AWARD OF EXCELLENCE: One Wall Centre

Category: Buildings

GLOTMAN-SIMPSON/ROWAN WILLIAMS DAVIS & IRWIN

In 1996 Wall Financial Corporation proposed to build the highest tower in Vancouver. Designing the structure meant taking care not to infringe upon certain protected view corridors of the city’s magnificent mountain and ocean scenery. The result was a very unusual slender glass tower, 450 feet tall, with a 70-foot wide elliptical footprint and a 7:1 height to width ratio.

The project was extremely complex. Not only was it to be one of the first highrises in Canada to combine both hotel and residential occupants, but also its extreme height-to-width ratio represented challenges for the structural engineers, Glotman Simpson of Vancouver. Rowan Williams Davis & Irwin (RWDI) of Guelph,Ontartio were engineering consultants on the wind and tower vibration aspects.

The idea was to design a large moment frame to work with a central shear wall. The 48-storey building contains a concrete core with walls up to three feet thick. Slabs are seven inches thick.

The original analysis of the structure in wind tunnel tests showed it to be far too flexible to be constructed as first conceived. Buffeting by extremely high winds such as occur every 10 years would have caused it to sway to an extent that would have caused discomfort for the occupants, perhaps even motion sickness.

To improve the building’s stiffness, outriggers were added. These beams connect the elevator core to the large columns at the building’s edge, spreading the axial load out to the columns. The outrigger beams were heavily reinforced with large rebar, resulting in the first use of 55M with Lenton Terminators known in Vancouver.

To control the vibration problems, the engineers designed an innovative and original damping system. Two tuned liquid column dampers “TLCDs,” each consisting of a four-storey high, 50,000 gallon water tank extending nearly the full width of the tower, were placed at the top of the building. The system not only solved the structural challenges but also saved an estimated $2 million dollars in construction costs compared to conventional damping systems.

Each liquid column damper has a broad horizontal chamber at the bottom with a column of water at each end, thus resembling a cup within a cup. The dampers work by allowing the water to move back and forth along the bottom chamber of the tank and up into the columns. When the building moves under wind loading, the water moves back and forth in the opposing direction, transferring its momentum to the building and counteracting the effects of the wind vibration.

The liquid column dampers had to be tuned exactly because once the tank 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 the analysis, physical and computer modelling were carried out, as well as field measurements. Research showed that the natural frequency of the building and TLCDs must match within 10% to be effective. The analyses and field measurements taken at level 41 were within 3.2% agreement.

To address the vertical (gravity) load, the tower relies on just 14 columns. Transfer beams 21 feet deep located at level four transfer this weight to just eight columns that pass through the lobby level, thus creating a more open span for the entrance to the hotel. The remaining eight columns make a strong architectural statement using Agilia, a self-compacting concrete.

Other structural features include the use of self-climbing forming to save construction time. To accommodate the 11,000-s.f. ballroom, five floors of parking and a 15-foot deep elevator pit, the foundations had to be excavated to 75 feet below grade into sandstone. Like the outrigger beams, the core footing extends the full width of the tower and at 75 feet below grade is said to be the deepest in Vancouver. The ground under the tower was dense sandstone, resulting in a tedious excavation.

The building’s combined use as private residences and commercial hotel raised a myriad of security issues that affected everything from the acoustics, to the energy control and mechanical systems, to the development of the elevator core. Separate lobbies and elevator shafts were assigned for each user group to ensure privacy and security for guest and resident alike.

Completed in September 2001, the project has won a 2002 Award of Excellence from the Consulting Engineers of B.C., and the 2001 skyscraper of the year award by an independent and international vote on the Internet through skyscrapers.com

Name of project: One Wall Centre