CROSS Safety Report
Insufficient attention to the construction of tower crane bases
While working on construction sites a reporter was involved with tower crane bases supported on piles with pile caps and pedestals with cast-in anchors. Unfortunately, several crane bases had significant construction defects and the reporter gives three examples.
The importance of identifying and correcting defects in foundations before proceeding with other work that relied on these load-bearing elements was recognised as a priority over tight deadlines and potential financial penalties.
Key Learning Outcomes
For structural and civil engineers:
Defects found in early stages of construction must be corrected before further work is carried out
Pile foundations located in positions outside of allowable tolerances may require re-design of pile caps and changes to other structural elements, e.g. anchors for crane masts
For construction personnel:
Significant defects found when removing formwork from in-situ concrete must be reported and rectified before further work is allowed to continue
Construction deadlines and potential financial loss must not have a priority over safety in construction
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While working on construction sites a reporter was involved with tower crane bases supported on piles with pile caps and pedestals with cast-in anchors for the tower crane. Unfortunately, several crane bases had significant construction defects and the reporter gives the following examples.
Large void found in concrete base
The first involves concrete workmanship where the concrete to a pedestal was found to have a large void after stripping the formwork and the subcontractor wanted to move right ahead with erection of the crane due to tight deadlines for crane erection. The reporter notes that in such cases tight deadlines are usually agreed, with road closures and traffic control in place for agreed dates and periods of time. This places pressure on construction personnel and in this example on the engineer to allow crane erection to proceed prior to completion of the rectification. However, in this case the erection of the tower crane had to be called off until the base was rectified.
Pile positions outside acceptable limits of tolerance and reinforcement errors
The second example involved problems with the piles and reinforcement to another crane base. The reporter notes that although the piles had a 75mm positional tolerance, 3 out of the 4 piles were at least 100mm out of position; and the projecting starter bars from 2 piles into the pile cap were too short and had to be extended with couplers. Furthermore, there were errors with the reinforcement scheduling such that the diameter of the reinforcement cage was 100mm smaller than that specified. This meant that the fabricated steel tower crane anchor assembly that required 600mm square clearance could not be placed in the reinforcement cage of the pedestal. The anchor manufacturer and the temporary works designer were able to find a way to modify the anchor so the crane base could be constructed; however, the crane erection was delayed on this occasion as well.
Major errors in pile positions lead to re-design of pile cap
The reporter's third example involved piles that were installed so far out of tolerance that the whole pile cap had to be completely re-designed once this was discovered. And again, the crane erection was delayed. In the reporter's opinion these examples demonstrate that there was a lack of supervision knowledge combined with bad workmanship and poor-quality control in place in these instances. Furthermore, the reporter observes that the importance of temporary works is often overlooked because it contains the word temporary and there have been many failures of tower cranes due to various reasons. Although there were no failures on this occasion, the reporter is concerned that construction personnel are often under pressure to meet programme deadlines resulting in poor execution of construction works.
Expert Panel Comments
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While we acknowledge that construction is never perfect and groundworks can present particular difficulties, it is regrettable that the examples reported appear to be all too common in the construction industry.
Faults identified and action taken
However, in this case, it is encouraging that the defects were identified and corrected before significant loading was applied or further damage occurred. While this is not ideal, it demonstrates what should happen in such cases, viz. that there was a competent person on site with the integrity and standing to resist the commercial pressure and to ensure that rectification was carried out before further works proceeded. The question arises what would have happened if the temporary works engineer had not been on site? Would the time and cost pressure on the builder to minimise any delay to the project, mean that the defects would have been covered up? It is not uncommon for investigations undertaken in the aftermath of incidents at construction sites to reveal construction programme scheduling and pressure as a causal factor.
Examples of crane failures
CROSS-UK has examples of crane failures and you can search for safety information on them on the CROSS website. An extreme example is given in report 316 Tower crane collapse through foundation failure.
The CIRIA report Tower crane stability (C654) gives recommendations on the design of foundations.
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