• 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).