Tips for Specifying Circumferential Prestress

Now-and-then I see a reservoir tender that reminds me that prestress reservoir design is foreign to many Engineers. I will not attempt here to educate on reservoir prestress design. Actually, it is not necessary to be a prestress Engineer to specify reservoir prestressing – just go the design-supply route and let others do what they know best. Here are some helpful tips for specifying circumferential prestress for design by specialist:

  • Prestress all reservoirs above 5Ml. This is for economy, practically of reinforcing and reliability/water-tightness. Reservoirs under 5Ml may also be prestressed, but it may not be the most economical solution, depending on the degree of applied prestress and other factors.
  • Specify a sliding joint at the lower and upper boundary of the wall. If the wall is radially restrained, especially at the bottom, effective circumferential compression may not be achieved. Furthermore, restraint will induce very high vertical bending from the dominant prestress loads.
  • Specify appropriate wall thickness. Dependant mainly on required prestress, 350mm to 450mm is generally suitable for reservoirs between 10Ml to 30Ml.
  • Specify appropriate buttresses. The number of buttresses required may range from 2 to 8.  4 is usually required for a 10Ml and 8 for a 50Ml reservoir. For fitment of anchorages and anchor zone rebar, the buttress projection should not be less than 350mm – preferably 400mm. The buttress length should not force tendon reverse curvature.
  • Avoid excessive horizontal casting joints. These pose a higher risk than vertical joints in prestressed reservoirs and they interfere with anchorage vertical spacing.
  • For design-supply by specialist, it is not essential to specify the prestress force profile, unless there are special project requirements. Unless stated otherwise, the specialist prestress designer should assume that full prestress is required and the design should comply with relevant codes of practice. However, vertical rebar design is especially dependent on the degree of applied circumferential prestress force, so if the vertical design is to be completed first, the prestress profile should be specified.

The specialist prestress designer should supply a set of calculations and working drawings for approval by the Engineer on Record. – Tim Dubber