Just to update this thread, we completed the 250 feet of steel cabling
today by lashing the two ends together using these cable clamps:
To keep the cables from cutting into the trees, and to allow the trees to
grow outward, we put up a series of these wooden standoff blocks:
You'll notice that we doubled the cables as they wrapped around the trees
so that the strength is always two time 14,000 pounds, at all times:
Here, you can see the two cables, hanging as two catenaries, from which
we will hand the suspension bridge:
We're starting to get used to working in the heights, as you can see by
this photo of my neighbor coming down from disentangling the lines:
As you can imagine, we wear harnesses and we have static lines hanging
from all the trees, as you'd be amazed how many times you need them:
In fact, my unenviable job today was to stand at the TOP of this ladder
and position the cables, which I did with two hands on the cables so I
had to be wearing a harness or I would have fallen off in no time:
I'll let you know when we drill the redwoods to put in the tree bolts,
which will anchor the house; but first, we're working on the suspension
bridge (you can see our cargo netting in some of the pictures above).
Tomorrow we're putting up WiFi on a neighbor's roof, so we wont' be
working on the treehouse until next week.
Thank you for that safety suggestion!
That is a good point. Safety is paramount.
This treehouse 50 feet in the air in the redwoods has to outlast us
and it has be safe at all times.
Since we didn't use lock washers on the steel clamps, I will advise
my neighbor and I will snap a picture of the results for you.
You will notice that we doubled up the two ends of the steel cable
as they wrapped around the tree, so that we'd always have two cables
supporting the bridge.
On the big tree, 125 feet away, we will add a wraparound additional
steel cable, so that the middle also has two cables.
Any other safety ideas are welcome, as we're just at the point now
where we can start hanging the suspension bridge from the two steel
For example, you will notice that we followed the rule as shown here:
Following that diagram, we put the "saddle" of the clamps on the
"live end" (the mnemonic we used was "don't saddle a dead horse").
Any other tips are welcome, as we're just now at the stage where
we have the ability to build the 125 foot long bridge starting
about 15 feet up in a pine, and then going straight across a
steep slope through the set of two redwoods, and then on to the
really big redwood 125 feet down the slope.
The treehouse will be in the middle of the bridge.
I don't believe there's any reason to think the environmental thermal
cycling has any chance of loosening those sufficiently to worry over
from that standpoint.
We've got several miles of cable in feedlot fences with the same style
cable clamps with tension on them sufficient for retaining cattle while
working them. They've been installed w/o lock washers for some 60 yr in
SW KS which is quite extreme in both temperatures and particularly in
changing in extremes over very short time frames relative to CA redwood
country. Not a single one has come loose on its own in that time.
What I'd suggest and use would be
a) at least two/ location, preferably three, and
b) for looped connections (very few in this application; the cables are
terminated at the clamps which are welded to rod)(+), we also used
compression connector at the end to hold the cut end to the running cable.
(+) The rod is then connected via a turnbuckle for takeup tensioning to
a second rod which penetrates the end post/tie.
Are you saying that the tree house will be in the middle of a 125 foot
suspension bridge. How much will the tree house weigh when fully loaded
and do you have any idea of the forces that may be in the cables?
Dan Coby wrote, on Sun, 28 Sep 2014 12:34:27 -0700:
The tree house will be where our cargo net currently is.
That's roughly half way from the uphill pine to the downhill big redwood
(with two little redwoods, side by side, in between).
The steel cables can handle 14,000 pounds each.
That's 28,000 pounds (because we maintain a double cable throughout).
We don't know how much the bridge & treehouse will weigh, but if it's
close to or greater than 28,000 pounds, then we have a problem.
How much do you think a treehouse will weigh?
You also have to consider the geometry of what you are creating. If you
are tensioning the cables for very little sag then the forces in the
cable can be many time the weight of the tree house. Without knowing
exactly what you are creating then I cannot guess. That is why I asked
if you had any ideas of the forces in the cable.
I do not know what you are planning upon building. That was why I asked
you. If you are talking about the tree houses that we built as kids with
a plywood floor and a few boards and a tar paper roof then only a
couple of hundred pounds. If you are talking about some of the multi
story creations that I have seen on TV then many tons.
I agree with Dan's concern. Someone with good trig math skills needs to
figure this for them. I would "eyeball" calculate that the load on the
cables would be at least 3 to 5 times the weight of the structure and the
added weight of the occupants and furnishings. It could be much higher than
It would appear to me that you are beginning to cross into the zone of
pushing the design strength of the cable.
Another engineering principle here would also be involved. That of safety
design limits. A totally dependant system such as this, where failure of
one system could and would probably cause loss of life would be designed
with a 150% safety factor.
That means that a structure weighing 4200 pounds would be the maximum, if
you used the figure of the tension increased by the geometry from the load
by five times.
You seriously, seriously need to get a structural engineer involved before
you proceed. I believe you are playing with fire, that could get someone
baldy hurt, or worse.
If his eyball sag estimate is accurate, the geometry could be multiplying
the forces by a factor of ten. Four average 180 lb adults + weight of
cables and structure and it already looks marginal. Also, real suspension
bridges dont anchor the cables to the end towers. They take the cables over
saddles to properly engineered ground anchors (grouted into bedrock if
possible) to mimimise or eliminate the bending stress on the end towers.
A static side load of 10000 lb or more 50' to 70' above ground level on a
tree growing from a steep slope that may well have an assymetric root ball
or have major roots compromised by having grown round boulders is *NOT* a
good idea. Dynamic loading is going to make the situation even worse.
I wouldn't be surprised if the whole lot comes down in the first big storm
taking several of the trees with it. I wouldn't want to be within 50
yards of any of the cables or anywhere downslope if anything fails.
Also the lifespan is going to be pretty short. I wouldn't trust it more
than 5 years later unless the cables and their fixings are professionally
inspected and maintained and cable replacement every 10 years will probably
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
Have you allowed for a windstorm where the trees may be moving in
opposite directions to each other?
Temperature affects the length of the wire rope, have you allowed for
maximum and minimum temperatures?
You want some sort of shock absorption built in too. Old antennas used
porcelain blocks for joining cables, the porcelain would shatter under
unexpected loads giving the cables a chunk of extra slack to avoid their
collapse by stretching beyond limits.
May I suggest you find an engineer to look over your design? I'm not
one, but can think of a few ways for this to go wrong already including
the clamps failing etc.
Suspension bridges are close to what you are building - read up on the
design criteria for these. Seat of the pants design may give you another
PS, it looks like a lot of fun though!
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