Re: Seismic Provisions 97-OCBF(ASD)

• To: yosi_h(--nospam--at)msn.com, SEAint(--nospam--at)seaint.org
• Subject: Re: Seismic Provisions 97-OCBF(ASD)
• From: Mlcse(--nospam--at)aol.com
• Date: Fri, 1 Jun 2001 21:25:59 EDT
Hi,

You may want to go to the AISC website to get the latest revisions (Seismic
Provisions for structural steel buildings, Supplement No 2).  I have provided

Michael Cochran S.E.

In a message dated 6/1/2001 3:38:27 PM Pacific Daylight Time, yosi_h(--nospam--at)msn.com
writes:

Hi,
1. Using part III of Seismic Provisions 97-OCBF(ASD)section 14.1 mentions
1.5 times the strength. Does it mean the formulas 4-a & 4-b have to be
magnified by 1.5? If so, what is the rational behind it?

The 1.5 is for the Chevron type of brace member only and applies to only
the E portion of formulas 4-a and 4-b.  It is due to the poor performance of
the chevron bracing system in high seismic regions and is to help prevent
premature buckling of the brace which will lead to failure of the beam when
the brace buckles.  See the SEAOC Blue Book for discussion about Chevron and
Diagonal bracing.

2
. Does formula (6-1) of pert I apply to part III of Provisions, and what
other formula is used in conjunction with (6-1) to get the tension force in
the OCBF?

Formula 6-1 of part I does apply to part III for OCBF's you need to include
the Ry to have a realistic expectation of what the capacity of the members
are in tension or compression when designing the connections.  The tension
capacity of the member is Ry Fy Ag, see AISC supplement #2 for additional Ry
values.  The 1997 UBC equation allowing the connection design capacity to be
not more than Omega times the seismic force should not be used since it
typically underestimates the potential seismic demand on the connection.

Thanks,

Joseph Harouni