On 10/27/2014 10:56 PM, Danny D. wrote:
dpb wrote, on Thu, 23 Oct 2014 08:13:06 -0500:

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.

Here's the response from the neighbor building the deck...
https://c4.staticflickr.com/4/3953/1...1f28763d_b.jpg

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.

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
--