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:
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
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)
(Please post followups or tech inquiries to the newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
You just don't get it. 28,000 is how much the cables will support if the
weight were hanging straight down towards the center of the earth. If you
pull the cables parallel to the center of the earth, with some sag even,
then pull down on the middle of the cable with the weight of this deck, the
pull on the cables is going
to be 10 times or more the force (weight of the deck) you are pulling down
That comes before the working load is decreased because of turns in the
cables, or weakening because of being clamped with a cable clamp.
If I was out there, I would turn you in to the authorities to get a stop put
to the project.
Someone is going to get hurt, or die. Mark my words. It will fail. It has
This email is free from viruses and malware because avast! Antivirus protection is active.
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.
> That gives us 22,780 lbs for the house and occupants, assuming the only > support is the cable.
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,
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...
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...
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.
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.
Where are the supporting cables in the picture--on top or just hidden by
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..
| X |
____ C xxxx _____
A and B represent the two tieoff points, the relative height between
them and then the distance from the lowpoint C in the middle somewhere
to either of those plus the horizontal distance from C to the two trees
represented by the vertical lines.
With that could at least crudely approximate the loadings and the
tension amplification from the angle of dangle, so to speak.
As you can see from the previous as the angle gets smaller, the
multiplier gets large quickly because there isn't much of a component of
the total tension in the vertical direction to support the load. You
also need to consider how much side load this is putting on those trees
and how large they are at that point to handle the lateral load plus how
well their root systems are capable of that additional continuous load,
particularly when the ground gets saturated when it finally does rain
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...
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
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 ...
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).
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!
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
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.