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EXTERNAL ENVELOPE
back to the main concrete structure of the building.
As balconies typically pass through the building envelope, the fixing detail requires special attention to minimise thermal bridging and its various consequences
Since the reinforcement passes through the insulation, it is no longer protected against corrosion by a concrete cover; it is therefore important to use a highly corrosion-resistant material at that location – such as stainless steel. An added advantage of using stainless steel is that its thermal conductivity is about a quarter that of carbon steel, which greatly reduces heat loss through the reinforcement. A robust, Eurocode-compliant design methodology has been developed for the use of insulated balcony connector elements. In addition, manufacturers can provide suitable guidance to specifiers on the most efficient connector layouts, taking into consideration the position of post- tensioning cables.
An increasingly popular method of construction, post-tensioned concrete slabs allow thinner slabs, longer spans, reduced deflections, reduced costs, and will usually result in a more sustainable design solution. Typically, balconies on post- tensioned concrete frames are constructed of conventionally reinforced concrete. Although a viable method, designers and installers can encounter rebar congestion and clashes at the joint between the balcony and main slab.
Post-tensioned connectors The issue of rebar congestion has led to the development of innovative balcony connectors for post tensioning applications – the result of contractors looking to include balconies in the post tensioning process. Such insulated balcony connector systems allow balconies to be cast together with the main slab and then simultaneously stressed – something that was never possible with conventionally reinforced concrete balconies. Transition pieces placed between standard load bearing balcony connector elements enable post-tensioning cables to run directly through the structural thermal break at the interface between the balcony and supporting slab, enabling stressing to take place at the edge of the balcony rather than at the thermal break location. This considerably relieves reinforcement congestion and speeds up the installation process. Transition pieces are designed for use with all types and configurations of post tensioning cables currently available on the market, using
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either bonded or unbonded strands. Post tensioning performed onsite in cast-in-place applications gives designers flexibility, allowing them to create original concrete components that are thinner, longer and stronger. Architects can take full advantage of the plasticity of concrete in their designs to create truly amazing spaces.
Accelerating programmes By using a post-tensioned balcony connector, balconies and the main slab are treated as one area, which results in a number of benefits for the contractor. Firstly, formwork can be installed and subsequently removed at the same time and then be reused on the next level or phase. In addition, concrete is poured simultaneously and tensioned all at once, driving significant efficiencies on construction sites. Balcony connectors provide continuity to both the concrete reinforcement and the thermal insulation of the building envelope, protecting against the effects of cold bridging, eliminating the risk of condensation and mould growth, and helping ensure the comfort of the building’s occupants.
Specifiers need to be confident that the insulated balcony connector system complies with the UK’s latest guidance and Building Regulations, with particular reference to fire resistance. Manufacturers have addressed this issue with mineral wool insulation for optimum thermal efficiency and fire resistance. The mineral wool specification is identical to that used for standard insulated balcony connector elements, affording some systems an REI120 fire resistance rating. Post tensioning strengthens concrete by exploiting its properties when in compression and helping to overcome its inherent weakness in resisting tension. As a critical structural component, balcony connectors transfer tension and compression forces in addition to bending moments and shear forces, maintaining structural integrity and minimising heat loss at balcony locations. With growing pressure on architects and specifiers to make sure that the buildings are safe and comfortable, these high integrity structural solutions offer significant benefits while enabling users to build better, and faster.
Nicolas De Vrieze is product manager, structural thermal breaks & acoustics at Leviat
ADF JUNE 2023
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