July 2006
H-18-8
6.0 COMMENTARY
Section 1.0
Section 1.2: Critical and Essential Facilities are those where most operations and
functions must be able to resume immediately, while repair is required to restore
some non-essential services due to a limited amount of seismic damage.
Section 2.0
Section 2.1: Previous editions of H-18-8 have defined, and had different seismic
design requirements for, "Essential" and "Ancillary" buildings. Other documents
such as FEMA 356 use somewhat similar definitions with the terms "Operational"
and "Immediate Occupancy" performance for both building and nonstructural
systems and elements.
Presently, the seismic design requirements of H-18-8 do not distinguish between
the Critical and Essential categories. Since VA Medical Centers may be able to
better withstand non-functioning buildings in the Essential category, opportunities
may exist for the VA to exercise flexibility in meeting the requirements of H-18-8
for these buildings, such as using a phased or incremental strengthening
approach. Additionally, it should be anticipated that the existing structure should
have limited disruption, but that the building continues to remain occupiable and
essentially operational after the earthquake.
Section 3.0
Section 3.3: The design engineer shall provide multiple lines of resistance when
selecting a lateral force-resisting configuration. Redundancy of frame lines is
intended to avoid concentration of seismic force demands in the structure and/or
foundation system. Lines of lateral force resistance shall be located at major
areas of plan irregularity such as reentrant corners, so that the aspect ratio of the
resulting diaphragms is limited to as prescribed in Table 6.
Section 3.4: In order to obtain written approval for use of an alternate structural
system, a complete cost estimate and an estimate of the probable maximum loss
must be prepared for the alternate structural system and submitted to VA.
Estimates for cost and probable maximum loss using an appropriate system that
is listed in this section must be submitted for comparison. Approval will be
granted only if the alternate system is cost effective and has the same or lower
probable maximum loss.
Section 3.5.c: Because seismic joints have a serious impact on exterior siding,
floor joints, and interior construction and utilities, they should be avoided if at all
possible.
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