# Re: Steel rules of thumb

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Steel rules of thumb
• From: Daryl Richardson <h.d.richardson(--nospam--at)shaw.ca>
• Date: Tue, 26 Feb 2002 20:32:17 -0700
```Charlie,

Here's a rule of thumb that's very handy for quick and dirty
calculations for beam deflection.

Delta(max) = M*L^2/(10*E*I)

where	Delta(max) = maximum deflection of simple span beam.

M = Maximum bending moment.

L = Length of beam.

E = Young's Modulus.

I = Moment of Inertia.

Derivation:
5*W*L^4/(384*E*I) = M*L^2/(9.8*E*I) = (very accurate)

P*L^3/(48/E/I) = M*L^2/(12*E*I) = (lower bound)

For equal and opposite end moments
Delta(max) = M*L^2/(8*E*I) = (upper bound)

For continuous spans the above equation applies with the following

L = length between inflection points.

Delta(max) = Maximum displacement of the beam from a straight line
joining the inflection points.

I've found this to be quite useful from time to time.  At the very
least it can divide your deflection problems into three categories: 1.)
deflection is no problem; forget it; 2.) deflection is a problem; take
corrective action; and 3.) deflection needs to be checked in more
detail.

I hope someone finds this useful.

Best regards

H. Daryl Richardson

Carter, Charlie wrote:
>
> A few good steel rules of thumb have now been posted. I have been keeping a
> file of them, including the Ioannides and Ruddy article set, with the idea
> that it would be useful to all to have a single sheet with rules of thumb
> for quick reference. Probably published in AISC's Modern Steel Construction.
>
> What other steel rules of thumb do you all use? If you'll share them, I'll
> consider including them. Please feel free to reply publically or provately.
>
> Charlie
>
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