CROSS Safety Report
Telecommunications towers and resin anchors
This report is over 2 years old
A correspondent shares their concerns following the collapse of a telecommunications tower.
Key Learning Outcomes
For the construction team and design engineers:
Connections can often be the weak link in structures and attention to detail is required
Manufacturer’s requirements and guidance for the installation and storage of fixings should be followed
Consider having representatives of the manufacturer attending site to train operatives on best practices. They can help 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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A telecommunications tower, held down by resin anchors which apparently passed pull-out tests of 125kN (although it is not known how these were carried out), failed six years later with little resistance in light winds, says a correspondent.
The collapsed tower was a replacement, and the original tower was half the height of the replacement. The engineers of the original tower had gone to great lengths to ensure that there was no chance of a failure at the steel to concrete interface at the base.
Although the prime cause of the collapse was poor workmanship, the correspondent have several concerns:
There is a growing tendency for towers and elements to be held down or secured by resin anchors and risks may not have been adequately considered
Stability should, in the opinion of the correspondent, never rely on just a chemical bond without assessment of the risks
No data appears to be available on pull out tests carried out after a few years
The companies who provide the anchors may not have been informed of failures
Resin anchors could be used in high-risk environments such as for towers positioned beside railway tracks
Pull-out tests at the time of construction might not be relevant after a few years, particularly if water can penetrate and freeze
There will be a need for continued and regular testing and replacement of anchors that rely on non-mechanical bonds
Some of these anchors do not allow for testing and replacement of critical bolts at the superstructure/substructure interface because of levelling nuts below the base plate
The correspondent concludes by saying that they has reservations about the long-term behaviour of resins for exterior use.
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It is my understanding that most, if not all, Commercially available Off The Shelf (COTS) resin anchor systems rely largely upon a mechanical bond after being set. The liquid resin will flow in to and out of the internal surface depressions of the hole, albeit at a microscopic level. If installed correctly, the resin will then act in shear and continue to perform until a traditional cone failure occurs in the base material. The rougher the internal surface of the hole the better. For this reason diamond cored holes undertaken by rigid track mounted coring machines are likely to be a higher risk due to the smooth internal surface that is created. Resin anchors do not rely upon chemical bond alone. I do concur with the majority of this article.
Failure of resin bonded anchors reminded me of an experience in the 60s with MIG welding wire drive feed rolls where repeated and unexplained failure of the epoxy joint between the Bakelite centre and the steel outer occurred. The loading of the joint was shear combined with fluctuating compression with every rotation of the roll. Fatigue was not identified at the time but it now seems it was the most likely cause. I believe it is clear work need to be done on fatigue strength of resin bonded joints.
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Designers of safety-critical elements have to avoid the possibility of failure wherever reasonably practicable. This is particularly so in cases such as this when the long term behaviour may be affected by a harsh environment and the quality of workmanship which is critical in post-drilled fixings. Anchor failures of various kinds have been reported including those in the spate of ceiling collapses sent to CROSS and in some serious collapses of tunnel linings.
This is particularly so in cases such as this when the long term behaviour may be affected by a harsh environment and the quality of workmanship which is critical in post-drilled fixings.
There is also an issue with the use of resin anchors to hold down structures such as freestanding towers which are predominantly subject to fluctuating wind load. It arises when the hole is completely filled with resin and the bolt is bonded along its full length. In such a situation, there may be strain incompatibility at the point where the resin and bolt meet. Over time, under fluctuating load, the bond between bolt and resin may have a tendency to weaken. This is in addition to the valid concerns raised by the reporter.
There is a case for assembling a database of resin anchor performance both short and long term and for developing more guidance on best practice which will need to include installation and testing. CROSS would therefore be interested to hear about other cases, particularly where there have been unexpected or premature failures.