PT design is sometimes foreign to the Structural Engineer-generalist. There are two options available to the Engineer of Record (EOR):

1) Full PT design

2) Design-supply by PT Specialist Contractor.

Option 1: The EOR will need to perform the full PT analysis and design including specification of strand grade and size, tendon size and ring levels. This will include prestress loss and elongation calculations.

Option 2: The EOR will need to specify general and special design parameters and criteria for PT design-supply by specialist PT Contractor.

For both options the PT Contractor is responsible for design of the local anchorage zone and the EOR is responsible for the general anchorage zone design.

Option 1 tends to lock the design to a particular PT system and limits competitive commercial bidding.

Option 2 is proven to be the most economic route.

Here are some helpful tips for specifying circumferential PT for design-supply 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 high vertical bending from the dominant prestress loads.
  • Specify appropriate wall thickness. Dependent 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 specified 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 maximum 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