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Steel
Thursday | 23 June, 2016 | 9:34 am

Making mayhem

Written by By Gretchen Salois

Six Flags’ latest roller coaster, The Joker, is a hybrid of wood and steel

June 2016 - A reinvention—rather than a demolition—is how Rocky Mountain Construction approached its latest roller coaster, The Joker, at Six Flags Discovery Kingdom in Vallejo, California. Engineers and designers transformed an aging wooden apparatus into a hybrid beast. 

Taking 15 months to complete, the DC Comics-themed attraction required the skills of 40 welders, laborers and plasma table operators, forklift drivers as well as 25 “coaster climbers” who were tasked with the perilous job of assembling at high heights. 

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Having to use an existing wooden structure as the foundation of the ride limited ride layout options, but Scott Voyles, designer/draftsman at the Hayden, Idaho-based Rocky Mountain, says designers were able to create inversions and banking turns not possible with a traditional wooden coaster. 

Rocky Mountain Construction used grade A36 steel plate, including 1/4-inch and 3/8-inch-thick stock to construct the sectional profile of the track, says Voyles. “The pieces were cut in long ribbons out of 8-foot by 40-foot steel sheets using a CNC plasma torch. To maximize our steel usage and minimize steel waste, we designed all standard parts for connecting the track to structural supports,” he adds.

“All our parts were also manufactured using A36 steel including track ties and formed angle iron, which maintains the correct distance (gauge) between the two track rails,” says Voyles. “Our ledgers, which support the track and transfer loads to the wood structure, are 10-inch-deep channels formed from A36 plate.” 

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The track is coated with a two-part epoxy paint and all other parts were hot dipped galvanized. Steel connections were made using grade A325 hot-dipped galvanized bolts with direct tension indicator (DTI) washers to meter the appropriate torque, Voyles says. “Our steel-to-wood connections use an A307 hot-dipped galvanized bolt with a tri-lock nut to keep the connection tight.”

The vehicles transporting passengers are made from a number of materials. The chassis is assembled from various thicknesses of A572 Gr50 steel. Lap bar arms are milled from 2-inch-thick plate of grade 6160 aluminum. “The critical housing blocks containing the chain dog, anti-roll back dog, and the train hitch are milled from round stock of aircraft quality 4340 steel.”

No room for error

The overall project required 1,522,188 pounds of steel. The track and structural parts are welded plate construction using a flux-cored arc welding process. “Once the track and parts leave the manufacturing shop, the remaining assembly consists of bolted connections,” says Voyles.

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Parts made from plate steel were cut using a CNC plasma torch; pipe and tubing were cut using a band saw. “We also sheared and punched lengths of bar stock to create custom washers for our bolted assemblies,” he says. “The ends of the track were pre-fitted in our manufacturing shop. The splice joint is detailed to ensure a smooth transition between the pieces by hand-milling the ends using a profile jig to the correct track length and dimensions.”

Addressing challenges

The Joker itself is relatively shorter than projects Rocky Mountain has completed in the past. Designers found the provided as-built drawing/blueprint did not reflect all structural modifications made throughout the years; neither did the document account for warped or deformed wood members. “Our survey of the existing footing elevations and structural leg location may be misleading,” Voyles says. “We frequently needed to make design modifications to supporting structure components to address these issues as they were uncovered during the demolition or rebuild process.

“Designing accommodations in our steel connections—to account for survey error, warped wood members, and the expansion/contraction of the steel—is a consideration we were continuously addressing,” he continues. Installation moves more quickly when the installation crew can easily adjust components to match up bolt patterns.

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During construction sequencing, the installation team needed to ensure equipment accessibility to certain areas of the job site for later stages of construction. “We were fortunate during this project to be located in an area with mild weather conditions,” Voyles says. 

One takeaway from working with long, ribbon-like pieces of steel to manufacture track pieces, is milled flat plate is preferable over sheets that come off a coil. “The coiled sheets we used in the past contain internal stresses that begin to dissipate [due to heating during] plasma cutting and welding processes,” Voyles says. When the material releases stress, the shape of the ribbons changes. “While this is not an issue during the manufacturing of smaller items, it has proved to be a major consideration in working with the longer, narrow pieces of steel” during track construction.

Since it opened at the end of May, The Joker, which reaches 53 mph and climbs 100 feet into the air, delights thrill-seekers with its heaves and twists. MM

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