Need a book? Engineering books recommendations...

# Re: Alternate Design Method

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Alternate Design Method
• From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
• Date: Wed, 2 May 2007 16:15:42 -0400 (EDT)

```Michael,

Correct...for pure bending.

If you including an axial load, then you need to include the P load in the
free-body diagram along with the T (tension steel) resultant and C
(concrete compression) resultant.  It is not just C=T.  But P=C-T.  And
the moment equation of the FBD also needs to include the moment due to the

Regards,

Scott

On Wed, 2 May 2007, Michael Hemstad wrote:

> On Tue, 1 May 2007, Jim Bessley wrote:
> > Scott,
> > I understand all of that regarding your comments. I would still like to
> > find an example.
> >
> > I do, however, appreciate your comments and thoughts on the matter.
> > Jim
>
>
> Working Stress Design is not dead yet.
>
> If you're analyzing an existing section:
> 1.  Figure out the depth to reinforcing, d.
> 2. Calculate rho.
> 3. Find N, the ratio of Young's Moduli.  For your purposes, use N = 12 - fc' (about 9).
> 4. Calculate k, the depth of the compression wedge (to the neutral axis)
>
>      k = sqrt(2 rho * N + (rho * N)^2) - rho * N   (usually about 0.3)
>
> 5. Calculate j:  j = 1 - k/3   (usually about 0.9)
> 6.  Calculate allowable moment:
>
>     M = As fs j d/12
>
> If you know the reinforcing grade (yield), use fs = 0.5Fy with a maximum of 24 ksi.  If unknown:
>
> If the structure was built prior to 1954, Fy = 33 ksi; use fs = 18 ksi.
> If the structure was built after about 1985, you can safely assume Fy = 60 ksi and fs = 24 ksi.
> Between these dates, assume Fy = 40 ksi and fs = 20 ksi.
>
> The 1954 date I lifted from an old (1983) copy of the AASHTO Manual for Maintenance Inspection of Bridges.
>
> To cut down on the blitz of emails screaming about how Grade 60 steel was common before 1985, yes, it was; but there was still design being done with Grade 40.  For instance, by me, as directed by my first boss.
>
>
> If for some odd reason you're designing a new section by WSD, it's a little harder to explain by email.  When we were doing WSD, we used tables; you'd calculate M/bd^2 and enter the table, pull out a value for rho, and off you'd go.  When I programmed WSD into my calculator, it involved solving a cubic equation based on first principles.  Good fun, that.  However, if you really, really want to do it, there's a shortcut equation, hitherto unpublished, that will get you remarkably close:
>
> for M = the design moment PER FOOT OF WIDTH, in foot-kips; fs in ksi; and d in inches,
>
>    As = (M^1.04) * 11.84/(fs * d)  giving you square inches of reinforcing.
>
> I'm pretty sure it's unpublished because I curve-fit the thing, and I don't think I've told anyone.  It works great for spreadsheets.  If you have trouble, Scott can help you.
>
> HTH,
> Mike
>
>
>
>
> Meyer, Borgman and Johnson, Inc.
>
> 12 South Sixth Street
> Suite 810
> Minneapolis, MN 55402
> (612) 338-0713 (main)
> (612) 604-3621 (direct)
> (612) 337-5325 (fax)
> www.mbjeng.com
>
>

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
*
*   This email was sent to you via Structural Engineers
*   Association of Southern California (SEAOSC) server. To
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
*   send to the list is public domain and may be re-posted