Natural frequency and vibration behaviour of free cantilever steel balconies


Lightweight support structures, such as free-cantilevered steel balconies, can be prone to vibration when people move about on them more heavily than usual.    As designs become ever more lightweight and competitive in cost terms, particularly with very large cantilevers, the vibration behaviour of a structure takes on even more importance. 

On this scheme, the balconies are an unusually demanding 2.3m in depth
On this scheme, the balconies are an unusually demanding 2.3m in depth

So when considering the development and structural design of steel balconies, there are a whole host of influencing variables that should be taken into account.  In selecting the method of connection to the building slab, the challenge lies not only in choosing a component that ensures effectiveness as a thermal break; but one that offers a safe structural design solution as well.  An additional factor is that it also needs to be compliant with the necessary serviceability requirements.

Acceptability of vibrations

The Building Regulations Part L defines the limit values for thermal bridges; and Eurocode 3 specifies the required verifications, such as for vibration, in serviceability limit state. Such verification is the responsibility of the structural engineer, whose function it is to calculate the natural frequency of the construction depending on its utilisation.    That being said, when considering the design of the balcony construction and thermal insulating element, it is prudent to ensure that the natural frequency is greater than the limit frequency specified by the structural engineer.     Generally, vibration of floors is considered to be a serviceability issue, primarily related to discomfort.   As the perception of discomfort varies from one individual to another, no precise limit can be imposed that will guarantee satisfaction for everyone during the lifetime of the balcony.    Assessment of acceptable vibration is therefore not straightforward. However, a logical approach is to design structures so that their natural frequency is sufficiently beyond potential excitation frequencies.

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. 

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The Schöck Isokorb performs above recommended limit frequencies

If planned properly, the Isokorb type KS for concrete-to-steel thermal connectivity allows almost any balcony geometry to be designed without constraints.     To assess how prone to vibration separated balconies using the Isokorb type KS might be, Schöck has employed the latest Natural Frequency Calculator software package.  This is a free service which uses geometric and material variables and enables project teams to identify the optimum solution for steel cantilever balcony connections early in the design process.     In most cases it will be found that the natural frequencies of normal balcony constructions are still above the recommended limit frequencies when utilising the Isokorb type KS.  

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The Schöck Isokorb type KS incorporates 80mm of insulation and is 180 mm wide and between 180 and 280mm high, to allow flexible adjustment for differing slab thicknesses.     It can be prefabricated, reducing assembly time on site and can bear extremely heavy loads.    This combination of features makes the unit ideal in meeting the various thermal and structural demands involved in the design of modern balconies. Which in the case of cantilever steel balconies, with a thermally broken connection to a concrete slab, normally sees the elements exposed to both vertical and horizontal bending moments and shear forces.   The KS has a shear-bending interaction which, based on project specific loadings, allows for a much more flexible design and results in greater tolerance when designing steel balconies. 

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Recommendations for planning and designing steel balconies

The following factors influence the natural frequency and therefore the vibration behaviour of free cantilever steel balconies. Particular attention should be paid to these factors when planning and designing thermally broken steel balconies:

·     Balcony geometry, especially 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

Demandingly large balconies at Rathbone Market

A good example of unusually large cantilever steel balconies being incorporated is on the Rathbone Market scheme, part of the Canning Town and Custom House regeneration programme. A three-phase development delivering a new library, around 652 new homes and a new market square at the heart of the scheme.  Here the balconies are an unusually demanding 2.3 metres in depth.

www.schoeck.co.uk

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