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Re: Lateral Brace Deflection

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> 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.

> From: "Ed Fasula" <tibbits2(--nospam--at)metro.lakes.com>

> With a 6' cantilever, this is quite the beam.  If this is correct, I have
> some serious re-thinking to do!

Yes, your bracing stiffnesses can be huge if you are not used to doing
the numbers. Bear in mind that required strength and stiffness are
inversely proportional. So, the more stiff the bracing, the less strong
that it needs to be. In your situation where a member is required to
brace in bending, the strength is inherent to the stiffness. Where
tension/compression is used to brace, the stiffness can be quite high
for relatively small members. This is one reason why diagonal tension
members are so efficient and common for bracing.

Note that it is possible to require bracing for flexural or compression
bracing members.

The whole thing is kind of like an Escher drawing ....

-- 
Paul Ransom, P. Eng.
Civil/Structural/Project
Burlington, Ontario, Canada
<ad026(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>