associated with the nominated load combinations diminishes as their
severity is progressively raised.
Three nominated load combinations are sufficient for almost all cir-
cumstances. In ascending order of severity they may be designated as
normal(sometimesusual),unusualandextremeload combinations, here
denoted as NLC, ULC and ELC respectively, or by similar terms (USBR,
1976, 1987; Kennard, Owens and Reader, 1996). A tabular summary of
nominated load combinations derived from representative UK (and US)
practice is presented in Table 3.2.
In studying Table 3.2 it will be observed that a necessary element of
flexibility is ensured by the provision of note 3(b). The nominated load
combinations as defined in the table are not universally applicable. An
132 CONCRETE DAM ENGINEERING
Table 3.2 Nominated load combinations (after Kennard, Owens and
Reader, 1996)
Load source Qualificationa Load combination
Normal
(or Usual) Unusual Extreme
NLC ULC ELC
Primary
Water At DFL ✓
At NML ✓✓
Tailwater At TWL ✓✓
Minimum ✓
Self-weight – ✓✓✓
Uplift Drains functioning ✓✓
Drains inoperative ✓* ✓* ✓*
Secondary (if applicable)
Silt – ✓✓✓
Ice Discretionary ✓✓b ✓
Exceptional
Seismic SEE (discretionary) ✓
a DFLdesign flood level; NMLnormal maximum level, i.e. maximum retention level of
spillweir (or gates, if fitted); TWLmaximum tailwater level; SEEsafety evaluation
earthquake (see Section 2.7.3).
b ice load should normally include both thermal expansion and wind drag but for the unusual
case with a reservoir at flood level, only wind drag need be considered.
* the possibility of blocked uplift relief drains and the degree of blockage is a matter of
judgement as to whether it is ‘usual’, ‘unusual’ or ‘extreme’ in likelihood. If deemed ‘usual’
or ‘unusual’, in that it could happen more frequently than ‘extreme’ events, the effect
should be studied but an appropriately lower shear friction safety factor (in the range
1.25–2.00) is often accepted.
Note:US practice is in broad accord with the above, with controlling maximum earthquake
(CME) substituted for SEE and the additional consideration of concrete temperature
under secondary loads (USBR, 1976, 1987).
