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RE: Wood column with steel flitch plate

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Thanks for the responses.  The flitch plate is mainly there for bending loads, but I need to figure a way to account for it under axial load since there will also be an axial load.  It is a post in a 25 ft tall unbraced wall (cathedral ceilings) with a glulam ridge beam sitting on it.  So, I will have some wind load putting it in flexure and the axial load from the glulam ridge member reaction.  My gut response was to use a wide flange or tube with wood "nailers" attached to it (it is a post in a SIP wall), but it was initially requested as a built-up set of LVLs with a flitch plate.  I have not delved in to the numbers as of yet, but wanted to first make sure I was heading in a "sane" direction if I went down the flitch plate path.  I am not really much of a fan of flitch plates and kind look to them as last resorts if I use them at all.
 
Thanks,
 
Scott
Adrian, MI
-----Original Message-----
From: Jordan Truesdell, PE [mailto:seaint1(--nospam--at)truesdellengineering.com]
Sent: Tuesday, October 16, 2007 9:20 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Wood column with steel flitch plate

Your steel will probably take a great deal of the axial load. Do an A*E comparison between your wood and your steel. You didn't mention sizes, but a 3/8" x 5.5 plate has more axial stiffness than a 6x6. You're right that buckling does not make the equations that simple. To do it right, I think you'll have to go back to mechanics and do the math from scratch, then figure your "real-world" answer based on your experience (aka black magic - I have a 8-ball in my office for such occasions). Euler buckling of non-isotropic composite members was never my strong point, though, to be honest.

If you want my real opinion, your posts that want to be wood should probably be a 3.5/4/5" square HSS with lumber pinned to the outside. I feel your pain; sometimes it's easiest (and most economical in the long run) to just pull engineering rank and throw a thousand dollars worth of steel at it. I've got a lot of gray hairs from the times I didn't. ;-)
Jordan


Scott Maxwell wrote:
I just wanted a "sanity" check to make sure that I am thinking correctly...
 
I have a rather tall post that "wants" to be wood, but must be of limited depth and to a lesser degree limited width.  To accomplish that in wood, we are looking at a flitch plate.  I have no problem designing the flitch plate/wood for bending, but there will also be an axial load.  So, I want to make sure that my thinking is on point.  I am thinking that the flitch plate would strictly add to the buckling length calculations (i.e. the Le/d) of the wood portions.  But I cannot quite seem to see an easy way to account for that since in wood it is purely a brace length over depth of member calculation, not a L/r calculation (i.e. calculated section properties).  I was thinking of calculating the transformed area section properties and then figuring out an "equivalent" depth of wood to get similar section properties and then using that "equivalent" depth in the Le/d calculation for the wood.  Or should I just assume that the steel is really taking the axial load and do standard kL/r calculations and assume that the axial load gets transferred from the wood into the steel?  In one direction, it is moot since post is embedded in a wall...thus the weak axis is basically fully braced...it is the out of plane length that is the issue.
 
Or should I go to the Le/(r*sqrt(12)) that is indicated in H.3 of the 2005 NDS for "other column shapes" and use the transformed area value for r?
 
I am also looking at just using a steel shape with wood nailers, but want to have the flitch plate option available if they REALLY push for it.
 
Thanks,
 
Scott
Adrian, MI
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