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Re: Re: "Flitch-plate" design

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seaoc(--nospam--at),Internet writes:
I have done a cursory search in my two main timber reference texts:

Timber Construction Manual, AITC, 3rd ed.(don't laugh, I know I'm an old
Design of Wood Structures, Donald E. Breyer, 3rd ed.

I could find no references to "flitch plates" or any other connection with
metal side plates used for the purpose of resisting bending moment.

Bill Allen

     Author - J. D. Wiesenfeld, Professor in New York Institute of Technology
School of Architecture

Journal of Light Construction Magazine Sept. 1992 - "FLITCHPLATES"
     Author - Robert Randall, P.E. from Monhegan Lake, NY

Since nobody has talked specifically about theory of flitch plate design,
just personal preferences, I thought I would tackle it.  It has been  several
years since I last designed a flitch plate beam but the theory behind it is
basic strength of materials.  You need to calculate a transformed section
based on the modular ratio.  From that transformed section you calculate an
effective moment of inertia and section modulus.  The design of the bolting
is based on how you expect the beam to behave.

The conservative method is to take all of the bending and shear into the
steel plate.  Therefore the bolts only have to transfer the gravity shear to
the steel.  No interaction of the wood and steel is assumed.  This also means
you only use the moment of inertia and section modulus of the steel plate,
NOT the transformed section properties.  The wood is basically along for the
ride and really only needs to be designed to span between bolts.  Sometimes I
have used this method.

The pure flitch plate design would include the transformed properties of the
steel and the wood.  The bolts have to be able to transfer the horizontal and
vertical shear between the steel and the wood.  This is done with the
traditional shear flow equation of q=VQ/I, where q is the force per unit
length, V is the maximum shear, Q in the static moment of the area beyond the
fastener and I is the transformed moment of inertia.  You also must check
bending stresses to ensure that neither the wood or the steel is

Some word of caution.  Always consider shrinkage of the wood in sizing the
steel plate.  If you are using a 2x12, don't make the steel plate 11-1/4"
high because the wood will shrink and the steel won't.  I always made the
steel plate at least 1/4" smaller than the wood.  This in critical at bearing
conditions as well as at midspan where a joist may bear on top of the flitch
beam.  I always use a load path of transferring the load from the floor/roof
into the wood side plates and then transfer into the steel plate with the
bolts.  Therefore the bolts have shear in 2 directions, both horizontal and
vertical and must be sized for the resultant force.  I also always use 2 rows
of bolts, one near the top and one near the bottom, and stager the spacing.

I know I have seen this presented in text book form, I just couldn't put my
finger on it today.


Richard Lewis, P.E.
Missionary TECH Team

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