Innovative axle

19 March 2008

Solving the bridge weight problem was a race against time. The solution was attaching additional set

Solving the bridge weight problem was a race against time. The solution was attaching additional sets of wheels to the outriggers

Fast track job, Hawaiian Crane & Rigging (HCR) was commissioned to complete the assembly of a missile tracking radar at the Pacific Missile range in Kokee on the island of Kauai, along with the help of military contractor Oceantronics Inc.

Due to heightened military sensitivity in the Pacific theatre of operations the radome had to be up and tracking by the customer's deadline.

HCR had to transport its Demag AC 120 all terrain hydraulic telescopic wheeled mobile crane from Honolulu to Kauai, traveling an unfamiliar route from the Kauai Harbor to the top of the Kokee mountain range and finally erect the dish and radome.

A last minute obstacle emerged, however, when HCR's permit to cross the Hanapepe Bridge over the Kauai Harbor was rescinded following a sudden decision to reduce the maximum load allowed to cross the bridge.

Kerwin Chong, HCR vice president, explains, “This de-rate was the state and county's reaction to a dam failure that occurred earlier in the year, which cut of a portion off the island as the breach washed away the only road connecting the two halves of the island – and they didn't want this to happen again in the event we damaged the bridge.”

HCR had just three days to cross the bridge and load the equipment on to the inter-island barge before it departed. The quick response engineering department was called in to solve the problem.

The Demag AC 120 has five axles, each with a loading of 26,000 pounds (12 metric tons) in traveling configuration. The Hanapepe Bridge had been de-rated from 28,000 pounds (12.7 metric tons) maximum axle load to 21,000 pounds (9.5 metric tons) per axle.

According to Chong the only “conventional”options were to lighten the crane or procure a boom dolly.

“Nothing could be done to lighten the crane, short of removing the boom,” says Chong. “There was no support equipment on Kauai capable of removing our boom and shipping an additional crane for this would have severe impacts on the budget.”

Procuring a boom dolly was also out of the question as there were no boom dollies in the state, because up until then, such a machine had never been needed, Chong says.

“Bringing one in from the mainland was time and cost prohibitive – we did not have the luxury of time,”Chong explains.

The only solution was to somehow add an additional axle to the crane, distributing the load and reducing it to 18,000 pounds per axle – 3,000 pounds less than the bridge's maximum weight allowance.

“But how could one economically graft an additional axle onto a crane?” Chong asks. Answer: “By using the outriggers as the wheel platform.”

An under-the-bridge survey was carried out to locate the bridge girder and the spacing was found to fit perfectly to the crane's short outrigger configuration width.

HCR already had walking outriggers for its Manitowoc 3900T truck cranes, so its “Skunk Works”department spent a day modifying a pair and test loading them.

“Fortuitously, the Demag AC 120 has pressure sensors on the suspension, so converting the pressures to forces was a simple calculation – and our preliminary calculations could be verified,”Chong says. “We re-submitted our travel plan back to Kauai County and the state traffic and bridge design department along with our calculations and field data for back-up. The traffic engineers scrutinized the analysis, and the permit was approved.”

The crane made the barge as scheduled and the job was completed on time and on-budget.


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