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RE: Beam design

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Martin,

First, this answer assumes that STAAD.Pro makes use of the "main body"
code provisions of ACI 318.

The second assumption is that you are in a pure "tension-controlled"
situation (i.e. that you will end up at "pure" beam end of the "sliding"
phi factor) and thus end up with phi = 0.9.

And last it assumes that something like minimum reinforcement does not
control (i.e. the amount of reinforcement is dictated by the good old
Mu<=phi*As*fy*(d-a/2)).

If so, then more than likely the difference is due to the fact that in the
2002 edition of ACI 318, they "moved" the ASCE 7 (and AISC) compatible
load factors into the main body of the code and moved the old load factors
to a appendix.  This means that for your typical gavity load situation
(i.e. dead load plus live load), one uses Mu=1.2*D+1.6*L rather than the
old Mu=1.4*D+1.7L.  They also moved the ASCE 7 compatible phi factors, but
also made at least one adjustment.  Prior to 2002, the phi factor to be
used with the ASCE 7 load factors for beams in the appendix was 0.8, but
when they moved it into the main body of the code in 2002, they changed it
to 0.9.  This in effect reduced the "factor of safety" (roughly an average
of the load factors divided by the phi factor), which caused some concern
to people, but the decision was based upon reliability studies that showed
that there was ample factor of safety.  But if you look at an "average"
factor of safety (assume equal dead and live loads...thus an overall load
factor of about of 1.4 for the new load factors and 1.55 for the old load
factors), then you end up with about a FS of 1.55 for the new load factors
and a FS of 1.72 for the old load factors.  That is about a 11% decrease
which matches relatively close to you 15% decrease in rebar (more than
likely your dead load to live load ratio is not 1 to 1 AND keep in mind
that you maybe getting technically more rebar in each case than you really
need as you have to get "real" bar numbers and sizes...i.e. you might need
say .18 sq in but use a #4 which gets you .2 sq in...and that "shift" from
pure theoretical to real bars can be different for each case).

This is where I get on my "soap box" and assume a lecturing type tone and
wag my finger at you...If you don't know the code well enough to do that
calculations by hand and understand what is going on in those
calculations, then you really should not be letting a computer do the
design work for you.  And I would say that may be the case if you are not
aware of such a major change (i.e. the load factors) in ACI 318.

OK, sorry for the "holier than thou" soap box speech.  <grin>

Regards,

Scott
Adrian, MI


On Wed, 11 Jan 2006, Martin Li wrote:

> I used STAAD.Pro to design a rectangle section concrete beam per ACI 318-99
> & ACI 318-02.  I found that the req'd reinforcing area per ACI 318-99 was
> about 15% more than per ACI 318-02.  Any comments?
>
> Martin
>
> -----Original Message-----
> From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu]
> Sent: Tuesday, January 10, 2006 5:23 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Beam design
>
> To be precise, the switch to the "sliding" phi factor, previous referred
> to as the "Unified Design Method"  (i.e. determining if it is
> tension-controlled or compression-controlled as Mr. Fu put it but
> really determined by the strain levels) has been in the ACI code for
> quite a while.  It is true that it was moved into the main body of the
> code recently, but this actually happened in the ACI 318-02.  Prior to
> that, this method was in an appendix for at least a couple of cycles.  In
> the 2002 edition, this method was placed into the main body of the code
> and the "old" method was placed into the appendix.  I will note that you
> are still permitted to do it per the old method since it is still
> contained in an appendix (Appendix C to be precise), unless the locally
> adopted code or model code specifically states an exception that disallows
> the use of that appendix (ACI 318 allows it, but can be "over-ridden" by
> the legally adopted code or a model code the references ACI 318).
>
> Regards,
>
> Scott
> Adrian, MI
>
>
> On Tue, 10 Jan 2006, TJ Fu wrote:
>
> > We know R.C. beams always subjected to some kind axial loads and bending
> moments. It doesn't matter whether that beam is horizontal or vertical, you
> have to design the beam for whatever loads you have along the beam according
> to ACI 318-05 9.3.2 thru 10.12.3.2 based on (1) stress and strain
> compatibility or (2) using ACI 10.3.6 approximate equation.
> >
> >   Due to beam self-weight, for horizontal beam, you'll have additional
> beam moment (M) to consider along the beam and for vertical beam, you'll
> have additional axial load (P) to consider at the bottom.
> >
> >   We used to apply strength reduction factor differently where you treat
> that member as beam (0.9) or column (0.70 tied or 0.75 spiral).
> >
> >   New ACI 318-05 has all these factors built in. You don't need to worry
> if it is a beam or a column now. You only need to determine if it is
> tension-control or compression-control based on the applied factored loads
> (Pu & Mu). It also tells you how to apply the reduction factor if you have
> in-between situation. Please note that the strength reduction factor for
> compression-control sections has been reduced from 0.70/0.75 to 0.65/0.70.
> >
> >   I have use MathCad to come up a Beam-Column design procedures per ACI
> 318-05 for rectangular section subjected to axial load, uni-axial or
> bi-axial bending. It is free if you like to try that out.
> >
> >   T.J. Fu
> >   Louisville, KY
> >
> >
> > Santhosh Kumar Yedidi <sant527(--nospam--at)gmail.com> wrote:
> >       I am sorry, I am not getting the replies.
> >
> > What I meant was, Generally members lying horizontally are designed as
> > beams, ofcource but columns are defined as those members irrespective
> > of its direction if there is some significant axial load acting.
> >
> > So in my case if the beam is subjected to axial load then we cant use
> > the beam theory to design it.
> >
> >
> >
> > On 1/9/06, David Fisher wrote:
> > > Gerard:
> > >
> > > Thanks for saying that...
> > >
> > > When I see questions like that, it really gives me the creeps.
> > >
> > >
> > >
> > > David L. Fisher SE PE
> > > Senior Principal
> > > Fisher + Partners Structural Engineers inc
> > > 372 West Ontario
> > > Chicago 60610
> > >
> > > 312.573.1701
> > > 312.573.1726 fax
> > > 312.622.0409 mobile
> > > www.fpse.com
> > >
> > > David L. Fisher SE PE
> > > Director
> > > Head of Design and Construction
> > > Cape Cod Grand Cayman Holdings Ltd
> > > 75 Fort Street
> > > Georgetown Grand Cayman BWI
> > > mobile 312.622.0409
> > > www.ccgch.com
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > -----Original Message-----
> > > From: Gerard Madden, SE [mailto:gmadden(--nospam--at)maddengine.com]
> > > Sent: Monday, January 09, 2006 10:53 AM
> > > To: seaint(--nospam--at)seaint.org
> > > Subject: RE: Beam design
> > >
> > > Is this on the midterm? What material? Is it open book?
> > >
> > > Good luck.
> > > -gm
> > >
> > > -----Original Message-----
> > > From: Santhosh Kumar Yedidi [mailto:sant527(--nospam--at)gmail.com]
> > > Sent: Monday, January 09, 2006 1:41 AM
> > > To: seaint(--nospam--at)seaint.org
> > > Subject: Beam design
> > >
> > > How to do beam design with axial forces acting.
> > >
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