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FW: SCBF question

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I do not understand how the exception does not apply as they
are listed in both the UBC and AISC under the SCBF sections.

Mark Pemberton

-----Original Message-----
From: Mlcse(--nospam--at)aol.com [mailto:Mlcse(--nospam--at)aol.com]
Sent: Thursday, December 21, 2000 9:05 PM
To: Markp(--nospam--at)lbdg.com
Subject: Re: SCBF question


In a message dated 12/21/2000 8:18:52 AM Pacific Standard Time, 
Markp(--nospam--at)lbdg.com writes:

<< 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.
  >>
Per AISC and the UBC, I believe the single story exception is only for the 
OCBF.  If you want to use a SCBF, then you can't use the single story 
exception since you are using a larger R value (R= 6.4) for the building
base 
shear calculation.  Per section 14.5 for OCBF if you use the load
combination 
4-1 and 4-2 you can ignore all of the OCBF requirements for design and 
detailing, but you are designing using a smaller R value (R=5.6).
Equations 
4-1 and 4-2 are the same (similar) to the load combinations 1612.4 in the 
UBC.  Note that the beta factor for the chevron brace also does not apply 
when using AISC section 14.5 (UBC 2213.8.5), since omega = 2.2 is greater 
than the 1.5 penalty factor used on chevron bracing.

For the single story building I would recommend using the OCBF and design
the 
brace, beam and connections for equations 4-1,4-2.  The beam will be larger,

but not as large as when using the unbalanced brace force.  Omega is 2.2, 
which is essentially the same as having designed the buildng system using an

R = 5.6/2.2 = 2.5 and supposidly this would be an essentially elastic 
structure taking into account strain hardening of the steel and global 
behavior of the structure.  Personally, I think the Omega  value should be 
increased, so the effective building R would be 2.0 and not 2.5.

If you use the OCBF as described in section 14.1 -14.4, beware that section 
14.3c basically requires using the 2t offset on the gusset plate, though no 
one seems to pay attention to this (Note that the wording in section 13.3.c 
and 14.3.c are basically identical).  When supplement #2 to the AISC seismic

provisions come out early next year there are significant changes and 
limitations restricting the design and use of the OCBF design.

Mike Cochran