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RE: "Apparent Rigidity"

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Bill,
I hope you get this straightened out.  Just last Friday I ordered new NDS
manuals ('97 edition is falling apart) and the WFCM.  I have seen the WFCM
referenced on the list and in particular for a question I asked last winter,
so I thought I would order a copy.  You can work through the bugs and then
educate the rest of us.
Joe Grill

-----Original Message-----
From: Bill Polhemus [mailto:bill(--nospam--at)polhemus.cc] 
Sent: Wednesday, September 15, 2004 3:40 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: "Apparent Rigidity"

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
is the answer.

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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