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Re: Steel rules of thumb

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> Working stress concrete design (OK, all you new guys,
> flame away): As = M / 1.76 d    (M in ft-k, d in

"New guys"?!... You mean those of us who graduated after 1975?! :)

Only kidding, only kidding!!!

I learned ultimate strength design in the 70's for concrete, but my
brain is still "wired" for ASD design in structural steel - although I
am enjoying climbing the learning curve with my brand crispy new 3rd
edition AISC LRFD Manual!

Speaking of which,... I have another structural steel shortcut that I
use when I "QA" projects designed by others. The service level "bare
beam" (fully braced, 0.66xFy) moment capacities of most of the
economical W Shapes became embedded into my brain years ago. I have
found that the effective service level moment capacity of a composite
beam designed using LRFD is APPROXIMATELY twice that of the ASD "bear
beam" moment capacity.  WARNING WARNING!! THIS IS A VERY ROUGH
APPROXIMATION, and varies depending upon beam size, slab thickness and %
composite action - but it is reliable for catching "blunders". !!!!!!!DO
NOT USE THIS APPROXIMATION FOR DESIGN!!!!!!

Actually, my most frequently used shortcut is for computing the required
area of reinforcing steel in concrete beams:  As = Mu / (4 x d) (yes,
the ultimate strength version of the equation above!) This equation is
quite accurate for 60 ksi rebar and concrete that is 3 ksi or higher. It
is conservative for small percentages of steel and is a little
unconservative for beams with lots of steel, but you usually like to
stay away from "loading up" a concrete beam with "too much" rebar anyway
so it doesn't matter.

Where is this equation from?... If you look in the ACI "design aids"
book there are tables with coefficients to help you quickly compute the
flexural strength of singly reinforced concrete beams. Those tables give
values for a coefficient "a"  that ranges from about 4.4 to 3.7 for 60
ksi rebar. (I'm doing this from memory while sitting at my home computer
so forgive me if the values are off by a little bit.) You plug the "a"
value into the equation As = Mu / (a x d).

Cliff Schwinger

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