ice pressures may generate a considerable horizontal thrust near crest
level. The pressures exerted on the dam are a complex function of ice
thickness, scale and rate of temperature rise resulting in expansion, and
the degree of restraint existing at the perimeter of the ice sheet. They may
be increased by wind drag effects.
An acceptable initial provision for ice load, Pice, where considered
necessary, is given by Pice145 kN m^2 for ice thicknesses in excess of
0.6 m (USBR, 1976). Where ice thicknesses are unlikely to exceed 0.4 m
and/or will be subject to little restraint, as on a sloping face, ice load may
be neglected.
In the infrequent circumstances where ice load is deemed critical,
expected pressures can be estimated by reference to the charts presented
in USBR (1976, 1987).
THERMAL AND DAM–FOUNDATION INTERACTION EFFECTS
Cooling of large pours of mass concrete following the exothermic hydra-
tion of cement and the subsequent variations in ambient and water tem-
peratures combine to produce complex and time-dependent temperature
gradients within a dam. Equally complex interactions develop as a result
of foundation deformation or by load transfer between adjacent blocks of
the dam. The prediction of such forms of interactive load response lies
beyond the scope of this text. Secondary loads in very large dams can be
comparable to the primary loads in order of magnitude. Their influence
upon deformation and stress distribution in such cases is significant, and is
discussed comprehensively in USBR (1976).
(c) Exceptional loads
SEISMICITY AND SEISMIC LOAD
A general introduction to seismicity and to concepts in seismic analysis is
contained in Section 2.7.3, to which reference should be made.
Concrete dams are quasi-elastic structures and are intended to
remain so at their design level of seismic acceleration. They should also be
designed to withstand an appropriate maximum earthquake, e.g. CME
(controlling maximum earthquake) or SEE (safety evaluation earthquake)
(Charleset al., 1991) without rupture. The possibility of structural reson-
ance must also be investigated for higher dams, although the risk of
serious resonance is considerably reduced in practice by damping effects.
Seismic ground motions are in any event irregular in their magnitude, peri-
odicity and direction. They are therefore unlikely to sustain resonance for
durations much exceeding a few seconds.
The natural frequency of vibration, fn, for a triangular gravity profile
of height H(m) and base thickness T(m) constructed in concrete with an
128 CONCRETE DAM ENGINEERING
