Question about W Shape Steel Beams 2

[Tioch]

I'm preparing a table on allowable simply supported loads on A36 steel beams for a project and I've run into a problem. While doing a calculation for a simply supported load on a 1 ft. W shape (sizes between W8x15 and W6x9) beam, I noticed something rather odd. The loads between a 1 ft. and 2 ft. span changed rather drastically (between 45 kips for a 2ft span to 92 kips for a 1ft load). I was wondering if there was something I was missing in the equation between those two sizes. I've been using the AISC Manual of Steel Construction: Allowable Stress Design to find the equations for it. Is there a different factor I'm suppose to multiply the load by because of such a short span? Also, is there any particular form of moment equation I should use with the method in the AISC manual? Any help on this would be greatly appreciated.

 

[Taro]

For spans that short, the allowable moment should not change. The spans are both less than Lc (the length for allowable stress = 0.66Fy). Make sure you are also checking shear. The AISC manual contains tables of allowable load already, so you shouldn't need to reinvent the wheel.

 

[JStephen]

For a short span, the allowable stress won't change, it will be the 0.66Fy. For a single load at the center, the moment is proportional to the length, so twice the length gives you half the load.

Check shear as Taro mentions.

Also, for high loads on small beams, you'd want to check web crippling and web yielding.

If you get the right loading on a short piece of beam, you can just crush the beam- in that case, look at the web as a short column.