Veering Towards a Unique Building Structure
September 2020 – by Neb Erakovic
I began my career as an academic at the University of Sarajevo and progressed as an engineer and project manager with major Canadian engineering firms–including Halsall and Yolles. Over the course of my 25+ year career, I’ve been fortunate to cultivate numerous and major market sectors, both locally and internationally, and enjoy delivering complex “first-of-a-kind” structures. One of them being the Veer Towers at CityCentre, on the Las Vegas strip, completed in 2010.
Above: The Veer Towers, situated on Block C of the CityCenter development on the Las Vegas Strip. The design team included both local and Toronto structural engineers.
The Veer Towers are twin high-rise residential buildings that each lean at opposing five-degree angles from the vertical. The leans are the defining architectural characteristic of these 37-storey, 480 feet tall towers and required the sophisticated application of conventionally reinforced concrete to the structural building systems. The towers house a combined total of 670 condominium units, rooftop amenity spaces, and podium lobby spaces.
Challenges
Numerous factors contributed to the leaning effect, demanding the towers to resist veering of its intended geometry. These include:
Overturning effects due to eccentricity of the building mass along the height
Lateral earthquake and wind loads
Differential axial shortening of gravity load carrying structures
Differential foundation settlement
The structural design ensures that the towers remain stable, the leans remain within a controlled range, all building movements within acceptable building code limits, and comparable performance to vertical buildings is achieved at all times.
Left: Veer Towers during construction; Right: Building cross-section
Structural Features
Inclined Lobby Columns
The residential lobbies of both towers are located at the sloping South facades adjacent to the elevated roadway. These 80 ft tall lobbies feature exposed concrete columns that are inclined to follow the leans. Located at the base of the towers the columns have significant axial forces, particularly the West Tower, which has a lobby situated on the down-lean side. To minimize column sizes and enhance slenderness performance due to their height, the composite columns utilized large embedded ‘W’ steel sections encircled with 8% of section with reinforcing bars. Temporary shoring was required to hold the columns in their correct position during construction until the tops of the columns could be tied into the transfer floor level above the podium.
Branch ‘Y’ Shaped Column Transfers
Although the building core and internal columns are concentric and vertical, the leaning geometry of towers required use of inclined columns along the sloping building facades at the North and South side. As the floor plates shift a total of 35 feet in the North/South direction, the ‘Y’ shaped columns at 6th, 19th and 32nd floors were required to branch-out where the new internal vertical columns are required to maintain feasible spans for the 8” thick flat reinforced concrete floor slabs in the residential areas.
Post-Tensioned Transfer Beams
Eight major post-tensioned (PT) transfer beams are located at the transfer level separating the lobbies/podium and the condominium levels. Due to space planning requirements, a number of residential columns were terminated at the transfer level in order to open up the building common spaces below. These PT transfer beams support 34 storeys of the towers above. The parallelogram-shaped floor plate and architectural layouts resulted in some transfer beams intersecting others, creating complex multi-staging stressing and reinforcing details.
Above: Veer Towers as seen from The Area Resort and Casino
The End Result
The conventional structure, incorporating reinforced and post-tensioned concrete, was innovatively applied in the extreme application to design the structure of the Veer Towers.
P-delta effects due to the eccentric gravity loads, environmental and seismic lateral loads, differential axial shortening of vertical elements, and foundation settlements tend to significantly impact the tower lean. This requires a detailed verification utilizing complex and sophisticated design tools and processes to ensure the intentional architectural geometry is controlled.
Branched ‘Y’ shaped and inclined columns can be applied to reinforced concrete buildings. Structural steel ‘W’ sections can be embedded into tall, heavily loaded and highly reinforced concrete columns to optimize section dimensions and enhance performance.
With careful attention to reinforcing detailing, large intersecting PT transfer beams with offset loads and supports are viable options to increase usable building spaces by terminating columns above and transferring their loads to adjacent structures.
Fast-track projects shall be continually monitored during construction to ensure the intended geometry is achieved. Super-elevating construction techniques and expansion joint detailing at the adjacent buildings shall be used to accommodate movements during and after construction. The architectural success of the striking leaning Veer Towers is reliant on an advanced structural engineering solution utilizing conventional construction with a stiff and strong reinforced concrete lateral load resisting systems to withstand a natural tendency to lean further than intended.
For more information contact: neb.erakovic@piccogroup.com
Principal, Building Structures at PICCO Engineering
Neb is a recognized industry leader, creative technical expert and collaborator, who thrives on challenging roles within projects of all scale and complexity. He is focused on results and committed to excellence in delivery of broad project objectives; adaptable and driven by an enthusiasm for creative solutions that deliver value to his clients and society. Neb has been an invited speaker at major AEC industry conferences and has contributed to the development of the overall industry as a past president of the American Concrete Industry (ACI) Ontario Chapter.
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