29.07.2009

Operator saves the day

A first class tower crane operator helped prevent a structural failure becoming a catastrophe earlier this year, preventing a near miss turning into an accident.

The crane, a Potain MR605 luffing jib tower crane, was one of two working on the Blue Cross/Blue Shield building in downtown Chicago, where 24 floors were being added onto the existing 33 storey structure. Both had over 200 metres of tower under them and were of course tied into the steel framed building.
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The two Potain luffers on the BCBS Building

The crane operator was operating the crane, but with no load on the hook, when he heard a loud pop or bang and felt the crane shudder. The crane still felt perfectly normal though, with no signs of anything untoward. However the crane operator shut everything down and investigated.

Some twenty metres of so down the tower from the slew ring a sheared connector spigot was discovered, effectively meaning that the crane was only fully supported on three of the towers four corner chords. The crane was assessed as safe, although the equivalent of a splint was welded to the tower until it could be dismantled.

With only a week left before it was due to come down anyway, the accident did not disrupt the build schedule too much.
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The sheared spigot of the conector

The cranes owner Amquip, in conjunction with Manitowoc/Potain carried out a full inspection, after which the manufacturer decided to issue a Technical Service Bulletin, asking owners to check the connectors on all their MR605 cranes at Manitowoc’s cost. We understand that so far no further such fractures have been discovered.

As to what caused the failure, the most likely theory is that the crane may have been distorted by being tied rigidly to the building while its ‘plumb line’ changed as construction progressed, exerting stresses on the cranes tower that were not intended nor allowed for.

A second theory relates to the fact that two cranes were tied rigidly to the same – relatively flexible building and may have helped caused some unusual stresses as they carried out their lifts, pushing and pulling the structure and each other.

.Vertikal Comment

It would have been so easy for the operator to have ignored the noise he heard and carried on lifting or at best conducted a cursory look-over the machine. If he had it is quite likely that the crane would have collapsed later that day and given its height over city streets what might have been does not bear thinking about.

Our hat is off to the operator, we could use a few more like him hopefully his efforts and quick thinking were fully appreciated.