Beam selection (steel I-beam)

[Jaydee]

TL;DR Summary

Increase loading on a beam by using steel instead of wood and proving suitability.

Hi all,
I have calculated that a wooden rectangular beam has a max bending moment of 1780N/m2 and I want to increase loading by 10times and use the table for a replacement steel i-beam.

Steel max stress = 275MN/m^2

and

safety factor of 2

How would a suitable beam be confirmed given the data? perhaps I'm making silly errors but I'm just not understanding how its done to the values found in the tables.

So far I've got:

275/2 = 137.5MN/m2 = max allowable stress
Max bending moment = 17800N/m2
How do I go about picking a suitable I beam from just this data?
Any help would be great.

Thanks

 

[jrmichler]

You need to go back to the beginning, and use a step by step approach.

1) Start by reviewing bending moment and stress, and the units for each. Use consistent units throughout, such as MPa for stress.

2) Forget the "increase loading by 10 times". Recognize that the wood beam and steel beam are two different problems, each with its own loading and allowable stress.

3) Calculate the maximum bending moment for each beam. Refer to your course notes on moment diagrams.

4) Pick a steel beam from the table, then solve for maximum stress using the depth and $I$ value from the table. Compare to the maximum allowable stress, then pick another beam from the table, and repeat the calculation. Iterate until you find the best beam for your loads. There is a shortcut, but do it this way until you fully understand the process.