Earthquake Considerations for High Performance Cantilevered Balconiesfrom Dieter Hardock
Cantilevered balconies are common in residential structures, yet their traditional design is a slab of continuous concrete passing through the building envelope. This concrete slab balcony creates one of the most significant thermal bridges with excessive heat loss.
As buildings improve with higher performance walls and windows, the amount of heat loss at the balcony slab is increased. Therefore, designers are looking for solutions to thermally separate the interior slab from the exterior balcony slab.
From a structural point of view, balconies have to resist several loadings conditions like permanent loadings (dead load), variable loadings (live loads, wind loads, snow loads) and rare loadings resulting from earthquakes. Seismic considerations could have a relevant influence on the design of buildings depending on the geographic location of the building (seismic hazard), soil characteristic, stiffness and weight of the building, the assemblies and so on.
Building codes address the dynamic behavior of earthquakes by providing clear requirements for design procedures, and procedures for dynamic analyses or equivalent static loadings caused by the movement of the building as a reaction. See the illustrations below.
The balcony, as an external structural element with a specific weight, also reacts on dynamic seismic effects which can be taken into consideration when designing the balcony and its connection.
Thermally Separated Balconies in Seismic Areas
Generally, manufactured structural thermal breaks, like Schöck Isokorb®, provide two main features. First, Isokorb® thermally separates the external structure from the interior structure. At the same time, and this is second major feature, Isokorb® ensures the structural integrity of the assembly.
The solution for the most common thermal bridge, the cantilevered balcony, is the Schöck Isokorb® Type CM. The product transfers the typical vertical loads caused by permanent and variable loading. In addition, Schöck Isokorb® Type CEQ complements the Type CM to resist loads caused by earthquake or wind conditions.
Isokorb® Type CEQ absorbs horizontal forces as well as uplift bending moments due to seismic loadings and ensuring the structural integrity of the balcony. Schöck Isokorb® Type CEQ absorbs the loads from earthquakes in interaction with the Schöck Isokorb® Type CM, see illustration below.
Further detailed information on the load bearing behavior due to earthquake can be found on pages 24-29 of the Technical Manual.
Isokorb® type CEQ follows the functionality of Isokorb® Type CM. The main body consists of expanded polystyrene of 80mm thickness, with reinforcement made of high strength stainless steel which traverses the insulation body to resist the specific forces.
The required number of Schöck Isokorb® Types CEQ has to be determined in accordance with the earthquake load analyses. The structural capacity of the product is determined in accordance with the relevant structural codes, see US technical manual pages 121-128 or and Canadian technical manual pages 93-98.
This solution has been included in recent projects such as the Westerleigh Senior residence, as you can see the Isokorb® type CEQ placement in the installation below.
This solution to thermal bridging is also current installating at the LIDO project in Vancouver Canada. Pouring on the sixth floor balconies has been completed, with 15 more stories to go.
The Isokorb® Type CEQ is a solution to thermally separate concrete balcony slabs from the building structure to safely reduced heat transmittance and energy costs in special seismic load combinations.
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.