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Log trellis/pergola load questions

Started by michaelvp, July 29, 2008, 10:51:40 AM

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michaelvp

I'm building a large pergola/ramada over the front area of my house with green lodgepole 5-10" diameter.  The longest span I have to cover is 12' and even though there will be no roof on this structure (only a series of skinny 2" poles spaced at 2" laid across the top for shade) I am still uncertain about how big the  spanning logs should be.  I've heard I need an inch of diameter for every foot of span.  Is this true even without any roof load?  If possible, I am hoping to use the 8-10" logs for the spans (with knee bracing) and the smaller logs for the posts.  I live in central Oregon and we get some extreme weather here, but not too much.

Also, because this will be a freestanding structure, I would be interested in any suggestions for foundation and anchor ideas.  I don't really want a big simpson tie strap running up the log and have thought a pier, sona tube, rebar configuration would be best, or perhaps a knife plate.    I've looked a several postings about this topic but haven't seen any specific pictures/details for log structures.

Thanks for any ideas or suggestions you can send my way. 

Michael

Raphael

That inch per foot rule of thumb includes a roof load so your 8-10" logs should be plenty fine over 12'.
I built a similiar free standing arbor over a walkway that uses 6-8" diameter 10-15' long ERC.
Ten years later it is now holding a significant mass of Wisteria with no problems.
... he was middle aged,
and the truth hit him like a man with no parachute.
--Godley & Creme

Stihl 066, MS 362 C-M & 24+ feet of Logosol M7 mill

jfl

Hi Michaelvp,

There is a lot of missing information in your request.  I'll just try to do something similar with the number of my own pergola (and I'll then figure out if it is going to fall down!), then you can plug in your numbers.

First question: what is the surface? In which direction is the beam, in which is the "joist".  I guess from your message the beam is 12 foot long.  My beam is also 12 foot long.  The "joist" or "rafter" on top of the beam are 8 foot long.

Next thing, what is the load?  Normally, dead load is assumed to be 10 pound per square foot (psf). You should compute the volume of wood per pole, multiply by the total of pole and divide by the total square footage to find your dead load. To find the weight of a volume of wood, you might want to check the wood handbook, chapter 3:
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm
Which brings the question: what species of wood are you going to use?

You mention 2 inch pole with 2 inch between.  This looks to me that snow can build up on top of that. Now you have to decide: do you want to have to remove snow if it ever accumulates beyond the maximal load of the structure, or do you want to build strong enough that it doesn't matter.  You are  in the best place to judge that...

Let's say it is 10 psf without any snow load.

There are nice calculators here:
https://forestryforum.com/members/donp/beamindex.htm

Close to the end, you'll find the log beam calculator.

There you enter the total load on the beam.  In my case, the pergola is 12ftx8ftx10psf = 960 pounds.  But since there are 2 beams, then each carry 480 pound, so I punch that in.
Next the span: 12 feet is 144 inches.
I select the specie: cedar western no.2 and click show result:

I get that the mid-span diameter required for bending (not breaking the wood) is 4.31 inch.

The deflection calculator tells me that I need a 5.6 inch log.  On the other hand, for a pergola, it doesn't matter if it is springy: there is no gypsum panel attached that must not crack...

The shear calculator asks for a 2.5 inch diameter minimum, so that's not a problem.

So how does the number fits with your pergola?

jf







Mr Mom

Ok i know what a trellis is but what is a pergola???


Thanks Alot Mr Mom

beenthere

Not much different from a trellis...posts and open joists above for some shade. Water in the joints is one of the enemies of a long-lasting pergola.

link here
http://www.gardenstructure.com/
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

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