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Strengthen wood beam -help

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The original questioner stated he wanted to add flitch plates to the
sides of a wood beam.  He also stated he was designing the plates to
carry the full moment on the beam.

If the flitch plates and wood beam have the same neutral axis (i.e. if
they are the same depth, or the steel plates are shallower than the wood
beam but mounted concentrically), then there is no composite effect.
The fasteners, in the general case of shared load, would be sized to
transmit the flitch plates' share of the load, based on their stiffness
relative to that of the wood beam.  The force on them would be vertical.

In this case, the designer wants the flitch plates to carry all the load
(let's say 500 pounds per foot).  We assume the load is actually coming
in through the wood beam, i.e. we asume the floor boards won't bear
effectively on the new flitch plates; and we assume all the load is
removed while the plates are installed.  So, he needs to design bolts or
screws to transfer 500 pounds per foot into the plates.  That's all.

If, instead, the designer wanted to use composite action, he would put a
plate on the bottom (presumable he can't get to the top).  He would
calculate composite section properties using some reasonable guess for
the ratio of moduli of elasticity between steel and wood--maybe 20.
Then the VQ/Iy formula comes into play.  Shortly thereafter, he finds
out that the shear is so high he can't get enough bolts in, even if he
can convince himself they all come into bearing at once.  That's why
they use (non-composite) flitch plates.

As an added note, I would hesitate to use plates this way, due to
compression side buckling.  Neither would I try calculate an unbraced
length to hold based on screw spacing, because the wood beam is probably
not perfectly straight and flat on its sides.  I would use channels.

Mike Hemstad, P.E.
St. Paul, Minnesota

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