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RE: Moment of Inertia using Autocad

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AutoCad calculates the Mass Moment of Inertia for solids, so with mass and sidelengths equal to;

M = 200,  X = 10,  Y = 20,  Z = 1

you get

About x-axis:  Ixx = 1/12xMx(Y^2 + Z^2) = 1/12x200x(20^2 + 1^2) = 6683.333

About y-axis:  Iyy = 1/12xMx(X^2 + Z^2) = 1/12x200x(10^2 + 1^2) = 1683.333

About z-axis:  Izz = 1/12xMx(X^2 + Y^2) = 1/12x200x(10^2 + 20^2) = 8333.333

Hope this helps
Gunnar Hafsteinn Isleifsson
Denmark



-----Original Message-----
From: Gerard Madden [mailto:GMadden(--nospam--at)mplusl.com]
Sent: Wednesday, January 31, 2001 02:33
To: seaint(--nospam--at)seaint.org
Subject: RE: Moment of Inertia using Autocad


Michael V & Michael B.

Thanks for the advice, it definately makes a difference - However, if I draw a box 10,20,1 and move it so the centroid is a 0,0,0 MASSPROP is telling me that Ix=6683.333 not 6666.667 that 1/12*10*20^3 would produce. Any Ideas ?

HERE IS THE OUTPUT FROM MASSPROP:

----------------   SOLIDS    ----------------

Mass:                    200.0000
Volume:                  200.0000
Bounding box:         X: -5.0000  --  5.0000
                      Y: -10.0000  --  10.0000
                      Z: -0.5000  --  0.5000
Centroid:             X: 0.0000
                      Y: 0.0000
                      Z: 0.0000
Moments of inertia:   X: 6683.3333
                      Y: 1683.3333
                      Z: 8333.3333
Products of inertia: XY: 0.0000
                     YZ: 0.0000
                     ZX: 0.0000
Radii of gyration:    X: 5.7807
                      Y: 2.9011
                      Z: 6.4550
Press ENTER to continue:
Principal moments and X-Y-Z directions about centroid:
                      I: 6683.3333 along [1.0000 0.0000 0.0000]
                      J: 1683.3333 along [0.0000 1.0000 0.0000]
                      K: 8333.3333 along [0.0000 0.0000 1.0000]


Gerard Madden, P.E.
Civil Engineer

Middlebrook + Louie, Structural Engineers
71 Stevenson Street, Suite 2100
San Francisco, CA 94105
Tel: 415.546.4900
Fax: 415.974.3680
Email: gmadden(--nospam--at)mplusl.com