Seattle, WA At SAFECO stadium, Mariners players and fans enjoy open-air games under clear blue skies. But when the weather turns foul, they aren't left out in right field. Rather, a computer-controlled, steel-wheels-on-rails overhead-crane-type mechanism rolls the massive three-section roof securely in place, protecting all inside from the elements.
Regardless of the weather it's a good bet that the engineering challenges associated with combining a building structure such as the roof with the overhead crane's machine-like structure eludes most attendees. As may the super-sized shock absorbers that help the stadium's 13,000-ton roof stand up to Seattle's extreme environmental conditions. That's because the dampers had to meet strict architectural requirements for aesthetic appearance, as well as mechanical stress and deflection constraints. In fact, architects limited the diameter transitions along the damper to not more than a 25% change at any point.
Taylor Devices (North Tonawanda, NY) supplied eight 24-ft long dampers weighing 4.5 tons each, which were more than twice the size of any the company had previously manufactured. Serving as a buffer between the stadium's side columns and the retractable roof sections that they support, the dampers reduce stress and deflection simultaneously so that the 20-million-lb roof won't collapse in the event of an earthquake.
In addition, Taylor also supplied smaller, 9-ft long dampers, designed to cushion contact between each movable roof section. Due to the often "drizzly" conditions of the Seattle area, the crane's motors, drives, and gearboxes must achieve roof speeds (approximately 6 inches/sec) that enable quick opening and closing during inclement weather changes. This movement, combined with the weight of the structure and sometimes high wind gusts, requires enhanced energy management and the subsequent use of shock absorbers to buffer the impacts between the roof sections and at the end stops.
Stress vs. strain. Interfacing between the stadium's roof and the crane mechanism joins concepts and products from mechanical engineering with the civil engineering structure. "Normally civil engineers tend to use stress-based calculations," explains Taylor Devices President Doug Taylor, "while mechanical engineers use strain-based calculations. Stress-based designs minimize the use of material, and result in low cost, lightweight structures that are aesthetically pleasing. Strain-based designs typically yield much stiffer structures."
Although the roof is intended to cover, not seal, the stadium, the design must stand up to tough conditions such as 70 mph winds, and earthquakes measuring 8.5 on the Richter scale with lateral ground accelerations of up to 1/3g, and six-ft snowdrifts. Initial testing indicated that the roof flexed too much under seismic forces, and started showing failure points where it connects to its supporting columns. "A seismic input perpendicular to the axis of motion of the roof tended to buckle the structure at the hinge or knee connection between the roof and the support trusses that actually connect it to the moving runway," explains Taylor.
So mechanical engineers came up with a solution that involved adding some six million lbs of steel to the structure, which was unacceptable to the civil engineers and architects because it would be too heavy and unattractive. Taylor engineers came back with a damping proposal to reduce loading in the structure under seismic input. Designed to sop up the energy produced by an earthquake, the dampers deflect up to 2.5-ft of movement, and absorb 1.1 million lbs of force.
Taylor's dampers are essentially a pressure vessel filled with silicone fluid. "We use silicone because it's non-flammable, non-combustible, and non-toxic to humans," Taylor says. The stainless steel cylinder and rod resists corrosion, and a bushed sleeve conceals the rod reducing bending moments compared to exposed rod designs. "By doing that, we change the end conditions from one end fixed, one end free, to both ends fixed," Taylor explains. "This helps the damper resist buckling, and allows long and slender configurations."
"Damping not only reduces loading in the structure and keeps the design attractive," says Taylor, "but it saves an estimated $5 million in net project costs. So instead of adding an extra $6 million in steel costs, the design requires only $1 million worth of dampers to reduce the loads down to a point where we have an optimal design."