Moment of inertia of asymmetric beam

[moyseh]

Consider a typical I section beam with a larger top flange than bottom flange.
Does this beam have the same moment of inertia about its major axis if you flip it upside down?

For example if you had a beam that was a cantilever. Would it be better to have the top flange thicker or the bottom flange thinner?

Thanks

 

[JStephen]

The moment of inertia is the same either way.
The section modulus is not, since the centroid of area is no longer midway between the two faces.
If allowable stress in tension and compression are the same, it wouldn't matter.
If criteria are different for compression and tension, there would be a preference.
If the compression is limited by buckling, it may be advantageous to put the wider flange on the compression side.
Generally, you'd have to work through the numbers to confirm.

 

[SWComposites]

if you had a beam that was a cantilever. Would it be better to have the top flange thicker or the bottom flange thinner? - this all depends on the loading. there is no one "best" for all cases. and must check buckling on the compression side(s) in addition to basic strength.

 

[moyseh]

Thanks for your responses. How does one calculate the section modulus for an asymmetric beam. Say for example, the tension flange is the bottom (positive bending). Is the positive section modulus the moment of inertia divided by the distance to the extreme fibre of the tension flange?

 

[SWComposites]

Yes, but the distance is measured from the neutral axis, which is not at the center for an unsymmetric beam section.

 

[BridgeSmith]

Thanks for your responses. How does one calculate the section modulus for an asymmetric beam.

This is one of the most basic concepts of structural mechanics. I don't wish to be rude, but if you don't know, you need to find a qualified structural engineer to do the beam design.

 

[rb1957]

an I beam with one flange thicker than the other is not (necessarily) an asymmetric beam (it isn't if it still has an axis of symmetry).

an I beam with one flange thicker than the other will have a centroid closer to the thicker flange (whether that is top of bottom on the section) and both beams will have the same stress magnitude, but will have opposite signs ... if one beam has 10ksi tension (on the thinner flange) and 20ksi compression of the thicker one then the other beam (with the flanges reversed) will 20ksi tension on the thicker flange (same location as the first beam's thinner flange) and 10ksi compression of the thinner.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.