All terrain meets crawlers31 July 2018 by Sotiris Kanaris
A Demag AC 350/6 all terrain and two Terex crawler cranes efficiently removed bridge girders at the Minnesota Highway 24 Bridge crossing the Mississippi River at Clearwater.
The 1,235ft (376m) long, 30ft-wide bridge was built in the 1950s. Like so many of that time, it is now functionally obsolete and had a restricted maximum load of 80,000lb (36.3t).
The price tag to demolish and replace the outdated Highway 24 Bridge came in at $17.4m. From August 2015 to August 2017, two Terex lattice boom crawler cranes – an HC 165 and an HC 110 – were tasked with completing the lion’s share of the heavy lifting to construct the new, modern two-lane highway with wide shoulders and a multipurpose trail.
Upon finishing the new structure, the general contractor focused efforts on the final contract phase, demolition and removal of the 60-year-old bridge structure. The delicate task had to be completed without damaging the new bridge or contaminating the river, and work had to be completed within approximately 10 months.
For deck girder removal from the center and east bridge spans, a specialized piece of crane equipment not in the contractor’s fleet was required. With the east causeway being short and narrow, the company needed a compact mobile crane with high capacity and long boom to help with center span lifts and one that could navigate the road embankment to access the causeway.
St. Cloud, Minnesota-based Landwehr Construction was called in for consultation. The bridge’s four centre I-beam girders were cut to a 98ft length and weighed 42,900lb each. The plan called for a tandem lift between the HC 165 crawler crane and Landwehr’s crane, which required working at a 120ft radius. In addition to assisting with centre span removal, Landwehr was contracted to remove four east-span girders, each 137ft long and weighing 67,000lb. For these jobs Landwehr chose the Demag AC 350/6.
Girder weights and boom working radius required the AC 350/6 to be equipped with its Superlift structure to meet capacity. However, causeway size didn’t allow enough room for the Superlift to be installed at river level. “Due to overhead obstructions and tight conditions, the crane’s Superlift structure had to be installed at road level with the assistance of the HC 110,” said Herron.
Landwehr mobilized the crane plus six truckloads of counterweight, Superlift structure and miscellaneous lifting materials for the short 13 mile trip to the jobsite.
After installing 257,300lb (116.7t) of counterweight, the final Superlift connections were made within a matter of minutes to complete rigging.
For center girder removal, the AC 350/6 crane’s boom was telescoped to 177.8ft. Working at the 120ft radius offered a crane capacity of 39,900lb, more than enough to remove the center girders in the tandem lift with the Terex HC 165 crawler crane.
The girders were hoisted onto trucks staged on the new bridge, secured and transported the short distance to a processing area, where the HC 110 crane removed the beams. Within a day, the centre girders were completely removed and positioned in the processing area.
The following day, Landwehr’s crew was back at the jobsite to remove the four east steel girder spans. Working with a 163.1ft boom length and a 76ft radius, the AC 350/6 hoisted the final 137ft long girders weighing 67,000lb each in less than eight hours.