Prestressed Concrete Final Part

Tendon Layout

Tendons can have the following type of arrangement on plan

Distributed - distributed (common in MIddle east and Australia)

Tendons in the x and y directions are distributed over the length of the slab with a spacing not exceeding 1.5m. The issue with this arrangement is the tendon will be like a basket wieve and it’s difficult to know which one goes up and which one goes down.

Banded - distributed (common in US)

This will have the same distributed tendon as part 1 but one direction will be banded (usually grouped in a width 1-1.2m). No max spacing requirements for banded tendons
Very easy to install as all the banded will be install first then the distributed

Banded - banded

Not very common

Note: the horizontal spacing between tendons doesn’t affect the structures a lot, the most important thing is the tendon profiles (measured vertically)

Economy of PT system

PT slabs will commonly have up-to 30% less thickness, and complete flat plates. The deflection and cracks are significantly reduced as we are allowed to use the full E in the calculation of deflections (both short and long term)

Note: I have noticed many engineers and consulting offices in Ethiopia ignore to check the long term deflections, this is a wrong practice, and not allowed by any code I know. Some engineers might argue saying ‘well we have done this and that without long term and the building is ok’ this is all what I have to say for that, your building is ok for the short term you can’t see the long term effect in short term.

The other significant saving related to PT is rebar, even though practically (code accepted) we can make a slab without any bottom rebar it is common to use a very light mesh (T10@500mm or even more) for bottom mesh as integrity bar. With PT one can save a significantly high rebar, up to 70% saving in rebar is very common.

Cost comparison

The cost comparison listed below are based on real projects in Ethiopia where I have done the design comparison

The prices include the design, material and installation costs of PT, rebar, concrete and formwork

PT vs Flat slab

A cost saving of 540birr/m2 can be achieved with post tensioning. For a B+G+12 building with a floor area of 800m2, a direct saving of over 6million can be achieved only from slabs. More saving can be gain from foundation size (even more if mat PT is used), from speed of construction etc

PT Vs Beam - slab

Here I should begin by saying this, based on my assessment and some other engineers. We have not found any cost saving by Beam - slab system against rc flat sla. So the saving will be the same as the flat slab, even more saving in time and formwork will be there as well. The aesthetics of the two systems is also incomparable

PT vs Ribbed

For this comparison i only took a ribbed slab design from a consulting office ( I have not made any design check), and did the quantities my self. Surprisingly enough this system consumes more rebar and only saves a little bit on the concrete, but costs more on formwork.
So based on the quantities a saving of more than 570birr/m2 was calculated. So for the same project size one can save more than 6.4million birr.

Thanks for you attention, any questions are welcome

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