The problem with steel beam design is at first glance it is really really simple. Then you get into lateral torsional buckling, web buckling, etc and it's like you're trying to read a book while it's on fire. The 9th edition AISC steel manual did a decent job at breaking down how to analyze for the various failure criteria, but with the 13th edition it became very difficult to get things done right (not to mention the fact that at times the steel manual will reference other books if you'd like to proceed). But that is neither here nor there, I will do my best to explain how best to solve for a beam by hand.
Steel beam design has many different checks and criteria, the common ones will be explained further here:
Steel design is broken up into a variety of steps. You have to check to make sure the beam is braced often enough, you have to check to make sure the web does not buckle, you have to check to make sure there isn't too much shear going through the beam. All of these checks are normally done automatically with whatever structural design program you use, but if analyzing a beam by hand (which with today's Steel Codes have become quite the tedious process), it can take quite some time to find the actual capacity of a steel beam. That being said, the
Design of a simple beam (using ASD?) can be done one of two ways:
1) Find the allowable moment acting on the beam: For Compact Sections with adequate lateral and vertical support, the nominal flexural strength of a steel beam is:
The Nominal Moment will be modified by the Safety Factor (Ωb) to become:
Given an A36 W-member that is compact and properly braced, with a Plastic Section Modulus of 41 in3 find the allowable Moment.
2) Compare the stress acting on a beam to the Allowable Stress: The more common way to solve for the allowable bending in a steel beam is to check the beam against its allowable stress. Reworking the equation in Section 1 above you can come up with:
Most common simplified bending equation:
Given an A50 W-member that is compact and properly braced, with a Plastic Section Modulus of 18.5 in3 and a moment of 141 k-in. Does the member work? What is it's design ratio?