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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,

MA MIStructE, Associate,

Ove Arup and Partners, London T he site of the 30 St Mary Axe bu-ilding lies at the heart of the City’sinsurance district. The former bu-

ildings on the site, including the home ofthe Baltic Exchange, had been severely da-maged by a terrorist bomb in 1992. Thelocation and history of the site demandeda design of the highest design quality thatwould make a real contribution to the ur-ban environment of the City.

Swiss Re started developing proposalsfor the site in 1998. Swiss Re, being clo-sely involved in sustainability issues inthe realm of insurance risks resultingfrom global climate change, emphasisedin their brief the need an environmen-tally progressive design, together with a

high standard of internal working envi-ronment for staff.

Design, procurement and fabricationprocesses were integrated through theuse by the design team of three-dimen-sional modelling of the steel frame and aparametric approach to the design, enab-ling complexity to be managed with re-duced risk and greater economy. Theproject shows the ability of structuralsteel to enable radical architectural ide-as to be realised.

The Architectural formThe development of the building form isthe result of the synthesis of a number ofcriteria, many of which are a direct re-

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Swiss Re´s Building, London

Building at a lonely height;erection and adjustment

remains a human labour.

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sponse to the particular site and clientrequirements. In the case of the SwissRe building the principal formative ide-as can be summarised as:●A net office floor area within the

building of around 500,000 ft2

(46,450 m2)●The enhancement of the public envi-

ronment at street level, opening upnew views across the site to the fron-tages of the adjacent buildings andallowing good access to and aroundthe new development

●Minimum impact on the local windenvironment

●Maximum use of public transport forthe occupants of the building

●Flexibly serviced, high specification‘user-friendly’ column free office spa-ces with maximum primary space ad-jacent to natural light

●Good physical and visual interconn-nectivity between floors

●Reduced energy consumption by useof natural ventilation whenever suita-ble, low façade heat gain and smartbuilding control systems

Tall building designs offer the possibili-ty of reducing the footprint at street le-vel and help the office floors to be wellproportioned for natural light. The max-imum benefit to the urban environmentis achieved by avoiding the creation of ➤

Swiss Re building as seen from the Thames.

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windy conditions around the base of thebuilding, and keeping the perception ofthe building’s size in proportion with ot-her buildings in the area. The curvedform developed for the Swiss Re buildingachieves these two objectives simultane-ously by virtue of its streamlined aero-dynamics and in the nature of its convexsurface, which recedes from the eye sothat the building’s size is not fully per-ceived from street level. The diameter ofthe tower is reduced at street level tomaximise the external plaza circulationspace and open up the areas in front ofthe adjacent buildings. The reduction infloor diameter towards the plant floorsat the top of the building, culminating inthe glazed domed roof, ensures that thebuilding enhances but does not domina-te the London skyline.

For flexible and adaptable office spa-ce a regular internal planning grid is re-quired. The office floors are organisedinto six ‘spokes’ or fingers, arranged ona 1.5m grid around a circular service andlift core. Between the spokes are triang-ular zones that are used as perimeterlight-wells. The result is a maximum14m‘core to glass’ internal dimension, withall parts of the office fingers within 8.5m

of a light-well. The light-wells are offsetat each successive floor by 5 degrees.This twist creates balconies at each leveland opens up dramatic views throughand out of the building.

The perimeter ‘diagrid’ structureThe perimeter steel structural solutionwas developed specifically for this buil-ding in order to address the issues gene-rated by the unusual geometry in amanner that was fully integrated with thearchitectural concept and generated themaximum benefit for the client. The finalsolution was one of a number of appro-aches that were assessed in detail for ove-rall structural efficiency, internal plann-ning benefits, buildability, cost and risk.

The design avoids large cantileversand keeps the light-wells free of floorstructure by inclining the perimeter co-lumns to follow the helical path of thesix-fingered floors up through the buil-ding. A balanced diagrid structure is for-med by generating a pattern of intersec-ting columns spiralling in both directions.

The addition of horizontal hoops,which connect the columns at their inter-section points and resist the forces arisingfrom the curved shape, means that the

perimeter structure is largely independentof the floors. The hoops also turn the di-agrid into a very stiff triangulated shell,which provides excellent stability for thetower. This benefit of the diagrid meansthat the core does not need to resist windforces and can be designed as an open-planned steel structure providing adap-table internal space. Foundation loadsare also reduced compared with a buil-ding stabilised by the core.

Diagrid analysisThe unusual geometry of the Swiss Rebuilding and its perimeter structure givesrise to significant horizontal forces ateach node level, acting predominantly ina radial direction. These forces are bestunderstood in terms of three indepen-dent geometric effects. The resolution ofa vertical floor load into a raking co-lumn requires a horizontal restraint for-ce. Adding a horizontal curveature to adiagrid structure in which the columnsare wrapped around the plan form me-ans that the column loads change direc-tion at each node. Thus a cylindricalform of diagrid generates an outwardspreading force at node points. Further-more, if a vertical convex curveature is

Artist impression.

Architect’s concept sketch.

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introduced, this increases the change incolumn angle and with it the spreadingeffect of vertical column loads.

In the Swiss Re building all these ho-rizontal forces are carried by perimeterhoops at each node level, which alsoprovide equilibrium for any asymmetricor horizontal loading conditions. Thecombination of these geometrical actionsresults in compression in the hoops atthe top of the building, where the co-lumns are more steeply angled and ligh-ter loaded, to very significant tension

The real thing!

Structural plan near mid-height ofbuilding (showingarrangement ofclear-span radialfloor beams aligning with perimeter columnpositions and lightwell edges).

Plan of the 18th storey,with denotation of the grid

of the raised floor.

Office division (note: showing possiblevariations of office planning layout).

The 3D- model pro-ved to be indispensa-ble in the communi-cation. The structuralengineer made the initial coordinationmodel with centre-lines and sizing, the contractor andsubcontractors used it for detailing andinterfaces with cladding and MEPservices.

The shape of the tower is influenced by the

physical environment ofthe city. The smooth

flow of wind around thebuilding was one of the

main considerations.➤

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forces at the middle and lower levels.The sizing of the steel elements is gover-ned by strength criteria – the total swaystiffness of the diagrid is sufficient to li-mit the wind sway to 50mm over the full180m height and provides a very goodlevel of overall dynamic performance.

The development of the diagrid nodesIt was recognised at the outset that thenode connection detail would be funda-mental to the success of any diagrid sche-me. The local geometry of the connec-

tion varies at each floor level, due to thediffering floor diameters. The triangula-ted nature of the diagrid demanded a de-tailed consideration of the control of fa-brication and erection tolerances.

Two design approaches are possible:one can focus on the individual elementsand fabricate end details to suit each si-tuation, or use separate node pieces acc-commodating all the geometric variationand allowing simple stick elements to beused. The latter approach allowed asimplification in the connection geome-

The produced node is prefabricatedin 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.

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try to the consideration of three inter-secting planes relevant to a node, as opp-posed to six individual element orienta-tions. One plane is defined by the axesof the horizontal hoops, one is commonto the upper columns, and one to the lo-wer columns. Principal compression lo-ads are transmitted through milled endbearing surfaces, and tension throughbolted splices.

The significance of the diagrid conn-nection detail to the steelwork contrac-tor’s method of working in both shopand site made it important for all poten-tial contractor’s to be given the oppor-tunity to develop their own ideas andapproach. Steel sub-contractor VictorBuyck - Hollandia JV (VB-H) developedthe detailed node design to meet a num-ber of defined performance criteria, in-cluding:●Loading combinations involving pri-

mary structural actions, local flooreccentricities and cladding loads

●Robustness tying requirements●Movement and restraint require-

ments between the diagrid structureand floor slab

●Erection tolerances and fit withincladding geometry

The chosen approach followed the samebasic layout as had been defined in theinitial design. Great emphasis was givento the accuracy of fabrication of the pre-pared bearing surfaces of the nodes andcolumns, which were milled to a tole-rance of 0.1mm. This ensured a verygood level of fit with minimal site ad-justment needed. This was despite thefact that alternate bands of steelworkwere fabricated in separate yards andhad not come together until erected onsite. VB-H developed and tested an inn-novative tied corbel connection detailbetween the floor steelwork and the no-de which allowed the required radialspread of the diagrid during constructionwhilst providing a reliable degree of re-straint to the diagrid nodes. The detailalso provided for fine adjustment of thenode position during erection using ra-

dial tie bolts. This ensured that the ringof hoop tension elements could be clo-sed without the use of oversized holes orpre-tensioned bolts.

Floor framingThe circular floor plates are framed bet-ween the core and perimeter structureusing radial beams on 10° centrelines.This leads to a range of spans for thecomposite floor slab of up to 4.75m bet-ween beams at the perimeter on the lar-gest floors. Arup worked closely withRichard Lees Steel Decking to develop adesign based around the Ribdeck 80profile to achieve these spans withoutthe need for temporary propping. Theoverall slab thickness is 160mm with asimilar weight to the more conventional130mm, and also provides improvedoverall floor plate vibration dynamicsdue to the increased rib stiffness.

Beam depths are minimised by use ofwide flanged (European profile) beams.The beam depth is most critical in theprimary services distribution zone

Schematicrepresentationof the perime-ter diagridstructure.

Some of the many alter-native approaches consideredby VB-H for the node, including steel castings andversions requiring welding on site. The chosen option(bottom left) is a develop-ment of the solution initially proposed by the design team.

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dome

externallyexposed steelwork

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30

20

10

bg

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around the core, whilst there is a less cri-tical fit at mid-span. This enables the be-ams to be specified without precamber,whilst maintaining adequate clearancesfor services. Within the cores the beamspans are much reduced, allowing thehorizontal separation of structural andservices zones. The only area where be-am web penetrations are required isaround the perimeter where supply andexhaust air is ducted via plenum boxesconnected to the back of slotted façadetransoms.

Working in 3-DA fundamental characteristic of theSwiss Re building is the use of a con-sistent unifying system combined with aconstantly varying geometry verticallythrough the building. This type of geo-metry is particularly suited to a parame-tric design approach: many of the detai-led design conditions can be investigatedby setting up fixed mathematical rela-tionships between a relatively limitednumber of geometric parameters defi-ning the building shape.

This approach was used to drive opti-misation studies, to build up data bases ofvarious design conditions allowing ratio-nalisation of structural components anddetails, and to generate 3D model geo-metry for analysis, co-ordination andstructural design. An example of this app-proach is the analysis of the relationshipbetween perimeter column setting out andthe facetted cladding geometry which all-lowed the team to home in rapidly on theoptimum geometry for the diagrid.

A full Xsteel model, incorporatingcentreline geometry and sizes for allstructural elements was created by Arupduring the detailed design phase. Thisability to exchange data in 3D enhancedthe level of confidence within the teamthat the detailed co-ordination was acc-curate and provided a firm basis to de-velop the rest of the design documenta-tion. The model provided all steel sub-contract tenderers with comprehensivematerial list reports, ensuring a commonbasis for logistical planning and pricing.This alone represents a significant savingin effort for a building in which there isvery little repetition of beam lengths.

The 3D model was subsequentlyadopted and developed by the steel sub-contractor to generate fabrication infor-mation. The continuity of model infor-mation from analysis through to fabri-cation greatly reduced the scope for err-rors in interpreting the design require-ments. The steel 3D model provided the

basis for detailed coordination of seve-ral trade interfaces including claddingand building services.

DomeThe upper three levels of the buildingfrom level 38 provide corporate faciliti-es for Swiss Re and other tenants, inclu-ding private dining rooms, restaurantand an upper viewing mezzanine offe-ring 360° views over London. These le-vels are enclosed with a steel and glassdome structure of 30m diameter, rising22m from its support on the top of theperimeter diagrid. The dome steelworkis a fully welded lattice of intersecting fa-bricated triangular profiles. The effici-ency of this structural arrangement re-sults in very minimal steel elements thatare only 110mm x 150mm in section.

Steel erectionWith planning permission granted, enab-ling works for the single level basementwere able to start on site in December2000. Steel fabrication started in Holl-land and Belgium in July 2001, withsteel arriving on site in October of thatyear. The erection sequence progressedin two-storey bands in the followingpattern:➊ Erect core steel complete with access

stairs and a small amount of tempo-rary bracing

➋ Deck core and establish survey points➌ Erect diagrid columns and nodes as A-

frames (pre-assembled at ground level)➍ Erect radial beams and plumb A-

frames, install hoop members to com-plete diagrid

➎ Complete floor framing and decking,including crane tie bracing where re-quired

➏ Concrete floor

The steel erection progressed at approx-imately one band per fortnight, withconcrete poured 8 storeys below the coreerection front. The last diagrid A-frameto level 38 was erected in October 2002,to an overall plumb tolerance of lessthan 10mm over the 160m height.

The erection of the fully welded free-standing dome lattice steelwork requireda different erection approach from Waag-ner-Biro. Off-site welding of transporta-ble sized sub-assemblies ensured that si-te welding was kept to a minimum. Jigswere set up on the plaza slab allowingtwo adjacent sub-assemblies to be joinedtogether to form sections of the domemeasuring approximately 12m by 8m,which were then erected onto temporarylocating jigs at the top of the building. Si-

Looking through the dome fromthe inside during installation ofthe top doubly-curved glass‘lens’ that crown the building.

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Arial view during installation of the glazing to the dome, showing external access hoist and platforms. View from street level during main façade installation. The speciallydesigned safety fan allows steel erection to continue above.

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te welding the members between erectedsections completed the dome framing intwo level stages, before the removal ofthe temporary supports. The top ‘spider’section was erected in one piece in March2003. ❏

Facts and figures

DimensionsHeight to top of dome: 179.8 mHeight to highest occupied floor level: 167.1 mNumber of floors above ground: 40Number of basement levels: single base

ment across whole siteLargest floor external diameter (lvl 17): 56.15 mSite area: 0.57 hectares (1.4 acres)Net accommodations areas:➤ Office 46,450 m2

➤ Retail 1,400 m2

Office floor-floor: 4.15 mGross superstructure floor area (incl. lightwells): 74,300 m2

Tower Structural SteelworkTotal 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 348Total length: 54.56 kmDiagrid column sizes: ➤ Ground – level2: 508mm f, 40mm thick➤ Level 36–38: 273mm f, 12.5mm thickHoop design tension at level 2: 7 116 kNPerimeter column maximum design load: 15,460 kNCore column maximum design load: 33,266 kN

Foundations750mm diameter straight-shafted piles into London ClayNumber of piles: 333Total length of piles: 9 kmTotal design capacity: 117,000 Tonnes

Credits

Client: Swiss ReProject Manager: RWG AssociatesArchitect: Foster and PartnersStructural Engineer: ArupBuilding Services Engineer: Hilson Moran

PartnershipCost consultant: Gardiner & TheoboldFire Engineering: Arup FireMain Contractor: SkanskaStructural Steel sub-contractor:

Victor Buyck – HollandiaDome sub-contractor: Waagner-Biro

The elements of the facade.➤Openable glass screen.➤Perforated aluminium

louvers (internal sun-screen)➤A column casing of

aluminium➤Façade frame of

extruded aluminiumfloor beam

air duct

horizontal hoop-tie extract duct

suspended ceiling

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