A tuned mass damper for a critical facility at one of America’s busiest airports
Airports continually update their facilities and infrastructure. At McCarran International Airport in Las Vegas, it was especially urgent to build a new air traffic control tower (ATCT) because nearby development had created blind spots in the old one, which had been erected in 1983. Standing 352 feet high, McCarran’s new ATC tower is twice as tall as its predecessor.
Like other tall, slender structures, ATCTs can be subject to wind-induced motion, swaying enough to make building occupants aware of the motion – and even sometimes causing nausea. Occupant comfort always matters, but it’s especially critical for ATC professionals to be able to focus on their work without the slightest distraction. Whatever solution we developed to limit the tower’s movement in the wind had to be designed to fit the extremely narrow space available, inside the core of the slender structure. In addition to accounting for wind effects on the tower, designers had to take into account the facility’s location in a highly seismic zone.
We carried out wind tunnel testing of the tower’s design to understand how it would be affected by local wind conditions. We determined that the tower would be susceptible to vortex shedding – and that adjustments to its shape, aimed at improving its aerodynamic performance, would not be enough to solve the problem. In order to ensure that the ATC team would be comfortable and free of distraction in all wind conditions, a damping system would be necessary.
The tower had distinctly different frequencies in the X and Y directions, so our team decided on a device that could be accurately tuned to each one. We determined that the simplest – and most easily optimizable –motion-control device available would be a tuned mass damper (TMD) that functioned as a simple pendulum, and could be bi-tuned.
Having identified the best damping solution, we faced our next technical challenge: the tower’s space constraints. We needed to add 60 tons of mass (the equivalent of a fully loaded Boeing 737 airliner) into a space 16 feet long by 6 feet wide by 7.5 feet tall. Only a lead-filled steel box could provide enough mass in such a small volume. Even within the limited space available, it was essential that there be enough clearance left to allow the mass to perform its essential function: swaying in response to the tower’s own movement. At the same time, we had to prevent the swinging TMD mass from overtravelling and damaging itself or the surrounding tower structure during a severe earthquake. We installed a floor-mounted snubbing system to prevent this excessive travel.
The new McCarran Air Traffic Control tower became operational in August 2016 and has been operating successfully since that time.