For some reason, balustrade failures – especially glazed ones – have been prominent over the past year or so.
• there seems to be plenty of room for a difference of opinion about the type of glass, fixings, rails and supports.
• the long-term durability of elements has been an issue in respect of paint finishes and metal treatment;
• support fixings into the structure have been the source of leaks;
• cost-issues have driven design solutions which are incompatible with the potential risk;
• minimum balustrade heights have not accounted for the deck falls and finishes;
• inconsistencies between a “generic” design, or the designer’s details and those of a specialist trade design/install contractor.
• poor workmanship or inadequate quality control has led to sudden or potential failures
• spontaneous failure of toughened glass as a result of: poor edge finishing or hole formation, damage in delivery or installation, uneven shading, localised fixing or shape issues, imperfections and inclusions in the glass at the time of manufacture, and unknown or a combination of all of these, perhaps beyond the control of the designer or contractor.
At the heart of the matter is the functional purpose of a barrier: irrespective of compliance minima, if the human consequences of barrier failure are significant, then that is the first priority for design, installation, and site observation.
Guides to the design of balustrades and barriers:
• The Building Code (primarily sections B1, B2, F2 and F4) and relevant determinations
• New Zealand Standards (e.g. NZS 4223 Glazing in Buildings Part 3)
• MBIE Guidance Documents
• Specific Design (usually by the sub-contractor)
• Rules for barriers around swimming pools.
The Building Code:
Section B1 (Structure), clause 22.4.3 amends what is set out in NZS 4223, and so it’s worth familiarising yourself with this if you are designing structural glass barriers. Also watch out for any change of use that might impact on the loads that your structural engineer uses. The MBIE Guidance on Barrier Design is a useful document to accompany B1, and Figure 3.1 in the Guidance document sets out a useful design flowchart.
Section B2 (Durability) accepts a 15-year durability for a barrier infill but there is a 50-year requirement for the handrail and its supporting structure. We can sometimes think of barriers as part of a cladding system but this makes it clear that the structural aspects need to match the life of the building.
Section F2 (Hazardous Materials) references NZS 4223: Part 3. A word of warning here is that if you apply some decorative film – say for graphics or privacy purposes – to toughened glass, you may be inadvertently turning the glass into something more hazardous than if it were able to break into many small pieces.
Section F4 Safety from Falling is obviously the most relevant part of the code and it’s worth re-reading this to make sure you don’t miss something. A few aspects to note include the following.
Table 1 sets out minimum barrier heights, however it would be a good idea to dimension your documents with extra height to take care of falls, finishes, construction tolerances, unexpected build-ups etc. There is no obvious provision in the code for inspectors to allow for “tolerances” when they check measure on site. Preferably dimension the height as a minimum in relation to the finished floor/deck level. (It’s not unknown for owners to change deck finishes without informing the designer!)
A commentary in F4 notes that there are more relaxed requirements where a building won’t be frequented by children under 6. But you cannot control who uses the building and how: we recommend caution before relying on this exemption, and drawing the issue – and inherent responsibility – to the client’s attention.
Figure 3 shows that any projecting lip of greater than 15mm in a barrier design is considered a foothold, and therefore the barrier height needs to be measured from this projection. It’s easy with a parapet flashing build up to get caught out by this rule and have to measure the barrier height from an upstand rather than a terrace level.
Figure 4 shows the requirements for stair barriers. Don’t be caught out by the 150mm diameter rule – it’s tested in 3 dimensions by a 150mm diameter sphere: if the handrail is set out from the stairs it may need a smaller opening than a 150mm elevational diameter.
Figure 5 is a reminder of the importance of preventing a barrier from being able to be used as a seat. This requirement does not apply to housing but in all other cases something like a top edge wider than 100mm could be considered a seat and so would not be code compliant, as well as being hazardous.
There are a number of requirements in situations where there is a toilet pan or any other fixed feature (e.g. seating) within 500mm of a window. It would be easy to forget about including a restrictor or barrier in these situations.
Landscaped areas may require a barrier. MBIE Determination 99/012 notes that “Barriers are required above retaining walls exceeding 1 metre in height, where people, particularly those unfamiliar with the area, would frequently be expected to be close to the top of the wall in the course of their normal activities.”
Some detailing issues:
• Try to avoid barriers going across backgrounds with different light reflectance that may create temperature differentials that cause stresses in the glass, leading to breakages. (East elevations can produce rapid temperature rises).
• Detailing and dimensions should allow for generous construction tolerances. Glass fixings where there is little to no gap are more prone to breakage.
• Conservatively detail the base plates of stanchions bearing in mind the potential risks from corrosion and surface water: consider the B2 50-year durability requirement.
• Make your compliance pathway clear in your building consent documentation, so that an inspector on site interprets things the same as the consent processing officer.