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Re: Deflection of Wood Studs for Brick

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Well, to be a pain, it's because controlling the slope at the support 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

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