all the parts until they are in place.  In other cases, a mound of soil is used instead of a frame. Once the arch is built, all the supports or soil can be removed.

These same methods could be used to erect a shell structure, especially when using a material that has no strength until it cures, like shotcrete. Clearly, if a frame already there, like your bamboo, can hold the entire weight of the uncured shell (as Monolithic does) you have what you need. Likely the cured shell will only add strength, and in some cases, even the bamboo will no longer be necessary. Propping up the bamboo shell, as suggested by Ashok, is likely a good idea unless you are very sure it is unnecessary.

Dx G

[Bernardo Guarderas]

Hello,

Sending some updates on this dome and the shotcrete process. We are on the 21st day of concrete curing, and we are starting to remove the formwork.

During the concrete application we had some hiccups and the failure of one strut member, where the bamboo broke under bending force, but besides that, it was smooth sailing.

We reinforced every panel and strut significantly and used pallets for framing/spreading the load onto the formwork.

We also used 1.5-inch spacers to make sure the thickness was consistent and the rebar mesh equally spaced. However, in the concrete application, some panels were bent inwards, and it resulted in slightly concave panels, as well as inconsistent thicknesses in some areas.

Here's videos where you can see the formwork from the inside and shotcrete application.

Video 1

Video 2

And here is a image from today:

Would like to hear some thoughts on the structural stability as we take out the formwork.

[Dx G]

Bernardo,
  Looks like quite a project.  I'm not a structural engineer, but my guess is there are others who know more about how long you should wait, or what indicators there are to tell you when the shell strength is high enough to make the inside supports unnecessary.

 If you are going to pull those prop beams out, I'd say safety is your number one concern.  

1) Best not to have anyone inside, or even close to the dome when you do that.  So you might want to tie rope to a prop and pull it from outside the door (avoid doing this suddenly if possible) with no one inside.
 
2) Another thing you could do is tie some string or flexible wire to several reference points on the inner surface, and let them hang down to the ground. A bolt or small weight at the bottom would keep it straight down. Set them so they just barely touch the ground. As you pull a prop, if they go limp and bend, you can see that without going inside. These will tell you if anything has moved, where, and how much.  Don't assume it's safe if none of them indicate motion, since its possible for something to move very suddenly with little or no warning.  This method is not terribly precise, so small movements could be happening that the strings won't show.  
 
3) In addition, you could always place another support beam close to the one you are going to pull out. If it is just slightly shorter than the original, there will still be space above it if the dome didn't move after the original support is pulled. If the dome does move, it won't go very far before it reaches this shorter support.

There might be better ways to do this. There may even be good reasons not to use the approaches I describe.  Others may know more about what you need to watch and how (cracking?), but it seems these could help you avoid big trouble.

I sure hope it goes well for you.

Dx G