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Tuesday22 July 2014

STEEL FOCUS

20 Fenchurch Street by Rafael Viñoly Architects

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Rafael Viñoly’s Walkie-Talkie tower is now flaring out over the London skyline, thanks to 13,000 tonnes of structural steel and 37 unique floor plates

Proclaimed on its hoarding as “the building with more up top”, Rafael Viñoly’s 177m-high tower 20 Fenchurch Street is rapidly taking shape on London’s skyline. Now almost fully clad, the 64,140sq m tower is due for completion next year.

The skyscraper’s controversial, distinctively flared shape — less like its nickname Walkie-Talkie and more like a pint glass — has been realised with the use of 13,000 tonnes of structural steel provided by steelwork contractor William Hare.

With such a distinctive form, this building will always divide opinion as it joins the growing number of unconventional towers now jostling for attention in the capital’s financial heartland. Viñoly maintains that the design – criticised by some as overwhelming – respects the City’s historic character by “following the contour of the river and the medieval streets”.

Commercially, the swelling form makes sense, maximising the footprint by creating larger floor plates as the views get better. And at the very top, the joint developers Land Securities and Canary Wharf Group promise a publicly accessible sky garden with spectacular views over the City.

20 Fenchurch Street 'Walkie-Talkie Tower' by Rafael Viñoly Architects

The tower’s form presented a tough challenge for engineer Halcrow Yolles and contractor Canary Wharf Contractors since each floorplate was a unique shape and size due not only to the widening floor plates but also the concave curve on the north and south elevations and the convex curve on the east and west.

Creating and delivering the solution to a tight 38-month timetable was only possible, according to Canary Wharf Contractors associate director Charlie Paul, with the use of what he terms 4D BIM modelling — the fourth dimension being time. This gives complete coordination of all aspects of design and construction using software compatible with the Revit programme used by the design team, Sketchup Pro used by the contractor and Tekla used by the subcontractors.

This real-time model allowed the contractor to fully coordinate the design team and specialist contractors so that they could anticipate any clashes and hiccups within the various interfaces during the construction programme and act accordingly to eliminate them. In this way, it also enabled those tendering to give a more accurate price, according to Paul.

“We had very little argument and debate with William Hare. The modelling we did allowed us to have a much closer relationship,” he adds.

20 Fenchurch Street 'Walkie-Talkie Tower' by Rafael Viñoly Architects

North section: 20 Fenchurch Street’s pint-glass profile is created as the floor plate flares and then tapers back on the upper storeys.

20 Fenchurch Street 'Walkie-Talkie Tower' by Rafael Viñoly Architects

West section: The steel frame cantilevers after the 25th floor. The upper three storeys are taken up with the sky garden.

Steel was the only viable choice for 20 Fenchurch Street’s structure, lowering the weight of the building and allowing the engineers to use existing foundations on the site, according to Halcrow Yolles buildings team lead Jason Guneratne. It also met the requirement for long spans in order to maximise lettable office space.

The structure consists of 22 box columns arranged on a 9m grid around a central core. Columns were constructed from fabricated sections ranging from 525mm x 525mm square box sections with 100mm-thick plates to 525mm x 350mm I sections with 40mm-thick plates at higher levels.

Each beam was kept to a 600mm depth whatever the span, except at the uppermost levels where they were increased to 1,150mm. All are fixed to the concrete core using embedment plates. Most are fabricated plate girders with web penetrations to accommodate services within the beam depth. Typical floor beam spans range from about 11m at level 2 to 18m on the upper levels.

According to Guneratne, the geometric changes were the crux of the structural design challenge. “Architectural aesthetics were the main driver. When the shape of the building changes, it fundamentally changes every beam a little bit. So we formulated an algorithm that automatically recalculated the positions of all the beams on every floor.”

Bifurcating columns were initially considered in order to achieve the flaring shape but this would have taken up too much space within the plan. Instead, the solution was to facet the steel structure up to the 25th floor, with the facets occurring first every six floors, then every four and finally every two on areas of high curvature as the tower neared its widest “bulge” point on the 27th floor.

However, the maximum beam span that could be tolerated was 20m. So from level 25, the structure’s flare was created using a cantilever to give the final 4m on the north and south faces beyond the column line. On the east and west sides, the columns remain in the facade, which is triple-glazed with panelised aluminium cladding and vertical louvres.

20 Fenchurch Street 'Walkie-Talkie Tower' by Rafael Viñoly Architects

Source: Halcrow Yolles

The steel frame is made up of approximately 8,500 major structural members.

In order to avoid using the three tower cranes needed to erect the structure during the worst of the winter weather, Canary Wharf Contractors asked William Hare to deliver the steel installation in just 36 weeks from May 2012 to January 2013, rather than 41, by working on accelerated hours for 17 weeks. This included the installation of the approximately 8,500 major structural members that made up the steel frame.

Having installed the vertical cladding, the contractors are now building the sky garden. When complete in April 2014, the Breeam “Excellent” building will contain 61,000sq m of offices up to the 34th floor as well as 1,200sq m
of ground floor retail, plus the sky garden, which is intended to be a public space with bars and restaurants served by its own dedicated lifts.

Development costs for 20 Fenchurch Street will total £239 million. So far, with completion still a year away, office accommodation is 56% pre-let.

Faceted structure

20 Fenchurch Street 'Walkie-Talkie Tower' by Rafael Viñoly Architects

Source: William Hare

The columns are faceted up to the 25th floor

To create the flared shape the columns are faceted up to the 25th floor, after which the top of the flare is achieved with a cantilever, both of which simply delivered the desired profile.

The angle of the columns is changed in a concealed, bespoke spigot connection welded to the top of each column during fabrication.

This allowed the next column to be positioned at the correct angle to achieve the facet while avoiding the need for an external flange.

The impact of the bolt head is minimised by the fire-protective covering.

With such complex column geometry, the use of 3D project information was hugely important, according to steelwork contractor William Hare.

Sky garden

20 Fenchurch Street 'Walkie-Talkie Tower' by Rafael Viñoly Architects

The sky garden is a 50m x 60m, double-height glazed space at the top of the tower with clear spans and a giant full-height window on the north and south elevations.

The original intention had been to use a space-frame construction but this was changed to a more economical and faster-to-erect portal frame with 34 structural fins spanning east to west, including two central 20m sections 1,200mm deep. Each is fixed by William Hare into a base connection — a concealed steel “shoebox” typically 750mm x 400mm x 100mm deep. This takes the horizontal forces into the steel structure and accommodates the transition from the aluminium fins running up the side of the tower into the steel roof structure.

Mitre connections on the four corner fins are also accommodated in more heavily loaded “shoeboxes”.

PROJECT TEAM
Developers
Land Securities, Canary Wharf Group
Architect Rafael Viñoly Architects
Executive architect Adamson Associates
Structural and facade engineer Halcrow Yolles
Contractor Canary Wharf Contractors
Steelwork contractor William Hare

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Readers' comments (2)

  • Personally I think it should be named the "Sore Thumb" for very obvious reasons.

    "It respects the City’s historic character by “following the contour of the river and the medieval streets”

    Please spare us this patronising and condecending drivel.

    At least the previous occupant of the site was not overbearing in the way this lump is.

    The only saving grace might be this sky garden, if indeed it is accessible to the public for a reasonable price and at least from there, you won't be able to see it...

    Unsuitable or offensive? Report this comment

  • Irrespective of whether you like, love or hate it, you'd have to be drunk to suggest it follows 'the contour of the river and the medieval streets'.

    Unsuitable or offensive? Report this comment

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