Technology Corner
Concrete Vision Image courtesy of WSP Cantor Seinuk
Engineering the United Kingdom’s tallest residential post-tension concrete building
The tower has a 3-metre-deep concrete raft foundation that sits on sandstone rock
W
ith a slender tower measuring 155metres high, the 48-story Beetham Hilton Tower in Manchester features 279 hotel rooms and an additional 219 apartment units. A glass and steel blade tower with an adjacent five-story steel-frame podium structure that houses the hotel ballroom, restaurants, and bars extends an additional 15 metres above the main roof for a total height of 170 metres from ground level, making it the tallest post-tension concrete residential building in the United Kingdom. The tower foundation is a 3-metre-deep concrete raft foundation that directly bears on sandstone rock. Using a raft design instead of piles delivered significant cost savings. Pad foundations support the lower structural steel podium block, and the perimeter walls for the basement are contiguous bore pile
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walls that retain the surrounding ground and external services adjacent to the site. The core walls provide lateral stability against wind and dynamic performance, and outrigger shear walls act as a cantilever from the basement raft. Two concrete cores that measure 8 metres by 9 metres with shear walls running from front to back extend through the building’s full height. One of the cores incorporates stairs, lifts, and services; the other is purely structural. Reinforced concrete columns around the building’s perimeter supplement the shear walls. The different functional areas within the building – hotel, residences, and the ground-floor lobby area – required the column layout to change from the top level to the ground level. Walking columns are used as transferred structural systems for the two short edges of the building. The edge columns walk in two directions at two different levels over several metres. Below the 23rd floor, the maximum width of the tower is 16 metres. The height-towidth slenderness ratio of about 11 is relatively high. Above the 23rd floor, the floor extends a further 4 metres beyond the north face of the main core to accommodate different functional requirements for residential use. A combination of post-tension slab and in situ concrete cantilevered beams extend from the main core walls to support the extended cantilevers.
Crowning glory
The building design and signature blade feature posed a number of challenges for WSP Cantor Seinuk. The firm provided all engineering services for the building, including structural, building services, fire, and environmental engineering. WSP relied on an assortment of structural design and analysis software applications, including STAAD.Pro for transfer structure, feature blade frame, and spiral-stair analysis and design, and RAM Frame for lateral analysis of the core and frame. The 22-metre-tall structural steel feature blade forms a striking visual statement for the building. WSP used STAAD.Pro to model the structural framing for the blade and to analyse the design. The team studied the structural forces and dynamic behaviour of the structure, with STAAD.Pro allowing easy changes to
section properties, support conditions, and connection options during various stages of the design development. This enabled the engineering team to quickly assess the steel weight of the frame and study temporary stability conditions to determine constructability and the best method for erecting the blade on top of the building. Also, engineers could make late modifications to the design to accommodate construction methods and the lifting capacity of the tower crane. Additionally, unexpected wind-noise effects surfaced during installation of the roof blade. To devise a solution to the problem the engineering team used STAAD.Pro to study the dynamic behaviour of individual elements and identify the natural frequency of the members. Analysing the walking columns also proved challenging. The engineering team again used STAAD.Pro for the finite element analysis of the walking columns and loadtransfer system. For the core wall and framing of the building the engineering team deployed RAM Frame. During conceptual design, the program was used for varying concrete properties and crack factors and for assessing wall thickness and geometry required for the core. During design development and detailing, RAM Frame evaluated and adjusted the design to accommodate changes such as new openings, wall lengths, and slab openings. During construction, the program helped assess temporary conditions caused by changes in construction sequence and out-of-tolerance conditions. The products’ ability to easily modify input data and produce adequate and sensible output helped the team make informed decisions during various stages of the project. This proved critical for late changes that required rapid decisions and affected cost and project schedule. When the landmark building opened for guests in October 2006, it was regarded as a symbol of Manchester’s recent economic resurgence. In fact, the Council on Tall Buildings and Urban Habitat selected the Beetham Hilton Tower as the Best International Tall Building for 2007. A Bentley Systems Pty Ltd