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.
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.
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 ...
dpb wrote, on Fri, 24 Oct 2014 08:22:06 -0500:
I'm forwarding all this to the owner.
The only thing I can say is that it "seems" sturdy when we're both on it, and that's almost 500 pounds of people alone ...
It's being supported, at the moment, by 6 separate winched vertical cables (the winches are for level adjustments).
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.
Ian Malcolm wrote, on Tue, 28 Oct 2014 00:21:08 +0000:
We actually joked, a few times, about the Tacoma Narrows bridge, and, yes, I think just about everyone has seen that video of the car on the bridge and the person getting out and making it just in time.
This bridge isn't nearly as long. It's only about 100 feet long, by 10 feet wide, supported on one end on the ground and on the far end about 40 or 50 feet up in a tree on a (very) steep slope.
Here's what the first two 16-foot-long sections looked like today, when we ran out of oiled wood:
I'm currently learning how to wash good clothes to get the oil out! :(
Where are the supporting cables in the picture--on top or just hidden by the view?
What I'd like to know is can you measure the height differential between the upper and lower mounting locations and the approximate distance from the straight line between them to the low point and where that point turns out to be between the two end points. In a _very_ crude sketch..
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dpb wrote, on Thu, 23 Oct 2014 08:13:06 -0500:
Here's the response from the neighbor building the deck...
The 3/8" steel suspension cable is a good deal higher than 10 feet above the deck at the ends. Currently that 3/8" steel cable is anchored at a tree about 25 feet above the deck at the beginning end of the deck.
Also, the 100 foot final length of the deck was a guess that is probably a bit high, where perhaps 80 feet might be closer to the final length. So the 11 degrees may no longer still be the result of the calculations.
Looking at the photos, the angle of the cable looks like around 30 degrees to me. If the height of the suspension cable above the deck is 25 feet, and the length of the deck is 80 feet, we now have 32 degrees.
Using his tension formula, we get a tension of 4,718 pounds. So, I believe, that means the 3/8" suspension cable can support almost six times what we are assuming.
While the deck was originally supposed to be free floating, since we decided to anchor the close end of the deck on the dirt path (so that people could just step onto the deck from the path), that end of the deck is now supported by the two fence posts, so half of its weight goes away.
If, additionally, we add another fence post, in a "T" shape support, at the end of the first 16-foot-long section, then the weight of that first
16-foot-long section goes away completely, as does half the weight of the next 16-foot-long section. And we still have the option of supporting the other half by attaching it to the small redwood trees, along with half of the third section.If we really did get to 28,000 pounds of tension, the trees would pull closer together, reducing the distance, and making the angle steeper. It thus gets asymptotically harder to actually put that much tension on the cable.
The treehouse itself, when it's built, will be supported mostly by the redwood trees.
Arfa Daily wrote, on Tue, 28 Oct 2014 02:05:05 +0000:
That's a good suggestion, as we have tools all over the place!
Usually, we tie in when we're out on the cable, with no firm footing:
And, we tie in when we're working on the steep slope below the deck:
The harnesses we use are the same ones we use for climbing the hills:
Here's my setup, for example, as I was digging the fence post holes:
And, here's what I used when I had to double-line rappel downslope:
Even so, we've had a few of the typical gotchas, from dropping the spool of wire (where it rolled a hundred or so feet downhill before getting wedge under a fallen tree) to the inevitable search for missing glasses and hammers.
Such is the nature of working in the trees...
dpb wrote, on Mon, 27 Oct 2014 21:06:58 -0500:
All the supporting cables are above the deck.
It's a bit hard to see in that picture from below today, but the 3/8" steel cable is glinting in the sunlight a few feet *above* the deck.
The deck is not attached to *anything* other than the cable on the free- floating end, as we build it out. In fact, while it's hard to tell from the angle of this picture, but the end of the second 16-foot-long section is still shy of the leftmost redwood by about a foot, and maybe it's two feet shy of the right-most redwood tree:
Here is a picture taken earlier in the day, before we planked the second
16-foot-long section, showing how the 10-foot-wide deck is suspended from the steel cables (this is the second of the two 16-foot-long sections):dpb wrote, on Mon, 27 Oct 2014 21:06:58 -0500:
This is a good point, in that sometimes it pours out here, and this is a steep slope, so the runoff could be great.
We just had a half inch of rain over the weekend, and this is a shot of the cables and the people on the decking (the dog won't go on the deck):
We're still about a foot from the left-most redwood here, and about two feet from the right-most redwood, so we "can" attach to them (if decided) with the third 16-foot deck section:
Some validity to the above but...
While it is true that the end that is resting on the ground does have that support, it's not necessarily so that the remaining load on the cable is only half; it depends on the actual geometry of the configuration. It _might_ be half; could be more, could even be less.
The previous "analysis" was simply intended as a demonstration of the effect geometry has on the overall tension required in the cable (actually, any supporting member--that it is cable in that sense is a distraction) is that which will, for the given arrangement, provide a vertical component that balances the gravity forces plus applied loads. The key lesson intended to be shown is that the vertical component is dependent upon the angle of the tension force.
Thinking that "it's harder to put that much tension on the cable" isn't really so--while the trees may move some, the actual counteracting force is whatever it needs to be given the loading. If the distance is shortened enough, yes, the angle will increase but that's going to exacerbate the other issues raised of whether the root structure can sustain that continued side load (increased moment at the base) indefinitely, particularly with the addition of wind load and ground saturation and all the other things that go on.
I do agree that with the one end supported that certainly helps and the idea of the second ground support at the outer end is also a good one but I'd still think it only prudent to get the input of a competent engineer to evaluate the overall structure, including the questions of the ability of the soil to hold and what would be realistic wind loads and so on. While one would presume nobody would be silly enough to be out there during a storm, it seems a sizable investment in both time and money to risk losing it all the first thunderstorm or the next coastal front with a good blow. It's certainly not unheard of uncommon for them to have 90 mph in WA where daughter is; don't know how far this is from coast nor elevation and such but would think it not unreasonable guesstimate.
It's just imprudent imo to not have more than just a "seat o' the pants" look at something this ambitious and that has such a potential for serious consequences if these guesstimates are wrong. Given the $$ that have and are obviously going to be expended, a few more thrown at the design/safety issues would seem a good investment. (Or, is the issue that if does that it'll get stopped because it isn't within whatever covenants are in place for the location?)
Anyways, I've had my say; simply hoping nothing does go wrong but it seems a very risky venture as is...
(*) The unique thing about cable is that unless the loading is such as was presumed before that the cable is essentially straightened by the load, the tension is variable along the cable
Arfa Daily wrote, on Tue, 28 Oct 2014 09:53:11 +0000:
We're all retired, and, getting a bit complacent, so, we tend not to own (real) work clothes.... :)
But, we're getting real good at buying army surplus static line!
Maybe we should pick up a pair of these Ghillies as fitting work clothes, since we're always hanging around in the trees:
On 10/28/2014 9:21 AM, dpb wrote: ...
Ignore the above; I thought I had deleted it before posting...I was beginning an involved discussion regarding multiple loading points and all and decided against it...
First and foremost, I think what you're doing is awesome. I love it.
But I do have to wonder what your insurance company would say. Couple years ago, the Safeco underwriter drove by my house and noticed my decks were 25-32" above ground level and I didn't have a proper railing around them. Safeco gave me 180 days to bring it to code or they were going to refuse to renew.
I think the Safeco guy would literally shit if he saw your deck! ;-)
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