Important Aspects of a Balcony Structure

Balconies are a popular essential for residential developments, and legislation changes following some high-profile historic failures means the ‘bar has been raised’ and the structures are now designed to resist a significant floor load of 2.5kN/m2, equivalent to over 3 persons per square metre. In addition, a lot of science goes into the design of connections between a balcony and the building to make sure they are adequate to deflect forces and maintain as stiff a connection as possible to resist movement on the balcony, while maintaining a thermally efficient and waterproof structure.

The new British Standard 8579: 2020 sets out the deflection parameters of a balcony. All aspects of balcony design are considered, including structural, acoustics and wind effects. The finished product should not deflect more than 5mm under a 2kN concentrated load.

Mitigating vibration

A resident standing on the edge of a balcony doesn’t want to feel movement underfoot.  We often use a 2kN point load test to verify our designs (rather than the difficulty of trying to measure natural frequency on site), which gives a very good indication of the what the balcony will feel like to the residents when they walk on it. Where possible, the balconies are designed so that deflection under this load should measure no more than 3mm at any point in the balcony, so significantly better than the 5mm recommended by BS8579. This, of course, becomes a challenge with a large projection balcony where there is potential for significant movement.

It is important to design balconies that also reduce horizontal vibrations. These most often occur from the prevailing wind directed at the building. Both directions of vibration need to be looked at in detail as the consequences of vibration are not just an annoyance to residents, they can cause damage to the strength of the balcony and the structure of the building.

The noise created by residents’ footfall and rain on balconies is less of an issue and are likely to be a minimal nuisance. What is critical is flanking or acoustic transmission. This is the noise created from an upper balcony, a chair scraping on the decking for example, that travels to the apartment below. Designs should include a degree of resilience to dampen the sound transmission. Good thermal insulation can also to dampen surrounding noise. The main thing is that the balcony remains lightweight to keep transmission as low as possible.

Balconies around the world

Loadings vary from country to country, in fact in Canada balconies are required to resist nearly 0.5T per square meter! Balconies in geologically unstable areas such as New Zealand and parts of Canada require particular attention.  Seismic action creates an acceleration of vibration on the balcony, both in horizontal and vertical directions. Various multipliers can occur depending on the proximity to the epicentre of the earthquake. Because Sapphire balconies are of a lightweight, rigid aluminium structure, they fare much better than concrete, where there is a greater mass that has to be resisted. Forces from concrete going back into the structure are far greater than they would be for a lightweight aluminium balcony.

No radical changes need to be made to lightweight aluminium balconies in extreme weather conditions, as they are so accommodating of extremes.