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Re: Reinforcing Wooden Beams

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<I've seen used the individual tension and compression resultants that
correspond to the *peak* bending stress in the reinforcing plate for design
of a line of connection bolts along the upper half to resist the tension
resultant and another line of bolts along the lower upper half to resist the
compression resultant (which is equal to the tension resultant), but I am
not sure about this. Again any references that address this connection
design ?>

Sorry if this is a bit pedantic.

My understanding is that the way tension and compression stresses develop in
beam flanges is through the transfer of stress through shear flow. The
"tension resultant" in the flange of a simple beam is the sum of the shear
flow from the support to the point in question. The connection of the steel
reinforcing plate to the beam is for shear flow stresses only and the
required connection will vary along the length of the beam depending on the
shear in the beam. In regions of high shear near the supports the shear flow
is higher and the connection must be stronger. At the mid point where moment
is maximum and shear is zero, there is theoretically no requirement for
connection since the shear flow is zero.

Depending on the magnitude of the shear flow you may need very strong
connectors. That is why I suggest you may want to consider shear plate
connectors rather than through bolts or lag screws alone. These are 2 1/2
inch diameter steel "pucks" which are routed into the wood beam and greatly
increase the bearing capacity of bolts in wood. There are design values for
them in the timber codes. Through bolts and lag bolts might be too flexible
and move too much due to crushing and splitting of the wood.

In steel beam/concrete slab composite beams the total shear connection
(number of studs) required from the support to the point of maximum moment
is determined and then the shear connectors are uniformly distributed along
the beam between the point of support and the point of maximum bending. I
don't know if this approach would be directly applicable to wood beams. I
would personally stick with connection proportional to shear flow. Or
connect everything for the maximum shear flow calculated depending on the
numbers. Or step the connection down along the beam like stirrups in a
concrete beam.

If you are significantly increasing the load on the beam a critical area may
be shear capacity. If the existing beam by itself can handle the new total
shear then you're okay. If not you'll need to do a more detailed check
and/or reinforce for shear.

Hope this helps,
John MacLean



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