One of the most exciting new office projects in Canada in recent years, the BOW attracts attention for its 58-storey height (the tallest building in Canada outside of Toronto), stunning architecture (it was rated among the Top 10 designs of 2012 by Azure Magazine), and distinctive curved glass and steel exterior. However, one of the most ground-breaking aspects of the BOW can’t be seen by the casual passerby.
For the first time in a curved skyscraper in North America a triangular diagrid system was used. The precise calculations and controls used to create and manage this system are technically challenging and conceptually creative. As office and condo towers continue to proliferate in densifying North American cities, this geomatics expertise will become a must-have for design teams.
A Calgary Landmark
The $1.4-billion BOW, the Calgary home of Encana Natural Gas and Cenovus Energy, broke ground in 2007. Over the course of five years, the design and construction team executed the Foster + Partners design. The distinctive crescent shape is aerodynamic to reduce wind resistance, down draft, and urban wind tunnels, as well, maximizes daylighting and allows for centralized spaces such as a sky garden system. The tower’s form and mass were developed to adapt and utilize Calgary’s climate, including seasonal sun paths, rainfall, wind, temperature and humidity.
MMM Group (MMM) provided geomatics services for numerous aspects of the landmark building’s construction, including the establishment and maintenance of the precise project control framework, the determination of real-time building movement and the positioning and layout of building structural components using precise motorized electronic theodolites and RTK GPS techniques. The BOW project features leading-edge geomatics methodologies and technologies never before used in Canada. In addition, MMM’s strategy of employing a network of precise inclination sensors for building monitoring is a practice employed by some of the world’s most exciting skyscraper construction projects, including the Burj Dubai.
The Tilt Factor
The geomatics survey methodology had to also accommodate building tilt. During the construction of a skyscraper as complex as the BOW, the structure will temporarily lose its exact verticality and the building will shift and move. To accommodate this tilt, innovative and leading-edge survey methodologies for the establishment of building control and for precise survey layout of the structure, were developed.
MMM installed three fixed-bracket permanent markers compatible with both conventional and GPS survey equipment to provide the framework control. They were installed at locations far enough from the BOW site to ensure stability. These markers were monitored to ensure disturbance had not occurred.
Further, MMM provided detailed topographic and legal surveys of the BOW site and the proposed connections to adjacent buildings. MMM placed and coordinated 12 Leica professional-grade 360 degree prisms, tribrachs and carriers at strategic locations around the site. The setup allowed for the co-location of a GPS antenna on top of the prism. At three-month intervals, MMM performed a complete static GPS control survey involving simultaneous occupation of all rooftop prisms capable of GPS occupation and the framework control markers. The GPS surveying involved the overnight occupation of rooftop locations, followed by two 10-hour GPS static sessions completed over two days. The survey enabled the assessment of the stability and accuracy of the horizontal network.
With leading-edge technologies such as a Nivel 220 inclination sensor network, visionary advances in geomatics methodology (such as building monitoring in real time and layout of steel members employing GPS), and rigorous quality control and quality assurance procedures, the geomatics team allowed the BOW skyscraper to reach new heights.