Load Combination for Design 

Load Combination for Design 

The design of a gravity dam is based on the most adverse combination of the loads/forces acting on it, which includes only those loads having a reasonable probability of simultaneous occurrence. The combination of transient loads such as those due to maximum flood and earthquake are not considered because the probability of occurrence of each of these phenomena is quite low and hence the probability of their simultaneous occurrence is almost negligible. Thus for the design of gravity dams according to Indian Standard is specified as the following load combination: 

  1. Load combination A (construction condition or empty reservoir condition): Dam completed but no water in the reservoir and no tail water. 

  2. Load combination B (Normal operating condition): Full reservoir elevation (or top of gates at crest), normal dry weather tail water, normal uplift, ice and uplift (if applicable) 

  3. Load combination C (Flood Discharge condition): Reservoir at maximum flood pool elevation,  all gates open, tail water at flood elevation, normal uplift, and silt (if applicable)  

  4. Load combination D - Combination A, with earthquake. 

  5. Load combination E - Combination A, with earthquake but no ice 

  6. Load Combination F - Combination C, but with extreme uplift (drain inoperative) 

  7. Load Combination G - Combination E, but with extreme uplift (drain inoperative)

Reaction of the Foundation 

The foundation should provide the required reaction to the resultant force for the dam to be  stable.

Foundation reaction for reservoir full and empty loading

Requirements for stability 

A masonry of plain concrete dam must be free from tensile stress, i.e. neither P’ nor P” shall be 

negative, or 

 e ≤ B/6  (law of the middle third) 

To limit compressive stress with in the dam body use: 

P‟, P” if uplift always acts to the fullest extent. 

P‟v , P”v  if uplift does not act always. 

Horizontal forces must be resisted both by shear and friction in the dam joint or in the foundation.