What are the basic components which affect structural design?
Structurally, the design of balconies comprises two main components: the support arms, which include the thermal break and support connections, and the Cassette® including the balustrade.
The Cassette® forms the skeleton of the balcony. This provides a stable platform to transfer the forces applied to the balcony into the arms, via adjustable connectors. It comprises a computer designed and optimised aluminium monocoque framework formed from a lattice of members folded from 3mm structural grade aluminium, strategically reinforced with 6mm bolted gussets to minimise local stresses and redistribute them evenly throughout the framework.
The balustrading, whether connected via regular bolted clamps or wedged into a structural glazing channel, forms an integral part of the structural system.
Key requirements for structural design of balconies
The primary forces balconies should be designed to resist are taken from the relevant Eurocodes and typically include all of the key forces listed below:
The deadweight of the balcony. Typically around 0.6kN/m² for the Cassette® and up to 0.8kN/m for the balustrade
- The live loads to the balustrades are generally 0.74kN/m horizontally and 0.6kN/m vertically
- Wind loads to the balcony including both pressure and suction. Typically these forces are up to 2mPa.
- Floor loads the balcony should withstand. Generally 2.5kN/m²
- Point loads, typically 2.0kN to the deck, and 1.0kN to the balustrade
The complete structure should be analysed using the Finite Element Analysis (FEA) method using relevant factors and load combinations to arrive at the ultimate expected forces in each member and connection. It is important to consider that local areas of higher stress should also be subjected to detailed scrutiny. Sapphire’s standard Cassette® sizes have already been subjected to this testing along with further UKAS test houses carrying out a spectrum of related tests.
While, as with all structures, and in particular cantilevers, it is inevitable there will be some movement experienced under load, the structure should be analysed to ensure expected deflections are within acceptable limits to users.
Consideration needs to be made not just for what achieves the requirements set out in BS EN 1090, but also for what the average user would feel comfortable. Otherwise, the end user may have a perception that their ‘balcony is bouncy’, which can be a common market problem. Sapphire recommends designing a balcony significantly beyond the basic requirements.
Deflection from live load
Deflection due to live load is more difficult to quantify as there is little definitive guidance given pertaining to the loads under which this should be considered. Eurocodes define the allowable serviceability load deflection… as 22mm. This may seem excessive, however, typical loads applied to balustrades are very small and balustrades designed to meet this are generally very acceptable to users.
The only guidance on allowable deflection of the main structure however, could be the rather generic L/180 rule for structural cantilevers (i.e. L=Length so….) which, if followed can give rise to some significant allowances, particularly on larger projection balconies. Following this generic guidance could be misleading when you consider the deflection under full design loads, as it could mean an unlikely scenario needs to be met. For instance, designing for a 6m2 balcony to have 19 persons on it, in a force 10 gale, with a balcony deck deflection of 20mm.
Perhaps a little more relevant however is the expected movement in a situation where an occupant sitting on the balcony on a calm day is joined by a couple of friends, where even a 5mm movement would be discernible and 10mm could be alarming.
Following consultation with leading engineers and specialists, Sapphire have their design target to effectively double the stiffness criteria, aiming for L/360 under a full floor load or a maximum of 5mm under a 2kN point load applied within the workable area of the balcony (200mm inside the balustrade line).
While the serial size of the main steel members (i.e. Stubs and Arms) will contribute to this allowance, by far the biggest factor in most application is the moment generated at the thermal break connector. Not only is this located where the moment is greatest, by definition, the reduction in tensile area to achieve the desired thermal resistance comes at the expense of potential lowering the strength.
The measure of the movement at this location is defined as the ‘torsional spring strength’, measured in kNm/° (the moment of magnitude required to rotate by one degree. Some manufacturers use the unit of kNm/radian, which can be compared by dividing this result by a 59.3 factor) and the higher number, the stiffer the connection.
While many manufacturers achieve figures in the range of 20-50kNm/°, Sapphire’s M30 connectors have commonly achieved stiffness of up to 200kNm/° by combining the mechanical advantage of increased lever arm and over engineered tension member size with a large compression zone formed from high strength insulators.