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Re: Steel flitch plates

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I tend to discourage the use of steel flitch plates, primarily because they're terribly inefficient. Most times, I find that a steel member (W or TS) can take the load for about 60-75% of the steel. For lateral wood connections (diaphragms, primarily), I have a 2x bolted to the top flange, but for simple bearing connections, I recommend PAFs - they're quick and easy. The total steel/lumber/fastener is much less than the cost of the flitch plate materials, and for residential, the weight difference can mean a three man crew can set it by hand, vs needing a boom lift or a bobcat to set it. Also, around here plate is 36ksi, but w shapes are almost all 50ksi. Doesn't help deflection, but gives a bit more length for unbraced spans

When Ive done them, I design the flitch plate as a sandwich (wood-steel-wood-steel-wood) with the wood on the outside to pick up a little support for the compression flange, and bolt liberally. As for the bearing condition, I usually assume the wood hits the bearing first, and the load gets transferred through direct bearing of the bolts - pretty good capacity in dense woods, even on side grain. If the steel happens to hit first...well the wood will hit soon after :-) (except on cmu, of course, then its a whole different game).

Actually, I just had a request for one last week, and the result of the calcs was - use a 3 ply LVL instead.

Jim Wilson wrote:

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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