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Design Rationale: Structure
721.4.1
Seismic Zone 1 (least susceptible to earthquake)
Buildings in Seismic Zone 1 fall into a code exception to the moment-
resisting frame requirement and the system could be used intact to any
height, limited only by the available shear walls' capacity to withstand local
wind pressures.
721.4.2
Seismic Zone 2 (moderately susceptible to earthquake)
A ductile moment-resisting space frame is required to resist 25% of
seismic forces in buildings over 160 feet. The moment-resisting frame
could probably be incorporated into the concentrated shear element
locations due to the small seismic force factor required. Thus each shear
element would be designed for 100% of the required seismic or wind force
working as a wall or braced frame, and 25% of the required seismic force
working as a ductile moment-resisting space frame.
721.4.3
Seismic Zone 3 (most susceptible to earthquake)
A ductile moment-resisting space frame is required to resist 25% of
seismic forces in buildings over 160 feet. Due to the large seismic forces
generated, the entire building frame (i.e. all columns) would probably be
required to satisfy the 25% moment-resisting frame requirement. This
would cause no difficulty in the direction parallel to the girders, as the
already required vertical load-carrying girder-column space frame would
merely have to be designed for the additional lateral loads. However, in
the direction parallel to the beams, difficulty would be encountered due to
the long span, shallow beams, and lack of a rigid beam coincident with the
columns. This could probably be overcome by shifting the beams to align
with the columns and making the aligned beam heavier than the typical
beam.
These deviations from the basic design will affect typical interface
conditions with other subsystems, and will necessitate reconsideration of
certain precoordinated details.
It should be pointed out that all members being used as part of the
moment-resisting space frame must be ductile, either steel or ductile
concrete, as defined by UBC.
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