How to ensure your high-rise won't be gone with the wind.

High rise building design for wind considers strength, stiffness, occupant comfort and building/wind interaction. Building shape can affect wind loads from vortex shedding: wind whirlpools form at building corners and then peel off, imparting crosswind forces. Four recent Thornton Tomasetti buildings illustrate these key attributes.

The 1483-foot high Petronas Towers in Kuala Lumpur, Malaysia, are the world's tallest twin buildings. Vortices from the upwind tower could increase wind load on the downwind tower, but wind tunnel testing showed that 'points and arcs' on tower perimeters minimized crosswind excitation.

Occupant comfort also relates to stiffness and damping. Concrete core walls, columns and ring beams provide strength, stiffness and an inherent critical damping ratio of 2%. Ten-year peak accelerations of 14-18 milli-g and 17-20 milli-g from aeroelastic and force-balance tests respectively are below a 21 milli-g comfort limit for office occupancy.

Slender steel pipe Skybridge legs with little damping could vibrate several inches from vortex shedding. To reduce crosswind leg vibrations, compact spring-assisted pendulum Tuned Mass Dampers inside each leg boost leg damping from 0.15% to 0.5% of critical.

At 1667 feet tall, the Taipei 101 office tower evokes jointed bamboo and pagoda tiers. Very large wind loads can result if the vortex shedding rate matches the building sway rate. Initial wind tunnel tests found wind forces for Taipei 101 several times greater than indicated by conventional formulas.

The solution was shaping the building to inhibit vortex shedding. 'Sawtooth' or 'double notch' corners with facets 8.2 feet wide reduce wind forces. Even so, typhoon wind loads require a steel braced core, outrigger trusses every eight to 10 floors and concrete-filled box columns. Per code, perimeter moment frames provide a 'dual system' for seismic resistance.

The steel frame estimated inherent damping of 1% meant unacceptable lateral accelerations. Overall building comfort is improved to an acceptable level by a Tuned Mass Damper: a 730 ton steel ball (0.24% of building mass) suspended as a pendulum and surrounded by telescoping dampers.

The 815-foot tall Bloomberg Building in New York has concrete flat plate apartments over steel-framed offices. Occupant comfort criteria are stricter for residences than for office space and wind tunnel testing showed comfort would not be satisfactory based on preliminary design properties. Comfort can be achieved by adding damping, but a pendulum TMD would require a costly 60-foot tall space that also increases wind load. Instead a 'tandem Tuned Mass Damper' fits in a 25 foot tall room. A 320 ton mass on a short pendulum is linked to a lighter, 220 ton mass on jointed 'table top legs.'

Random House, a 675-foot tall New York building, has 25 concrete flat plate residential floors above 25 steel-framed office floors. A steel transfer truss transitions column layouts and engages lower perimeter columns as an outrigger.

Strict residential criteria and steel lower framing with low inherent damping required two Tuned Liquid Column Dampers (TLCDs) at the top floor. Large U-shaped tanks oriented at right angles address two sway directions. Water moving back and forth rises and falls in upstanding legs of the U, flows through adjustable louvers that create turbulence and provide damping by turning water motion into heat. Total supported weight is greater than for a two-way TMD: moving water mass is 550 tons, or 0.33% of building weight, in each tank, and the concrete tanks themselves add weight and require floor space. But installed cost is less than for a pendulum TMD.

Choosing between TLCD, TMD, tandem TMD requires careful consideration of space, height, supported weight, maintenance, reliability, cost and other factors. All are viable approaches to consider.