Accounting for Sway during the Design of Elevator Systems in Ultra Tall Buildings
From the Desk of Rick Sayah – Senior Vice President
Is that Building Moving? Yes, it is!
Steel, glass, and concrete might not seem like flexible materials, but when considering tall buildings, the engineers must consider the subtle effects of how these materials interact and move over the entire height of the building. The building also must withstand the forces of extreme weather. Many major cities lie on the coast and are exposed to tropical cyclones somewhat regularly, and this risk may grow with climate change. Buildings are designed to expand, contract, and flex along with the demands of the environment.
“The green reed which bends in the wind is stronger than the mighty oak which breaks in a storm.”
Elevators are especially affected by these conditions. Where structures and facades can be designed with expansion joints, many elevators in tall buildings require a continuous unbroken line from the base of the building to the topmost floor.
A building may be designed to flex at 1 foot for every 300 feet of height. This motion may be imperceptible at street level, but for a building of 1000 feet in height, this can be 3 feet of movement off-center. This means the total swing of the building could be as much as 6 feet. When an elevator shaft is typically only 8 or 9 feet wide, it becomes inevitable that ropes which hang in the shaft will be subjected to extreme motion and possibly strike the walls or other elevator elements.
Further – each building has its own natural frequency. Like the rocking of a boat, but much more subtle. Elevator designers must be aware of the building frequency and adapt their systems to prevent harmful harmonics from developing that can excite the cables and cause damage to equipment.
When considering a tall building, it is important to think beyond the elevator systems and treat the building itself as an integrated system. Choices in structural design will affect how the elevator systems perform. Architectural strategies involving sky lobbies can break up the elevator runs. These strategies need to be considered early in the design of any project, as fixing these problems once the building is built may not always be feasible.
VDA has worked on the “Design Team” for many “Ultra Tall Buildings” throughout the globe.
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