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Re: 1997 AISC seismic - IBC seismic design category D - OBF brace connections - column strength

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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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