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RE: Increaing double-angle brace compressive strength

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Have you looked at how SJI and Vulcraft does this?  It might be worth your
time.  If the angles have a space between them, you can place a plate
between the two and get them to work as a single unit.  If they are
together, stitch weld them together.  The specs for doing both are in the
SJI stuff and the LRFD manuals.

Good Luck,

Jake Watson, P.E.
Salt Lake City, UT

-----Original Message-----
From: Dave Evans [mailto:DEvans(--nospam--at)tnh-inc.com]
Sent: Thursday, August 01, 2002 8:57 PM
To: seaint(--nospam--at)seaint.org
Subject: Increaing double-angle brace compressive strength


We need to increase the compressive strength of some double
angle braces. Y-Y axis buckling is the problem. Our first thought
was to bolt a channel to each one so that the channel web is in
contact with the angle outstanding legs and the angle outstanding
legs nestle between the channel flanges thus (hope this art comes
thru):
------------------
|  ----   ----   |
       | |
       | |

Call this Case A.  It is easy to analyze using the "lean-on" bracing
concept.

Where this arrangement is physically impossible because of
attachments to the brace, it would be possible to invert the channel
and bolt it to the back-to-back legs thus:

   ----   ----
       | |
|      | |        |
------------------

Call this Case B.

I like Case A much better than B, mostly because in Case A, the
shear centers of the channel and double angles are relatively close
to each other.   In Case B, they are quite distant.  The problem is
figuring out whether or not there is a significant difference in the
effect of the reinforcement.

I've considered the column-constrained-to-buckle-about-a-fixed-axis
concept for Case B, but that's not completely accurate.  Might be
good enough, though.  Any comments or other ideas on how to
approach Case B would be appreciated.

Dave Evans, P.E.
TNH, Inc.

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