building structural systems with irregular shapes (L, U, T, E, H, or cross),
setbacks, or other unusual features will be avoided. If these building layout
shapes must be used, the structural system will be divided by expansion
(seismic) joints into regular rectangular (plan view) shapes.
Structural System Type.
Ductile moment-resisting space framing systems are adequate, but
the large floor-to-floor heights and long spans necessary in some modern
health facilities may make adequate drift control difficult and expensive.
Pure shear wall box systems provide excellent seismic resistance
but are generally restrictive and inflexible from a planning point of view.
Braced frames, both concentric and eccentric, provide good
strength and drift control characteristics. They are more restrictive from a
future planning perspective than moment frames, however, since the frames can
be strategically placed to lessen the restrictions, they can be significantly
less restrictive than shear wall box systems.
Dual bracing systems, combining complete moment-resisting frame
system with shear walls, or braced frames have good drift control
characteristics. Space frames offer stiffness and tie the building together.
Individual space frame members must resist at least 25 percent of the
required lateral load. Shear walls or braced frames must resist 100 percent
of the lateral load. Resistant frames must resist forces based on their
relative stiffness and must satisfy deformation compatibility requirements.
Seismic Structural System Considerations.
All health care facility buildings must have a
complete lateral force resisting structural system that provides a continuous
and direct load path with members and connections that possess the strength
and ductility to transmit seismic forces to the foundation. This structural
system shall be capable of withstanding design earthquake ground motions
while, (1) remaining within prescribed limits of strength, (2) maintaining
deformation limits, and (3) providing adequate energy dissipation capacity.
Innovative Systems. Both base isolation and passive energy
dissipation are considered to be innovative seismic force resistant structural
systems. Innovative systems shall be considered for major health care
facilities in high seismic risk areas, where the design spectral response
acceleration at short periods (SDS) is equal to or greater than 0.50. The
specific types of base isolation systems that are considered for use in health
care facilities must have been researched, tested and proven to be acceptable,
based on sound engineering principles and experience. Base isolation
materials must be durable, i.e., have minor aging and temperature effects and
have reliable, long term performance characteristics Selection considerations
shall include a life-cycle cost comparison between a conventional, fixed base
system and the base isolation system. If a base isolation system is proposed,
it must be submitted to HQUSACE, CEMP-E for approval along with data
supporting and justifying the selection.
The designs of innovative systems are often specific to the
device, which may be a proprietary item. The entire design, including the
choice of device and the detailing of the entire structural system, shall be
completed by the building designer. The completion of the design of the
structural system shall be extended to the construction contractor by the use