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Re: How to calculate capacity of two Log beams, one on top of the other ?

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Helen,

If you have two flat 2x4 pieces, one on top of the other, they will both share in resisting the load. You will NOT have the equivalent of a 4x4, since the members are not joined and the ends are not constrained to move together, but you still have more than a single 2x4. The load is shared through compatibility of deflection. The load distribution is not unknown, it is a function of displacement.

The common assumption is to assume the beams will share equally in resisting the load. This is not entirely correct, and becomes less so as the members get deeper. The radius of curvature for each member will be slightly different, and the lower member must deflect further for the same net displacement. With 14 inch deep elements, the lower element will have slightly higher stress than the upper element, but the difference may be negligible. The lower element will not take 100% of the load. For round elements, I would question what keeps the two pieces in the same plane without any interconnection, but then I am not at all familiar with log construction detailing.

Paul Feather PE, SE
www.SE-Solutions.net
pfeather(--nospam--at)SE-Solutions.net
----- Original Message ----- From: "Zhou, Helen" <HZhou(--nospam--at)stlouisco.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Thursday, May 19, 2005 8:41 AM
Subject: How to calculate capacity of two Log beams, one on top of the other ?


When you have two beams with same sizes and simply supported, one on top
of the other with no connection between, I always think the bottom beam
shall be designed for almost 100% load, because load distribution is
unknown.

I am reviewing plans of the log home, have 23.5 ft span beam with 6000
lb concentrated load in the center and 240plf distributed load, M=51810
lb-ft. The engineer use two 14" dia Engelman Spruce, select structure
(fb=1100, Mu=24750 lb-ft, E=1,300,000), one beam sits on the other with
no any connection, the engineer calculation claimed total Mu=2x24750
lb-ft.

Please help me.


Thank you !


Helen Zhou
Saint Louis, MO

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