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Re[2]: Reinforcing Wooden Beams

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Actually, in thinking about it some more, the design basis for the
connection
of reinforcing plates (steel or wood) attached to both faces of a member is
fairly straight forward and illustrated by the following example. Consider
a
simply supported wood beam with some defect over a portion of its length
that
requires  reinforcing plates attached to the faces of the beam over the
middle third of the of the beam. At the third point locations, the wood
beam
is fully effective, and assume the reinforcing plate is terminated in this
region. Because of the defect in the beam, the connection of the
reinforcing
plate has to be in solid wood, outside the defective region. The moment and
shear in the reinforcing plates have to be transfered to the beam at the
location where the reinforcing plates terminate. The connection of the
reinforcing member to the beam is a "moment connection" in the sense that
the
simple beam moment (and shear) at this point in the reinforcing plate has
to
be transfered to the wood beam. So this would require a connection pattern
(bolt group) designed for this moment, properly considering bearing, edge
and
end distances. Why a moment connection and not a connection just required
to
carry shear ? Because with a shear connection, there are then simple hinges
at the third points, and the reinfoced beam could only carry the load by
the
beam by developing membrane tension, not by bending, and this is not the
design intent. This concept is completely different from composite members
and their connection.

Questions on the steel shear pucks please. Are there some manufacturers
catalogs
available that give allowable loads for different size pucks ?  Do these
type
connectors have a torsional moment capacity (I'm picturing teeth located at
the
outer diameter of the puck) or are they usually just credited for shear ?
Finally, are these the same as "split-ring connectors" ?  Thanks.

Jim Fulton


______________________________ Reply Separator
_________________________________
Subject: Re: Reinforcing Wooden Beams
Author:  seaint(--nospam--at)seaint.org ("John MacLean" <john_maclean(--nospam--at)pomeroy.ca>) at
ROH
Date:    10/19/2001 1:45 PM





<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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