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#33167 - 09/16/15 11:47 AM Using felled trees (logs) as long span beams
LongLog Offline

Registered: 09/15/15
Posts: 1
Hi, I am building a tree house in a remote area of Vancouver Island, BC, Canada without any road access for miles. I would like to use the trees on-site as beams (20+ foot spans) for a tree house deck which I will then set atop a geodesic dome. I am in the designing stage and have the following questions. I am hoping someone can direct me to the answers!
1. What is the allowable bending stress for a western red cedar or Douglas Fir (Coast-type) log used as a beam.
2. What is the max. allowable deflection for the above trees ?
3. Does anyone know where to find information about how to make the following connection: A cut tree at say 10 used as a column supporting a log beam?

#33168 - 09/16/15 05:03 PM Re: Using felled trees (logs) as long span beams [Re: LongLog]
Jay White Cloud Offline

Registered: 11/06/08
Posts: 479
Loc: Vermont
Hello LongLog,

This is getting into some really deep and heady engineering, as well as some advanced rigging necessary, with further challenges of wood degradation rather rapidly because of the location.

Most of it sounds plausible (with caveats) yet without blueprints/CAD/diagrams it is hard to speculate beyond that.

The "load criteria" is not typical as it reads like you are going to be using round logs?

Allowable deflect is base on the "lumber species" but a set IBC standard.

As for #3...???...I could begin to go there without blueprints/CAD model to look at, perhaps others would venture there from just a verbal..

There are way more questions here than answers, at this time, for me. I am have concerns about "tree selection" mounting system, general design, decay issues, etc.

I will check later, and perhaps others would/could be of more assistance...

#33698 - 05/28/16 03:13 PM Re: Using felled trees (logs) as long span beams [Re: LongLog]
Jon Senior Offline

Registered: 05/04/11
Posts: 112
Bending stresses may be harder to calculate, but they will be much higher than for sawn (or hewn) sections. The second moment of area can be easily calculated, but I don't know where you'd get the Young's modulus values.

The allowable deflection would depend on your plans. L/200 to L/400 are all values that I've seen thrown around. If the beam is not supporting a finished (plasterboard) surface and you can tolerate some movement then you could comfortably go with L/200 or even less.

As Jay says, you do have a potential degradation problem. While both Douglas and Red Cedar are rot / insect resistant, this applies only to the heartwood. And since you're incorporating the sapwood into the joints you'd need to be sure that if the sapwood was attacked, the remaining heartwood - heartwood connections would remain resilient. To my mind this would mean cutting shoulders through the sapwood to ensure that load doesn't end up on the pegs when a wood borer munches it's way through the outer 4cm of your trees!

3. I was asking similar questions not that long ago. Mortise and tenon joints with scribed shoulders to mate the round exterieurs of the timbers. Have a hunt on youtube. Or buy Ben Law's book.

Contemporary Norman longhouse in Normandy


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