Usual. Usual loads refer to conditions which are
related to the primary function of a structure and can be reasonably expected
to occur during the economic service life. The loading effects may be of either
a long term, constant or an intermittent, repetitive nature.
Pile allowable loads and stresses should include a conservative
safety factor for such conditions. The pile foundation layout should be
designed to be most efficient for these loads.
Unusual. Unusual loads refer to construction,
operation or maintenance conditions which are of relatively short duration or
infrequent occurrence. Risks associated with injuries or property losses can be
reliably controlled by specifying the sequence or duration of activities,
and/or by monitoring performance.
Only minor cosmetic damage to the structure may occur during
these conditions. Lower factors of safety may be used for such loadings, or
overstress factors may be applied to the allowables for these loads. A less
efficient pile layout is acceptable for these conditions.
Extreme. Extreme loads refer to events which are highly
improbable and can be regarded as emergency conditions. Such events may be
associated with major accidents involving impacts or explosions and natural
disasters due to earthquakes or hurricanes which have a frequency of occurrence
that greatly exceeds the economic service life of the structure.
Extreme loadings may also result from a combination of
unusual loading effects. The basic design concept for normal loading conditions
should be efficiently adapted to accommodate extreme loading effects without
experiencing a catastrophic failure.
Extreme loadings may cause significant structural damage
which partially impairs the operational functions and requires major
rehabilitation or replacement of the structure. The behavior of pile
foundations during extreme seismic events is a phenomenon which is not fully
understood at present.
The existing general approach is to investigate the effects
of earthquake loading at sites in seismic Zones 1 or 2 by applying psuedostatic
forces to the structure and using appropriate subgrade parameters.
In Zones 3 or 4 a dynamic analysis of the pile group is
appropriate. Selection of minimum safety factors for extreme seismic events
must be consistent with the seismologic technique used to estimate the
earthquake magnitude. Designing for pile ductility in high risk seismic regions
is very important because it is very difficult to assess pile damage after
earthquakes and the potential repair costs are very large.
