Engineering Tall in Historic Cities: the Shard

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Title: Engineering Tall in Historic Cities: The Shard

Authors: Kamran Moazami, Director, Head of Building Structures, WSP Group Ron Slade, Director, WSP Group

Subject: Structural Engineering

Keywords: BIM Structural Engineering

Publication Date: 2013

Original Publication: CTBUH Journal, 2013 Issue II

Paper Type: 1. Book chapter/Part chapter 2. Journal paper 3. Conference proceeding 4. Unpublished conference paper 5. Magazine article 6. Unpublished

© Council on Tall Buildings and Urban Habitat / Kamran Moazami; Ron Slade Structural Engineering Engineering Tall in Historic Cities: The Shard

Can historic cities preserve their character and identity while embracing the modern tall building phenomenon? Two engineering-team leaders behind the Shard discuss the creative solutions needed to insert a massive project into the dense historic and infrastructural fabric of London.

The Shard and The Place at London Bridge One of the key challenges of building tall in (see Figure 1), projects by developer Sellar historic cities is keeping streetscapes Kamran Moazami Ron Slade Properties / Qatari Diar and architect Renzo permeable and delicate. Certainly, tall Piano, demonstrate the special challenges buildings require large cores and huge Authors faced by engineers and architects to preserve columns to take lateral and gravity forces, but Kamran Moazami, Director, Head of Building Structures and develop the character of historic cities, they must also respect people and public Ron Slade, Director while properly embracing the tall building spaces at street level. Historically architectural WSP UK phenomenon. Both buildings work within the development has not paid suffi cient attention WSP House, 70 Chancery Lane London WC2A 1AF ancient street pattern of Southwark and to the interface of tall buildings and life at United Kingdom demonstrate how modern development can ground level. Now it is generally accepted by

t: +44 20 7314 4654 add new life to an area without destroying the the best designers that the eff ect at street f: +44 20 7314 5111 local environment. Southwark, like other parts level is equally as important as the impact on e: [email protected]; [email protected] of London, suff ered heavy damage during the the skyline, and is critical to a project’s www.wspgroup.co.uk Second World War, but retained its historic integration into the culture and lifeblood of a street layout. In fact, the shape of The Shard city. Kamran Moazami site was created by bombing during the war. With over 33 years of structural engineering experience, Kamran works between the United In recent decades, the planet has urbanized Kingdom and United States, ensuring that clients The site was redeveloped in 1975 by the on an unprecedented scale. We need to involved in property development derive the full benefi t of WSP’s internationally renowned, specialist construction of a rather insensitive, inward- create cities that can cope with these expertise in structural engineering. looking building, but now, with the help of demographic changes and accommodate the Kamran received his BSCE and MSCE from Columbia far-sighted designers, the new buildings are inevitable pressure on transport, energy, University and his professional engineering licences in New York. In 1989 he set up WSP Cantor Seinuk’s on the verge of changing the whole area into water, and living spaces. In an age when more London offi ce and since then has been involved in an iconic part of London, adding to the rich than half the world’s population lives in cities, more than 20 million square feet of mostly specialist structures throughout the United Kingdom. He texture of the neighborhood, rather than there is more demand than ever for space. Tall has also been involved in masterplanning and detracting from it. It was no accident that buildings off er the most effi cient use of land; development of such prestigious projects as Canary Wharf, Wood Wharf and Bishopsgate Good’s Yard. Renzo Piano acknowledged the history of building up, rather than out, must be part of London and the local area in his planning the solution. It is part of the responsibility of Ron Slade submission by referring to the infl uence of engineers to design the structure of tall Ron received his BSc in Civil Engineering at City 17th-century church and cathedral spires. buildings so that they meet the various needs University, London and became a chartered member of the Institution of Structural Engineers in 1971, when he was awarded the Institution’s A. E. Wynn prize. He was fi rst appointed as a director in 1982. Ron has special expertise in construction forms and methodologies and materials use. He has been involved in a wide range of jobs in various sectors, on both new-build and refurbishment projects. Over the past 13 years, Ron has specialized in high-rise construction. His recent work includes concept design for several London projects, including both the Shard and the Place at London Bridge, and the Milton Court development, which incorporates the Guildhall School of Music and Drama. Ron has published papers in The Structural Engineer, Civil Engineering and elsewhere and chairs WSP’s Technical Strategy Group for structural engineering for the advancement of technical knowledge and design skills.

Figure 1. The Shard & The Place at London Bridge. © Sailko

44 | Structural Engineering CTBUH Journal | 2013 Issue II Figure 2. The Shard – winter garden. © Renzo Piano Building Workshop Figure 3. Existing and new piles at the Shard. © WSP of the urban population in a fl exible and for tenants. These spaces are outside the under-reamed piles founded in the London responsible way. Consequently, the challenges double-glazed part of the façade, but within clay. They extended only a few meters below for the engineer are numerous. the outer single-glazed skin. Ventilation allows The Shard basement slab, and so could not be fresh air into these spaces, and the steelwork reused to provide vertical support for the new detailing is visually enhanced and left on view building. It was not economical to remove the The Right Streetscape (see Figure 2). old piles, so the new piles were designed to pass between the old ones and their under- In the past, street design has not been a By breaking away from the standard, risk-averse reams (see Figure 3). priority for all stakeholders – the net result is approach to design, and instead following a that tall buildings have been planted on sites rational and creative process, we can achieve Existing piles were not the only constraint to without due regard for their surroundings. By high-quality places that embrace modern tall be dealt with at the Shard. Other existing engaging in a diff erent approach, through an building design. This leads to large buildings infrastructure in the vicinity included London evolving and collaborative process with that sit comfortably alongside buildings from Underground’s Jubilee line tunnels, which were professional designers and enlightened key diff erent ages. Instead of dominating historical about 5–10 meters from the northwestern stakeholders, a robust and respectful result is cities, they will complement and contribute to corner of the Shard basement, a disused stair achievable. Analyzing diff erent street the development of our cities in a way that shaft inside the site, a disused lift shaft typologies and public realm treatments and preserves and enhances urban character. In straddling the boundary, and a ventilation shaft assessing capacity for pedestrians, public well-considered schemes, all applicable to the west, in Joiner Street. The adjacent transport, and traffi c now contributes to engineering disciplines will have studied traffi c streets also contained the usual sewers, balanced streetscapes, in terms of movement and pedestrian fl ows, wind comfort, and the Victorian water mains, and other utilities. and the psychological perception of tall permeability of structures at ground level. buildings. Creating good-sized fl oor plates in the tower Dealing with a Spider’s Web of Existing called for some clever structural solutions and The concept of the “vertical city” is gaining Underground Infrastructure meant building as close as possible to the acceptance, resulting in more mixed-use tall underground infrastructure. Limiting ground buildings in recent years. The Shard recognized Unlike building on clean sites in suburban movement was critical to protect London that tall buildings need to deliver places that areas, building on historic inner city sites often Underground’s assets and to ensure no people enjoy for living, leisure, and shopping, has inherited complexities that must be damage occurred to the water main. Therefore, as well as for working. The wider economic, considered. New development is constrained 3D fi nite element analysis was used to predict social, and environmental benefi ts of tall by a spider’s web of ground-level and potential ground movement and convince the buildings set in a good public realm is beyond underground infrastructure, which presents statutory bodies that it was safe to work within question. A well-positioned tall building has a enormous challenges to the designer. sensitive close-proximity zones. regenerative eff ect on its surroundings. The Shard foundations in London were aff ected Movement monitoring, vibration, groundwater In some instances, the streetscape can be by the piles left in the ground from Southwark mitigation and possible re-use of old piles were taken into the buildings. At the Shard, each Towers, a 26-story reinforced concrete building taken into account in the design of the Place as offi ce fl oor is provided with three winter constructed in the 1970s. This building had no well as the Shard. These factors aff ect many gardens in order to improve the environment signifi cant basement, and was supported on other developments in London and elsewhere.

CTBUH Journal | 2013 Issue II Structural Engineering | 45 Made ground +4.7 m OD to 0.0 m OD techniques were used to adjust plant circumvented. This enabled the River Terrace Gravel 0.0 m OD to -6.0 m OD positions and to package the plant rooms superstructure and basement works to London Clay -6.0 m OD to -27.0 m OD more effi ciently, resulting in reduced proceed simultaneously (see Figure 4). The Lambeth Group Clay excavation and construction costs, and archaeological work, essential to our -27.0 m OD to -39.0 m OD (cohesive soil) program risks. understanding of historical cities, was Lambeth Group Sand -39 m OD to -45.0 m OD completed without compromising the job. (granular soil) As well as reducing the size of the basement, Thanet Sands -45 m OD to -57.5 m OD the Place team also had to fi nd a construction The team selected 1.2- and 1.5-meter Chalk below -57.5 m OD methodology that would minimize delay. Top- diameter rotary bored piles founded in the down construction was the answer to this top of the Thanet Sands to support the very Table 1. Ground confi guration at The Place site. challenge, enabling work to progress above large concentrated loads. ground while the basements were Navigating Around Sensitive Buried Secrets constructed below ground. Ground conditions at The Place are typical of inner-city development sites, consisting of The potential presence of archaeological Top-down construction is essentially a made ground over natural strata (see Table 1). artifacts also complicates building tall in time-saving methodology, wherein The areas to be piled were tackled initially by historic cities. Excavations have to be construction takes place from the ground the archaeologists. When their work was painstakingly inspected by archaeology level downwards, rather than excavating to complete, the pits were backfi lled and piling teams, and the unpredictable nature of the bottom of the basement and working up. could begin. archaeological excavation poses a big risk to At The Place, 500-by-500-millimeter steel the program. columns were lowered or “plunged” from The basement perimeter was piled fi rst, and ground level through empty pile bores into plunge columns were used to support the Smaller buildings covered the site for The freshly poured concrete. The ground level slab core and top-down slabs. There was so little Place at London Bridge Quarter, and Roman was then cast on grade. Once it had gained room on the southeastern side of the site that remains had been left undisturbed. The time strength, the slab was capable of propping a single 2.4-meter diameter pile was needed needed for the archaeological dig threatened the perimeter embedded walls and could be to support the load, rather than a more typical to delay construction in the ground. Engineers supported on the knew that time lost in the ground is rarely plunge columns, recovered. In the interest of minimizing risk to allowing excavation to program, the team responded through the commence below the rather drastic measure of reducing the depth slab. of the basement by one level. Through top-down How could the team deal with losing a whole construction, the risks basement in a project of this scale? In some posed by ways, the answer was Building Information archaeological works Management (BIM). The software and to the program were

Figure 4. Top down construction at The Shard. © WSP Figure 5. Avoiding the historic banana dock wall in London’s Docklands. © WSP

46 | Structural Engineering CTBUH Journal | 2013 Issue II construction continued beneath. At one stage, the plunge columns beneath the core were supporting 23 stories to Level 21. The The Shard lowest parts of the core walls were cast using “ self-compacting concrete, pumped from the foundations in London base of the shutters, up to the underside of the previously cast concrete. were affected by the piles left in the ground The foundation piles, constructed in similar strata as the nearby Place, were bored up to from Southwark 1.8 meters diameter, using standard auger rigs, to a depth of up to 53 meters from Towers, a 26-story ground level. The piles were cased through the alluvium and gravels, and bentonite was reinforced concrete used to support the bores in the water- bearing Lambeth Group beds and Thanet building constructed in Sands. the 1970s. A pile test, in accordance with the Institution ” of Civil Engineers (ICE) specifi cation for Piling Figure 6. The Place – raking columns and corbel. © WSP and Embedded Retaining Walls (ICE 2007) was More Plot Constraints at The Place conducted in the early stages of the piling group of three 1.2-meter diameter piles program, and the results were used to justify a In historic city settings, designers and spaced three meters apart, which would have factor of safety of 2.25. A vertical tolerance of engineers of tall buildings often need to cope taken more space in this narrow corner of the 1:200 was specifi ed for the secant piles in with highly constrained footprints. building. By agreement with London order to ensure that they remained Underground, the pile was formed only 1.5 interlocked for the full depth of the basement At The Place, not nearly as tall as The Shard at meters from the Jubilee Line tunnel wall. Piles – 98% of the piles achieved this tolerance. 17 stories, it was nevertheless necessary to close to the underground infrastructure were cantilever the frame outwards from the sleeved to minimize ground movement. Many of the bearing piles inside the basement ground level perimeter by up to 12 meters, in Because the piles were so close to sensitive perimeter contained plunge columns to order to maximize the available air space underground tunnels, best practice soil- support the core, superstructure columns, while leaving the underground infrastructure structure analysis was carried out to fully and upper basement slabs. They were unaff ected. Here, rather than opting for a explore likely ground movements and installed using a hydraulically controlled, typical space-hogging two-story high transfer convince the guardians of the assets that laser-guided rig specially developed for the structure, the structural engineer developed ground movements could be properly project. The plunge columns achieved a corbel arrangements to transfer the loads to controlled. Further confi dence was provided vertical accuracy of 1:400, and the tops were ground more effi ciently. by real-time monitoring of the surrounding within ±15 millimeters of the specifi ed area. positions. Raking columns extend from the cores to the façades, at Level 3 to Levels 11 and 12 and are Twin cores were “jump-started,” borrowing a All these measures are necessary tied back to the cores with specially designed technique pioneered on The Shard. Jump- consequences of building in historic cities. beams (see Figure 6). A strong, triangulated starting counters the costs inherent in Other examples include building transfer structure was created, with the lower fl oors building to reasonable programs on confi ned bridges over archaeologically important suspended from the nodes where the raking and constrained sites in historic city centers. In structures to leave them in place and columns meet the façade. This form of eff ect, the cores were also built top-down, undamaged for the benefi t of future construction required the use of extensive and therefore the superstructure could move generations. This approach was taken to temporary works to support the lower parts ahead without waiting for the basement protect the Grade 1-listed dock wall at Canary of the frame while the corbel arrangements “bathtub” to be completed. Wharf, where transfer structures were built to were being erected. divert load to safe positions on either side of Similarly at the Shard, the slip form was set up the wall, or building columns were sloped to The Place’s constrained site also required at the second basement level, and while the avoid the wall completely (see Figure 5). cantilevering over the bus station next to the core was being built upwards, basement building, so again temporary works had to be

CTBUH Journal | 2013 Issue II Structural Engineering | 47 located skillfully to keep the service impact on construction. operational. To add to the pressure, the bus Pre-sets were built into station had to be clear of temporary works in the frame so that fl oors time for the 2012 Olympics – all part of were eventually level. building in a vibrant, historic city!

One of the most unusual aspects of The Place More Plot Constraints at is the fact that the cores are subject to The Shard permanent unidirectional lateral loading. This is because the building and associated At The Shard, the vision cantilevers are asymmetrical, and the building of a “vertical city” within a pulls to the east. The cores are more heavily constrained area set into reinforced and have thicker walls than would ancient street patterns otherwise be required, to provide suffi cient inspired the tapered stiff ness. The cores were built with a “pre-set,” a shape of the building. Figure 7. Steel framed structure above the residential levels. © Terri Meyer Boake lean toward the west, and were gradually The engineering design pulled vertical as the building was construct- needed to bring this to reality. Large fl oor two fl oors to be added without changing the ed. plates were preferred for offi ces because of height of the building. The thicker, normal- their suitability for commercial operations. weight concrete slabs in the upper levels As the frame was built, the loadings on the Intermediate levels were allocated to provided additional mass, as well as the cores and individual steel members gradually restaurants, and the luxury ShangriLa hotel, necessary acoustic separation between hotel increased. WSP had to calculate the which required a corridor near the core and and apartment fl oors. movement of the building and predict its suites around the perimeter. Very large fl oor plates would not have been suitable for the Creative engineering can maintain strength hotel because each room required a view. and mitigate the horizontal forces due to Exclusive residential apartments were wind. By using concrete at the upper levels for positioned at the top of the building, the Shard, additional structural damping was The lower level occupying entire fl oor plates in most cases, delivered. Wind-induced horizontal “ and in some cases two fl oors. accelerations in the apartments were limited fl oors were framed in to 0.015 m/s² during a 10-year storm, steel to suit large The vision to create this mixed-use vertical following guidance published by the Council city and desire for the tapered shape informed on Tall Buildings and Urban Habitat. In early spans, while in the the choice of material and structural system. designs, the structure was conceived as steel At the Shard, the lower level fl oors were throughout the full height, but the low upper levels, the spans framed in steel to suit large spans, while in the damping of the steelwork required the use of upper levels, the spans are smaller, and a tuned mass damper. Removing the tuned are smaller, and post- post-tensioned concrete was more suitable. mass damper from the design saved money Spans were suffi ciently short in the top few and weight, while releasing a further two tensioned concrete concrete fl oors to allow the use of normal fl oors of useful space. In some ways, it was was more suitable. reinforced concrete construction. The spire counter-intuitive that concrete near the top of was framed in steel for ease of construction the building improved the dynamics. Spans were (see Figure 7). However, the relatively small area of the upper fl oors led to only a small increase in mass, and suffi ciently short in To create extra space in a constrained site, at the additional damping inherent in the the Shard’s upper levels there was no need for concrete-framed upper stories provided the top few concrete a large ceiling void, because the services adequate compensation. routes were above the corridor around the fl oors to allow the use core. A 200-millimeter thick post-tensioned Constructing the spire at the very top of the of normal reinforced fl at slab reduced the structural depth and this, Shard while 120,000 people per day were together with shallower fi nishes in the upper using the London Bridge station at its feet concrete levels, reduced the fl oor-to-fl oor height by presented a considerable challenge. The steel 550 millimeters, compared to the composite frame was more than 60 meters tall, and there construction.” steel fl oors in the offi ce levels. This allowed was no lay-down space at ground level to 48 | Structural Engineering CTBUH Journal | 2013 Issue II With this reassurance, London Underground was prepared to allow construction fi rm Mace to work within 1.5 meters of the sensitive Northern Line escalators. Work was constantly monitored for movement using real-time techniques. At The Place, the asymmetrical nature of the loads called for non-linear analysis to understand the direction and power of the forces in the building, and to develop an effi cient structural design.

Another aspect of designing any tall building in a prominent city like London is the higher risk of extreme events. To fully assess a building’s structural performance in an extreme event, a non-linear geometric/ material properties analysis is required. Non-linear analysis was pioneered by WSP immediately after the World Trade Center attack in 2001 and was used to understand the disproportionate collapse of tall structures. Figure 8. Installing the steel spire. © Mace This has helped engineers design more Figure 9. BIM used to position utilities and services routes for The Place. © WSP assemble structural elements. The spire was resilient buildings, and is now widely used for therefore designed so that it could be designing complex tall structures. These way of the secant wall at The Place. The shaft fabricated in modules; each module was the analyses provide a better understanding of turned out to be a forgotten entrance to the maximum size that could be transported by structural behavior and allow appropriate old City & South London Underground line. road and lifted by the tower crane. Because design to be carried out. BIM enabled a rapid redesign of the secant the spire contained the viewing gallery, the wall and associated structure and plant layout. structural engineer worked closely with the As well as being a core method used in steel fabricator to ensure that there was no overcoming construction confl icts and Using BIM also made it easier to position reduction in aesthetic quality by using constraints, BIM has a strong commercial role openings for utilities and services (see Figure modular construction (see Figure 8). to play. Sellar Property Group/Qatari Diar 9). Sellar/Qatari Diar had an agreement with wanted to use BIM on The Place to help Mace that no holes over 150 millimeters in A trial assembly of the spire was carried out at streamline its construction and operation. diameter would be cut in the structure after the fabricator’s yard in order to ensure the WSP worked with Mace specialist contractors the concrete had been cast; using BIM helped modules could be erected rapidly and safely and the client facility managers to enable BIM eliminate new hole cutting. when they arrived on site. to be part of the construction process as well as facility management (FM), making the For historic cities to grow and develop, and to design and delivery of the project more be leaders on the world stage, they must Modern Modeling Techniques Allow effi cient, and saving an estimated 10% on embrace social and cultural shifts and Historic Cities to Enjoy the Juxtaposition of build costs and on-going FM costs. welcome the opportunities that tall buildings Old and New provide. The benefi ts of modern modeling BIM was key to the process of removing a techniques such as BIM, fused with the Computer modeling plays a central role in basement level at The Place, in order to save creativity of tall building specialists, drive enabling the realization of complex projects. time and reduce program risk around developers to overcome signifi cant economic Computer analysis enabled the structural archaeological work. This was achieved by and physical challenges that are inherent in engineer to demonstrate to London modeling the plant and packaging it more the fabric of historic cities. As demonstrated Underground that it was safe to build within effi ciently. Without BIM, the developers would here, these challenges are not its usual 3-meter exclusion zone while not have had the confi dence to proceed with insurmountable.  working on The Place at London Bridge. this approach. For more information on The Shard see the The biggest ground movement predicted was BIM was also useful as a tool when Mace Talking Tall data box on page 57. only 4 to 5 millimeters on some of the assets. discovered a hitherto-unknown shaft in the

CTBUH Journal | 2013 Issue II Structural Engineering | 49