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Single or Double angle bending member

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The distinguishing feature of a double angle is that the two pieces
counter each other's tendency to rotate torsionally.

Let me add a distinction here:  in designing a single angle member, it's
not enough to consider LTB per se.  You also have to consider torsion.
An unbraced wideflange beam centrally loaded will fail in LTB, but has
no torsion.  A single angle will have torsion unless you take some very
extreme steps to eliminate it.  You have to factor that torsion into all
your analyses.  It really complicates a LTB analysis if you have torsion
to start with.

So, two angles back to back, connected every so often, and centrally
loaded, don't have torsion.  Two angles toe to toe don't either, if they
are connected adequately.  I don't have a quantitative answer for what
"adequately" is, but to me it would involve bars about as thick as the
angle, welded all the way across both horizontal legs, every couple of
feet.  Just having a block wall between the angles doesn't cut it;
putting expansion anchors though the angles into the block doesn't
either, because that system is neither strong enough or stiff enough.
Maybe a series of horizontal through bolts would be adequate if it
pinched the angles tightly against the block.

Assuming the angles are connected adequately, now you can analyze the
vertical legs for LTB failure.

Charley Hamilton was asking the right questions, in my opinion.  Bill,
does your MathCad worksheet take the actual torsion into account?
That's what's always scared me about single-angle lintels.

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

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