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RE: 70%-30% Bracing Load Distribution

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I got curious about the history of this provision. Near as I can tell, it started with the 1988 UBC, which mandated that not more than 70% of the horizontal seismic load be resisted in either tension or compression along any bracing line. The exception was if the tension members were designed based on overstrength requirements. The provision remained pretty much unchanged im all the UBC revisions through the 97 code, and applied to braced frames generally except for SCBF (special, concentrically braced frames) (see 97 UBC section 2213.8).
The language in AISC 341-02, section 13.2.c is about the same, but only seems to apply to special, concentrically braced frames. In SEAOC's 1999 Blue Book commentary, the lateral force distribution provisions in AISC 13.2c are intended "to prevent the accumulation of inelastic drifts in either direction. This accumulation is not determined by the elastic distribution of forces but rather by the ultimate (postbuckling) force capacity in each direction and the energy required to produce inelastic drift. ...An exception is made if the system is designed for amplified forces, as more energy is entailed in elastic deformation, leaving less energy to cause inelastic drift."
It's kind of a mystery to me why this provision now only applies to SCBF frames. It seems like it would be good practice to apply it to any braced frame.
-----Original Message-----
From: raincat1(--nospam--at) [mailto:raincat1(--nospam--at)]
Sent: Tuesday, April 26, 2005 10:05 AM
To: seaint(--nospam--at)
Subject: Re: 70%-30% Bracing Load Distribution

Bill/ James,
Could I arbitrarily assign 70% to the tension brace and 30% to the comp brace? This way my members would be smaller. Or assignment should be based on the stiffness of the braces? 
-----Original Message-----
From: Scott, William N. <William.Scott(--nospam--at)>
To: 'seaint(--nospam--at)' <seaint(--nospam--at)>
Sent: Tue, 26 Apr 2005 08:18:27 -0800
Subject: RE: 70%-30% Bracing Load Distribution

You should design the bracing for 100 % tensile capacity assuming buckling of the compression brace.
-----Original Message-----
From: raincat1(--nospam--at) [mailto:raincat1(--nospam--at)]
Sent: Tuesday, April 26, 2005 7:09 AM
To: seaint(--nospam--at)
Subject: 70%-30% Bracing Load Distribution

In AISC Seismic-02, 13.2c, up to 70% of the lateral seismic force could be resisted by tension braces.
Does this mean that if I have an X-Bracing I could assign one brace to resist 70% (tension) and the other brace to resist 30% (compression)? I will not get the 70-30 distribution by using Staad which will
have 50-50 distribution assuming the brace members are the same.
Rainier C. Catubig, S.E.
San Diego, California

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