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# RE: "Apparent Rigidity"

• To: seaint(--nospam--at)seaint.org
• Subject: RE: "Apparent Rigidity"
• From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
• Date: Wed, 15 Sep 2004 20:29:23 -0400 (EDT)

```Bill:

If your WFCM shows the units for "apparent rigidity" as in^2/lb, then
maybe you have one with a typo.  My copy (that I just ordered and received
a couple of weeks ago) shows the units as in^2-lbs, which is completely in
line with Rand's response.

If it helps, there is an example for using Table 2.13A and Table 2.13B in
the commentary (assuming that you got the commentary with the WFCM).  And
both examples are consistant with Rand's explanation of apparent rigidity.

(And to Joe:  I just ordered the 2001 NDS with LRFD and WFCM not because
my 97 NDS is "falling apart" but rather because it is now the "law of the
land" in my neck of the woods (Michigan just adopted the 2003 IBC as the
state building code this past Spring, which then references the 2001 NDS))
<grin>

HTH,

Scott

On Wed, 15 Sep 2004, Bill Polhemus wrote:

> Thanks for the reply, Rand, but hold everything.
>
> If the units given for "apparent rigidity" are "inches squared per pound),
> and the units for EI are "pounds-inches squared," then I don't see that this
>
> In fact, now I'm looking at it, the units for "apparent rigidity" appear to
> be the RECIPROCAL of those for "E", which is even MORE puzzling since "E" is
> typically a "big" number (order of magnitude six or seven or so), and so is
> the order of magnitude for "apparent rigidity" according to the WFCM tables.
>
> So I'm still left in the dark, unless you can help me see the error of my
> ways.
>
> > -----Original Message-----
> > From: Rand Holtham, P.E. [mailto:rand(--nospam--at)sigmaengineers.com]
> > Sent: Wednesday, September 15, 2004 3:02 PM
> > To: seaint(--nospam--at)seaint.org
> > Subject: Re: "Apparent Rigidity"
> >
> > Apparent Rigidity is the EI value required for a simple span live load
> > deflection meeting the criteria specified in the table. In other words the
> > rigidity (EI) required to limit the deflection to say l/240.
> >
> > HTH,
> >
> > Rand
> >
> > ----- Original Message -----
> > From: "Bill Polhemus" <bill(--nospam--at)polhemus.cc>
> > To: <seaint(--nospam--at)seaint.org>
> > Sent: Tuesday, September 14, 2004 4:30 PM
> > Subject: WOOD: "Apparent Rigidity"
> >
> >
> > > I have come across this term in perusing the WFCM, and I cannot figure
> > out
> > > exactly what it means.
> > >
> > >
> > >
> > > It occurs in particular in Tables 2.13A-B (CEILING FRAMING CAPACITY
> > > REQUIREMENTS). Footnote (1) to this table states:
> > >
> > >
> > >
> > > "Apparent rigidity capacities shall include the effects of both bending
> > and
> > > shear deflections. Apparent rigidity capacities have been adjusted for
> > > solid-sawn lumber to account for these effects. Contact the I-joist
> > > manufacturer for apparent rigidity capacities to be used for I-joists in
> > > this table."
> > >
> > >
> > >
> > > And Footnote (2) says:
> > >
> > >
> > >
> > > "Tabulated apparent rigidity requirements assume single span conditions.
> > For
> > > continuous span conditions, tabulated apparent rigidity requirements
> > shall
> > > be permitted to be multiplied by 0.75."
> > >
> > >
> > >
> > > The units of "apparent rigidity" are in.^2/lb., and a typical value is
> > > between 5 and 30 MILLION.
> > >
> > >
> > >
> > > So not sure how to use this information.
> > >
> > >
> > >
> > > Interestingly, the rafter span tables give you selection values in f-
> > sub-b
> > > for load and E for deflection. Don't know why the difference.
> > >
> > >
> >
> >
> >
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