Exploring Geometry and Form in Tall Buildings

Chris Wilkinson, CTBUH 2016 China Conference

This paper describes the design approach to four tower projects on four continents.  Each is different and relates to its particular brief and context.  Each explores geometry and form in a different ways ranging from the conventional rectangular vertical extrusion in Toronto, to the sculptural curvilinear form in Sydney, the deceptively simple looking trocoidal tower in Guangzhou and the understated stacked rectangles in London which relate to planning constraints. They are all designed by WilkinsonEyre.   

Guangzhou International Finance Centre

The first tower project was won in an international competition back in 2005 and, at that time, the practice was relatively small and had no previous experience in the design of tall buildings. However, the competition was run by the City of Guangzhou, who were relatively open minded about their procurement procedures.  

WilkinsonEyre, who were in a joint venture with Arup, were selected in August 2005 and work was expected to start on site by the end of the year.  A challenging programme by any standards, but it was achieved and the project was completed in 2011 for the Asian Games with the hotel opening in 2012.

The result is a 440m tower with 103 storeys, 165,000moffices on 69 floors and a Four Seasons Hotel on the top 34 storeys with their reception on the 70th floor. At that time, this building would have been one of the tallest buildings in the world but, of course, things have moved on and there are now many more taller supertowers.

In terms of the architecture, the desired aesthetic was for an ‘elegant simplicity’ with no frills or unnecessary decoration.

The curved triangular plan was chosen for its efficient layout and because, in studies, it proved to be good at shedding wind load.  Following wind tunnel testing, an estimated 20% was saved on the supporting structure by pointing a corner of it into the prevailing wind.

The geometry of the vertical form, looks simple but is surprisingly complex because it tapers out from the base to a maximum girth at a third of its total height then reduces again to a minimum at the top.  Each of the three faces of the building are set out on a radius of 5.1km, which was determined through 3D modelling and rapid prototyping models.  Many options were considered, but the final form was selected because it gives the most generous floor space for the offices at the lower levels and the minimum depth for the single aspect hotel floors at the top.

In plan, the outer surface of the three sides is also curved with a radius of 71m and the corners have a tighter radius of 9m, which is constant although each floor has a different area, because of the angle of the outer skin. 


In towers of this height, the central core cannot provide sufficient lateral stability and, with the hotel accommodation at the top, movement had to be restricted to a minimum. So a decision was taken with the engineers, Arup, to go for an exoskeletal structure.

Much analysis went into the design of this structure to achieve maximum efficiency in engineering terms but the visual aspects were also carefully considered.  The drawings explore the pattern of the diagrid in 3D in order to ensure the best node set out.

With the structural priority in mind, the diagrid proportions extended to 54m between diamond nodes and, for ease of construction, the exoskeletal structure was fabricated out of steel tubes filled with concrete.  The scale is of almost monumental proportions with the node connections extending over two floors and the tubes are 1.8m diameter at the base, reducing to 900mm on the upper floors.

Clearly, the setting out of the structure was extremely difficult because of the curvature, but the local Chinese contractors managed to achieve it within the required tolerances.

Much of the gravity load of the building floors is taken by the structural core, which links back to the perimeter diagrid structure via floor beams to create a stiff tube-within-tube system. Whilst the core is a straightforward vertical extrusion in reinforced concrete, it was planned for maximum letting flexibility.  Its complexity relates to the planning for the 71 lifts in which express double-deckers serve the skylobbies with shuttle lifts between.  The lavatories are located in the areas where lifts tail off.

In section, the central core stops at the 69th floor allowing for a spectacular 120m high top lit atrium for the hotel above the lobby reception on the 70th floor.

Within the atrium, the geometry of the hotel corridors has been designed on a twisting angular grid that helps to bounce the light down through the space like a kaleidoscope.  The roof is glazed and the top eight floors are enclosed with glass for smoke control.  

By contrast, the outer skin of the tower has been designed to achieve a simple clean line which emphasises the pure geometry of the building’s shape.  Floor to floor unitised glass panels provide maximum repetition that is only interrupted by the plant and refuge floors which are characterised by a vertical hit and miss glazing system.

The cladding is smooth, aerodynamic and easy to clean.  Its simplistic modularity is only interrupted by views of the diagrid structure that can be seen through the transparency of the glass skin.

Crown Sydney Hotel Resort

In 2013, WilkinsonEyre won an international competition for the six star Crown Resort Hotel on an important site at Barangaroo in Sydney.  The waterfront site on Darling Harbour called for a special approach and the WilkinsonEyre proposal was for a sculptural form.

The idea came from previous work on a landmark installation that had three petals which twisted through 90o as they rose up to the sky. This was really just a sculpture but it offered the potential to become a building by filling in the space between the petals with accommodation and growing it to become a tower.  In this instance, the twist has been reduced to 60o and the sculptural form rises up to a height of 275m.

The narrative about the petals proved to be an extremely useful means of describing such a complex piece of geometry and one that people could understand and appreciate.  The development and interpretation of the idea into an architectural proposal, however, was not easy, with every floor being a different shape and the twist causing the plan accommodation to spiral with the form.  This also complicates duct risers and partition wall connections to the exterior cladding.

The structure has been achieved with a vertical core and outriggers but the perimeter columns are helical, following the twisting form of the exterior cladding.  This may be one of the first times that this concept has been used in a conventional building.  Perhaps the biggest problem, however, is the design of the cladding and no single solution provided the answer, so three different systems were required.  A triangulated diagrid was selected for the west elevation facing Darling Harbour and where the curvature is greatest.  A stepped system of floor to floor rectangular panels was selected on the main facades where the curvature is less, but cold pressing will be required where the curvature pushes the glazing out of tolerance.  The third system is a more conventional curtain walling for the block of hotel rooms, which are vertically stacked.  Architecturally, this change of cladding systems also serves to emphasise the curvature and twist of the tower’s geometry which is so distinctive.

As with all towers, the roof top where it meets the sky is visually important and it is here that the three petals are expressed in their own right where they form a spectacular three storey sky villa.

As with many resort hotels, the Crown Sydney requires a large podium to house all the restaurants, cafés, beauty salons, spa rooms, shops etc. as well as the two casinos and this is clad with a veil of stone tracery tensioned up with steel cables which enables it to span up to 28m with intermediary connections back to the main structure. This is designed to be constructed out of CNC cut 1.5m long marble blocks, which are tensioned up with stainless steel cables. The curvilinear pattern of the tracery has been designed to have an apparently random appearance although it is made up of repetitive elements.

The veil, which provides shading to the perimeter terrace, is intended to give this resort hotel a distinctive and luxurious appearance.  This referencing back to European Gothic stone tracery gives an appealing historic narrative to an otherwise contemporary architecture.

A prototype has been built in a stoneworks outside the City which has proved that the system works and can be constructed in an efficient manner.

The concept for Crown Sydney is an ‘inhabited artwork’ that takes advantage of its spectacular site, and provides a visual marker to the northwest corner of the City’s main Central Business District and at the same time it provides spectacular views across to the Sydney Opera House and the Harbour Bridge. This project is due for completion in 2020.  

CIBC Square, Toronto

The third tower is a regeneration project in the centre of the City of Toronto in Canada.  Here, the brief was to extend the Central Business District towards the lake and beyond the railway lines.

Bay Street is in the heart of the financial district which is contained on the south side by railway lines leading to Union Station and the Gardiner Expressway, which is raised above ground level.  The site, which is currently partly occupied by the main city bus terminal, provides the opportunity for the new development of two towers, one on either side of the railway connected by a new public park above the tracks, and a newly configured bus terminal at ground and first floor level.

The first phase of this development, which is due to start on site later this year, is 45 Bay, a 54-storey office tower and associated podium, as well as tenant amenity space, conference facilities and the new bus terminal.

The tower has a simple but elegant rectangular extruded form with a central core and concrete frame.  Its distinctive appearance is achieved with a triangulated façade of shallow pyramids that catches the light differently on each facet.

Its strong diamond pattern extends over 10 floors and projects out by approximately 0.75m from the slab. The pattern is defined by an aluminium frame that visually overrides the vertical curtain walling system. 

The rectangular form is divided into two square sections with a recess between that helps to reduce the visual bulk and creates an increased number of corner offices. The two parts, which finish at different heights, are crowned with an open crenulated glazed section which is highlighted on the skyline.

In this project, the geometry and the form of the tower are much simpler than the Guangzhou International Finance and Crown Sydney but it is designed to fit into the Toronto city context which is generally more understated in its architecture.


150 Leadenhall Street Tower

The fourth and final project is in the City of London, which is where WilkinsonEyre is based, but it is perhaps the most challenging in terms of design. London, being steeped with history, makes new interventions interesting but also highly constrained, not just on height and gross floor area, but also by restricted view corridors, particularly related to St. Paul’s Cathedral.

The site, on the corner of Leadenhall Street and Bishopsgate, marks an important gateway into the insurance and financial district of the City.  It is located in what is known as the Eastern Cluster, where taller buildings are permitted.  It is close to Rogers Stirk Harbour’s recently completed tower the Leadenhall Building (commonly known as the ‘Cheesegrater’) and adjacent to a KPF design for a tower known as the ‘Pinnacle’ at 22 Bishopsgate.  However, the Pinnacle, which was under construction, was put on hold during the last financial crisis and now has a new design by PLP Architects for a different client. 

Whilst the context is important, the most defining constraint came from the Chief Planning Officer’s requirement for the new building not to intrude into the angled line of the south face of the Cheesegrater as seen from a prescribed viewpoint in Fleet Street.  Whilst this may seem perverse, it does relate to a heritage requirement to preserve visual space around the dome of St. Paul’s Cathedral from Fleet Street which is on the ceremonial route from Buckingham Palace and it was this requirement that shaped the Cheesegrater at the planning stage.

The view constraint heavily influenced the design of 150 Leadenhall Street and that determined its stepped form which was interpreted by WilkinsonEyre as a series of stacked rectangular boxes, connected with a side core on the northern boundary. From the top, the double height rectangular box creates a public viewing pavilion which sits on a narrow but deep stack of office accommodation that sits on and cantilevers forward, above a wider block, that in turn sits on a recessed plinth at street level.  The visual aesthetic is based on modernist principles where elements are expressed individually with clean undecorated appearance. Each of the stacked blocks has a slightly different character which is expressed in the cladding systems. The top pavilion has wide shopfront glazing to maximize the views and the narrow block below has a double skin curtain walling system with low iron glass.  Below that, the wider block is designed with a glazing module with an expressed steel structure that supports the cantilever to the block above.  A third block behind, has an even closer glazing module of 750mm.  The corner building has a change of cladding from glazing to stonework which relates to the adjoining historic buildings and helps reinforce the identity of the street.

In plan, each of the stacked blocks is set out on a twisting geometry related to adjacent buildings and road alignments, which sets up a dynamic composition from different viewpoints in the City.

Summary

So the four projects are each very different in character because they are designed to suit their specific site Context and Brief.  The Guangzhou IFC strives for an elegant simplicity in the new CBD that distinguishes it from the many other towers in China’s third city.  

Crown Sydney is designed as a sculptural form which creates a landmark on its important waterfront site at Barangaroo. The Bay Centre in Toronto makes a strong visual statement to extend the Financial District beyond the railway towards the Lake.

And the 150 Leadenhall Street tower in London, which marks a gateway into the financial and insurance district, takes a restrained architectural approach that sits comfortably in the City’s Eastern Cluster.

At WilkinsonEyre, there is no house style and each project relates to its particular brief and context, but the interest in geometry carries through to each project which provides the opportunity to explore something new.  It determines the form and appearance of the building which is expressed in the architecture, giving it a distinctive character and individuality.