In Section 13.2 various forces that are likely to act on the dam have been discussed.
But all these forces seldom act simultaneously on the dam.
There are some combinations of the loads that may act simultaneously.
Design of the dam should be based on the most adverse combination of “probable load condition”.
USBR has recommended two load combinations.
1. Standard load combinations.
2. Extreme load combinations.
1. Standard Load Combinations.
Under this heading three possible combinations are given.
(i) Horizontal water pressure, at normal reservoir level + ice pressure + silt pressure + normal uplift pressure. But this combination is adopted where ice pressure is considerable.
(ii) Horizontal water pressure at normal reservoir level + earthquake pressure + silt pressure + normal uplift pressure.
(iii) Horizontal water pressure at H.F.L. + silt pressure + normal uplift pressure.
2. Extreme Load Combination.
Maximum flood water elevation + silt pressure + extreme uplift with all the drains choked.
While designing the dam, whatever load combination might have been considered.
Indian Standard Recommendations of Loading Combination for Design
IS has specified A to G seven types of load combinations. Gravity dam design should be based on the most adverse load conditions and using prescribed safety factors.
Depending on the scope and details of the various project components, site conditions and construction programme.
one or more of the loading combinations A, B, C, D and E may not be applicable in reality and may require suitable modifications.
I. Load combination A (construction condition) — Dam completed but no water in reservoir and no tail water.
II. Load combination B (normal operating condition) — Full reservoir elevation (or top of gates at crest), normal dry weather tailwater, normal uplift, ice and silt, (if applicable).
III. Load combination C (Flood discharge condition) — Reservoir of maximum flood pool elevation, all gates open, tailwater at flood elevation, normal uplift and silt (if applicable).
IV. Load combination D — Combination of A and earthquake
V. Load combination E — Combination B with earthquake but no ice.
VI. Load combination F — Combination C, but with extreme uplift (drainage inoperative).
VII. Load combination G — Combination E but with extreme uplift (drainage inoperative).
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