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# RE: Lateral Brace Deflection

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Lateral Brace Deflection
• From: "Ed Fasula" <tibbits2(--nospam--at)metro.lakes.com>
• Date: Sun, 15 Aug 1999 10:56:22 -0500

```Mark,
>Salmon & Johnson has a good treatment of point bracing for
>beams.  You want
>to make the brace stiff enough to inhibit undesirable
>buckling shapes.  The
>strength of the brace is then calculated as a function of
>the crookedness
>of the brace.
They mention members braced by transverse members, but they only address

By their method,
The required lateral brace capacity, Q, is:
Q = 0.004*Kideal*L
Kideal = 4*Pcr/L  (with beta=4), Pcr = 156k (service) and L = 120" so,
Kideal = 5.2 k/in and
Q = 2.5k (conservative compared to 0.002*P, in this case)

2.5k is not hard to accommodate, but Kreqd:
Kreqd = 2*Kideal*FS  (FS = 2.12 for AASHTO) so,
Kreqd = 22 k/in

As I understand the derivation, Kreqd would simply be directly related to
the bending stiffness of the beam in this case, rather than the axial
stiffness of a brace for the case addressed in the book.  Given the
geometry, 1" vertical deflection of the beam = 1.7" lateral deflection of
the top chord.  Therefore, the beam's stiffness must be at least 22*1.7 =
37.5 k/in!

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

Ed

```