Avoiding bounce in balconies

Want less bounce in your balcony?

A technical paper

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Balconies have always been a desirable element to any residential property. They provide a link to the outside environment where direct access at ground level is unavailable and the addition of an ‘outdoor room’.

The increase in urban density in towns and cities and the drive for more lettable area in a multi-storey format has seen an increase in the inclusion of balconies as part of the design. These protruding structural elements, whilst simple in concept, pose complex issues in supporting loads, whilst not compromising the ever-increasing requirements for building envelope thermal performance and avoiding thermal bridging.

Problems with conventional thermally broken balcony attachment methodology

Any construction element passing directly between regions of differing temperature has the capacity to form a thermal bridge. High density inorganic materials such as steel and concrete typically used for structures of balconies have high conductivity and so need to be thermally broken or isolated so as to improve their thermal performance. Typically, low density, low conductivity materials perform well in limiting heat transfer and so can avoid the negative effects of thermal bridging but cannot perform as a structural element.

The balance between structural and thermal integrity is pushed to the limit in balcony design and as cantilever distances grow for increased usable space, other performance criteria can suffer. One side effect of bigger, lighter balconies is the propensity for them to oscillate or ‘bounce’ when subject to live loads.

Further architectural demands, such as slimmer slabs, greater cantilevers, and no vertical supports, can have an appreciable impact on the behaviour of the oscillations within balconies. Although these oscillations have no impact on the load-bearing behaviour of the system, they are perceived as disconcerting and unsettling for occupancy.

In the case of a concrete balcony, in order to minimise thermal bridges, projecting elements are secured to the reinforced concrete floor slabs of the building with the aid of cantilever slab connection elements using mechanical thermal break solutions. As these systems possess relatively little rigidity, cantilever structural elements such as balconies are highly susceptible to this oscillation and deflection.

An alternative approach

Lightweight balconies with stiff, point connections to the building shell, as a rule, have higher resonant frequencies and are therefore less susceptible to oscillations. These systems incorporate solid state thermal isolators rather than soft insulation and are installed both within retrofit projects as well as new builds. Due to the fewer penetrations required to support the balcony, smaller heat losses occur compared with conventional attachment methods.

The higher rotational stiffness and the favourably efficient, solid state thermal insulation and connections increases design freedom as well as improving energy efficiency. Reduced thermal bridge loses allows a designer more freedom with inclusion of other architectural features, such as large glazing areas and parapets. Furthermore, since these systems are less susceptible to oscillations they offer the designer individual configuration and dimensioning possibilities for their balconies.

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