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Excerpt from Metals In Construction Spring 2014
Renovating and expanding "the world's most famous arena" while it was still open and operating required close collaboration between everyone involved - and some creative engineering to add a new interior level for fans.
Perhaps the biggest single challenge, yet the most defining change to the arena, is the addition of two new promenade and seating bridges located at the tenth level - one on the north and one on the south side above the playing surface.
Because the bridges are ultimately supported by the existing cable roof system, rhythmic movements, such as those generated by an excited crowd during a rock concert or a sports event, can potentially case motions strong enough to make spectators uncomfortable.
Rather than adding brute force stiffness (and additional weight) to the roof, truss, and bridge decks, the project's engineers sought a more elegant and cutting edge solution.
Motioneering, based in Ontario, Canada, worked with the design team to devise a "tuned mass damper" (TMD) to dissipate the dynamic energy. Each of the two bridges received five TMDs, three on the front (arena side) to combat vertical motion and two on the back (street side) to combat horizontal motion, with all five working simultaneously to control roll.
Each TMD comprises 9,000 pounds of stacked lead plate, a crank-shaft and two hydraulic pistons (weighing approximately 1,000 pounds) that translate rotational motion into vertical motion (similar to the engine of a car). The lead plates are put into motion by the motion of the spectators during an event.
The entire TMD system is calibrated to oscillate (move) in the opposite direction as the loading frequency caused by the spectators. Thus, this opposing motion caused by the TMD will weaken the loading frequency, dissipating the energy and dampening the perceivable motion throughout the entire structural system. Monitoring the TMDs during events has verified their satisfactory performance.
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