BUILDINGS: ONE WALL CENTRE, VANCOUVER

One Wall Centre which opened this year in Vancouver’s downtown, is such a tall and slender tower that the original analysis showed it to be far too flexible to be built as it was first conceived.

To improve its stiffness, we added large “outriggers” — large beams that connect the concrete elevator shaft to columns at the outside edge of the building — to the design. Now the tower could be built, but there was still a question of whether the occupants would feel motion in a strong windstorm.

Wind tunnel tests indicated that storm winds coming across adjacent buildings would buffet the tower. Vortices trailing its leeward corner would suddenly move from one side to the other, causing a sudden lateral shift load that would start the tower vibrating.

The normal approach to solve vibration problems (when mass and stiffness cannot be changed) is to add artificial damping that acts like a shock absorber. The most common damper for this type of tower is the “Tuned Mass Pendulum Damper,” which is a large mass hanging in the form of a pendulum that sways in a direction opposite to the movement of the tower.

With the help of RWDI from Guelph, Ontario, we also explored the use of an alternative damping device employed in a communications tower in Australia. The device is a water column damper, apparently the only one in the world to control movement of a large structure.

We adopted a solution that varies on this principle. At the top of the 48-storey One Wall Centre there are two “tuned liquid column dampers” (TLCDs), each consisting of water tanks 16 m long x 4.5 m wide x about 8 m tall, extending nearly the full width of the tower. Within each tank is a long horizontal chamber at the bottom and two columns of water at each end.

The dampers work by allowing the water to move back and forth along the bottom chamber of the tank and up into the columns of water. When the building moves under wind loading, the water moves back and forth, transferring its momentum to the building and counteracting the effects of wind vibration.

The liquid column dampers had to be tuned exactly because once the tanks walls had been constructed, only the water volume and a gate through which the water flows could be changed to adjust the vibration characteristics. To confirm our analysis, models were made of the tank, computer models were made of the building, and field measurements were taken of the tower when construction had reached the 41st floor. Research showed that the natural frequency of the building and the tuned liquid column dampers must match within about 10% to be effective. Our analysis and the field measurements taken at level 41 were within 3.2% agreement.

Because the building is made very stiff to resist wind loading, it has more than enough strength to resist earthquake loading. Even so, it is designed to incorporate special techniques that will make the building ductile when it is “hit” with an earthquake. Special elements can yield without breaking so that the building can continue to stand in earthquakes even larger than those expected in Vancouver.

Other interesting facts:

Water tanks for the tuned liquid column dampers double as a source of water for fire fighting in the upper third of the tower. The tanks also enhance the efficiency of the building’s mechanical systems.

The strength of a tower depends crucially upon its foundation. Like the outrigger beams, the core footing extends the full width of the tower and at 23 m (75 ft) below grade is said to be the deepest in Vancouver to date. The ground was dense sandstone requiring tedious excavation. The elevator pits were 4.5 m (15 ft.) deep, contributing to a very deep footing.

Post-tensioned beams 2.4m (8 ft.) deep span the ballroom under the plaza deck, allowing for a heavy planting load and assembly area above.

The tower has just 14 columns. Transfer beams 6.4m (21 ft.) deep at level four transfer the weight to just eight columns that pass through the lobby level of the hotel. The remaining columns make a strong architectural statement using Agilia, a self-compacting concrete recently introduced by LaFarge. This concrete is produced with steel-faced wood formwork and has a surface so smooth and without blemishes that it feels like a painted wall.

Outriggers are placed as follows: 1.5m x 6.4m at Level 5 (also used as a transfer floor for tower columns); 1.8 m x 1.8 m at Level 21 and Level 31; and the deep walls of the rooftop tank at the roof level. The outrigger beams were heavily reinforced with large rebar. This is the first use of 55M with Lenton terminators known in Vancouver.