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Re: Torsion in wood beam

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Dennis,

How often are the treaded rods that connect the bottom track of the rail
to the 4x12 beam?  Where do they occur (i.e. at teh TJI location or in
between or a combination)?

In my opinion, most of any torsion induced by the rail into the 4x12 beam
will likely be taken out at the TJIs due to the straps.  Thus, to me, any
torsion that the beam will see will either be to the torsion load being
applied in between the TJIs (i.e. if the bolts for the bottom rail occur
at 2 ft oc and fall at the midspan of the 2 ft spacing of the TJIs) or due
to rotation of the ends of the TJIs just due to gravity loading (which
should be small since there should be very little rotation...but you are
in effect creating a rigid connection rather than a simple support at one
end).  In either case, I would have to think that the 4x12 should be able
to handle such torsion since it should be very small.  In the former case,
you are dealing with a 2 ft torsion span with 24 psf applied (I am curious
where that number comes from...I am used to 50 plf at the top of the rail
with some variations in the UBC).  In the later case, you likely would not
have much torsion as there will not be much need for rotation in the ends
of the TJIs and the overall span of the 4x12 would be the torsion span
for this situation, which would likely mean that you would have little
torsion as any rotation put into the overall 4x12 beam would likely be
"relieved" by little or no torsional restraint at the ends of the 4x12
beam.  Thus, the later case is likely not a problem at all.

As to how the wood member will behave under any torsion, I will claim the
fifth on that one.  My wood experience is rather limited and I have to
admit that I am not really sure how it behave under torsional loading.

HTH,

Scott
Ypsilanti, MI


On Thu, 2 Oct 2003, Dennis Wish wrote:

> I need to see a calculation that will help me solve a design problem. I
> have a glass railing that is approximately thirty feet long and three
> feet tall. The glass is continuous and the top, two sides and bottom are
> encased in an aluminum tract. The tract is secured to a 4x12 rim beam
> connected to the ends of an 11 7/8" TJI's spaced at 24 inch on center. I
> have specified an LTT Strap at the top and the bottom of the TJI's that
> are bolted through the 4x12. The bottom track of the glass railing is
> secured through the 15/32" sheathing and through the center of the 4x12
> with a threaded rod, recessed nut and bolt.
>
>
>
> The plan checker has required a calculation for the railing for 24 psf
> normal to the glass. The code requires a 600-lb load applied normal to
> the railing at the top.
>
>
>
> I anticipated the 600-lb lateral load applied 36" above the deck and
> rather than try to induce torsion into the beam, I resolved the
> reactions at the top of the TJI and the bottom to calculate the reaction
> that will be resisted by the LTT straps. The beam is not secured in the
> wall at each end of the beam Can anyone help me resolve the problem?
> Should I be considering torsion in the beam and if so, how do I prevent
> the beam from splitting down the center?
>
>
>
> Thanks in advance
>
> Dennis
>
>


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