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beam statics Question
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Posted by: smd79

11/17/2004, 15:52:47

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I have a question regarding the calculation of the max deflection for a simply supported beam (pinned at both ends). Actually I am confused on the moment of inertia used in this calculation.
When determining the moment of inertia about an axis means rotion about that axis? ie Ix means the beam spins around at the x-axis
In that case is the moment of inertia that I use that which acts in the same direction as the force?

I am using an 'I' beam in my calculations.

All input is greatly appreciated,Thanks.

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Re: beam statics
Re: beam statics -- smd79 Post Reply Top of thread Forum
Posted by: Cragyon
11/17/2004, 17:26:20

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Check out the following links: /standard_material/Steel_ibeam_properties.htm or /standard_material/Steel_ibeam_s_properties.htm for an I-Beam, W or S Flange, you would use the moment of Inertia Ixx if the load and supports are applied on the top or bottom of the section shown within the drawing graphics.  Use Iyy if the load and support is applied on the left or right side of the drawing graphics.  The distance "c" is normally from the center of the shape to the outside along the direction the load or support is located.


Hope this helps!

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Re: beam statics
Re: Re: beam statics -- Cragyon Post Reply Top of thread Forum
Posted by: smd79

11/18/2004, 12:05:44

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but just to be clear, the applied force is perpendicular to the axis of the moment of inertia?

my problem is for a rail which is supported on both ends with a pin. the force applied is uniform and all along the length of the rail (i have this set as the x-axis). this force is pressing down causing deflection in the negative y direction. this view of the rail system is the top view, therefore we are looking down onto the rail from above. the cross section of the rail is an I-beam, and so when we face the I-beam the height of the I is the z axis.

I hope that explanation is clear. and from the drawing graphics on the links you sent the force would be applied to the side of the I-beam. therefore i will be using Iz (as my coordinate system is set up, Iy on the graphic).

I need some reassurance and am hoping this is correct.

Thankyou once again for your help.


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