ANCHORS AND DEFLECTION

Deflection is the movement/bounce up and down of the balcony. The amount a balcony will deflect/bounce is often a key factor when it comes to balcony design, as even 10mm deflection on a balcony can be very noticeable to the end user and can feel unsafe. The measurement of movement at the thermal break is called 'torsional spring strength' and is measured in kNm/o (the moment of magnitude required to rotate by one degrees). The industry standard is 20-50kNm/o, in contrast sapphires m30 connectors achieve up to 200kNm/o!

The BS EN guidance (BS EN 1993) for balcony deflection is Length divided by 180 (L/180). Length here refers to the projection depth of that balcony. So in real terms with a balcony which has a projection 1800mm, 1800/180 is 10mm, which is still a noticeable amount of deflection. Additionally BS 8579 now recommends balconies shouldn't deflect more than 5mm under a static 2kN concentrated load, which is in line with our promise of L/360 or max 5mm under 2kN point loads applied to the workable area of the balcony.

Eurocodes states the primary forces that balconies should be able to resist are:

Deadweight is 0.6kN/m2 for the cassette and 0.8kN/m2 for the balustrade.

Live weight to the balustrade are 0.74 kN/m2 horizontally and 0.6kN/m2 vertically.

Wind loads up to 2mPa.

Floor loads 2.5kN/m2

Point loads 2.0kN to the deck and 1.0kN to the balustrade.

How do we minimise the deflection on our balconies?

Using aluminium, making the balcony a lot lighter than a traditional steel balcony.

Using a 30mm stud in our cast in anchor, (market place average is 18mm-22mm) which reduced stretch.

Having a 2 part thermal break, with the bottom half a non compressional Resin to counteract the compression forces at that point.

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Top fix anchors

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The majority of our work (and the subject of the above) is with cast-in anchors. However top fix anchors are another option. 

Like their name suggests, top-fix brackets are for balcony connections which are installed after the floor substrate has been installed.

The main driver for this is often offsite manufacture, whether it be a timber frame construction or a pre cast plank system. Another key reason is fixing to existing slabs on old buildings.