Ask the Expert: Does MMC Mean Bouncy Balconies?

Managing Director Tristan Parsons discusses designing and manufacturing balconies using MMC processes. He tackles common challenges such as bouncy balconies and provides recommendations for achieving the best structural performance and programme efficiency.

1. What is the biggest challenge with putting balconies on buildings using MMC?

The biggest challenge is transferring the structural loads into the building. On a traditional RC frame building the large cantilever forces from the balcony are taken by the RC floor slab. On other frames, consideration must be taken at the design stage to ensure the load is properly managed.

2. Which balcony materials should I consider?

Due to its weight concrete is a non-starter. Aluminium is much lighter than steel so reduces the forces going back into the structure, making it far better suited to MMC processes.

3. Should balconies be installed in the factory or on site?

We recommend installing balconies to the building on site. This avoids double handling, allows larger building modules to be transported, and reduces the lorry movements as balconies can usually be stacked on a trailer.

4. Should balconies be installed to the modular unit at ground level or installed afterwards?

Either is possible and will depend on the requirements of a particular project. Our standard recommendation would be to build the modules first, then install a large batch of balconies in a short space of time. This achieves the most efficiency and ensures there is no risk of interruption in the installation of the façade elements.

5. Is there a risk of bouncy balconies and how is this best avoided on MMC schemes?

A. Early design engagement – The risk of bouncy balconies is higher than with RC frames. So, it’s important to engage early on – Sapphire often get involved in early stage design work for MMC to ensure that the necessary structure is designed in to take the balcony loads. You may also want to consider doing a full-size mock-up.

B. Ensure structural brief includes deflection study – It is extremely important to ensure that the structural engineer designs the supporting elements not only to resist the loads (which is relatively easy) but also carefully assesses the deflection that will result from movement within the structure (much harder to assess but very important).

C. Balcony projection – especially with MMC, it works better to keep the balcony projection down where possible (e.g. a 6×1.5m balcony is better than a 4x2m balcony)

D. Recess the balconies – this depends on the architecture, but where it is possible to recess balconies into the façade, it allows a balcony to be supported on 3 sides rather than cantilevering from one side

E. Tie wires – this has a visual impact that needs considering, but diagonal wires/rods do substantially reduce the loads and increase rigidity

6. What are 3 key tips to have in mind?

Use aluminium balconies – the reduction in weight minimises the challenges faced on MMC schemes
Engage with a balcony specialist early in the design process to ensure all elements of the design facilitate the best outcomes.
Keep balcony projections down to limit the forces transferred back to the building and reduce balcony deflection.