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RE: Tension Only Braces Seismic Zone 3

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Using the 97 UBC you can design a steel ordinary concentrically braced frame (R=5.6, omega=2.2, height <= 160 ft).  Under the general circumstances the requirements in Sec. 2213.8.2 (for brace slenderness and lateral-force distribution) would preclude use of tension-only rod bracing.  However, Sec. 2213.8.5 indicates that Sec. 2213.8.2 (and 2213.8.4) need not apply to one- and two-story buildings if you design for omega times the design seismic forces. 

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Michael Valley, P.E., S.E.                       E-mail: mtv(--nospam--at)
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201

-----Original Message-----
From: Peder Golberg [mailto:peder(--nospam--at)]
Sent: Wednesday, October 03, 2001 11:59 AM
To: seaint(--nospam--at)
Subject: Tension Only Braces Seismic Zone 3

Are tension only braces allowed in Seismic Zone 3 anymore?   I have a large one-story building (covers a swimming pool) in design and the architect wants to have 100% windows on one side.  So far I'm losing the battle to get some wall piers.  The building is too large to assume that rotation of the wood diaphragm distributes forces to end walls (cmu walls).  The architect currently will only except diagonal rods in front of the windows (and states that other engineers, our competitors, wouldn't require anything along this wall - per similar previous projects before 97 code), and is against tube steel bracing (braced frame).   I can't find anything that either allows or prohibits this lateral system per the 97 UBC if we use R=2.8 and Omega = 2.2.   I know that the braced frame sections in the code prohibit tension-only braces but can't find any information on design of rod tension-only bracing.     I also see many issues with connections and transferring the loads from a glu-lam drag strut/collector into the steel rods with the omega factor making the loads very high.  I don't like this current system and am looking for justification to discuss alternate lateral options with the architect.  This architect is a big client of ours.
Thanks in advance for any tension-only input and comments.
Peder Golberg, P.E., S.E.
Portland, Oregon