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 for the tower crane. Unfortunately, several crane bases had significant construction defects and the reporter gives the following examples.

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.

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.

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.