Quick Calc: Beam Weight Based on Moment

Have you ever been in a project meeting and needed to make a quick estimation of beam sizes without any reference books?  Experience alone can sometime suffice, but a quick rule of thumb can be helpful to determine the beam weight needed for a given loading condition.

In the September 2023 SEU session, John A, Kennedy, SE, PE, from Structural Affiliates International, Inc.presented Rules of Thumb for Steel Design.  John shared some quick reference approximations for mechanical properties of steel sections.  He also identified some rules of thumb for the design of beams, columns, and trusses, and shared some design examples using these rules. 

John offered a quick, simple equation for determining the beam weight based on the moment and selected beam depth. For 50 ksi steel, the slide below shows this quick, useful equation which results in the beam weight per foot based on the moment in kip-feet and the beam depth in inches.

John walked through an example using this rule of thumb to determine the beam weight needed for a 16 inch beam which is 32 feet long with a dead load of 1 kip per foot.

As you can see, the rule of thumb was quite accurate to make a quick determination for the beam size needed for this given loading condition.  John noted that all rules of thumb are approximations and are not always conservative; however, they can be very useful when an on-the-spot intelligent decision is needed.  A rigorous structural analysis should always follow when more time allows.

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