Swiss Re´S Building, London

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Building at a lonely height; erection and adjustment remains a human labour.

Swiss Re´s Building, London

This article describes the development of the structural design of the new 40 storey steel framed landmark office building in London known as 30 St Mary Axe. The development of building’s unique form within its prime urban context, with its circular plan of varying size and spiralling lightwells, and the structural design solutions are explained from an engineering perspective.

Dominic Munro, he site of the 30 St Mary Axe bu- high standard of internal working envi- MA MIStructE, Associate, ilding lies at the heart of the City’s ronment for staff. Ove Arup and Partners, London T insurance district. The former bu- Design, procurement and fabrication ildings on the site, including the home of processes were integrated through the the Baltic Exchange, had been severely da- use by the design team of three-dimen- maged by a terrorist bomb in 1992. The sional modelling of the steel frame and a location and history of the site demanded parametric approach to the design, enab- a design of the highest design quality that ling complexity to be managed with re- would make a real contribution to the ur- duced risk and greater economy. The ban environment of the City. project shows the ability of structural Swiss Re started developing proposals steel to enable radical architectural ide- for the site in 1998. Swiss Re, being clo- as to be realised. sely involved in sustainability issues in the realm of insurance risks resulting The Architectural form from global climate change, emphasised The development of the building form is in their brief the need an environmen- the result of the synthesis of a number of tally progressive design, together with a criteria, many of which are a direct re-

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Swiss Re building as seen from the Thames.

sponse to the particular site and client ●Flexibly serviced, high specification requirements. In the case of the Swiss ‘user-friendly’ column free office spa- Re building the principal formative ide- ces with maximum primary space ad- as can be summarised as: jacent to natural light ●A net office floor area within the ●Good physical and visual interconn- building of around 500,000 ft2 nectivity between floors (46,450 m2) ●Reduced energy consumption by use ●The enhancement of the public envi- of natural ventilation whenever suita- ronment at street level, opening up ble, low façade heat gain and smart new views across the site to the fron- building control systems tages of the adjacent buildings and allowing good access to and around Tall building designs offer the possibili- the new development ty of reducing the footprint at street le- ●Minimum impact on the local wind vel and help the office floors to be well environment proportioned for natural light. The max- ●Maximum use of public transport for imum benefit to the urban environment the occupants of the building is achieved by avoiding the creation of ➤

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Architect’s concept sketch.

Artist impression.

➤ windy conditions around the base of the of a light-well. The light-wells are offset perimeter structure is largely independent building, and keeping the perception of at each successive floor by 5 degrees. of the floors. The hoops also turn the di- the building’s size in proportion with ot- This twist creates balconies at each level agrid into a very stiff triangulated shell, her buildings in the area. The curved and opens up dramatic views through which provides excellent stability for the form developed for the Swiss Re building and out of the building. tower. This benefit of the diagrid means achieves these two objectives simultane- that the core does not need to resist wind ously by virtue of its streamlined aero- The perimeter ‘diagrid’ structure forces and can be designed as an open- dynamics and in the nature of its convex The perimeter steel structural solution planned steel structure providing adap- surface, which recedes from the eye so was developed specifically for this buil- table internal space. Foundation loads that the building’s size is not fully per- ding in order to address the issues gene- are also reduced compared with a buil- ceived from street level. The diameter of rated by the unusual geometry in a ding stabilised by the core. the tower is reduced at street level to manner that was fully integrated with the maximise the external plaza circulation architectural concept and generated the Diagrid analysis space and open up the areas in front of maximum benefit for the client. The final The unusual geometry of the Swiss Re the adjacent buildings. The reduction in solution was one of a number of appro- building and its perimeter structure gives floor diameter towards the plant floors aches that were assessed in detail for ove- rise to significant horizontal forces at at the top of the building, culminating in rall structural efficiency, internal plann- each node level, acting predominantly in the glazed domed roof, ensures that the ning benefits, buildability, cost and risk. a radial direction. These forces are best building enhances but does not domina- The design avoids large cantilevers understood in terms of three indepen- te the London skyline. and keeps the light-wells free of floor dent geometric effects. The resolution of For flexible and adaptable office spa- structure by inclining the perimeter co- a vertical floor load into a raking co- ce a regular internal planning grid is re- lumns to follow the helical path of the lumn requires a horizontal restraint for- quired. The office floors are organised six-fingered floors up through the buil- ce. Adding a horizontal curveature to a into six ‘spokes’ or fingers, arranged on ding. A balanced diagrid structure is for- diagrid structure in which the columns a 1.5m grid around a circular service and med by generating a pattern of intersec- are wrapped around the plan form me- lift core. Between the spokes are triang- ting columns spiralling in both directions. ans that the column loads change direc- ular zones that are used as perimeter The addition of horizontal hoops, tion at each node. Thus a cylindrical light-wells. The result is a maximum14m which connect the columns at their inter- form of diagrid generates an outward ‘core to glass’ internal dimension, with section points and resist the forces arising spreading force at node points. Further- all parts of the office fingers within 8.5m from the curved shape, means that the more, if a vertical convex curveature is

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Structural plan near mid-height of building (showing arrangement of clear-span radial floor beams aligning with perimeter column positions and lightwell edges).

Plan of the 18th storey, with denotation of the grid of the raised floor.

The real thing! Office division (note: showing possible variations of office introduced, this increases the change in planning layout). column angle and with it the spreading effect of vertical column loads. In the Swiss Re building all these horizontal forces are carried by perimeter hoops at each node level, which also provide equilibrium for any asymmetric The shape of the tower or horizontal loading conditions. The is influenced by the combination of these geometrical actions physical environment of results in compression in the hoops at the city. The smooth the top of the building, where the co- flow of wind around the lumns are more steeply angled and ligh- building was one of the ter loaded, to very significant tension ➤ main considerations.

The 3D- model pro- ved to be indispensa- ble in the communi- cation. The structural engineer made the initial coordination model with centrelines and sizing, the contractor and subcontractors used it for detailing and interfaces with cladding and MEP services.

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The produced node is prefabricated in the factory. The heart consists of a solid block of steel of 240 by 140 mm.

The tower was assembled in construction cycles of two storeys, with one cycle every two weeks.

➤ forces at the middle and lower levels. tion varies at each floor level, due to the The sizing of the steel elements is gover- differing floor diameters. The triangula- ned by strength criteria – the total sway ted nature of the diagrid demanded a de- stiffness of the diagrid is sufficient to li- tailed consideration of the control of fa- mit the wind sway to 50mm over the full brication and erection tolerances. 180m height and provides a very good Two design approaches are possible: level of overall dynamic performance. one can focus on the individual elements and fabricate end details to suit each si- The development of the diagrid nodes tuation, or use separate node pieces acc- It was recognised at the outset that the commodating all the geometric variation node connection detail would be funda- and allowing simple stick elements to be mental to the success of any diagrid sche- used. The latter approach allowed a me. The local geometry of the connec- simplification in the connection geome-

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Some of the many alter- dome native approaches considered by VB-H for the node, 38 including steel castings and versions requiring welding externally on site. The chosen option exposed steelwork (bottom left) is a development of the solution initially proposed by 30 the design team.

Schematic representation of the perimeter diagrid structure. 20

try to the consideration of three inter- ments between the diagrid structure dial tie bolts. This ensured that the ring secting planes relevant to a node, as opp- and floor slab of hoop tension elements could be clo- posed to six individual element orienta- ●Erection tolerances and fit within sed without the use of oversized holes or tions. One plane is defined by the axes cladding geometry pre-tensioned bolts. of the horizontal hoops, one is common to the upper columns, and one to the lo- The chosen approach followed the same Floor framing wer columns. Principal compression lo- basic layout as had been defined in the The circular floor plates are framed bet- ads are transmitted through milled end initial design. Great emphasis was given ween the core and perimeter structure bearing surfaces, and tension through to the accuracy of fabrication of the pre- using radial beams on 10° centrelines. bolted splices. pared bearing surfaces of the nodes and This leads to a range of spans for the The significance of the diagrid conn- columns, which were milled to a tole- composite floor slab of up to 4.75m bet- nection detail to the steelwork contrac- rance of 0.1mm. This ensured a very ween beams at the perimeter on the lar- tor’s method of working in both shop good level of fit with minimal site ad- gest floors. Arup worked closely with and site made it important for all poten- justment needed. This was despite the Richard Lees Steel Decking to develop a tial contractor’s to be given the oppor- fact that alternate bands of steelwork design based around the Ribdeck 80 tunity to develop their own ideas and were fabricated in separate yards and profile to achieve these spans without approach. Steel sub-contractor Victor had not come together until erected on the need for temporary propping. The Buyck - Hollandia JV (VB-H) developed site. VB-H developed and tested an inn- overall slab thickness is 160mm with a the detailed node design to meet a num- novative tied corbel connection detail similar weight to the more conventional ber of defined performance criteria, in- between the floor steelwork and the no- 130mm, and also provides improved cluding: de which allowed the required radial overall floor plate vibration dynamics ●Loading combinations involving pri- spread of the diagrid during construction due to the increased rib stiffness. mary structural actions, local floor whilst providing a reliable degree of re- Beam depths are minimised by use of eccentricities and cladding loads straint to the diagrid nodes. The detail wide flanged (European profile) beams. ●Robustness tying requirements also provided for fine adjustment of the The beam depth is most critical in the ●Movement and restraint require- node position during erection using ra- primary services distribution zone ➤

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➤ around the core, whilst there is a less critical fit at mid-span. This enables the beams to be specified without precamber, whilst maintaining adequate clearances for services. Within the cores the beam spans are much reduced, allowing the horizontal separation of structural and services zones. The only area where beam web penetrations are required is around the perimeter where supply and exhaust air is ducted via plenum boxes connected to the back of slotted façade transoms. Looking through the dome from the inside during installation of Working in 3-D the top doubly-curved glass A fundamental characteristic of the ‘lens’ that crown the building. Swiss Re building is the use of a con- sistent unifying system combined with a constantly varying geometry vertically through the building. This type of geometry is particularly suited to a parametric design approach: many of the detailed design conditions can be investigated by setting up fixed mathematical rela- tionships between a relatively limited number of geometric parameters defi- ning the building shape. This approach was used to drive opti- Arial view during installation of the glazing to the dome, showing external access misation studies, to build up data bases of hoist and platforms. View from street level during main façade installation. The specially various design conditions allowing ratio- designed safety fan allows steel erection to continue above. nalisation of structural components and details, and to generate 3D model geo- basis for detailed coordination of seve- stairs and a small amount of tempo- metry for analysis, co-ordination and ral trade interfaces including cladding rary bracing structural design. An example of this app- and building services. ➋ Deck core and establish survey points proach is the analysis of the relationship ➌ Erect diagrid columns and nodes as A- between perimeter column setting out and Dome frames (pre-assembled at ground level) the facetted cladding geometry which all- The upper three levels of the building ➍ Erect radial beams and plumb A- lowed the team to home in rapidly on the from level 38 provide corporate faciliti- frames, install hoop members to com- optimum geometry for the diagrid. es for Swiss Re and other tenants, inclu- plete diagrid A full Xsteel model, incorporating ding private dining rooms, restaurant ➎ Complete floor framing and decking, centreline geometry and sizes for all and an upper viewing mezzanine offe- including crane tie bracing where re- structural elements was created by Arup ring 360° views over London. These le- quired during the detailed design phase. This vels are enclosed with a steel and glass ➏ Concrete floor ability to exchange data in 3D enhanced dome structure of 30m diameter, rising the level of confidence within the team 22m from its support on the top of the The steel erection progressed at approx- that the detailed co-ordination was acc- perimeter diagrid. The dome steelwork imately one band per fortnight, with curate and provided a firm basis to de- is a fully welded lattice of intersecting fa- concrete poured 8 storeys below the core velop the rest of the design documenta- bricated triangular profiles. The effici- erection front. The last diagrid A-frame tion. The model provided all steel sub- ency of this structural arrangement re- to level 38 was erected in October 2002, contract tenderers with comprehensive sults in very minimal steel elements that to an overall plumb tolerance of less material list reports, ensuring a common are only 110mm x 150mm in section. than 10mm over the 160m height. basis for logistical planning and pricing. The erection of the fully welded free- This alone represents a significant saving Steel erection standing dome lattice steelwork required in effort for a building in which there is With planning permission granted, enab- a different erection approach from Waag- very little repetition of beam lengths. ling works for the single level basement ner-Biro. Off-site welding of transporta- The 3D model was subsequently were able to start on site in December ble sized sub-assemblies ensured that si- adopted and developed by the steel sub- 2000. Steel fabrication started in Holl- te welding was kept to a minimum. Jigs contractor to generate fabrication infor- land and Belgium in July 2001, with were set up on the plaza slab allowing mation. The continuity of model infor- steel arriving on site in October of that two adjacent sub-assemblies to be joined mation from analysis through to fabri- year. The erection sequence progressed together to form sections of the dome cation greatly reduced the scope for err- in two-storey bands in the following measuring approximately 12m by 8m, rors in interpreting the design require- pattern: which were then erected onto temporary ments. The steel 3D model provided the ➊ Erect core steel complete with access locating jigs at the top of the building. Si-

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Facts and figures

Dimensions Height to top of dome: 179.8 m Height to highest occupied floor level: 167.1 m Number of floors above ground: 40 Number of basement levels: single base ment across whole site Largest floor external diameter (lvl 17): 56.15 m Site area: 0.57 hectares (1.4 acres) Net accommodations areas: ➤ Office 46,450 m2 ➤ Retail 1,400 m2 Office floor-floor: 4.15 m Gross superstructure floor area (incl. lightwells): 74,300 m2

Tower Structural Steelwork Total weight of steel (from Arup Xsteel model): 8,358 tonnes of which: ➤ 29% is in the diagrid ➤ 24% core columns ➤ 47% beams. Total number of primary steel pieces: 8 348 Total length: 54.56 km Diagrid column sizes: ➤ Ground – level2: 508mm f, 40mm thick ➤ Level 36–38: 273mm f, 12.5mm thick Hoop design tension at level 2: 7 116 kN Perimeter column maximum design load: 15,460 kN Core column maximum design load: 33,266 kN The elements of the facade. ➤Openable glass screen. Foundations ➤Perforated aluminium 750mm diameter straight-shafted louvers (internal sun-screen) piles into London Clay ➤A column casing of Number of piles: 333 aluminium Total length of piles: 9 km ➤Façade frame of Total design capacity: 117,000 Tonnes extruded aluminium floor beam air duct horizontal hoop-tie extract duct Credits Client: Swiss Re suspended ceiling Project Manager: RWG Associates Architect: Foster and Partners Structural Engineer: Arup Building Services Engineer: Hilson Moran Partnership Cost consultant: Gardiner & Theobold Fire Engineering: Arup Fire te welding the members between erected Main Contractor: Skanska sections completed the dome framing in Structural Steel sub-contractor: two level stages, before the removal of Victor Buyck – Hollandia the temporary supports. The top ‘spider’ Dome sub-contractor: Waagner-Biro section was erected in one piece in March 2003. ❏

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