25. April 2017

Can Isokorb® be used on balconies with post-tensioned (PT) slabs?

from Brent Chancellor

The architect’s goal is to create a beautiful building that functions well over the life of the structure and is within the developer’s budget. Our experience has been that when architects realize there is a good solution for the thermal bridging problems on the building envelope, they want to use it. However, one question that often comes up when discussing the benefits and value added by using Isokorb structural thermal breaks is: Can Isokorb® be used at balconies with post-tensioned (PT) slabs?

Modern concrete buildings in many cities in North America are often constructed using flat post-tensioned slabs. This method of construction simplifies the formwork in construction by eliminating beams and drop panels and allows for thin slabs with low floor-to-floor heights. The low floor height means a developer can get more leasable space (added floors) for the same overall height of the building, increasing the value of the site. However, this type of construction creates other challenges that have to be considered in design. One of those challenges is how to integrate structural thermal breaks on post-tensioned slabs with balconies.

Integrating a structural thermal break at the balcony on a post-tensioned slab is not difficult, but instead of the PT tendons terminating at the edge of the balcony, they must now terminate at the floor slab edge where the balcony is joined to the building. The main floor slab thickness must be a minimum of 6-3/8” thick (shallowest depth of Isokorb® available). The PT tendons must terminate at the edge of the floor slab because Isokorb® is not capable of carrying the very large compressive loads imposed by the PT tendons. If we were to strengthen the Isokorb® to carry these compressive loads, it would compromise the thermal performance. The new PT tendon termination point means that the construction sequence must follow one of two ways.


One possible construction sequence is to place the reinforcement for the main floor slab, the PT tendons, the Isorkorb®, and small pour stops at the location of the PT tendons. The main floor slab is then poured. After the slab has gained sufficient strength the pour stops at the end of the PT tendons are then removed, and the PT tendons pulled, anchored, and cut. At this point an insulation body (Z-type Isokorb®) is then placed between the structural Isokorb® modules at the location of the cut PT tendon. The balcony reinforcement is then placed and the balcony is poured.




One drawback of this construction sequence is that the balcony slab pour is one floor behind the main floor slab pour. If a contractor objects to this construction sequence, then the following sequence may be used.


In this second possible construction sequence, the reinforcement for the main floor slab, the PT tendons, the Isokorb®, and the balcony reinforcement are all placed. A pan leave out is put into the balcony slab at the point where the tendons in the main floor slab will terminate. This pan is just large enough to allow the hydraulic jack used to pull the PT tendons to fit into the balcony slab. The main floor slab and balcony slab are then poured. After the concrete has gained sufficient strength, the PT tendons are pulled, anchored, and cut. The pan in the balcony slab is then grouted.

This construction sequence allows the balcony and main floor slab to be poured at the same time, simplifying the formwork sequencing.

On a PT project, it is best for the design team to have a conversation with us early in the design process, even as early as schematic design. While we encourage this early communication on any project where structural thermal breaks will be used, it is especially important on PT projects. This early communication with the design team helps to address any issues with incorporating Isokorb® into the slab design so rework and delays are avoided in later design phases.

With this collaboration early in the design process and a few slight modifications to the construction sequence, incorporating Isokorb® structural thermal breaks into your next PT slab project is not only possible, it will help you in your efforts to create a high-performance building envelope that saves energy, increases thermal comfort, and avoids the condensation and mold growth caused by thermal bridging in the floor slab.


Visit the Schöck North America website for more information about structural thermal breaks for concrete and steel connections. Explore the full range of Isokorb® products for balcony, canopy, steel beam, exposed slab edge, parapet and rooftop connections.

Considering structural thermal breaks for an upcoming project? Have a Schöck Engineer call you to answer your specific design questions.  


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