Any standards for designs using live trees? 2

[MTEng1]

Does anyone know of any standards for using live trees in the design of a structure which carries people? It is for a zipline that would be used to carry one person at a time. I would recommend that the tree be load tested to 1.5-2 times the design load, but I don't think that is enough. There is just not a good way to be able to model a trees root system. Any suggestions would be appreciated.

"Structural engineering is the art of modeling materials we do not wholly understand into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess in such a way that the public at large has no reason to suspect the extent of our ignorance." -Dr. A. R. Dykes

 

[msquared48]

With the root system as a big unknown, why don't you take it out of the equation by extending another line from the head of the zipline at the tree to an embedded deadman anchor at the rear?

Please - no deadman jokes here...

Mike McCann
MMC Engineering

 

[BAretired]

Good answer, Mike.

BA

 

[SkiisAndBikes]

I believe that there are some code standards developed for zip line and tree top treking design, I just can't put my finger on them at the moment. They will likely be found with either amusement device standards or elevating device standards. In our region zip lines must be licensed by the Technical Standards & Safety Authority. (

http://www.tssa.org)

At time of initial design, they perform an engineering review before the feature can be licensed for public use. I believe they require an arborists report as part of the submission. (hopefully one experienced in this type of installation).

I would suggest that a factor of safety in the 1.5 to 2.0 range would be inadequate. Minimum I would consider for using a tree (or for that matter any elevated amusement device) would be 5.0 or higher (similar to suspended work platforms). There is also the concept of two independent means of support which should be considered.

Msquared's idea of using a deadman anchor is also a good idea for consideration.

 

[hokie66]

There are too many unknowns. A tree can be judged sound by a qualified arbourist, then blow over in the next storm, especially if the ground is wet. Use Mike's guywires.

 

[MTEng1]

In response to msquared48:

Thanks, that is one option I’ve considered. The issue is that access is tough for pouring concrete and one option I am considering is using guy wires tied to boulders that can be set and placed behind the tree. Even with the guy wires the problem is that it becomes a matter of compression on the tree, which I do not have a standard to design to. I’ve thought of just using a conservative compressive strength value?

"Structural engineering is the art of modeling materials we do not wholly understand into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess in such a way that the public at large has no reason to suspect the extent of our ignorance." -Dr. A. R. Dykes

 

[BAretired]

Maybe you should use 3 guys at 120 degrees apart so that the tree is laterally braced a little below the point of attachment of the zip-line. That would reduce the effective length of tree by a factor of two.

Or maybe it would be better to abandon the tree altogether and erect a guyed tower.

BA

 

[MTEng1]

In response to SkiisAndBikes:

Thanks, I will check out the TSSA link. The standards that are out there are from the ACCT (

http://www.acctinfo.org)

and the PRCA (

http://www.prcainfo.org),

and no this is not the rodeo association. I have both of these and have reviewed them. There is not any real guidance on these as far as design with live trees.

As far as factors of safety, yes I was planning on 5. In the initial post, I was only referring to load testing the tree at 1.5-2 of the design load.

"Structural engineering is the art of modeling materials we do not wholly understand into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess in such a way that the public at large has no reason to suspect the extent of our ignorance." -Dr. A. R. Dykes

 

[MTEng1]

BAretired,

I will look at the 3 guys idea, I think that would help. I would love to just use a guyed tower.

"Structural engineering is the art of modeling materials we do not wholly understand into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess in such a way that the public at large has no reason to suspect the extent of our ignorance." -Dr. A. R. Dykes

 

[msquared48]

No. Take a single horizontal guy back to the tip of a new pole and guy down to however many deadmen you see fit from that point. That will remove the compression force from the tree.

Mike McCann
MMC Engineering

 

[msquared48]

I guess the next logical question is, if you have to do that, why use the tree at all, other than the woods effect of the zipline...

Mike McCann
MMC Engineering