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Torsion in extruded alum tube sections

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Hello,

I am designing a window glazing aluminum framing system (6063-T6 Alloy).  It
is 20 ft wide by 26 feet tall.  It is anchored to the base steel support at
the bottom, to the ridge skylight framing at the top (skylight framing
consists of similar 5"x3"x0.125 tubes with TS3x3 reinforcement on the
inside).  At the midheight there are steel bent plates at each mullion for
out of plane anchorage.  These plates are anchored in turn with the single
bolt to the side of the concrete slab.

The architect added three glass sections protruding out of plane of the
wall, placed in between 2 alum. rectangular tube mullions.  These glass
sections supposed to look like sails.  The biggest sail weighs about a 1000
lbs and each of these sails consists of 4 foot tall panels in between
horizontal alum. tube beams. What concerns me is a torsion that sails will
apply on mullions.

Here are my questions:

1.  As the glass sail is sandwiched between two mullions, can I divide the
torsion between two members (one member 1/2 torsion, another shear in plane
of wall + 1/2 torsion)

2.  Do I have to worry much about warping in rectangular tubes?  Is it
pretty much pure torsion with concentration of stresses at the corners?  If
so how do I quantify these concentrations?

3. Can I rely on the horizontal beams as support members?  Beams are of the
same size as mullions, attached using shear block on the inside of beam.
Shear block connected to the mullion with three screws and to beam with two
screws on outside face only.  I am concerned that I don't have fixity in
this connection to resist torsion in the mullion.  It is not really a
question, I know.

4. I couldn't find any references for calculating allowable shear stresses
due to torsion for rectangular tubes in Aluminum Design Manual, only for
round and oval tubes (section 4.2).  Do I use full allowable shear stress,
or am I to calculate the salloable shear stress for unstiffened flat webs?

I'll be happy to e-mail some sketches if anyone wants more info.

Any takers?

Thanks in advance,

Sasha Itsekson, SE



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