Posted by David Sherman on Monday, April 17, 2006 at 10:55PM :
In Reply to: Re: not a doctor posted by john foster on Monday, April 17, 2006 at 7:18PM :
Assuming a simply-supported beam and either a concentrated load located at the center of the span or a uniform load distributed across the span, you should be able to get it right out of the AISC tables without too much math. Unfortunately I'm away from my office or I'd look it up for you. By the way, a beam can carry twice the load if it's evenly distributed as if it's concentrated at the center, so the latter gives a worst-case scenario. Bridges and floors are usually calculated with some combination of the two, but an I-beam trolley, for example, would definitely be a concentrated load.
The basic design procedure is you pick the beam formula that applies to your situation (simply supported, concentrated load at center), plug in the length, the load and the max allowable stress (generally 20,000 psi for structural steel, which gives about a 5X safety factor), crunch the numbers (simple arithmetic) and come up with a "moment of inertia" which is a number that you then take and go through the steel tables looking for an I-beam that you can easily obtain that has that value or greater. If you still haven't figured it out by next week, send me an email and I'll calculate it for you and copy some pages from the AISC steel handbook for you.
I took a quick look at the AISC website (www.aisc.org) and noticed they have downloadable "beam calculator" that runs as an excel spreadsheet. I took a quick glance at it and it looks useful, but too complicated for what you want and you'd have to understand some technical terms to make it work.
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