Determination of Actual Design Moment in Ultradeep Reinforced Concrete Rafts

This paper deals with determination of actual design moment in the flexural design of ultradeep reinforced concrete rafts. Due to diagonal crack in concrete, the workpoint of the internal tension-compression couple resisting the applied moment is shifted by a distance equal to the effective depth of the member. This shift in the workpoint necessitates that the design moment at a particular section shall also be shifted accordingly. The case is highlighted with the example of a 9.0 m (29.5 ft) deep circular raft that is acted upon by upward buoyant forces as well as downward gravity loads. Design moments in various regions of the raft increased by 1.3 to 248% after application of the moment shift rule described in the paper. The substantial increase in moment values signifies the importance of the phenomena for ultradeep reinforced concrete rafts.