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RE: Autocad Section Property Calculation

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You might want to be careful about using the principal moments.  If
the principal axes are not parallel to the X- and Y-axes, the moments
about X and Y will be different from those about I and J.  For many
practical problems (L-shaped members for instance) the principal axes
are rotated.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Michael Valley, P.E., S.E.                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.              Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699      Fax:        -1201

-----Original Message-----
From: Brent Koch [mailto:kochengr(--nospam--at)pacbell.net]
Sent: Wednesday, May 31, 2000 8:46 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Autocad Section Property Calculation


The results from executing the Acad MASSPROP command should give you
the
moments of inertia about the origin (wherever you choose to locate it)
and
ALSO the principal moments about the members centroid.

For expample, here is the text screen for the MASSPROP command run on
a
12x24 rectangle
with the UCS origin set at the lower left corner of the shape ...

Command: _massprop
Select objects: 1 found

Select objects:

 ----------------   REGIONS   ----------------

Area:                    288.00000
Perimeter:               72.00000
Bounding box:         X: 0.00000  --  12.00000
                      Y: 0.00000  --  24.00000
Centroid:             X: 6.00000
                      Y: 12.00000
Moments of inertia:   X: 55296.00000
                      Y: 13824.00000
Product of inertia:  XY: 20736.00000
Radii of gyration:    X: 13.85641
                      Y: 6.92820
Principal moments and X-Y directions about centroid:
                      I: 13824.00000 along [1.00000 0.00000]
                      J: 3456.00000 along [0.00000 1.00000]



Here it is again with the UCS origin set at the center of the
rectangle....

Command: _massprop
Select objects: 1 found

Select objects:

 ----------------   REGIONS   ----------------

Area:                    288.00000
Perimeter:               72.00000
Bounding box:         X: -6.00000  --  6.00000
                      Y: -12.00000  --  12.00000
Centroid:             X: 0.00000
                      Y: 0.00000
Moments of inertia:   X: 13824.00000
                      Y: 3456.00000
Product of inertia:  XY: 0.00000
Radii of gyration:    X: 6.92820
                      Y: 3.46410
Principal moments and X-Y directions about centroid:
                      I: 3456.00000 along [0.00000 -1.00000]
                      J: 13824.00000 along [1.00000 0.00000]


As you can see, the first set of results lists the centroid at (6,12)
from
the lower left hand corner origin. Right where we expect it. The
second set
of results lists the centroid at (0,0) from the center of the
rectangle.

As expected, for the second case, the calculated monents of inertia
and the
principal momoents of inertia are one and the same since we located
our
coordinated system origin at the centroid of the section.

Hope this helped.

Brent R. Koch, P.E.
Koch Engineering, Inc.
Livermore, CA
mailto:kochengr(--nospam--at)pacbell.net