Spreading the load

20 March 2008

Traffic backs up as the Demag AC 120 cautiously makes its way over the Hanapepe bridge

Traffic backs up as the Demag AC 120 cautiously makes its way over the Hanapepe bridge

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.

HCR had to transport the Demag AC 120 from Honolulu to Kauai, travel an unknown route from Kauai harbour to the top of the Kokee mountain range and finally erect the dish and radome.

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

A last minute obstacle emerged, however, when HCR's permit to cross the Hanapepe Bridge over the Kauai harbour was rescinded following a sudden decision to reduce the maximum load allowed to cross the bridge.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 off a portion of 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 tonnes), in travelling configuration. The Hanapepe Bridge had been derated from 28,000 pounds (12.7 tonnes) maximum axle load to 21,000 pounds (9.5 tonnes) 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. 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 are no boom dollies in the state, as up to this point none was ever needed. 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 (8.2 tonnes) per axle - 3,000 pounds (1.4 tonnes) less than the bridge's maximum weight allowance.

“But how could one economically graft an additional axle onto a crane?” Chong questions. 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.

Chong said: “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.

“We re-submitted our travel plan back to the Kauai County and State Traffic ' Bridge Design Department along with our calculations and field data for back-up, the traffic engineers scrutinised 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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