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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: Mark Pemberton <Markp(--nospam--at)lbdg.com>
- Date: Thu, 21 Dec 2000 08:12:45 -0800
This is a single story building with a horizontal girt to cut down the stud spans and in some areas to support lower roofs. We felt the unbalanced post buckled requirement would not apply per the exception for single story structures. Mark Pemberton, P.E. -----Original Message----- From: Mlcse(--nospam--at)aol.com [mailto:Mlcse(--nospam--at)aol.com] Sent: Wednesday, December 20, 2000 7:42 PM To: Markp(--nospam--at)lbdg.com; seaint(--nospam--at)seaint.org Subject: Re: SCBF question In a message dated 12/20/2000 11:11:38 AM Pacific Standard Time, Markp(--nospam--at)lbdg.com writes: << 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.. >> The condition I imagine you have is a brace frame beam along a floor opening at the beam midspan so you can't physically place a brace at this location in the out-of-plane direction. I am not sure if you will have a problem making the beam work strength wise for the condition you described using just the special load cases, but the overall stiffness of the brace is another issue. For strength requirements, both the AISC provisions and the UBC require that you check the SCBF beam for an unbalanced force when the compression brace buckles and the tension brace pulls down on the beam. The resulting vertical force component of the braces is very large since you are using (FyAgRy - 0.3 phi Pn) for the brace axial forces and I imagine the beam will weigh in about 200 lbs/ft to resist this vertical force at the beam midspan in the plane of the beam. If you are checking a W24x192 beam (Fy=50 ksi), in the strong axis you have an unbraced length "Lu " of 24.7 feet where maximum allowable design stresses are limited to 0.6Fy max and you have a reasonable ry value (ry = 3.07) for out-of-plane buckling of the beam as a column . If you have braces that have a small cross section area, then maybe the beam will be smaller, but the tensile force for a HSS 5x5x3/8 tube is AgFyRy = (6.58)(46)(1.3) = 393 kips and the vertical component, assumming 45 degree brace slope is 0.707 x 393 = 278 kips which is a huge force, which will still likely be above 200 kips after you subtract out for the compresson brace. I would not apply a force in the out-plane direction to the beam flanges (I assume it is a typo you have to use Fy(bf)(tbf) for the out-of-plane force which would be huge). When you check the interaction on the beam using the ramped up seismic forces by Omega for both the brace to cause bending in the beam and axial to cause compression in the beam I would think it would still work since you orginally sized the beam for the unbalanced brace buckling force. If it is a chevron brace, isn't half the beam length in tension on one side of the brace connection point to the beam midspan and the other half in compression, which is another reason for the out-of-plane brace of the beam. If you include an out-of-plane force, I wouldn't use more than 2% of the beam flange force, which may cause you to use a larger beam size since now you have biaxial bending. If you are having to check the beam for these forces, what does your beam to column connection look like with such huge forces resulting from the beam/brace midspan connection. If you have a two story x brace, then one leg of the two story brace is going help control the amount of out-of-plane buckling when the other two story leg starts to buckle in compression. Considering overall brace frame stiffness, if you have two identical brace frames, where one is braced at the beam midspan, and the other is not, then they will have different stiffnesses when loaded to brace buckling. I imagine the brace frame without the midspan out-of-plane brace will start to buckle before the other brace frame does since there is limited torsional stiffness at the beam midspan point, thereby shifting the force to the brace frame with the midspan brace point that provides torsional stiffness. If probably would take a non-linear analysis to determine the difference in overall brace frame stiffness. In your computer model, you might want to try eliminating the beam on this particular brace (assuming it failed at this level) and see where the loads go the other brace frames if they still work and see if you can use this as a rational approach. You might have to upsize the columns on this brace frame. Curious to know more about this, or how you resolve it. Michael Cochran S.E.
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