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Re: x brace question
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- 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?)
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