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RE: Log Beam-Girders

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What ICC and ASCE 7 needs is a response modification factor R value for stacked horizontal log bearing walls with drift pins, threaded rods or spike shear connections.  The rest is mechanics of materials and structural analysis.
 
 The were many log homes that withstood the 7.9 magnitude Mentasta earthquake with less damage then many light timber frame structures.  
 
This is ample empirical evidence that log homes perform as well or better than light timber frame homes.  Therefore it is my opinion the response modification factor for log homes should be the same as the response modification factor for wood panel light frame shear walls.
 
 


From: Jordan Truesdell, PE [mailto:seaint1(--nospam--at)truesdellengineering.com]
Sent: Saturday, July 22, 2006 3:55 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Log Beam-Girders

Well, what I have yet to find any engineering design (rigorous analytical) basis for logs, especially when it comes to lateral loads. ICC-LOG is useless for design except to provide material values, but you could probably deduce a set of reasonable values out of the NDS if you had to.

What you have envisioned is probably the best method to analyze the condition you have, as it matches the physical geometry. One thing to worry about is partial logs - those which do not span the entire girder length. You should carefully specify how many courses must be made of continuous members or you may end up with far less capacity than you expected.  It's easy to drive a mess of OLY-LOG screws from course to course, and that will give you the interlaminar shear. You could also cheat and put up one log, then bury a height-matched W, 2L, or WT section  in the next 1 or 2 courses by having them split the logs and rout for legs/flanges. I suppose an HSS rectangle might work without worrying about flanges, but they're not the most efficient used this way, and the per-pound cost will make most builders/owners faint.

I'm still trying to figure out how to justify transverse wind loads and detail them to be "foolproof". By following the loads, your sill and head act as king studs, spanning from corner to corner (which doesn't work in the Resort Lodges folks are building these days).  Interior walls could be used as intermediate supports, but that means shear capacity in the wall and OTM reactions at the wall ends, or first doorway. That's a tall order if the floor is a 24'+ clear span parallel chord truss. And that doesn't even address the suction C&C case where you need to secure the wall to the logs for tension and still allow for settlement. With some of the crews out there that have a pile o' logs and a floor plan, you may as well send the neighborhood kid up on your steel building to weld up your SMF with a bag of iron oxide and aluminum filings and a propane torch.

Anyway, you're on the right track, or at the least along the same path I would take. Most of the time I'll get bigger logs to work with (20+" butt diameter) and I can take the arithmetic stacked capacity of two logs to get to the load needed.
Jordan


Barry Welliver wrote:

Log home construction seems to defy neat structural solutions and I’m wondering if there are any design standards floating around. I am aware of the ICC draft standard on log structures, but as others have noted, there is little in the way of engineering and perhaps that is appropriate.

My question revolves around a log girder (supporting log purlins) which spans the width of a room. The girder is built from varying lengths of 8” logs so as to conform to the gable shape of the roof. Looking at this as a segmented member with horizontal shear transfer requirements seems somewhat crude. Any recommendations for educating this simple engineer?

Barry H. Welliver

barrywelliver2(--nospam--at)earthlink.net

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