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Steel rules of thumb

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Here are a couple of formulas I use to find required Ix and Sx of a
simply-supported uniformly loaded beam during preliminary calculations (it
uses ASD to find Sx, but it's easy for preliminary work, even for LRFD
designers - just substitue Zx).

Ix = wll * (L^3) * Delta / 15466
Zx/Sx = wtl * (L^2) / 20

For a typical floor load with max deflection = L/360 the first equation can
be shortened to:
Ix = wll * (L^3) / 43

wll = live uniform load in kips/ft.
wtl = total uniform load in kips/ft.
L = beam length in ft.
Delta = max deflection as a function of length (i.e. for max defl. = L/360,
Delta = 360)

Assumptions:
Fy = 50 ksi
E = 29,000 ksi
ASD at 60% stress

Example: Beam supporting masonry (max deflection = L/600), wtl = wll =
2.0klf, L = 25 ft.

Ix = 2.0*(25^3)*600/15466 = 1213 in^4 --- W24x55 <= Controls
Sx = 2.0*(25^2)/20 = 62.5 in^3  --- W16x40
Zx = 62.5 in^3  --- W18X35


-----
Jason Kilgore
Leigh & O'Kane, L.L.C.
(816) 444-3144
jkilgore(--nospam--at)leok.com


-----Original Message-----
From: Michael Hemstad [mailto:mlhemstad(--nospam--at)yahoo.com]
Sent: Wednesday, February 27, 2002 3:03 PM
To: seaint(--nospam--at)seaint.org
Subject: Steel rules of thumb


>From: Daryl Richardson
>Subject: Re: Steel rules of thumb
>To: seaint(--nospam--at)seaint.org

>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)

Daryl,
I've derived the same equation--it's very handy.  With
moment in foot-kips, span in feet, and deflection in
inches, and for a steel beam (E = 29,000 ksi), it ends
up:

        Delta = M * L^2 / 161 I

A few more:

Working stress design of E70 fillet welds, allowable
load = 0.928 kips/inch per 1/16 inch of weld size
(e.g. a 3/16 fillet is 3 x 0.928 kips/inch).

Working stress concrete design (OK, all you new guys,
flame away): As = M / 1.76 d    (M in ft-k, d in
inches), or, far more accurately (good for
spreadsheets)

Per foot width,
 As = ((M, ft-k)^1.04) x 11.84/(Fs (ksi) x d (inches))

(Charlie, don't put this one in MSC)

Mike Hemstad
TKDA
St. Paul, Minnesota



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