December 1, 2012
By Neb Erakovic, M.A.Sc., P.Eng., and Walter Koppelaar
With its iconic crescent-shaped, inversely curved form, the Bow is a striking presence on Calgary’s skyline. Designed by Foster + Partners of London, this 59-storey tower features a vast atrium partitioned in four clear-height sectors with the façade integrating an architectural exposed diagonal grid structure in six-storey segments. The perimeter ‘diagrid’ frame helps make up the building’s hybrid lateral force-resisting system (LFRS).
The Bow Tower serves as the new headquarters for Canadian energy corporations. The site includes two city blocks in downtown Calgary’s central area. At 238 m (780 ft), it is the tallest building in western Canada, and the second tallest commercial office in the country. The tower’s gross constructed above-grade area is 195,000 m2 (2.1 million sf), along with 97,000 m2 (1 million sf) at six below-grade levels, including 1375 parking spaces and loading dock and service areas.
All project team members successfully fulfilled roles in addressing unique design and constructability challenges, including the long-span structures’ irregular floor plates and sophisticated lateral system, along with delivering the fast-track project in a dense urban centre under harsh climatic conditions.
Other significant construction factors included the analysis and design of a temporary structure required during construction, superelevation and elastic column shortening, extremely tight erection tolerances, and a high standard of quality for the 8000-tonne architecturally exposed structural steel (AESS) atrium.
This article revisits the development of concepts, specifically design and construction details of the perimeter diagonal grid and the lateral system. It explores the logistics in the context of a ‘top-down’ construction technique for the substructure employed to accelerate the schedule. It also examines the above-grade tower superstructure, which is characterized with large and heavy building components and reflected in the fabricator’s perspective on production and erection of complex AESS elements to tight tolerances, along with a fast-track construction schedule. (The authors would like to thank H&R REIT, the tenant, Ledcor Construction, Matthews Developments Alberta, Foster + Partners and Zeidler Partnership, Supreme-Walters Joint Venture, Yolles–A CH2M HILL Company, Barry Charnish, and David Stevenson, and everyone else who contributed to the success of this project).
The Bow successfully fulfils its owners’ goals of a distinctive tower that is both a progressive and sustainable office space and a vibrant cultural, civic, and shopping destination for Calgarians. Three ‘sky gardens’ divide the building into distinct zones, forming a series of destination floors with lobby areas, meeting rooms, communal spaces, and a high-speed lift service running between the lobbies.
Early design meetings with the client identified the prime objectives for the development. These included a distinctive image, a ‘home-away-from-home’ ambiance, a desire to transform the local area in downtown Calgary and tie into the enclosed pedestrian path though the downtown buildings, and a way to introduce landscaping at the base to facilitate interaction with the public.
An assessment of the space planning and business units became the basis for the building’s architectural design. Foster + Partners explored various floor plate configuration and layouts for consideration by the client; each was studied for building efficiencies, functionality, economics and esthetics. Figure 1 (left) illustrates the final contestants in the design process, along with options for the building’s vertical profile.
As a result of the client’s office culture, there was a significant requirement for perimeter offices for their staff—as opposed to the recent trends in office layouts with ‘seas’ of office cubicles or systems furniture. Conversely, interior areas were needed to provide meeting rooms and support facilities. The most typical office floor is shown in Figure 2, with the elevator banks in a side-core position clustered with the washrooms and the north-facing stairs. The shape selected also had the benefit of minimizing the length of the perimeter and interior wall system surrounding the atrium area, and capitalizing on the view of the Rocky Mountains on the south.
The tower’s shape is more than a visual statement—it also helps with the pursuit of sustainable design goals. The curving bow shape meant wind loads were reduced when compared to a rectangular building. This improved the economies of the building, resulting in effective building material use. Studies were carried out to position the building to maximize sunlight to the office space and to internal gathering locations; the findings partially helped determine the crescent-like shape, with the interior curvature facing to the southwest (Figure 3).
Using this interior curvature captures sunlight; the shape also gave rise to the locations of the atrium along the southwest elevation. This atrium or plenum-type buffer zone is designed to absorb the heat from daily sunlight and use it to partially warm the building in the winter; it also buffers the solar gain in the summer. This orientation had the added benefit of maximizing unobstructed views of the Rockies to the west.
Structural design concepts
The development of structural systems and corresponding material selection were driven not only by the client’s building geometry and objectives, but also through close collaboration between the design team members to create an iconic building in which structural systems were an integral and natural part of the overall architectural expression (Figure 4).
Structural steel was a natural choice given the overall design objectives and building requirements, as well as the building geometry that had been developed, and fulfiling an important role in achieving sustainability goals. Structural steel’s high strength generally offers advantages over other construction materials, such as:
After considering various structural solutions for the building lateral force-resisting system, a dominant perimeter diagrid frame, coupled with other systems was selected. A six-storey vertical module was used, as it related well with the client’s internal space planning requirements.
This hybrid LFRS involved three primary braced frames on the curved south elevation and the two northerly facing ones, ‘coupled’ together with steel moment-resisting and braced frames. To augment the tower’s lateral stiffness between the six-storey-spaced ‘nodal floors,’ a secondary bracing system consisting of conventional steel braced frames at two remote finger-core stairs and around the main central elevator core were also provided.
Consequently, the lateral system comprises the following four principal components:
The alternative of using perimeter systems with closely spaced steel columns and variations of diagonal bracing schemes (and other hybrid systems, including ‘outrigger’ frames and ‘belt’ trusses) was studied before selecting this hybrid LFRS. (The architectural interpretation of alternate LFRSs is shown in Figure 5.) The curvilinear geometry of the floor plate, the cladding design, exterior and interior esthetics, space planning, daylighting, and economics of the steel framing scheme were primary decision-making factors for the building primary structural system.
The lateral system, and hence the atrium wall, was designed under wind, seismic, and thermal loads—wind being the governing case. Wind tunnel studies were performed to obtain the loads on the tower by creating a 1/400 scale model in a 600-m (1969-ft) radius environment. A separate test was performed on the atrium wall to determine the impact of wind loads on the long, unsupported atrium wall members. The resulting wind and seismic loads were applied to a finite element model.
In total, 180 ultimate limit states (ULS) and 64 serviceability limit states (SLS) load combinations were used. The deflection criteria considered were a maximum of h/400 at the top of the building, as well as a maximum inter-storey drift of h/350. For seismic loads, the inter-storey building drift was limited to h/40.
The gravity-load-carrying system was affected by the need to minimize the building’s height. Since the project is just south of the Bow River, Calgary’s urban guidelines required the building be low enough to avoid shadowing the river during the September equinox period. Consequently, a network of interior columns was added in a layout to ensure a depth of the floor beams below 485 mm (19 in.) underneath a composite floor slab construction consisting of a 75-mm (3-in.) concrete cover on 75-mm steel deck. High-strength steel—450 MPa (65 ksi)—was specified for all W-shape gravity columns above level 24 and for the heavy W360 members of the diagrid below this point.
The atrium screen wall was a very dramatic element in the architectural design as it was exposed to all building users. Structurally, the wall was important as the diagonal grid was involved in completing or closing the perimeter lateral load-resisting system. Complicating the screen wall’s structural aspects was the large, unsupported length of the compression elements and the the screen wall’s tendency to attract gravity load from adjacent floor plates (Figure 6).
The design options included rectangular or triangular steel elements, along with round pipe (with possible concrete fill). Various studies were carried out on the three possibilities, including:
On a material basis, the round pipe with flange plate connections proved to be the least for structural cost, but with consideration of the other aspects (particularly esthetics), triangular elements were chosen. They appear in building information modelling (BIM) images in Figure 7.
The concepts of connections were initially developed by the structural design team through a series of hand-drawn sketches, and details then further refined and fully engineered by the steel contractor. Having dimensions of approximately 2.8 m (9.2 ft) in width and standing 4.2 m (13.8 ft) tall, the nodes are challenging not only to design, but also fabricate. Working with the thick plate involved preheating the elements before welding.
Internal stiffening plates were employed to contribute to an efficient load transfer through the node. This added to the system’s complexity, creating additional challenges for fabrication. The nodes were fabricated in the shop and then field-welded to the diagonal and horizontal diagrid members (Figure 8).
Using triangular built-up sections had its challenges—in the modern engineering era with computer-aided design (CAD) tools used extensively, these sections, nodes, and connections needed to be designed ‘by hand.’ The team had to go back to first principles in engineering to design these structural elements.
Other aspects of the Bow’s construction brought unique challenges requiring creative problem-solving.
Top-down below-grade construction
Since the project’s design and construction schedule requirements had an aggressive timeframe, the client decided to bid the structural steel with a unit price contract based on schematic design phase documents. This design was based on a six-storey basement being constructed of reinforced concrete with the structural steel commencing at the ground floor level. Decisions by the project and construction managers led to the incorporation of structural steel to start from the raft foundation and with the concrete basement framing following after the steel was erected at grade (Figure 9).
The steel was also extended most of the north block to provide an ‘umbrella’ to assist in the structural steel erection of the tower over the deep basement. The intent was for the tower steel to proceed above, while the slower-paced reinforced concrete basement backfilled off the main critical path schedule.
To achieve this ‘top-down’ construction, the lowest lifts of columns were augmented with tie down anchors into the raft design for the lower level floors. Added bracings located within the basement area were reinforcement to support the building until the permanent below-grade shear walls and ground-floor diaphragm could be constructed. In some cases, this temporary bracing was embedded within the final shear wall construction.
The construction logistics developed by the fabricator and erector required the general office area with the service core and outside finger cores to be built before the atrium screen wall. This base construction could be used to establish the column and diagonal grid node locations to facilitate erecting the atrium screen’s long diagonal members.
It was determined early on that the project required three tower cranes to provide full coverage to the vast site, offer sufficient crane capacity for the mega-nodes and atrium framing (up to 65-tonne lifts), and support an aggressive construction schedule. All three cranes were internal climbers supported by temporary vertical braced cores to provide lateral support.
Unlike conventional structures with a reinforced concrete or structural steel core, the diagrid perimeter system did not have the advantage of a central erection base to which the perimeter framing could be anchored and adjusted. It was also anticipated the erection of the atrium screen wall would take longer than that of the office portion due to the 10.2-m (33.5-ft) offset of the atrium wall from the office space’s edge slab.
In design studies, a strong core scheme was found to trigger a dead load drift of the tower as a result of the side-core location. This saved steel tonnage.
To achieve the erection of the atrium screen wall at a later date, temporary frames were constructed to span the atrium plenum at various levels as a means of stabilizing the entire wall. These temporary frames were removed and reused as the atrium wall construction progressed up the building. Generally, the atrium wall erection was approximately six storeys behind the office area construction. This delay also helped avoid some of the gravity load creep from the office areas, which would happen with a structure of this nature (Figure 10).
Construction tolerances and atrium AESS
Foster + Partners had established a plan tolerance for all slab edges and the atrium diagrid of ±25 mm (1 in.) of the theoretical position over the tower’s full height. This criterion is to a large extent more stringent than the traditional +50/–75-mm (+2/–3-in.) tolerance specified by the American Institute of Steel Construction (AISC) for high-rises, and thus required highly accurate shop fabrications and special measures in the field.
To ensure the atrium was constructed to this tolerance, each AESS member between it and the tower was custom-trimmed before erection, and the atrium’s entire height was ultimately installed within 15 mm (0.6 in.) of the theoretical plan location.
As a Class A office tower, the finished floors also needed to meet very stringent floor flatness criteria. Due to the more heavily loaded interior, columns would ‘shorten’ to a greater degree than the perimeter framing; therefore, the interior columns were super-elevated by approximately 2 mm (0.08 in.) per floor. Adjustments to column elevations were made by adding shims to the bolted or welded column splices locations.
The architect required a very high standard of fit and finish for the fully exposed atrium diagrid framing structure (Figure 11). This was achieved by developing a system of jigs and fixtures at the fabricator’s manufacturing plants to ensure seamless joints where the members joined in the field.
Another unique aspect of this project was the provision of more than 100 fully pre-fabricated modular washrooms (Figure 12).
Steel frames were custom-designed by Feature-Walters (a member of the Walters Group) and fit-out with all interior finishes, vanities, ceilings, lighting, electrical, and mechanical services, granite flooring, and back-painted glass. These modules were then shipped from the manufacturing facility in Ontario to the construction site and hoisted by the crane to the appropriate floor level and moved into position.
This cost-effective solution ensured the highest standards of fit and finish, but more importantly moved a significant body of work off-location to help ease jobsite congestion and accelerate construction progress.
Building the Bow: The Project Team
At the early stages of the project, the client acquired the land for the development, selected the architect and consulting team, and developed the concept to the schematic drawing phase. Following a developer/owner proposal call, H&R REIT were successful in negotiating the purchase of the property and the project, with the client signing a long-term lease for its space. The conceptual development of the project started in late 2005, with the assignment of Foster + Partners of London as the signature design architect and Zeidler Partnership Architects of Toronto and Calgary as the executive architects. Other project team members include:
As of October 2012, the construction of the main building structure and the envelope is complete, along with the interior fit-out work, completion of the landscaping areas, and pedestrian path link bridges. The building has been signed off for occupancy to the 40th floor.
From the design team perspective, this unique building proves to satisfy the vision and objectives the client had when it entered into this venture. The unique design aspects of the project on the structural side include the unique crescent-shaped floor plate, sophisticated hybrid lateral system, exposed atrium screen, and numerous construction logistics aspects.
Neb Erakovic, M.A.Sc., P.Eng., is a principal in Yolles, A CH2M HILL Company’s Toronto office. He has successfully performed technical design and management roles on some of the firm’s largest and most notable buildings in Canada, the United States, and overseas. Erakovic’s work focuses on complex structures and collaboration with signature architects. Erakovic can be contacted via e-mail at email@example.com.
Walter Koppelaar is president of Hamilton’s Walters Inc., and directed the company’s transition from a regional steel fabricator to a vertically integrated steel contractor of architecture, infrastructure, and heavy industry projects throughout North America. He established the groundwork for the committee that completed the development of the Canadian Institute for Steel Construction (CISC) standard for architecturally exposed structural steel (AESS). Koppelaar currently serves as a board member of the Ontario Erectors Association, and has served as the past-president of the Hamilton Halton Construction Association. He can be reached at firstname.lastname@example.org.
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