CROSS Safety Report
High rise balustrade design
This report is over 2 years old
Wind loads used in design of balustrades on high-rise buildings (particularly those with glass infill panels) may need to be increased around the edges of the building.
The risk of spontaneous explosive failure of glass panels as a result of nickel sulphide inclusions can be significantly reduced by heat-soaking of glass during manufacture.
Key Learning Outcomes
For structural and civil engineers:
Wind loading at the edges of high-rise towers need careful consideration and may produce the most onerous loading in the design of balcony balustrades, especially those with solid in-fill panels (e.g., glass panels)
Be aware of the possibility of nickel sulphide inclusions within toughened (tempered) glass and insist on heat-soaking or consider an appropriate form of laminated glass
For construction personnel:
Recognise the safety advantages of having a single party to co-ordinate designers, suppliers and contractors when using glass (or other solid infill) balustrades in balconies within high-rise buildings
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It has come to this reporter's attention, over a number of recent cases, that designers of balustrades on high rise buildings (i.e., not Class 1 or 10 buildings under the NCC) may not be considering the more onerous case of wind load on the balustrade, particularly when the balustrade infill panels are solid (such as glass panels).
High pressure from wind loads at height, on edges of high-rise buildings
The reporter notes that the Live Load requirement for balustrades can be easily obtained from the relevant standard (i.e. AS1170.1) but for taller buildings the designer must also calculate the appropriate wind load and apply the appropriate net pressure coefficients (as can be found in the AWES Wind loading handbook (AWES-HB-001-2012) to the balustrade elements. In the reporter's experience, wind loads of 4 to 5 kPa (or more) on balustrades can easily be achieved on high rise buildings, particularly where balconies are placed at the edges of towers.
This is relevant to both new buildings and refurbished buildings where grill type balustrades are being replaced with glass infill balustrades.
Nickel sulphide inclusions and spontaneous explosive failures
In addition, the reporter would caution designers of balustrades that are to be placed at the edges of balconies, to not use monolithic toughened glass in these locations. This is due to the well-recognized risk of spontaneous failure of toughened glass due to nickel sulphide inclusions and the likelihood that the failed glass will vacate the balustrade and fall to levels below. The reporter also notes that this was the topic of a recent SCOSS Alert on Structural Safety of Glass in Balustrades (December 2019) but which had a UK focus.
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Expert Panel Comments
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This is certainly a matter of serious concern and we note that the Queensland Building and Construction Commission (QBCC) conducted an investigation in 2018 of glass-balustraded balconies at 18 multi-storey residential buildings in southeast Queensland. It was discovered that the glass used on many was not properly certified; and that certification paperwork was signed off by people with the wrong qualifications. The design of all such balustrades should be carried out by a qualified structural engineer.
Dynamic effects and very high wind forces
As the reporter highlights the very high local wind forces that can occur around the exterior of high-rise buildings may be the more onerous design case. Additionally, the dynamic effects from fluctuating forces around the corners of buildings must be considered.
Type of glass, supports, and fixings
The design and installation of glass balustrades must consider the total assembly, including glass type, method of supporting the glass and the fixing of supports into the base structure. Each of these areas must be addressed and the SCOSS Alert of December 2019, Structural Safety of Glass in Balustrades, noted by the reporter contains much useful guidance. The fasteners into the base structure should comply with AS5216:2018 - Design of post-installed and cast-in fastenings in concrete.
Failures arising from nickel sulphide inclusions in toughened glass have been well documented, e.g. Managing Health & Safety Risks (No. 53): Nickel sulphide failure in toughened glass, published in The Structural Engineer, August 2016; and, there has been a requirement in the Building Code of Australia (BCA) since 2011 for balustrade glass more than 5m above the ground to be heat soaked in accordance with AS1288 Amendment 2.
Single party needed to co-ordinate designers, suppliers and contractors
It is often the case that responsibility for the safety of balustrades is split between designers, suppliers, and contractors so there should be single party responsible for co-ordinating the structural aspects and all of those parties should share information, co-operate and co-ordinate activities to ensure the safety of those balustrades.