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RE: SCBF question[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: SCBF question
- From: Charlie Carter <carter(--nospam--at)aiscmail.com>
- Date: Wed, 20 Dec 2000 13:44:18 -0600
Title: RE: SCBF question
The lateral bracing requirement you quoted from the AISC Seismic Provisions is intended to "keep the assembly in line" as the system deformations occur elsewhere. The 2 percent rule gives you a fictitious lateral force that can be used in design. A more exact approach that considers strength and stiffness requirements could be taken, but we chose to keep things simple.
In any case, lateral bracing can be provided directly, torsionally, or some combination of both. A direct brace would be something like a kicker. A torsional brace would be something like that used in many railroad bridges ... the trains ride between the girders, the deck beams provide flexural resistance to girder rotation and the connection to the girder is such that the top flange of the girder can be restrained by the deck beams. Most practical details provide some direct and some torsional bracing.
If you provide a kicker, the 2 percent rule can be applied directly. If you go with a detail that creates torsional bracing to restrain the other flange, you should pay attention to the stiffness, which is not really addressed directly by the simple 2 percent rule, other than the force requirement. The stability bracing seminar that Joe Yura and Todd Helwig have put on for AISC and SSRC covers this in great detail (see shameless promotion below) and you can find this information in textbooks that get into stability. If all else fails, I can fax you some information.
Shameless promotion: my colleague Scott Melnick will be most happy if I note here that this seminar will be a short course at the AISC North American Steel Construction Conference, which he runs:
and in AISC continuing education programs, which I run:
From: Mark Pemberton [mailto:Markp(--nospam--at)lbdg.com]
Sent: Wednesday, December 20, 2000 1:07 PM
Subject: SCBF question
Section 13.4.a.4 of the 1997 AISC Seismic Provisions Manual states:
"The top and bottom flanges of the beam at the point of intersection
of braces shall be designed to support a lateral force that is equal
to 2 percent of the nominal beam flange strength Fy(bf)(tbf)"
If the beam cannot be directly laterally supported at this location
is it acceptable to design the beam using the special loading
combinations of chapter 16 (97 UBC) along with a lateral force
equal to Fy(bf)(tbf) in the weak direction at each flange and
check the member for the combined biaxial bending and compression
resulting? A plan checker at DSA suggested using 10% of the axial
force in the beam resulting from the special loading combinations.
Mark Pemberton, P.E..******* ***
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