LOADING: CONCEPTS AND CRITERIA 131
and acts at elevation 0.40z 1 above X–X. As an initial coarse approxima-
tion, hydrodynamic load Pewhis sometimes equated to a 50% increase in
the inertia load, Pemh.
The resultant vertical hydrodynamic load, Pewv, effective above an
upstream face batter or flare may be accounted for by application of the
appropriate seismic coefficient to vertical water load, Pwv. It is considered
to act through the centroid of area A 1 thus:
Pewv vPwv. (3.17)Uplift load is normally assumed to be unaltered by seismic shock in view
of the latter’s transient and oscillatory nature.
3.1.2 Load combinationsA concrete dam should be designed with regard to the most rigorous
adverse groupings or combinations of loads which have a reasonable
probability of simultaneous occurrence. Combinations which include tran-
sitory loads of remote probability, and therefore have a negligible likeli-
hood of occurrence in service, are not considered a valid basis for design.
Such combinations may be investigated when verifying the design of the
most important dams, but are generally discounted in the analysis of lesser
structures.
The loads discussed in the preceding section have differing but indi-
vidually distinctive operating envelopes in terms of probability of occur-
rence, intensity and duration. Individual load maxima which can
reasonably be anticipated to act in concert under service conditions can be
grouped into a structured sequence of defined load combinations for
design purposes. Within such a sequence the probability of occurrence
Table 3.1 Seismic pressure factors, Ce
Ratio z 1 /zmax Pressure factor, Ce
u0° u15°0.2 0.35 0.29
0.4 0.53 0.45
0.6 0.64 0.55
0.8 0.71 0.61
1.0 0.73 0.63
uis the angle of the upstream slope to the vertical.