under a uniform load, you control the rate of change of the slope, which
is somewhat analogous to the imposed strain at the mortar joints. Don't
feel bad, mechanicals do this too when designing for serviceability in
composite circuit boards - we just do it in two dimensions instead of one.
There are two issues - the first is that above, the second is that for
many years brick veneer was not held to such tolerances, and the
resulting stiffness of the backup is far greater than people "expect,"
especially for wood backup. This is a little easier to get away with in
steel, because you have several gauges and flange configurations in CFS
which allow you to "hide" the increased I. In wood, going from the L/72
I mentioned as allowable in the IRC (or about L/90 in the IBC, table
2308.9.1) to L/600 at design loads means going from a 2x4 stud to a 2x8
stud in a "normal" wall. Even with data, it can be difficult to argue
that someone should double or triple the cost of a wall, and have it be
no "safer" - just less likely to leak or show cracks. This is especially
true when we design for 50 year loads, and the statute of repose is a
small fraction of that. (Difficult to argue about, not difficult to come
up with an answer on paper - the engineering calcs are always easier
than telling someone they cannot afford to build their building, or
build it for a price which provides a competitive lease rate in the market)
I suspect the real answer you're going to get to your question is that
(a) it correlates well to the limit for damage - say, within 10 or 15% -
and (b) the additional time and effort it takes to calculate the exact
answer is generally not justifiable given (a). Some engineers, given
the L/600 number as a guideline will design to it, then look at the
answer they get and if the deflection calcs to L/500 or better for the
"worst case" call it good and go on. Until I start modeling the
stiffness of the sheathing on both sides, the semi-fixed rotation spring
constant for the track at the top and bottom, and account for the short
and long term friction between the nails and the plates, I'm going to
fall squarely in the "some engineers" camp above.
Jordan
Mark Gilligan wrote:
> I believe that part of the problem with establishing
> deflection criteria for masonry on metal studs has to
> do with the way we specify deflection criteria.
>
> When I see deflection criteria specified in terms of
> L/X I wonder why they are limiting the slope of the
> beam at the support. If you look at the equations for
> deflection and compare it to deflection criteria in
> the form of L/X it is clear that all beams with L/360
> have the same slope at the support.
>
> If you are interested in controlling the natural
> frequency you should specify deflection criteria in
> inches. Similarly if you are interested in limiting
> the curvature in your beam you should specify
> deflection criteria in terms of L^2/X.
>
> What we need to do is to decide what we need to limit
> and then specify criteria that controls that
> parameter. Until we find a rational way to specify
> criteria we will have difficulties whenever our spans
> are significantly different from those tested in
> establishing the criteria.
>
> The next time you are checking beam deflections ask
> yourself why you are controlling the slope of the beam
> at the support.
>
> Mark Gilligan
>
> ttp://www.seaint.org
> ******* ****** ****** ****** ******* ****** ****** ********
>
>
******* ****** ******* ******** ******* ******* ******* ***
* 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@seaint.org. Remember, any email you
* send to the list is public domain and may be re-posted
* without your permission. Make sure you visit our web
* site at: http://www.seaint.org
******* ****** ****** ****** ******* ****** ****** ********