Post-tensioning Does Not Mean 'No More Penetrations'

A common question we are asked by our clients is what happens if a penetration needs to be formed through a PT slab after the building is constructed.  

Post tensioned floor structures can be as flexible as any form of construction in accommodating the changing requirements of building tenants. There are a number of options available to create penetrations depending on the proposed size.

It is generally possible to create small penetrations up to 500mm in a slab without the need to cut the post tensioned tendons or add strengthening. Tendons, reinforcement bars and services can be easily located and marked on the surface of the slab using radar or x-ray. Penetrations can then be positioned and cored to avoid any adverse consequences. This method is common, inexpensive and suitable for most new core holes for power, communications, drainage and water.

Larger penetrations for stairs, lifts, travelators and service risers are likely to be larger than the spacing of the post tensioned tendons and will require tendons to be cut. MPN Consulting has structural details and methodologies for the cutting of post tensioning tendons. An engineering assessment is required to determine if any strengthening of the existing slab is required to support the remaining structure and new loads.

If the zones for future penetrations are known during the design phase it is also possible to design the floor structure to allow for the future penetrations without the need to cut tendons or strengthen the floor structure.

And what happens if a tendon is cut? If it is fully grouted then in the short term, not much. Longer term the strength of the floor may have been compromised and an engineering review will determine what remediation, if any, is required.

Post tensioned floors give building owners a structure that is cost effective to construct and has flexibility to accommodate changing usage over the building’s life cycle.

Author: Robert Burns – Senior Associate