Question

Which of the following criteria has to be satisfied for no tension at any point on a gravity dam?

(a) The resultant of all the forces must always pass through the mid-point of the base of the dam

(b) The resultant force for all conditions of loading must pass through the middle third of the base

(c) The resultant of all the forces must pass through the upstream extremity of the middle third of the base

(d) The resultant of all the forces must pass through the downstream extremity of the middle third of the base

I have been asked this question in an interview.

Question is taken from Gravity Dams in portion Gravity Dam Design and Construction of Irrigation Engineering

Answer 1

The correct choice is (b) The resultant force for all conditions of loading must pass through the middle third of the base

The best explanation: The minimum vertical stress Pmin is equal to zero in order to ensure that no tension is developed anywhere. If Pmin = 0, e = B/6 i.e. the maximum value of eccentricity that can be permitted on either side of the center is equal to B/6. This concludes the fact that the resultant of all forces must lie within the middle third of the joint width.