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Re: 1997 AISC seismic - IBC seismic design category D - OBF brac econnections - column strength

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Haan:
Orthogonal affects are not just an ugly unneeded growth, they are part of the
minimum force. I believe the code was assuming that orthogonal affects are
always insignificant. A local jurisdiction is able to prescribe more than the
code minimum. Why not ask for a demonstration of the significance of the
orthogonal affects, if they are significant, in your opinion, then do what is
needed.

David Merrick, SE

"Haan, Scott M." wrote:

> I had a private question about why I think the special load combinations
> should consider orthogonal effects.
>
> For a  multistory braced frame building the columns at the intersecting
> corners of an interior braced core can be critical.  Also brace connections
> are critical. For the situation I was specifically refering to the columns
> were designed for special load combinations with orthogonal effects.  The
> braces were designed for orthogonal effects with the regular load
> combinations.  While doing the check I saw connections were not designed for
> Ry*Fy*Ag.  I indicated the connections need to be designed for the lesser of
> Ry*Fy*Ag, special load combinations or the maximum force that can be
> delivered to the connection by the system.  The response was that most of
> the connections work for OMEGAo*Qe if you do not consider orthogonal
> effects.
>
> The direction of load used to hit a corner column should be for the worst
> case direction. The ground could move both directions at the same time. The
> OMEGAo special load combinations are supposed to approximate the maximum
> load that can be delivered to the element by the system.  It seems wrong to
> me then to not check the critical element for the maximum load that could be
> delivered to it.  If the column buckles the building doesn't have a lateral
> system anymore.
>
> The code now requires orthogonal effects to be considered for active seismic
> areas.  It seems wrong to me to design critical connections or elements
> considering loading along each perpendicular axis independently especially
> if there is a rigid diaphragm and torsion effects are significant.
>
> Scott M Haan P.E.
> Plan Review Engineer
> Building Safety Division http://www.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: Haan, Scott M. [mailto:HaanSM(--nospam--at)ci.anchorage.ak.us]
> Sent: Tuesday, May 01, 2001 2:28 PM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: RE: 1997 AISC seismic - IBC seismic design category D - OBF
> brac e connections - column strength
>
> --------------------------------------------------------------------------
> Your following message has been delivered to the list
>   seaint(--nospam--at)seaint.org at 15:41:17 on 1 May 2001.
> --------------------------------------------------------------------------
>
> Thanks David.
>
> Orthogonal effects are definitely significant for the specific case I am
> thinking about.  Braces and columns were designed considering orthogonal
> effects with special load combinations but connections were not.
>
> I agree and think the IBC intends orthogonal effects to be considered with
> special load combinations.  The problem is the IBC does not specifically
> amend the steel code which it specifically adopts.  The special load
> combinations in the AISC Seismic Provisions do not require orthogonal
> effects to be considered and this leaves room to debate.
>
> It is also interesting to note that the IBC special load combinations do not
> jive with the 1997 AISC Seismic Provision special load combinations.  The
> IBC special load combinations include a .2*Sds*I*D vertical component while
> the AISC provisions are based on an older code and do not include this
> effect.
>
> The only thing I have got a code person or standard person to say is: yes
> they don't jive, it seems like the IBC requirements should be used but it is
> not written that way. It is kind of hard to require anything with an
> interpretation like that because the local engineers are kind of smart and
> read the code.
>
> P.S. By my reading of ACI 530-99 and the IBC 2000  it is ok to construct a
> masonry shearwall with non-loadbearing glass block with type O mortar or
> plastic cement mortar in Southern California.
>
> Scott M Haan P.E.
> Plan Review Engineer
> Building Safety Division http://www.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: David B Merrick [mailto:mrkgp(--nospam--at)pacbell.net]
> Sent: Tuesday, May 01, 2001 1:48 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: 1997 AISC seismic - IBC seismic design category D - OBF
> brace connections - column strength
>
> Haan, Scott M.
>
> The new code makes more sense to me. Yes orthogonal affects should be
> considered. The issue is the insignificance of some orthogonal affects.
>
> Orthogonal affects are the direct results of the base shear. Orthogonal
> affects
> are part of the equation used to distribute forces to walls. It is that part
> of
> the equation ignored when walls are all parallel or perpendicular to each
> other.
> The issue is the significance of the orthogonal affects.
>
> The extreme and most significant affect is a triangular plan. Resulting
> loads
> can change from 0 to 30%.
> ROTATION changes.
> SHIFTING PERPENDICULAR to the global force changes.
>
> Assumptions can be made to avoid the complexity of the orthogonal set of
> equations. There are wrong assumptions that do not envelop the more
> accurate,
> detailed results. Some assumptions maybe acceptable only when the orthogonal
> affects are insignificant. These assumptions are useful to design the
> orthogonal-wall details. Consider walls any type of directional shear
> resistance.
>
> When orthogonal affects are insignificant, one needs to only consider the
> detailing of the one small orthogonal wall. A simple vector analysis, for
> that
> wall, is adequate. The perpendicular-to-force-walls must be significantly
> (maybe
> ten times higher) stiffer than the orthogonal-wall. If not, the
> parallel-to-the
> force-walls will not share the load with the orthogonal-wall, resulting in
> higher loads. A good rule is to not reduce shear by sharing loads with the
> orthogonal-wall. Share the load only to determine a shear for the
> orthogonal-wall itself. For flexible diaphragms, mostly/only, the
> perpendicular-to-force-walls that are IN LINE with the orthogonal wall will
> offer stiffness for the above considerations. I have seen cases where there
> are
> none in line.
>
> A test of theoretical limits is to consider a case of no
> perpendicular-to-force-walls. The orthogonal-wall moves in the direction of
> least resistance, tilting enough so that the vector of building movement is
> perpendicular to the face of the orthogonal-wall. The building is deflected.
> Parallel-to-force-walls are loaded. The orthogonal-wall moved but is not
> loaded.
>
> What is insignificant? I use 5% as a guide. Does anyone know of a code rule
> as
> to what is an insignificant error?
>
> Back to the question at hand. Consider the question, in reference to the
> principle axis: Can I use an arbitrary direction for the global (building)
> force
> that is not inline with the principle axis when used with special load
> combinations including OMEGAo*Qe? Well yes, if has an insignificant affect
> on
> results.
>
> It should not be allowed, to choose a non-principle axis for the global
> (building) design force. It will reduce some local wall loads. The change
> may be
> insignificant and acceptable if the angle from global force to the principle
> axis is small.
>
> To simplify the considering of the orthogonal affects: The method must
> envelope
> the two extremes.
> FIRST extreme is to not let the building shift perpendicular to the force
> (that
> is a simple vector analysis of the orthogonal wall itself),
> The SECOND extreme is to consider no perpendicular-to-force-walls, by not
> sharing the load with the orthogonal wall (if the
> perpendicular-to-force-walls
> are not significantly stiffer than the orthogonal wall).
>
> David Merrick, SE
> Mrkgp(--nospam--at)pacbell.net
>
> "Haan, Scott M." wrote:
>
> > Hello :
> >
> > 1997 AISC Seismic Provisions Section 4.1 indicates orthogonal effects are
> > not required with special load combinations including OMEGAo*Qe.
> >
> > IBC 1620.3.5 always requires orthogonal effects to be considered for
> Seismic
> > Design Category D.  IBC 1617.1.2 does not say that orthogonal effects are
> > not required when considering the special load combinations in the IBC.
> > 1997 UBC only required orthogonal effects for highly load columns at
> > intersecting lateral systems, non-parallel systems and torsional
> > irregularities and didn't say orthogonal effects did not need to be
> > considered with special load combinations for ASD steel design [it did for
> > LRFD].
> >
> > It seems the intent of the IBC is to use the orthogonal effects with the
> > special load combinations in Seismic Design Category D.  Is this an
> > oversight in the conversion to the IBC from the UBC, that the AISC
> > provisions were not amended?  Should orthogonal effects be included in
> > special load combinations for columns, and brace connections in Seismic
> > Design Category D?  For tension only braces?
> >
> > Thanks,
> >
> > Scott M Haan P.E.
> > Plan Review Engineer
> > Building Safety Division http://www.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
> >
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