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CROSS Safety Report

Cantilever signal base failure - holding down bolts in bending

Report ID: 627 Published: 1 October 2017 Region: CROSS-UK

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Overview

A railway asset owner noticed that a 9m cantilever signal structure was sagging alarmingly and stopped trains on the line to investigate the matter.

Key Learning Outcomes

For construction professionals:

  • Quality assurance and competent supervision on site can help to ensure that the structure is built in accordance with the design

  • Consider introducing a quality control procedure for the inspection of safety critical elements such as holding down bolts to ensure they are installed as specified on the construction drawings

For asset owners and managers:

  • Regular inspections and maintenance can help keep a structure safe and identity any obvious safety issues

Full Report

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A railway asset owner noticed that a 9m cantilever signal structure was sagging alarmingly and stopped trains on the line. It was observed that the baseplate of the signal was only in partial contact with the concrete base. The concrete foundation was intact showing no signs of movement, but it was noted that five of the twelve holding down bolts had failed.

These bolts were embedded into the concrete foundation and, prior to failure, held the signal in the correct position using nuts below and above the baseplate. Of the remaining seven bolts, the lower nuts had been displaced downwards and threads stripped on six bolts (Figure 1). A single nut on one bolt was holding the entire structure from collapse. 

Image
Figure 1: cantilever signal base failure

It was immediately apparent that the grout specified between the concrete plinth and the signal baseplate had not been installed. This is an important part of the installation which is required to reduce the bending stresses on the bolts in service. It also provides a secondary effect of encapsulating the bolts and excluding water/debris from the bolts.

It was immediately apparent that the grout specified between the concrete plinth and the signal baseplate had not been installed

The Denso tape also shown in the design for covering the bolts above the baseplate had also not been installed (another anti-corrosion measure). Three of the bolts appeared to have failed due to the surface corrosion whilst the other two were less corroded and probably failed when the signal sagged. It is suspected that the omission of the grout would have subjected the bolts to increased stresses, which over time led to fatigue cracks and failure.

Expert Panel Comments

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As in report 630 which discussed multi-storey car park foundation, this highlights the importance of column base anchorage. Several of the key themes set out in report 644 and report 630 are also applicable. In terms of safety another highlighted attribute is consequence. Whilst many failures might be classified a nuisance, with just commercial consequences, this failure had the potential to create severe consequences and so the design, site supervision, and site inspection, should have merited special attention

Unfortunately, there was inadequate quality control on site because the installer did not comply with the design drawings which showed grouting. As such it is probable that both the baseplate and the bolt sets were significantly overstressed compared with the design.

This construction error was not picked up on site and might have led to a major incident. It is essential that works are executed in accordance with the design or if varied this is only done with permission of the designer. Any reasonable inspection of the works would have revealed non-compliance with design, which should have eliminated the obvious risks.

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