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

[malua mada!]

Looking at the animated sequence,
the uphill end of the carrying wire is tied to a tree, not anchored in the ground otherwise? That tree is going to bend over.

Likewise, no matter how huge the downhill tree may be, watch the roots on the downhill side for signs of popping.

Good luck with the parade ground in the sky!

 

[Tom Miller]

"Danny D." <dannydiamico@gmail.com> wrote in message
news:m2n459$c0c$25@dont-email.me...

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/4/3953/15460990220_5b1f28763d_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.

Here are a few things that come to mind:

Has the owner checked with his insurance provider to see if he is protected
from liability? Things like this are known as an attractive nuisance and
everyone involved might be at risk should anyone get hurt.

Have you considered corrosion of the cable? Is it steel, galvanized, or
stainless?

Do you have an inspection plan in effect to detect future failure
conditions?

You might apply some paint to the cable clamps to serve as a witness mark to
see if anything slips.

It is pretty neat and will have all the kids in the area interested.


Regards

 

[Danny D.]

josephkk wrote, on Wed, 29 Oct 2014 21:11:45 -0700:

It never was a question about the strength of the trees. It has always
been about the cables and the load.

I would tend to agree, as the big redwood is massive (30 feet in
circumference).
https://c2.staticflickr.com/6/5607/15498557171_df86936bcb_b.jpg

For scale, there's actually a person, wearing blue, in the cargo net
right next to the tree, fixing the blocks for the cable that we later
wrapped around that tree.

BTW, even the little redwoods are not all that little:
https://c4.staticflickr.com/4/3910/15279581646_2753fa993e_c.jpg

 

[Danny D.]

Tom Miller wrote, on Wed, 29 Oct 2014 15:56:20 -0400:

Things like this are known as an attractive nuisance and
everyone involved might be at risk should anyone get hurt.

The owner is an ex Google executive, so, he has the disposition to have
lots (and lots) of "attractive nuisances" on his property! For example,
you can travel in another part of his property, high up, from tree to
tree to tree to tree (etc) by cargo net, for HUNDREDS of feet!

I always find a way to take my grandkids to his place for fun stuff.

Have you considered corrosion of the cable?
Is it steel, galvanized, or stainless?

Steel. When I asked, he said there's plenty of zinc fittings, so, he
wasn't worried about rust.

Do you have an inspection plan in effect to detect future failure
conditions?

Good question. I'll ask.

You might apply some paint to the cable clamps to serve as a witness
mark to see if anything slips.

This is a GREAT idea!
I will suggest that to the owner!

> It is pretty neat and will have all the kids in the area interested.

Kids love his place. I can't count the number of "attractive nuisances"
he has on his rather large (scores of acres) property.

 

[josephkk]

On Tue, 28 Oct 2014 20:48:36 +0000 (UTC), "Danny D."
<dannydiamico@gmail.com> wrote:

dpb wrote, on Tue, 28 Oct 2014 09:21:42 -0500:

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.

Thanks for all the insight.
Below is the owner's response to your concerns.

BTW, I created an animated GIF of the entire process, as I see it,
but I can't get Flickr to show the animations since Flickr turns
an animated GIF into a static JPG.

I'll post the animation separately, if I can figure out how to
preserve the animation, but here is the starting point static JPG:
https://c2.staticflickr.com/6/5605/15466740929_084969faac_z.jpg

Here is where we are right now:
https://c4.staticflickr.com/8/7575/15629669196_d8ed414ac3_z.jpg

And here is the penultimate ending point static JPG:
https://c2.staticflickr.com/6/5600/15466738719_4866bb66b1_z.jpg

Here's the owners response to your valid concerns ...

I wonder if they realize how huge the final redwood tree is?
The tree probably weighs in excess of 2,000 tons, and has a 30 foot
circumference.
The smaller set of redwood trees I would estimate weighs 15 tons.
In a wind of 50 mph, the small tree experiences 200,000 pounds of force
due to the wind.
The idea that 28,000 pounds of tension on a cable is more than it
encounters in a light wind does not seem tenable.
The root structures of both trees routinely handle much larger forces
during a typical day.

A wind blowing at 100 miles per hour generates 25 pounds of force per
square foot.
If that wind were blowing straight down on 800 square feet of deck, we'd
have 20,000 pounds of force.
I consider that unlikely.

Edge-on, we have 67 square feet, or 1,666 pounds of force. But that is
also somewhat unlikely.

Sideways forces will add a little to the cable tension, but will mostly
be taken up pushing against the trees and the support posts.

The deck will weigh in the neighborhood of 5,000 pounds, and has 800
square feet of maximum surface area. Lifting that, requires 6.25 pounds
per square foot, or a wind speed straight up of 50 miles per hour. But
the deck is held down at the ends and in the middle by either trees or
posts, which also limit the amount it can tilt or twist. The surrounding
trees limit the wind considerably.

The assumption that the engineering is "seat of the pants", or that the
mathematics have not been done is incorrect, but the ideas are all good
because I don't want to miss something, by not thinking about it at least.

Let them know that I appreciate their advice!
(Please invite them to lunch on Wednesdays in Redwood City if they're
local.)

It never was a question about the strength of the trees. It has always
been about the cables and the load.

?-)

 

[Danny D.]

dpb wrote, on Thu, 30 Oct 2014 08:13:07 -0500:

I don't need a picture of the big redwood; I'm perfectly willing to
allow as how they can and do get big; I've been through redwood country
a number of times. I was simply noting from the pictures posted near
that point in the thread there didn't seem to be much of any real size
and was more concerned of potential on the root system with the load
than whether the tree itself was sufficient presuming it was.

I apologize that most of my pictures were from the uphill side (where
the suspension bridge is currently forming), where those trees are puny
in comparison to "General Sherman" (which is what we call the big one).

The only time I climbed down the 100 feet to General Sherman was when we
were setting up the cables around it, and I was the gaffer who passed up
tools and supplies.

So my only pictures of General Sherman are the ones I showed, which don't
quite show the massive girth of the thing, especially at the bottom,
because what you see above is already split in two.

When the response to the question regarding angles for trying to
estimate tension needed to provide a given vertical force component
includes the justification that the angle will increase owing to the
tree flexure doesn't lend itself to thinking they're terribly big,
either. Just a "devil's advocate" position raising the question...

I agree with you, that when I first saw the angle stuff, I too wondered
about bending a tree that much to make *any* difference. I'll forward
your comments above on to the owner to see what he makes of that.

From a diagram such as that with a few measurements one could get at
least a reasonable approximation using simple-enough analyses as
outlined in the following (beginning at 7-30ff)--

http://isdl.cau.ac.kr/education.data/statics/ch7.pdf

I will forward that "Chapter 7: Forces in Beams and Cables" PDF to the
owner, who, while he isn't an engineer, he has multiple graduate degrees
and can handle almost anything we throw at him (he was an early Google
exec).