Strategies for tackling vibration in free cantilever balconies

With the growing demand for cantilevered balconies, architects and structural engineers need to ensure a structurally sound construction to provide safety and comfort. Chris Willett of Schöck examines the challenges they face.

In many sectors of the market, free cantilevered steel balconies are becoming larger, more lightweight and more cost competitive. With such a demanding specification these balconies can be prone to undesirable vibration when people move about on them more heavily than usual; and as a result the vibration behaviour of a structure is taking on greater importance.

Building Regulations Part L defines the limit values for thermal bridges, but Eurocode 3 specifies the required verifications, such as for vibration, in serviceability limit state.  Because vibration is considered to be a serviceability issue and the perception of discomfort varies from one individual to another, no precise limit can be imposed that will guarantee satisfaction for everyone during the balcony’s lifetime.  Assessment of acceptable vibration is therefore not straightforward.

Striking a balance between efficiency  and safety

A sensible approach is to design structures which have a natural frequency sufficiently beyond potential excitation frequencies.   Architects and structural engineers need to consider a whole host of influencing variables in the development and structural design of steel balconies to make sure the solution is both safe and efficient.  When selecting the method of connection to the building slab, the challenge lies in choosing a component that will ensure both an effective thermal break and a structural and design solution that is safe – and all still compliant with the necessary serviceability requirements.

In the case of cantilever steel balconies, with a thermally broken connection to a concrete slab, the elements are normally exposed to both vertical and horizontal bending moments and shear forces. To meet the various criteria demanded in these circumstances, the optimum solution is a heat-insulating load-bearing element that can bear extremely heavy loads for the thermal partitioning of free cantilever balconies.

Natural Frequency Calculator

To assess how prone to vibration the chosen solution might be, a software package which calculates the frequency of the steel balconies that can be employed. One example, the Natural Frequency Calculator, uses geometric and material variables. Depending on the type and utilisation of the structure, published data indicates limit frequencies of between 4 Hz and 7.5 Hz.  Experience has shown that adopting a limit frequency of 7.5 Hz for steel balconies not only eliminates the possibility of undesirable vibration, it also enables the design of cost efficient structures.

As has been established, a number of factors can influence the natural frequency and therefore the vibration behaviour of free cantilever steel balconies. It is recommended that particular attention should be paid to the following factors when planning and designing thermally broken steel balconies:

  • • balcony geometry: the cantilever length and spacing between the connections
  • • rigidity and proper execution of the stub bracket, especially on stepped thresholds
  • • sufficient transverse rigidity of the balcony construction
  • • incorporation of the stiffness of the chosen thermal insulating element into the planning process.

Case study: Rathbone Market, Canning Town

An example of large cantilever balconies incorporated into a project is the third and final phase of the Rathbone Market scheme in east London, part of the Canning Town and Custom House regeneration programme. The scheme is delivering a new library, around 652 new homes and a new market square bordered by around 35,000 ft2 of shops, cafes, offices and community facilities.  This final phase will see 216 new homes in a mix of one, two and three-bed designs within three blocks, ranging in height from six to 14 storeys. The apartments on the upper levels are designed with large 2.3 m depth balconies, which is a demanding specification, and use of natural frequency calculation was crucial in delivering the right balcony specification.