Author: Robert Quintal, MSc FE, Barch, UWE Bristol
Mentors:
Dr Francisco Sierra (Senior Lecturer – FET - Architecture and the Built Environment)
David Metcalfe (Director CWCT)
Karina Silverio (Senior Lecturer in Architecture and Construction Technology – FET - Architecture and the Built Environment)
For 15 years, CWCT guidance for glazed balustrades has been to use laminated safety glass capable of providing containment. This guidance has been widely followed. The introduction of the ‘ban’ on combustible materials in certain buildings prohibits the use of laminated glass as a balustrade, with the risk that designers will instead turn to monolithic toughened glass, which does not provide containment when broken.
A ban on the use of laminated glass as a balustrade at a balcony in a high-rise building was introduced in 2019 due to the interlayer material being combustible. This ban potentially increases the use of monolithic toughened (MT) glass within a balustrade in high-rise buildings which increases the risk of a fall from height in the post-breakage mode.
This potential increase of MT glass within a balustrade and the lack of containment in post-breakage due to the breakage characteristics of MT glass, has led to the questioning of its safety against human impact. Historically, there have been accidents due to MT glass breaking, people being injured or falling through due to no containment provided after the breakage. The breakage of glass in a balustrade should be safe, and post-breakage should still provide containment.
In addition, it is noted in the BS 6180 standard that when a test is required, the test assembly should be representative of the actual building construction.
This leads to the question: How is a balustrade with MT glass infill tested and representative of the conditions on the building site? Currently, there is no test standard for glazed balustrades. The only test procedure and classification of glass is according to BS EN 12600, this test is for a specific size and for glass in four-sided framed supports. Therefore, it does not represent as-built conditions. In addition, how are alternative glass fixing methods accommodated in the test?
The purpose of the dissertation was to examine the above issues and to determine if a balustrade with MT glass infill, supported with alternative fixing methods, is safe as a balustrade glass. Can it withstand an impact load when designed and installed according to the Building Regulations, British Standards, and other guidance?
The best-suited method to prove the above was to conduct an impact test, and a multi-method procedure was undertaken:
1) Quantitative data was collected from suppliers and site surveys conducted by the author on relevant balustrades, representative of what is installed in the UK. Data was collected to determine the design of the balustrade samples, the support system, pane sizes, glass thickness, and fixing methods.
2) The two most popular balustrade glass fixing methods were utilized: the clamp glass support method, and the through-fix glass support method.
Fig 1: Clamp glass support method.
Fig 2: Through-fix glass support method.
3) A test rig specified in the BS EN 12600 was used as this was the only test noted in the regulations, but due to the rig not being able to accept the balustrade described above, an additional support beam was designed, manufactured, and bolted to the BS EN 12600 rig.
Fig 3: Bespoke support beam bolted to BS EN 12600 rig
4) A double tyre impact test was conducted according to the BS EN 12600 guidance, and any unbroken panes were used in the next higher drop height. If the glass broke under impact, the test ended, information captured, and the following test sequence was started.
Fig 4: Section indicating drop heights
5) If an accidental impact occurred, the impact location from a person would be random on the glass and cannot be predicted. The BS EN 12600 impact is only to the centre of the glass (refer to fig 5). But during the impact test, more onerous impact positions were included, these being an impact adjacent to a fixing (refer to fig 6) and to the edge of the glass between fixings (refer to fig 7).
Fig 6: Impact test 3 and 4, impact adjacent to the fixings
Fig 7: Impact test 5, impact to glass edge, middle between fixings.
Breakage of MT glass occurred when impact testing according to BS EN 12600 test method, confirming that the fixing methods do indeed affect the outcome of the impact test and clamp or point-fixed glass increase the risk of breakage.
In addition, the impact position considerably affected the test results. The worst result was a low energy impact at a drop height of 190mm, resulting in the thickest tested pane of 12mm with through-fix fixings breaking on impact. This impact energy is similar to a person bumping into the glass at the location close to a fixing. The photos below were taken during one of the three impact tests resulting in glass breakage.
Fig 8: Worst-case impact resulting in glass breakage
In conclusion, the results from the impact test provide evidence to demonstrate that a balustrade with MT glass infill fixed with alternative methods could potentially be unsafe. Even though the design process was followed according to BS 6180 and BS EN 16612 and using Finite Element Modelling, the calculations showed that the balustrade samples can withstand the relevant applied loads to the centre of the pane. Therefore, this further justifies the requirement for testing an as-built system and not relying on calculations alone.
From literature reviews and the test outcome, preventative recommendations should be considered to increase awareness and mitigate future possible accidents. The recommendations can be divided into two sections included below.
-
Existing installation of a balustrade with MT glass supported with alternative fixings:
1) A risk assessment should be conducted on in-service balustrades to determine their safety.
2) Maintenance inspections should be conducted as necessary, and a method statement to be considered, including maintenance, cleaning, repair, and replacement, added into the operation and maintenance manual. This should include safe method of carrying out the works.
3) Where existing unsafe balustrade installations are located, alternative solutions should be investigated, replacing the glass or the complete balustrade with a safer option.
4) The cause of breakage should be investigated, and where possible, the broken glass to remain at the location for further investigation. Failure could be due to other causes, for example Nickel Sulphide inclusion failure.
5) Post breakage, there should be a plan of immediate action to add a temporary barrier and replace glass as soon as possible, due to the MT glass not providing containment.
-
New design and installation of a balustrade with MT glass supported with alternative fixings:
1) Further testing should be considered and design adjustments can be made for different scenarios, boundary conditions, and specification checks, utilising Computer Aided Engineering programmes.
2) To alert designers when designing and installing balustrades with MT glass, to consider containment after breakage, to preserve a life from an accidental fall from a height.
3) Legislation and impact test guidance, including a CWCT Technical Note for all types of glass installed in a balustrade should be considered, examining the process of achieving regulation compliance which should include alternative fixing methods and alternative impact locations. All performance issues, including aesthetics, building operations, construction, and safety, should be considered.