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RE: Wood column with steel flitch plate

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Hi, Scott.

 

Others have offered their “sanity check”. Here is my version.

 

Suppose you go through all the mental gymnastics and find a really creative solution which saves the owner from installing an HSS column, but down the road there’s a problem with the column. What is your defensible position? Here in California, there doesn’t even have to be a problem with the column, it could be a problem anywhere on the project. When the owner sues, s/he will sue everyone. One of my colleagues will turn into an expert witness adversary (at $200/hr or more with no limit) to find anything wrong with the structural drawings regardless of whether it’s related to the problem or not. Then, by arbitration, since the expert witness couldn’t be convinced that my flitch plate column idea is something of an “industry standard”, I’m found to be 10% responsible for the heretofore unrelated problem. Wait. There’s more. No one else in the lawsuit carries E&O insurance. All of a sudden, I’m the deep pocket! Yikes. I now have to pay 100% of the settlement. Now, the HSS column isn’t looking too bad to me.

 

I know you are in Michigan which isn’t nearly as litigious as California, so let me take a different tack. How much time are you willing to invest to come up with an alternative to an HSS column? What is your time worth? Is your client willing to pay for you additional time? If not, what is your goal?

 

One more approach. How much labor will it take to mate the flitch plates to the wood post? Subtract that cost from the HSS column and how much have you saved?

 

Finally, from a technical perspective, how are you planning to attach the flitch plates to the post? If they are not interference type fasteners (i.e., screws), I would be suspicious of how well the two materials will work together, regardless of all the calculations you can generate. It’s the same issue I have with flitch plate beams. Search the archives on “Bob Powell” and you will find more.

 

Here’s what I’ve told my clients over the years: “The wood post that is required to support the load won’t fit in the wall. Do you want a bump in the wall or do you want me to use an HSS column?”

 

I’ve offered a legal, business, labor and technical sanity check. Take your pick.

 

:o)

 

Good luck,

 

T. William (Bill) Allen, S.E.

ALLEN DESIGNS

Consulting Structural Engineers
 
V (949) 248-8588 F(949) 209-2509

-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)umich.edu]
Sent: Tuesday, October 16, 2007 12:14 AM
To: seaint(--nospam--at)seaint.org
Subject: Wood column with steel flitch plate

 

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