How are concrete balconies constructed?
Concrete balconies are usually constructed one of two ways: precast or site-poured.
Precast balconies are cast offsite and brought to the site where they are connected to the main superstructure. This is usually achieved by either leaving a pocket for the connecting ribs or having the balconies arrive prior to that level of slab being poured.
Alternatively, non-precast balconies are poured at the same time as the main superstructure directly on site.
How are concrete balconies connected?
When connecting balcony slabs to the main superstructure, the connectors need to withstand shear forces and bending moments, depending on the actual balcony design. One such system is AVI Thermokorb, consisting of a structural truss made of independent ribs and of an 80mm thick expanded polystyrene panel (EPS W30 according to EN13163). The system consists of several variations of individual ribs running through the EPS panel. All individual ribs are designed in a way that they can transfer both positive and negative bending moments and shear forces. This enables a proper and bespoke design for any balcony application.
Often, balcony connectors will also be referred to as thermal breaks. This is because the structural connection between balconies and the main superstructure serves two purposes:
- Structurally connect the two elements.
- Reduce heat loss and the risk of condensation.
The thermal break aspect of the balcony connector is required to reduce heat losses that arise from joint-induced and geometric thermal bridges. Without a thermal break, uninsulated joint areas could lead to significant reductions in the surface temperature of a building's component and increase the risk of condensation and mould formation. This is achieved through the Thermokorb keeping the heat inside the main structure and pushing the dew point out.
Fire safety considerations for balcony connectors
Given the structural importance of the balcony connectors, it is crucial that these components are designed appropriately and still provide adequate support in the event of a fire. As such, many balcony connector systems are fire-tested. For example, Thermokorb is rated REI120 in the standard range, where a 30mm fireboard is provided as standard. If required, this can be removed and the system is then rated at R60.
What does R60 and REI120 mean?
The letters at the start refer to the fire-resistance rating of a structure and the different elements:
R – Load bearing. The ability of an element to preserve its structural properties and the relevant load capacity during a normal fire.
E – Integrity, considered as the structure's ability to not allow the passage or production of gas or vapor to the area not exposed to the fire
I – Thermal insulation. The ability of a structure to limit the transfer of heat to the unexposed side. This usually is within a temperature limit which is often 140℃.
The number that follows relates to the associated time that the resistance applies and therefore what the system is rated at.
So, what this means is that R60 fire rating, the system will preserve the same structural properties for a minimum of 60 minutes. For REI120 the system will preserve the same structural properties and load capacity, maintain its integrity by not allowing the passage or production of gas or vapor away from the fire, and also maintain the same thermal insulation for a minimum of 2 hours.
Designing concrete balcony connectors
Having worked on many balcony projects, such as Lombard Wharf, or Battersea Powerhouse Phase 3a in London, CFS has developed a deep technical knowledge of balcony structural design. As part of our service, we offer to design balconies based on your drawings. We are also able to assemble the balconies connectors on our site at Medmenham, meaning our lead times are very short, just 8 weeks. Combining the design and manufacturing capabilities means we can minimise any risks linked to the product delivery and incorporate last-minute changes if needed.
