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Re: Lateral Brace Deflection

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Paul

The difference between our statements is based on the assumption of the
buckled shape.  You are assuming that the brace suppresses all displacement
in the brace in which case your statement is true.

On the other hand if you use more braces, each somewhat less stiff, you
will get a different buckling mode shape that has displacements at the
bracing points.  Salmon & Johnson 3rd Edition has a figure that shows this.
  The capacity of the chords will depend on the size of the chords and the
stiffness and distribution of the braces.  With buckling it is not
necessarily all or nothing.

The standard formulas are based on the assumption of a stiff brace.  What I
am refering to might be thought of as distributed braces.  It is the
difference between a simple beam and a beam on elastic foundation.  To get
an answer for a system with more but fewer braces you will probably need to
perform a more general stabilkity analysis.

Mark Gilligan


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Message text written by INTERNET:seaint(--nospam--at)seaint.org
>> From: Mark Gilligan <MarkKGilligan(--nospam--at)compuserve.com>

> you could sustain the same chord forces with smaller values of bracing
> stiffness if the braces were placed closer together.   if one wanted to
> explore this approach the standard formulas are probably not directly
> applicable.

If you put in more braces (e.g. reduce your unbraced length), the
buckling mode increases (e.g. from 1/2 sine wave to 1 sine wave, etc.).
With each added brace the stiffness required to hold that point in place
against the new out-of-plane buckling mode will increase but the
restraint force may decrease. This is easy to see since the reduced
allowable lateral deflection is related to the reduced unbraced length
although the compression load is unchanged. Check the work by Yura et
al.

I fail to see why the standard formulas would not apply. Adding more
bracing against "compression buckling" can become very inefficient but
designers frequently do this because if one brace is good, more can only
be better. This is acceptable to the extent that you do not assume a
reduced unbraced length and you provide adequate strength/stiffness at
the points of original bracing. So, in other words, the added braces,
which may support a nominal load, become aesthetic - we sleep better at
night and architects/laypeople are happy that all appears to be right in
the universe.