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RE: QUERY: Use of "Beta" and "Theta" in VisualAnalysis

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Bill,

I am sure that the guys from VA can give you a better description, but I
will give it a shot.  When you draw a member, you are defining the local x
axis of the member.  The y and z axis are of course perpendicular to the x.
If for some reason you need to rotate the member about the primary axis, you
would use the theta angle.  I typically do this if I am doing a rigid frame
analysis, and I need my columns to align in the strong direction of the
beams (girders).  This is theta.

Now remember when you apply a load, you can choose to apply the load along
the global or local direction.  The local direction is defined by the beta
angle.  If you set up a 3D model, draw a beam, and put a point load on it,
through the local direction.  Look at it, then change the beta angle to like
22 degrees.  You will see the load rotate 22 degrees, because it is still
through the "local" section.  Now from here you can change theta all you
want, hence rotating the beam about the x direction, but the load still is
applied through the local (beta) direction.  Now set beta back to 0, and
play with theta again.  You will see that the load never moves.

One reason that the beta angle never showed up on your tubes is because you
were in a 2D mode instead of 3D.

Oh and by the way, I thought I was in the IES group, but I have never
received any email.

Brian K. Smith, P.E.

> -----Original Message-----
> From: Bill Polhemus [mailto:bpolhem(--nospam--at)swbell.net]
> Sent: Thursday, August 03, 2000 10:01 AM
> To: iesug(--nospam--at)polhemus.cc; seaint(--nospam--at)seaint.org; structx(--nospam--at)polhemus.cc
> Subject: QUERY: Use of "Beta" and "Theta" in VisualAnalysis
>
>
> Sorry to cross-post, but I realize there may be users of VA who
> are not on the
> IESUG mailing list, and I want to understand this.
>
> I note that VA uses botha "Beta" angle and a "Theta" angle to describe the
> orientation of the local coordinate system axes with respect to
> the global.
>
> I am a little puzzled by this, as the description in the dialogue
> box doesn't
> really help me understand the distinction between Theta and Beta. To make
> matters more unclear, I note that the input field for Theta is
> "grayed out"
> disallowing input, at least when the member is an HSS section.
>
> Can anyone shed some light on this for me?
>
> Thanks.
>
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