Fiberglass advise for structural strength

[mvoltin]

The question is not related to boats but there is tons of wonderful information about fiberglass on this site and hope to get some guidance since my project has dimensions similar to smaller boats. I am building a space-station themed tree-house for my kids that is about 18ft in diameter and 7ft tall. The enclosed image is fairly close but it will have flatter top and bottom with a railing and seating area on the roof. Most fiberglass DIY sites seem to focus on smaller projects that may not be applicable to this project. Here is the brief description of the non-fiberglass structural plans and would really appreciate hearing guidance on the fiberglass considerations.

The base of the structure will be well supported with a round wooden frame of 10ft in diameter.The sides, starting from the 10ft diameter base to the 10ft diameter seating area on the roof, will be reinforced by 1/2 metal tubes bent to match the side profile (they will be oriented vertically) and spaced 2ft apart (approximately 28 total for 56ft circumference). 1/2 foam insulation will be glued between the metal tubes that will help both with insulation and serve as a mold. Fiberglass will be placed both outside and inside of the treehouse separated by 1/2 insulation.

• Is this approach reasonable (does it pass the laugh test)? Would you recommend an alternative approach?
• Would alternating 3 layers of each CSB and 3.7oz E-cloth (total of 6 layers) suffice? Considering that I am applying this on both sides of 1/2 insulating foam?
• The reason for using 1/2 tubes is that the roof will be supporting with metal posts directly from the floor frame (as described in the next paragraph) and the sides are only intended to support kids leaning on it or running into it at "low speeds", etc.
• Should I add additional support structure such as "ribs" to reinforce the sides or the fact that it is curved will be enough to ensure rigidity?

As far as supporting the roof seating area, I planned having 4 metal posts go from the edge of 10ft round base to the ceiling. Thus, there would be 4 support points spaced evenly in a circle with a round wooden frame to support the fiberglass ceiling that will also serve as a floor for the roof seating area. This seems easy but any suggestions will be appreciated.

 

[Woodonglass]

CSB??? Did you Mean CSM? Are you Planning on using Polyester Resin? It's Not compatible with the 1/2" Plank Foam. It will "EAT" it. Epoxy would work but very pricey for a project this large. You could cover the foam with cheap packing tape and then glass over it. CSM offers NO structural Strength Just helps aid the bonding process. You do realize that a project this size will require approx. 20 gallons of resin and 25-30 yards of both CSM and Fabric which in dollars with shipping is close to $1,000 bucks. Again Polyester resin is NOT the ideal resin for this. Epoxy would be much better due to its flexibility but your costs would prolly double.

 

[Bondo]

Ayuh,.... Welcome Aboard,.... Pretty Cool project,.....

Like W'nG says, with foam, 'n the loads yer lookin' at, I see epoxy, 'n woven rovin' cloths,.... Heavy stuff,...
Then top it off/ smooth it with light weight cloth, 'n fairin',...

'n for the surround wall, strips of plywood, rather than metal tubin' would be easier, 'n just as effective,....

 

[mvoltin]

Thanks for the advice. Yes, completely agree that I will need epoxy instead of polyester resin if working over 1/2 insulation. The cost is no problem since doing this with project from wood will cost me around $3,000 or more. Plus, the fiberglass will look much better.

the one last thing wanted to confirm that the three layers of cloth (or 1 layer of woven and then 1 layer of cloth) will suffice if I do this from both sides of the 1/2 inch insulation? again, these walls will be curved and will not bear weight, they just need to support kids leaning on it.

thank you

 

[DeepBlue2010]

As an engineer and a father, I would strongly encourage you to slow down a little and think about this project from the point of view of kids safety. Fiberglass schedule (what fabric to use, how many layers and in what order) is done by engineers after estimating all acting forces on the object they are designing.

In a project like yours, where the safety of my children and possibly their friends depends on the structural integrity of this structure, I certainly would not want to "experiment" with structural engineering. As a rule, we never design anything to barley stand the "normal" use cases of it. We have to account for misuse and abuse use cases whether intended or accidental. For example, the normal use case for the walls is for the kids to lean on them slightly or run to them with moderate force.

What define moderate force and how can you make sure your laminate schedule will support it?
What is the kids start fighting and pushed each other around?
Will this structural stand to a wind storm?
Will these walls protect them from a flying debris or a falling tree branch?
Will these walls support their own weight given the spherical shape of the structure?
Resin is flammable, so what is the fire retardation plan and the escape/evacuation plans in case of fire.

IMHO, your perception of cost saving with the fiberglass as a building material could be wrong especially when it comes to applying the final finish. Painting a hand laid fiberglass requires specialized material and lot of fairing and sanding. What is your assembly/building plans would be? Are you doing all of this on the ground and then left the structure above the tree or you will build on the spot?

Finally please understand that I am not by any chance trying to persuade you not to do it. This is not my intention. I am just trying to help you think through the project and plan accordingly.

If I were you, I would – if I don’t have the design skills required – buy plans from a company and follow them. I can change minor details that are not structural by nature if I choose. Lots of thinking goes into these plans and good engineering efforts. Just my 2 cents

 

[sphelps]

I would think a structure like this will need permitting of some sort .. I agree with DB on the engineering .
I hope ya get it all figured out . What a cool thing to do for your kids !

 

[Scott Danforth]

I agree with DeepBlue as well.

Also being an engineer that gets involved with structures supporting people there are certain minimum guidelines

For example, residential code alone requires calculation of both live and dead loads for the floor and roof. some structures fall under commercial requirements, depending on zoning laws. for most residential code, the floor loading is 20psf (# per square foot) dead load (stuff), and a 25psf live load (people) If it is considered a balcony, residential code is 60 psf - 85 psf. for your 10' diameter, you would need to calculate for up to 6700# people load if the requirement is 85psf. will you ever have 6700# of people in there, probably not, however then again, it is possible. For the roof, you need to design for 15-20 psf (depending on the slope of the roof, and assuming non-drifting)

For a structure in the air, you have wind-loading. for most residential applications, this is for a 90mph sustained wind (about 20psf). In certain locations, you need to design for a 140mph wind load, in others a 200mph wind load.

for certain locations, you need to design for seizmic requirements as well.

The structure needs to be anchored to the earth, so ground loading is a requirement.

I also am not saying dont do it. I am saying, you need to be aware if the requirements, because your local AHJ (Authority Having Jurisdiction) does know the requirements.

BTW, if you look at the picture you posted, the footings alone are probably 12' deep. the tubular framework below appears to be 3" minimum heavy tube. The materials to build something along the line of what you have in the picture is probably in the $10k-$15k range.

 

[mvoltin]

Thanks again for your advice and agree with everything said and safety is paramount and I am all about planning and researching to make informed decisions. At the same time, I am not sure if it will necessarily need to meet commercial requirements that are often driven by litigious considerations - I just wanted to make it reasonably safe (big proponent of the 80/20 rule) that would be at least as safe as 95% of DIY treehouses built by homeowners.

I had no problem building a two story tree-house from wood 5 years ago because I could guesstimate what would be needed to support the structure and the kids. It was very helpful to have a general guidance for building decks or framing a house - for example, maximum length of 2x8 beams or recommended spacing between joists based on both joist sizes, 2X4 spacing for a supporting wall etc. etc. These guides distill all the complex considerations mentioned above into actionable set of recommendations.

Unfortunately, I have zero experience with fiberglass to do any informed guesstimation and was hoping to find a general guidance for fiberglass structures/frames.

Thanks again for your help and hope to have plans ready fairly soon for posting here.