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CROSS Safety Report

Foundation base plates for tower cranes

Report ID: 257 Published: 1 October 2011 Region: CROSS-UK

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Overview

On two different contracts it was observed that the foundation base plates for tower cranes had suffered from low compaction of the concrete on which they rest.

Key Learning Outcomes

For construction professionals:

  • Quality assurance and competent supervision on site can ensure safety critical elements such as foundation bases are built in accordance with the design. This should be ensured by appropriate supervision and post-concreting checks.
  • Extra care should be exercised when concreting near cast-in inserts, including anchors, and the subject should be addressed in a method statement
  • Failure of a tower crane, or indeed any crane, can be catastrophic and this type of event, and its management, is covered in the CIRIA report Guidance on catastrophic events in construction

For civil and structural design engineers:

  • Raise awareness of the importance and requirement of full compaction around baseplates and cast-in safety critical components with the contractor

  • If possible, attend site to inspect the installation and construction of these works

  • If you are unable to attend site, ask the contractor for site photos for these works

Full Report

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On two different contracts it was observed that the foundation base plates for tower cranes had suffered from low compaction of the concrete on which they rest. It was noticed by the reporter in one instance when ripples were seen in standing water around the base of the mast as a crane slewed. Closer inspection resulted in the detection of a small amount of 'vertical play'.

The type of anchor used for this crane depended on threaded bolts and not a steel stool. It was seen that the nuts above the base plate had not rotated so the concrete below must have moved. The bolts had lost some of their pretension so they were re-torqued to the correct value and regular checks were made.

There was no need to remove the crane or reduce its working capacity. It would appear that the concrete team who cast the base plates onto the foundation did not take sufficient care to ensure all the air under the plate was removed and that the concrete was well compacted. Sometime later at a meeting of construction engineers, continues the reporter, the topic of tower crane foundation was discussed and it was found that another company had suffered the same type of problems but they used a steel stool and not bolts.

It would appear that the concrete team who cast the base plates onto the foundation did not take sufficient care to ensure all the air under the plate was removed and that the concrete was well compacted

This meant that they were unable to re-torque the fixing and the crane had to be removed to cure the problem. Concrete teams need to take particular care when casting reinforced concrete crane bases and the engineers supervising the works should find out if the team in question has performed this type of work before and make clear the need for full compaction under the base plates.

Expert Panel Comments

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There are many examples of cranes failures: particularly tower cranes and mobile cranes, which tend to topple, and the importance of a stable base and substrate cannot be overstated. Failure of a tower crane, or indeed any crane, can be catastrophic and this type of event, and its management, is covered in the CIRIA report Guidance on catastrophic events in construction.

In planning such work, any assessment of risk should identify ‘safety critical’ considerations and one of these would be the quality of concrete and its placing and compaction. This should be ensured by appropriate supervision and post-concreting checks. Extra care should be exercised when concreting near cast-in inserts, including anchors, and the subject should be addressed in a method statement by the contractor.

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