720
Design Rationale: Structure
The second is the rigid frame system, in which the members and joints are
capable of resisting lateral forces by bending.
The shear wall system is considered the more applicable for the structures
of this study for the following reasons:
1. In general, for buildings up to approximately ten stories in height, shear
wall lateral systems are more economical than rigid frame systems.
2. The size of vertical shafts normally required in hospital buildings
suggests a concentration of lateral stiffness at these shafts, leading to a
shear wall system. Often these shafts have concrete walls for fire
protection, acoustical or maintenance reasons. In this case, the
stiffness of rigid frames is incompatible with the stiffness of the shafts.
3. The spans generally contemplated for planning flexibility are not
conducive to development of an economical rigid frame.
4. Prestressed concrete members cannot be used in rigid frames
designed to resist earthquake forces because of the unknown ductility
of the material. This would indicate shear wall systems wherever
prestressed members are used to carry vertical loads.
721.4
BUILDING HEIGHT LIMITATION
Since the structural lateral force resisting system has been defined as a
"box system", a 160-foot height limit was set to coincide with the height at
which the building code design requirements change.
In seismic zones 2 and 3, the Uniform Building Code requires buildings
over 160 feet in height to have a ductile moment-resisting space frame
capable of resisting not less than 25% of the required seismic force. This
moment-resisting frame requirement would change the structural priorities
and therefore affect the final system.
However, if for some reason it is extremely desirable to go over 160 feet
with the system, it could be done, and the following directions should be
considered.
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