CROSS Safety Report
Failure of epoxy fixings due to high temperature
This report is over 2 years old
Two mobile phone towers erected on the roofs of nearby buildings collapsed when the epoxy anchors pulled out of the drilled holes.
Key Learning Outcomes
For construction professionals:
Connections can often be the weak link in structures and attention to detail is required
Manufacturers requirements and guidance for the installation and storage of fixings and resins should be followed
Consider having representatives of the manufacturer attending site to train operatives on best practices. This can help to raise industry standards.
Where fixings are key components and part of the quality assurance procedure consider carrying out site testing to ensure their strength capacity
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Two mobile phone towers were erected on the roofs of two nearby buildings using drilled epoxy anchors into the roof slabs. During a storm of moderate intensity, both towers collapsed; the failure mechanism being the pull out of the anchors from the drilled holes. The recorded wind speed was below the design wind speed for the site (a built-up area) and there were no other structural failures in the vicinity.
On examination, it was identified that the epoxy grout used had a limiting temperature of application of 35 degrees. Although the temperature at the time of construction was marginally below this, the capsules had been left out on the roof for the best part of a day in the direct sunlight on the black tarmac covered roof. The grout had therefore been subjected to conditions that were well outside its operating limits.
The corrective action identified after the failure was to monitor and control the temperature of all chemical fixings, with recognition that ambient shade temperature and exposed surface temperatures on site can be vastly different. The product labelling was not particularly clear on this, although with hindsight, the failure could easily have been avoided through better preparation of method statements and a focus on environmental effects at the time of construction.
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Thank you for this report. A comment I would offer re this mobile phone tower failure incident is that in addition to questions of what was the temperature of the epoxy grout material and the receiving concrete substrate at the time of installation, note that just about ALL installed chemical anchoring products start to lose strength once they get to about 80degC. Even if installed within the stated temperature limits, a black tarmac covered concrete roof with a steel threaded anchor element embedded into the epoxy might attract, hold and conduct high temperatures into the epoxy on a sunny day & possibly for some time after that when storm conditions gather. On that basis I would think a mechanical anchor is a better potential solution.
I was reviewing some drawings and had reservations about epoxy anchors in tension in high thermal conditions, so it was timely to comes across the fact that CROSS had noted this in the Newsletter No 24, October 2011. I will use this to help make my point, so that the detail is properly thought through.
Expert Panel Comments
Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.
The reporter succinctly summarises the situation and the required actions. The strength of epoxies is generally related to temperature. Engineers know about the fatal consequences of the Boston Big-Dig when epoxy fixing failures led to fatalities and massive civil claims. The Standing Committee on Structural Safety (SCOSS) has issued an Alert on The Selection and Installation of Construction Fixings.
The construction industry uses some sophisticated materials and compliance with manufacturer’s instruction is always essential, not least as otherwise the supplier’s liability will be invalidated. Detailed advice on fixings generally can be obtained from the Construction Fixings Association.