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Re: Dual System-Shearwall Boundary Element as Column for Conc Moment Frame?

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See responses below:

Paul Feather PE, SE

> Can a dual system shearwall with concrete moment frame have the shearwall
in line with the moment frame such that the end column of the moment frame
is also the boundary element of the shear wall?


>If so, how would this be distinguished from a shearwall-frame interaction
system? Is the only difference the amount of seismic load that the moment
frame can resist(i.e. 25%)? (Zone 4)

I am not sure I understand this question, the system is dependent on
shearwall - frame interaction so I am not sure what you are trying to
distinguish it from.  The classical distinction for tall buildings is that
the frame deflects in a shear mode while the shearwall deflects
predominantly by bending as a cantilever.  The interaction of the two
systems is dependent on compatibility of deformation. The shearwall frame
interaction will have a tendancy to behave similar to a propped cantilever,
limiting the drift.  Where the frame elements are integral with the wall
elements the frame analysis must account for this and the resulting frame
beam / column forces outside of the wall.

There is an excellent discussion of this exact situation in Dr. Taranaths
book "Structural Analysis and Design of Tall Buildings"

The 25% rule is simply an arbitrary minimum value the frame must be capable
of resisting, the actual value depending on geometry may be higher .  This
is a hold-over from the older code provisions that the shearwall portion be
designed for 100% of the required lateral force.  (I am not advocating the
reduction in the 25% minimum)

> SEAOC's 1999 Blue Book indicates in the commentary on Dual Systems:
C104.6.5.2 and C104.6.5.3, Item 2 "...Columns of the frame system may also
function as boundary elements of shear walls. As such, these columns must be
designed to resist the vertical forces resulting from overturning moment in
the shear wall along with the load effects associated with the frame
system." The commentary, however, does not distinguish whether the boundary
element (when permitted to be used as part of the moment frame) has the
moment frame perpendicular to the axis of the shearwall or whether it could
be in line with the shearwall.

Refer to the example above.  This is quite common to have the frame element
and the shearwall on an integral line.  As an additional note, in my
personal opinion and experience, the beauty of these systems (dual systems,
core systems, tube systems...) is that they are intended to be conceived and
viewed as truly 3 dimensional systems with an overall perspective.  The
basic principles in tall building systems like this grew out of a total
system concept where all the components are participating.  The relative
rigidities of the frame and shearwall systems require an implied degree of
redundancy (adequate frame lines) for the system to behave as desired.
Similar to a tube concept, the entire system should be frames with the
addition of the shearwalls, not just a frame tagged on to each shearwall
line and the rest simply gravity framing.  Without the implied redundancy
the basic precepts start to fall apart. (IMHO)

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