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RE: Need "Flitch-plate" design example

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It would be interesting to see a free body diagram which shows how the load
goes from the wood member to the steel plates and then, at the ends of the
steel plates, from the steel plates back into the wood member. If the steel
plates are supported by the same structural element as the original wood
beam and the two (wood beam and steel plates) are fastened together, it
would make sense that the load would be distributed proportional to the
relative stiffness of the members. If the load is actually applied to the
wood member, then the connection between the steel plates and wood member
can be determined by the equivalent load required to force deflection
compatibility.

I can see how easy it is to extrapolate a design methodology from a cover
plate design for example to a problem such as this, but I do not thing this
extrapolation is proper. Obviously, based on my responses, I am hesitant to
use this repair method based purely on "home brew" calculations without some
sort of computer modeling and long term testing (including creep of the
wood).

In my opinion, this repair method saves a modicum amount of construction
costs and may very likely invite the engineer of record into a courtroom.

Regards,
Bill Allen

-----Original Message-----
From: Roger Turk [mailto:73527.1356(--nospam--at)compuserve.com]
Sent: Friday, May 15, 1998 9:47 AM
To: seaoc(--nospam--at)seaoc.org
Subject: RE: Need "Flitch-plate" design example


Bill Allen wrote:

. > Sure, steel is stiffer than wood, but wouldn't the steel plates have to
. > span to the reactions in order to get deflection compatibility?
. > Otherwise, how does the load get into the steel plates?

Yes, Bill, the steel plates do have to be supported at their ends, but many
things can constitute a "support."  The steel plates could be extended to
the
same element that is supporting the wood member, or, if the wood member has
the shear capacity, the ends of the steel plates can be supported by bolting
to the wood member, or, a steel saddle could be placed over the wood member.
By connecting the steel plates to the wood member as near as possible to the
end of the member, it is equivalent to having the steel plates supported by
the same support as the wood member.

I have found it virtually impossible to get composite action with a wood
member using mechanical fasteners, even with a wood-to-wood combination such
as an inverted "T" and (for retrofit work) even if composite action is
achieved, the neutral axis is shifted so much that the flexural stress in
the
top of the wood member exceeds the allowable.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona