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

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Just to keep you informed, and to get your advice on better ways to tackle the engineering problems:
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After a lot of initial setup of ladders strapped to the trees:
https://c3.staticflickr.com/3/2948/15314802989_a57dd072da_c.jpg
And setup of the many utillity ropes, and tree-to-tree netting:
https://c4.staticflickr.com/4/3928/15501340972_e31032dcd7_c.jpg
We've finally got the 100-foot-long 3/8" steel cables hung well:
https://c4.staticflickr.com/4/3948/15314983930_3c606db7b4_b.jpg
The next step, was building the first 16-foot long set of the 10-foot- wide decking from the top of the hill, which will eventually connect over 100 feet downslope to the gnarly big redwood at the bottom of the hill:
https://c2.staticflickr.com/6/5607/15498557171_df86936bcb_b.jpg
It's amazingly ungainly tying all the 16-foot long 10-inch wide boards to the 100-foot long swinging 3/8-inch cables as we try to build the bridge:
https://c4.staticflickr.com/4/3949/15314983180_78420114c9_b.jpg
It took two people and a lot of rope to get things squared up at first:
https://c4.staticflickr.com/4/3944/15314801909_48b46268b6_b.jpg
In fact, it was so ungainly, that we put in temporary crossbeams, just to hold the bridge square as we were assembling it on the hillside:
https://c4.staticflickr.com/4/3940/15314801829_a3eeb9395e_b.jpg
Even steps as simple as screwing in the self-tapping screws is difficult when the entire 16-foot long 10-foot wide bridge-like structure is swinging freely and nowhere bolted to the ground:
https://c4.staticflickr.com/4/3942/15315124677_c121c8def1_b.jpg
But, eventually, we managed to assemble the first 16-foot long 10-foot wide lengths from the cables between the groups of redwood trees:
https://c4.staticflickr.com/4/3943/15315306567_5e25779b8b_b.jpg
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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:
https://c2.staticflickr.com/6/5604/15558189375_03e664d327_b.jpg
Right now, we just "hung" the first 16-foot section.
https://c2.staticflickr.com/6/5598/15372553580_9751d293de_b.jpg
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:
https://c2.staticflickr.com/6/5608/14937422664_148d70ba58_b.jpg
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!
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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:
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On 10/17/2014, 5:39 PM, Danny D. wrote:

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:
http://stoneycreeker.com/knots/knots.htm
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...
http://www.engineeringtoolbox.com/wire-rope-strength-d_1518.html
...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.
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On 10/17/2014, 8:47 PM, Danny D. wrote:

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:
http://www.safetysling.com/wr2.htm
and this:
http://www.thetreehouseguide.com/bridges.htm
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:
http://www.engineeringtoolbox.com/green-kiln-dried-pressure-treated-lumber-weights-d_1860.html
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:
https://c2.staticflickr.com/6/5608/14937422664_148d70ba58_b.jpg
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.
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On 10/21/2014, 1:10 PM, Danny D. wrote:

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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On 10/21/2014 3:10 PM, Danny D. wrote:

...

The problem as others have been saying generally and that Morgans points out in another posting is this doesn't account for the geometry.
Consider for simplicity the condition if the cables were mounted on telephone poles on level ground 100-ft apart. Say you tension the cable so there's 10-ft sag in the middle. Using the tabulated weight for 3/8" cable, I estimated that it takes only about 30 lbf to achieve that sag so we'll ignore that for the time being.
With 10 ft drop at midpoint of 100 ft run, and considering that the applied load will essentially straighten the cable, if the load were at the center rather than distributed the angle between the horizontal and the cable is invtan(10/50) --> angle ~11 degrees.
Now to support that load, the vertical component of the tension has to balance the weight of approximately 5000 lb. That vertical component is Tv=T sin(angle) or the cable tension T is Ty/sin(angle). Substituting numbers and noting that for small angles sin(theta)~theta, the tension to support that 5000 lb is
T=W/sin(angle) = (W/2)/sin(11) --> 2500/0.2 --> 12,500 lb
So, you've taken up roughly half the total strength of the cable simply by the decking. That's only a safety factor of ~2X and minimum generally accepted is 3X while for overhead rigging and personal safety of support 5X is considered prudent.
As said, this isn't going to approach that kind of margin and is extremely risky going forward without more serious engineering than has happened to date.
There are excellent design guides in the various handbooks that have been linked to extensively before in the previous threads; no point in reproducing them yet again if they're not going to be heeded.
But, your friend really needs some input from an engineer who understands statics well enough to make some reasonable calculations for the actual geometry and design.
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> That gives us 22,780 lbs for the house and occupants, assuming the only
> support is the cable.
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On 10/23/2014 7:36 AM, dpb wrote: ...

...
ERRATUM: I forgot to divide the cable limit by the two above -- the actual limit per cable is (optimistically) as used by your friend 14000, not 28000.
So, the decking alone is roughly 90% of the rated breaking strength and adding a 200 lb person is 100/0.2-->500 lb.
You'll gain a little by considering the decking as a distributed instead of point load, but that'll be only a marginal improvement and you'll likely lose some (and potentially a lot) for the non-uniform geometry on the downhill side as that side may be almost perfectly horizontal so the amplification factor of 1/sin(theta) goes way up as Morgans feared (hence his 10X estimate).
THIS IS VERY BAD...
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On 10/23/2014 8:13 AM, dpb wrote:

...

...

...
On the last point -- the angle at which the total tension amplification factor reaches 10X the applied load is invsin(10) --> 5.7 degrees as opposed to the 11 degrees. That's not a lot of difference.
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"John Robertson" wrote:

---------------------------------------------------------- You can buy them books, BUT if they eat the covers.
Lew
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On 10/21/2014 4:35 PM, Lew Hodgett wrote:

Now days Lew you buy them books and they eat the Teacher!
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The lawyers are going to love it. Everyone from the wire rope manufacturer to the water-boy will be sued.
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Good analysis. Unfortunately Danny D and Friends just don't seem to understand what the effect of the catenary configuration has in increasing the tension in the cable. I really hope someone convinces them to have a engineer look at their exact config and do some simple calcs as you have done above.
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On 10/23/2014 8:05 AM, CRNG wrote:

...

Excepting I divided the load by 2 for each cable but then compared that to the total rating of the two cables. So actually, the decking is 12,500 lb out of 14,000 breaking strength not 28,000 for the two.
See my followup to myself wherein I noted the fault...
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On 10/23/2014 8:21 AM, dpb wrote:

ERRATUM 2:
I outsmarted meself...I left the other 2X out on purpose originally and then whiffed when came back. The free body diagram has T1 and T2 supporting the load, the tension force in each direction. Hence the actual magnitude _is_ half the total, one half going each way.
So we're back to the ~2X SF for the decking which, while likely not collapsing while they're building the basic deck, isn't a good starting point for the overall structure.
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I don't think they are taking heed. They have been successful with their little neighborhood projects, and they are starting to get over confident.
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On 10/23/2014 12:40 PM, CRNG wrote:
...

I think Danny D is reading responses but the friend-in-charge is apparently bullheaded-enough to not be willing to reconsider there's anything to consider other than tensile strength in straight loading.
I do suggest that Danny not be committing himself to relying on this for support from this point on as it's getting at least marginal already. Of course, if the droop angles are significantly larger than the assumed 10 degree or so, then they gain a fair amount by there being a larger vertical component but it's too spooky by far as described and shown.
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+1 on that
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