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CROSS Safety Report

High mast light poles removed from UK site

Report ID: 610 Published: 1 January 2017 Region: CROSS-UK

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Overview

Five high mast light poles (HMLPs) were removed from a public site as it was considered that there was a possibility of collapse.

One was a particularly obvious risk because about half of the nuts on its anchor bolts were not fully engaged.

Key Learning Outcomes

For asset owners and managers:

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

  • Safey critical fixings should be inspected as part of the ongoing maintenance programme by a person who is competent to inspect them

For civil and structural design engineers:

  • Selecting the correct fixings and their corrosion protection for the given environment is important to ensure they perform as expected

  • The anticipated life span of the fixings should be noted in the operation and maintenance manual

Full Report

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Five high mast light poles (HMLPs) were removed from a public site as it was considered that there was a possibility of collapse. One was a particularly obvious risk because about half of the nuts on its anchor bolts were not fully engaged. The reporter believed that the remaining four were also a high risk because of the same inherent design features and the inability to inspect critical parts.

HMLPs usually consist of a tapered polygonal mast fixed to a concrete foundation using multiple anchor bolts connected to a circular flange (Figures 1 & 2). The flange is fixed using two nuts on each anchor bolt. One nut is located below and one above the flange (Figure 3). The lower nut is often referred to as the levelling nut and this design is known as a stand-off base plate.

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Figure 1: top view of circular baseplate with holding down bolts
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Figure 2: close-up of nuts
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Figure 3: side view of circular baseplate with holding down bolts

The most safety critical features of an HMLP are its baseplate and anchor bolts. Failure of any one bolt will transfer load to adjacent bolts. If adjacent bolts were not able to withstand the increased load, then collapse could occur.

Potential design weaknesses

In the reporter's opinion, the design of stand-off base plates as used on HMLPs have several severe inherent design weaknesses that could lead to collapse even when installed as designed.  Concerns expressed by the reporter about this case, but also about such structures in general include:

  • Inadequate design of baseplates

  • Baseplates and fixings obscured by finishes or soil/vegetation

  • Nuts not fully engaged

  • Corrosion of bolts

  • Bolt fatigue due to stress concentration in the threads

  • No pre-tensioning of bolts

  • Thread type

The reporter takes the view that not enough is being done to ensure safety and says that this report has been produced in the interests of public safety. He wants CROSS to consider the matter.

Expert Panel Comments

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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 is quite right to be concerned about risk of corrosion to holding down bolts, especially when hidden in 'porous' materials that can hold air and water against steel.  In this case the poor quality of erection demonstrated by the inadequate anchor nut installation was sufficient reason to take down the columns.

Issues with fatigue

There have been many fatigue failures of this family of structures (lighting columns, cantilever gantries, etc...) but failure has been in the structure, not the holding down (HD) bolts. The aerodynamic damping in these structures means fatigue damage due to wind buffeting is highly unlikely. However, CROSS  knows of a case where improvements in quality of fabrication led to a vortex shedding problem which caused large amplitude motion resulting in fatigue. 

Failure of masts due to corrosion

In the past, there has always been some movement in the slip joints between stacked sections that dissipated energy and so improved structural damping. There have also been historic cases of lamp posts failing due to corrosion of the pole.

There are several practical points to be considered such as:

  • The design of the fixings and their ancillary parts to provide security against loosening

  • The ability to take all applied forces (which may include shear and bending as well as tension)

  • Protection against corrosion

  • The possibility of inspecting the assembly in future

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