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# Re: x brace question

• To: seaoc(--nospam--at)seaoc.org
• Subject: Re: x brace question
• From: "Bill Sherman" <SHERMANWC(--nospam--at)cdm.com>
• Date: 11 Mar 98 22:05:13 -0500

```Arenson wrote:

> In order to displace, because of deformation compatibility with the
tension
brace (assuming they are connected), wouldn't the effective radius of be the
combination of both members, i.e., ry1 + ry2? <

I'm not so sure - I don't see that the ry's can be added necessarily.

> In the theoretical case where compression members are fixed-fixed, the
LRFD
uses k=0.65 rather than k=0.5 as a recommended effective length factor.  By
the same reasoning, it would make sense to use k=0.65 for out of plane
buckling of concentric x-braces. <

Naeim states that a case can be made for "significant end restraint in the
in-plane direction"; but in my opinion, in the out-of-plane direction the
gusset plates would not provide much end restraint and braces are rarely
connected with full end fixity.  Thus K=1.0 makes sense from an end
restraint
standpoint.

> Maybe this is where Naeim gets his k=0.69 ( he actually uses 2/3 which
could
be interpreted as a rough approximation of 0.65) <

Actually Naeim states his basis in article 8.4.5 of The Seismic Design
Handbook:
"For the out-of-plane direction, it has been shown that the tension brace
provides an elastic lateral support for the compression brace so that the
effective-length factor is greater than 0.5, but less than 1.0.  A value of
K=0.67 has been used by some designers as a reasonable compromise."

I agree that the real value is between 0.50 and 1.00 - but the value to be
used in design seems to be "by judgement" - why K=0.67 instead of 0.75
(midway) or 0.85?  (Are you an optimist or a pessimist?)

```