We started the 100-foot long 10-foot wide deck high up in the California redwoods

What are the two vertical posts made of, and how are they secured into the ground/stone??

Arfa Daily wrote, on Tue, 14 Oct 2014 11:35:38 +0100:

It's going to be a nice, 100-foot long, 10 foot wide, treehouse strung between the redwood trees on a steep slope when it's done.

Here's the varnished decking laid in place but not attached yet:

Right now, we just "hung" the first 16-foot section.

It's 17 feet on the left and 18 feet on the right to the next straddling set of redwood trees, which is too bad, given our beams are 16 feet long:

After the straddling redwoods, it's 20 feet to the big fat giant redwood.

So, we're not sure if we'll string four 16-foot lengths, or maybe change the plan to go for one fewer section but of a longer section length.

It's a design-as-you-go project, where advice is always welcome!

Danny D. wrote, on Fri, 17 Oct 2014 21:28:15 +0000:

redwood.

Ooops. It's around 40 feet to the final tree, far downhill.

So, we think, the total length is much less than the length of the 3/8- inch steel cable, which was 250 feet long (and we had to cut a few feet off the end).

The bridge itself starts about 15 feet from the end of the cable (simply because that's a convenient point to get ON the bridge, at the crossing of the old footpath).

The first section is 15 feet long. The next set of redwoods is 18 feet away. The big redwood is another 35 to 45 feet from there.

So, the total length of the bridge is gonna be more like 80 feet or so. Once the bridge is done, then we can put the building up.

We're told that we don't need any permits for tree structures, so, we can even put a bathroom, kitchenette, bar, etc., there, and not have to worry about meeting code requirements.

So, it's probably not 100 feet long, but something like:

I love the idea but think there are design issues that need serious attention.

I always thought that most code requirements were to prevent people from building structures that would kill them or their guests...

I still don't like the cinched cables as shown in the pictures earlier, those are stress points and the wire rope is quite a bit weaker now for them. Have you never looked at how cables are secured when done by professionals? Clevis hitches, or good splices are best for securing the rope end. Found a nice page on splices you may want to look at:

I know you aren't the designer (you are one of the helpers), and I am no expert on wire rope, but I do have a lot of respect for the people who make it and when they say to follow certain rules I would think they are the better adviser.

3/8 inch wire rope is rated at 2440 lb safe load...

...using well designed cinching. Now you have two ropes, however each of them must be able to bear the entire load (if swinging for example) so you really are limited to around 2500 lbs (a bit over a ton) for the total weight of the rope, structure, and guests. If I am wrong then the most safe weight load could only be 4800lbs. I'm not an engineer and thus don't know how to work out the loading factors, but I can add.

Guests have weight.

Have they made allowances for any people to visit? Looks like you can put five people in it plus a 1200 lb structure (if 2400 lb safe load), if the average guest is around 200lb. Ten people and you can only have a simple wooden platform that weighs 400 lbs...

John :-#(#

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John Robertson wrote, on Fri, 17 Oct 2014 18:29:39 -0700:

I see you noticed that the cables are currently bending in a sharp "V" shape where they are holding up the first gang plank.

We all agree that any sharp bend in the cable is a bad thing, but, that "V" is supposed to be temporary.

The end result "should" be either a catenary or a parabola (depending on the amount of force downward per linear distance of cable). If that weight (i.e., force downward) is evenly distributed, the cables will form a catenary; otherwise they'll be a parabola.

In our case, the owner has stated that each cable can hold something like 7,000 pounds (IIRC), so that's 14,000 pounds.

The *original* plan was to have the bridge free swinging, but, we attached one end of the bridge to posts cemented into the hillside and, we probably will attach the other end to the big redwood, and, in the middle, we will probably make use of the two skinny redwoods.

So, exactly how much weight the cables will hold is not really going to be something we'll be able to calculate on this home DIY project.

I do know that this lumber is very heavy! I helped carry it, so, I'd know. We lugged those 16-foot ten-inch wide beams hundreds of yards into the woods along a narrow foot path, and my hands were hurting by the time we put them down. Lugging the cement bags was easier! So, each bridge section is probably something like

500 pounds, and there will be a minimum of four 16-foot sections, maybe 5, depending on how far it is to the big tree.

We don't actually know the distance because the hillside is steep, so, you can't just run a 100-foot tape measure, but, it's less than 100 feet by ten or twenty feet.

Better check the specs of the wire rope, the owner may be going by the rated maximum capacity (12,200 lb for 3/8" wire rope) and only derating it 50%. Note the manufacturer recommends only a 20% load factor for safety. I don't think that is so they can sell thicker rope...

It also depends on how the load is distributed on the rope, and any pressure points, bends etc. All of these are part of the 20% safety factor, or may derate it further.

Please read this article:

and this:

note the recommendation that only one person at a time is on the suspension bridge...

Weight of the wood used...

So, lets say you are using 2 X 6 pressure treated wood that is 10 feet wide. 2200 lbs equals only 68.75 boards - or about 35 feet if laid side by side. Check the math yourself:

All this is based on 3/8 inch wire rope with a safety factor of five.

If these guys can afford to build this, they can afford to get a qualified engineer to aid in designing it to be safe enough that you would be willing to risk your children on.

I wouldn't let any kids near this design based on what I've seen so far. Adults can take their chances...

John :-#(#

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John Robertson wrote, on Fri, 17 Oct 2014 23:46:50 -0700:

I asked the owner about the weight of the deck, where this came back:

Hello Danny:

Thanks for your help last week. I couldn't have done it without you.h

You seemed skeptical about my estimate of the first bridge section. Given a 2x10 weighs 3.37 lbs per foot. Given a 2x6 weighs 2.00 lbs per foot.

3x16x3.37 = 162 lbs for the three long boards 2x10x3.37 = 68 lbs for the two end boards

2x6 2.00 lbs/ft

10x2x32 = 640 lbs for the decking

870 lbs per 16 foot section.

So if there are 5 sections (80 feet), or 6 sections (96 feet) we have

4,350 lbs or 5,220 lbs for the deck.

The cables can support 28,000 lbs.

That gives us 22,780 lbs for the house and occupants, assuming the only support is the cable.

If we allow that some of the weight is supported by the posts and the trees, we have even more leeway.

If the house is 24 feet long and ten feet wide, it will weigh about 8,000 pounds.

Add 4,000 pounds for furnishings and appliances. That gives us 10,780 pounds of leeway for occupants.

But I plan to have most of the house weight supported by the redwoods, not by the cable.

It again all hinges on the diameter of the cable (wire rope) they are using and how it is secured. If it is 3/8" then I wouldn't go near it assuming he is going by the maximum load.

Based on his reasoning (28,000 load spread over two cables) he needs at least 7/8" (12,900 lb safe load each), but 1" (16,700 lb) would be better.

Attaching the house to the tree will save a lot of weight, however trees grow larger in diameter so the support must allow for that somehow.

Also his securing of the wire rope must be flawless, and the fact that he has already stressed the rope in a few places with the clamps has weakened the original wire rope significantly.

However he doesn't appear to be willing to get an inspector, so I'll assume I'll read about this upcoming disaster in the newspaper in the near future (next couple of years).

Just because you are clever doesn't mean you are right. Some of us can be quite smug...(ducking)

John :-#(#

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derating

8,000

I see a complete failure to account for wind loads. Wind loads on this tree house will be larger than the dead loads. Ask the owner to calculate the wind loads as well.

?-)

Danny D. wrote, on Tue, 21 Oct 2014 20:10:10 +0000:

We finished rigging up the second 16 foot section, which missed the next set of redwood trees by about a foot or two.

Unfortunately, those two redwoods straddling the end of the 32-foot suspended section are just a tad under ten feet apart.

So, we're gonna have to engineer a slight bevel inward, to squeeze in between those two trees, and then it's on to the next three or four 16-foot long 10-feet wide sections, all of which is suspended by ropes and temporary cables, at the moment, as we build it as we walk out to the edge...

josephkk wrote, on Thu, 23 Oct 2014 18:23:37 -0700:

This is a good point so I will mention it to him.

We worked on the second floating 16-foot section today, by the way.

So now we're suspended 32 feet straight out.

Only 60 or so feet to go!

dpb wrote, on Thu, 23 Oct 2014 07:36:12 -0500:

Wow. Those were wonderful calculations. I forwarded it all to my friend, and will reply back with his response.

Meanwhile, we worked on the second section today, and we ended up stopping about 2 feet away from redwoods which we need to squeeze through.

CRNG wrote, on Thu, 23 Oct 2014 12:40:15 -0500:

I think the owner is taking heed, it's just that he's a third party to this conversation (he doesn't know Usenet). :)

BTW, here's a view from below today, when I dropped my glasses and had to climb down the steep hill to retrieve them.

dpb wrote, on Thu, 23 Oct 2014 13:35:20 -0500:

I'm sorry I haven't responded in a while. I hurt my back and was laid up but hopefully I'm better now ...

The whole thing is supposed to hang from the cables, but we did anchor one end because we needed a way for people to get "on" the decking.

Here's where we left it today...

CRNG wrote, on Thu, 23 Oct 2014 08:05:45 -0500:

I'm sorry I haven't been able to respond lately.

We had to readjust all the cables today, with a set of 5 winches, as we had to re-balance everything once the second 16-foot section was planked.

Unfortunately, I ruined my clothing, as I hadn't expected the oil to still be soaking wet ... even though it was drying outside for a day ...

VinnyB wrote, on Wed, 22 Oct 2014 05:58:58 -0500:

You forgot the screw manufacturers!

Notice we gave up on the lower screw (the one with the longer thread).

It was just too hard to drive into the wood.

Even with this nice pile driver thing from Harbor Freight!

The screw on top, with the shorter thread, goes in without pounding!

"Danny D." wrote in news:m2mk88$c0c$ snipped-for-privacy@dont-email.me:

See The chances of your bridge deck fluttering in a blow are fairly high.

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