CROSS Safety Report
Tower crane bases
A reporter came across a situation in which one of the anchors into a concrete crane base appeared to be loose.
Key Learning Outcomes
For the construction team:
- Quality control and competent supervision on site can help to ensure that temporary structures such as crane bases are installed in accordance with the design
- Temporary works required for cranes must be designed and constructed with the same degree of competence and quality as required for permanent works
- Consider appointing a competent temporary works coordinator (TWC) on site who should be able to ensure all temporary works are carefully considered, planned and installed
- Connections can often be the weak link in structures and attention to detail is required to ensure what is designed is installed correctly on site. Any site constraints should be communicated to the primary designer.
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A reporter came across a situation a couple of years ago in which one of the anchors into a concrete crane base appeared to be loose. The whole cast-in leg of the anchorage appeared to be moving up and down a few millimetres as leg load oscillated from compression to tension.
Unfortunately, this was never fully investigated, but what appeared to have happened is that;
The cast-in legs of the anchorage assembly were Universal Column sections with an end-plate top and bottom. The reporter is not sure how the four legs were connected to make an assembly but it certainly seemed that any such connections either broke off under shear load within the concrete base or punched out of the bottom; it was uncertain as to whether there were properly designed shear keys.
In construction, the fabricated anchorage had been stood onto the blinding; then the rebar was fixed and the concrete cast around it.
Concrete compaction under the top plate, to which the crane legs were bolted, was poor. The top plate was level with the top of the concrete.
The downward force of the crane broke the adhesion between UC and concrete, and compressed the poorly compacted concrete under the top plate slightly; one imagines that the bottom end-plate punched into the blinding under the base; any shear keying from cross members failed.
When the upward leg load came into effect, the cast-in leg could pull back up until the bottom end-plate contacted the underside of the base.
The question of how shear keys are designed warrants attention. The load will not be distributed equally between them due to compatibility issues. The top shear keys will attract most load – and if this is sufficient to break them off, a 'zip-type' failure ensues. While it was not investigated, it appears that the cross members connecting the 4 legs of the anchorage into an assembly were not designed to take the shear loads that occurred, and the welds broke.
The situation was carefully monitored, and no worsening occurred over around 18 months of crane operation. It was checked that the welding on, and size of the bottom end-plate, was adequate to hold the upward force. It was also checked that the crane would stand on only 3 legs just in case the loose leg broke free. The primary concern was that repeated impact loads would cause fatigue in the welds to the bottom end-plate.
The lesson: use shear keys and design them properly, don't rely on a bottom end-plate to prevent upward pull out as it will loosen by pushing down first. Having said that, a bottom end-plate is a good ultimate safety measure.
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