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RE: Steel flitch plates

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We use flitch plates to strengthen existing wood beams from time to time.  We provide enough bolts along the length of the steel plate to transfer the portion of the load resisted by the steel plate from the wood to the steel.  We provide enough bolts at the end of the steel plate to transfer the plate end reaction back into the existing wood beam which bears on the supports.  We usually stop the steel short of the end bearing and check the existing wood beam for the entire shear load. 

 

We check the compressive stress in the plate due to the bending moment in the plate and look at kl/r to get an allowable compressive stress with k=0.5 (plate is continuous in the maximum moment region).  We typically have bolts at about 12” o.c. 

 

You may run into problems with fit-up when the existing wood beam is built with through-bolts… makes it hard to add the steel plates.

 

Consider adding additional laminations of 2x lumber or microlams.  We are currently working on a project where we are adding microlams, carbon steel and stainless steel flitch plates to various existing wood beams.

 

BDH

 

 

-----Original Message-----
From: Jim Wilson [mailto:wilsonengineers(--nospam--at)yahoo.com]
Sent:
Wednesday, July 27, 2005 7:46 PM
To: seaint(--nospam--at)seaint.org
Subject: Steel flitch plates

 

I'm reviewing a design to strengthen an existing

triple 2x wood beam by adding steel plates to the

outside faces.  Buckling is a concern for plates in

compression.  Bearing is also a concern because any

capacity developed into the plates has to be balanced

by a support reaction of the plates at the bearings.

If the plates do not bear on the support, then any

load in the plates is transferred in and out of the

members through bolt shear alone.

 

So, I've convinced myself this is not an acceptable

detail.  However, its been done many times before and

I'm just wondering if there aren't any old-school

"design guides" that might indicate ways to properly

bolt these plates in place so that they do actually

support some load.  This might save me raining on my

architect's parade.

 

The same question would also pertain to using steel

channels, except that possible buckling would be much

easier to eliminate.

 

Any thoughts would be appreciated.

Jim Wilson, PE

Stroudsburg PA

 

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