Same as the effect on RC members plus about 10% but no one thinks about it for RC members .
The normal procedure would be to provide a pour strip in the slab between the 2 codes allowing the compression from the PT to get into the concrete. Then closely spaced reinfrocement (same as RC, about 200 centers) should be provided to give crack control for shrinkage etc.
rapt - I didn't get your first sentence about RC. But I agree that there must be some space around the lift shaft to ensure that during post tensioning of the slab there will be some movement and therefore the slab is correctly post tensioned. I would say in practice this space would be expected as there will need to be anchorages (either dead or stressing end)
RC members between stiff elements experience nearly as much shortening induced stress as PT members from shrinkage and temperature etc. Everyone ignores it in their calculations and detailing.
Regarding "space" around the shaft, not normally. The tendons would normally be anchored into the shaft. Normally a pour strip will be placed somehwere between the 2 shafts at a logical low stress location such as near a point of contraflexure in a span (assuming there are multiple spans between the shafts). This allows the slab around each shaft to shorten towards the shaft it is connected to. The actual arrangement would be specific to each project.
