Return to index: [Subject] [Thread] [Date] [Author]

RE: 2000 IBC seismic design category D - orthogonal effect requi rement

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Hello Mark and anyone else who cares:

1994 UBC 1628.1 also says, "the design seismic forces may be assumed to act
nonconcurrently in the direction of each principal axis of the structure,
except as required by Section 1631.1."  1994 UBC 1631.1 requires considering
a direction other than the principal axes only for torsional irregularities,
non-parallel systems or when a column is part of an intersecting lateral
force system where the seismic axial load is more than 20% of the column's
capacity.  

The IBC now requires you to consider orthogonal effects for Seismic Design
Category D, no matter what.  This is a departure from the UBC and makes a
difference for the brace sizes and column sizes on the building that I am
reviewing.  It would make a difference on story drift also.  The new IBC is
not clear  that orthogonal effects need to be considered for story drift.
If orthogonal effects are required it will make a difference in the brace
and column sizes because this building is already close on drift limits.  I
might add the building has not been submitted for a permit yet and the city
is working out Code issues on the computer model with the designer before
the contractor orders the steel.

I have a side question: Is it required to combine loads from both directions
when calculating story drift?



Thanks!!

Scott M Haan  P.E.
Plan Review Engineer
Building Safety Division http://muni.org/building, 
Development Services Department,
Municipality of Anchorage
phone: 907-343-8183   fax: 907-249-7399
mailto:haansm(--nospam--at)ci.anchorage.ak.us

> -----Original Message-----
> From:	Mark Gilligan [SMTP:MarkKGilligan(--nospam--at)compuserve.com]
> Sent:	Tuesday, March 13, 2001 5:59 AM
> To:	INTERNET:seaint(--nospam--at)seaint.org
> Subject:	RE: 2000 IBC seismic design category D - orthogonal effect
> requi  rement
> 
> 
> 
> The 1994 UBC section 1628.1 states "Structures shall be designed for
> seismic forces coming from any horizontal direction."  This is a nice
> simple declaritive statement that applies to all structures.
> 
> The orthogonal effects provision is simply a way to account for this
> without requiring analyses at multiple directions.
> 
>  The excemptions are an attempt to list common situations where for one
> reason or another the code authors felt that this effect would be small
> enough to ignore.  If this  effec t is significant for your structure then
> it should be adddressed.
> 
> Mark Gilligan
> 
> 
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> +
> +++++++
> >Do you need to consider orthogonal effects now when calculating story
> drift
> and building separations with the new 2000 IBC?  This is pertinent to a 10
> story building that I am reviewing under the IBC.
> 
> > -----Original Message-----
> > From: Haan, Scott M. [SMTP:HaanSM(--nospam--at)ci.anchorage.ak.us]
> > Sent: Monday, March 12, 2001 5:09 PM
> > To:   Seaint list (E-mail); Se-Practice List (E-mail)
> > Subject:      FYI: 2000 IBC seismic design category D - orthogonal
> effect
> > requi rement
> > 
> >
> --------------------------------------------------------------------------
> > Your following message has been delivered to the list
> >   seaint(--nospam--at)seaint.org at 18:19:51 on 12 Mar 2001.
> >
> --------------------------------------------------------------------------
> > 
> > 
> > 1997 UBC 1633.1 required orthogonal effects to be considered in seismic
> > design for columns at intersecting LFRS grids when Pu_seismic>.2*f*Pn.
> > The
> > 20% column capacity rule appears to be gone in the 2000 IBC.  The The
> 2000
> > IBC now appears to require orthogonal effects to be considered for
> seismic
> > design category D for all conditions.
> > 
> > For seismic design category D, IBC 1620.3.5 indicates that considering
> > each
> > direction independently per IBC 1620.1.10 is not considered acceptable.
> > 
> > Also for design category D, IBC 1620.3.5 indicates for any structure
> that
> > considering SSRS or 1*EQx+.3*EQy or 1*EQy+.3*EQx per 1620.3.5 is
> > acceptable.
> > 
> > In English: orthogonal effects need to be considered in design category
> D
> > for columns, braces and beams.
> > 
> > 
> > Scott M Haan  P.E.
> > Plan Review Engineer
> > Building Safety Division http://muni.org/building, 
> > Development Services Department,
> > Municipality of Anchorage
> > phone: 907-343-8183   fax: 907-249-7399
> > mailto:haansm(--nospam--at)ci.anchorage.ak.us
> > <
> 
>