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proceedings

Civil EngineeringVolume 165 Issue CE2

Cannon Place, London: design andconstruction over a live railway stationFraser, Seel, Chadwick, Valambhia and Offiler

Proceedings of the Institution of Civil EngineersCivil Engineering 165 May 2012 Issue CE2Pages 74–81 http://dx.doi.org/10.1680/cien.2012.165.2.74Paper 1100034

Received 17/06/2011 Accepted 23/12/2011

Keywords: buildings, structures & design / steel structures / temporary works

ICE Publishing: All rights reserved

Cannon Place, London: design andconstruction over a live railway station

1 Jim Fraser BEng (Hons), CEng, MIStructEStructures director, Foggo Associates, London, UK

2 David Seel BEng (Hons), CEng, MIStructEDirector and UK manager, Robert Bird Group, London, UK

3 Jonathan Chadwick BE (Hons), GIPENZStructural engineer, Robert Bird Group, London, UK

4 Ketan Valambhia MEng, CEng, MICEDesign leader, Laing O’Rourke, London, UK

5 Alister Ofler BSc (Hons), CIOBDeputy project director, Laing O’Rourke, London, UK

1 2 3 4 5

This paper describes the design and construction of a dramatic 50 000 m 2 ‘air-rights’ development above CannonStreet main-line and underground station in London and a scheduled ancient monument. The complex building

structure includes two 21 m deep cantilevered wings, requiring a 38-stage structural analysis and full-height pre-cambering of façade structures. Careful phasing and meticulous planning were required to segregate and shieldpassengers from construction activities. The challenges were successfully overcome by fully integrating design andconstruction, permanent and temporary works designs, and working in a truly collaborative environment.

1. Introduction

This paper describes the design and construction of Cannon Place,a 50 000 m 2 ‘air-rights’ office development above Cannon Street

main-line and underground station in the City of London.On a broadly rectangular site measuring 68 m by 87 m, the project

comprised the demolition of a series of undistinguished 1960sconcrete slab blocks, the construction of a new building and thesubstantial remodelling of both the main-line and underground partsof the station. Up to 60 000 passengers pass through the station on adaily basis.

This paper illustrates how the challenges of working over a livestation were overcome by fully integrating the design and constructionprocesses and working in a truly collaborative environment createdwithin the design and construction team.

2. The brief

The existing buildings (Figure 1) did not make particularly gooduse of the air rights volume over the station as well as nearing theend of their useful working lives. The client’s brief called for abuilding which would appeal to financial, legal and corporate tenants,be capable of multiple subdivision, would optimise the amount of

lettable space on the site while at the same time maximise value,quality and, as a key element of these parameters, floor-to-ceilingheights.

Through the client’s partnerships with rail operators Network Rail

and London Underground, the brief also included improving thestation by increasing connectivity between the main-line platforms,the street and underground platforms. The underground part of thestation was also to receive new accommodation, improved disabledaccess, a new ticket hall and an improved presence on CannonStreet.

3. Site constraints

3.1 DesignThe site sits in the foreground of protected views of St Paul’s

Cathedral. While the existing building did not actually respect thissubsequently imposed limitation on height, the restriction had theeffect of ‘capping’ the height of the new scheme at 51·3 m aboveordnance datum (AOD). Combining this with Network Rail’srequirement that a minimum height of 5·1 m above the main-linerunning tracks be maintained results in a height of just 32 m withinwhich to plan the eight floors of office space necessary to make thescheme commercially viable (Figure 2).

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Civil EngineeringVolume 165 Issue CE2 May 2012


The site is also encumbered by

n the inability to found any vertical structure over the shallowcut-and-cover underground railway tunnels running along on thenorthern edge of the site

n the limitations on column setting out imposed by the through-siteservice road it was necessary to maintain and by the platformsand running tracks of the main-line railway

n the previous Cannon Street facing slab block was founded onrelatively shallow 6 m square under-ream pile foundations, thebases of which cover a large plan area

n the original Victorian brick viaduct remains over much of the siten the southern half of the site is part of a scheduled ancient

monument, protecting archaeologically sensitive remains of aRoman Governor’s palace.

Regarding the last of these, consent from English Heritage wasrequired for any excavations or foundations. English Heritage’spolicy, in keeping with current planning policy guidelines, is to

3 2 m

51 .3 m AOD

Main-line platformsService road

Undergroundplatforms

Settlement-reducing piles

Reused 1960s under-ream pilesScheduled ancient monument

Minipile and ‘solid earth’ caissons

3 5 m

Figure 2. Three-dimensional section of new building structure andrestricted foundations

Figure 1. Aerial view of the site looking north east, showing previouslyexisting buildings over the Cannon Street main-line and underground station

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various constraints. Within the northern (12 m wide) core strip, theexisting under-ream piles have been reused supplemented by straight-shafted settlement-reducing piles at the east and west perimeters.

The southern foundations are generally minipile groups where theexisting viaduct limits headroom, and where the scheduled ancientmonument imposes constraints on foundation size and accuracy ofinstallation.

Only the south-eastern service core, located in an area wheredemolition of the viaduct was permissible, was founded onconventional under-ream piles drilled with a standard piling rig.

4.1 Construction stage analysisThe nature and scale of the structure and the construction sequence

presented particular challenges in respect of the foundation and framedisplacements and locked-in stresses.

A ‘wished in place’ form of structural analysis, where the completestructure is modelled and all building loads are applied in oneinstantaneous step, was unsuitable and had to be replaced with asequential analysis model, which mimicked the main constructionstages by applying the appropriate loading at the appropriate stage.

The analysis delivered the distribution of elastic stress and strain forwhich the elements were designed. It also delivered, with allowancesfor fabrication and erection tolerances and pin-slip added to the elasticdeformations, the total deformation of the structure.

From the analysis a deformed pre-set shape was determinedto which the structure had to be fabricated and erected (and for aperiod of time maintained) so that, at the end of construction, apredetermined line and level was achieved.

It was critically important to the success of the project that thepredetermined line and level be achieved as it allowed for thesubsequent installation of the cladding and the alignment of theinternal elements, such as the raised floor and suspended ceilings, inthe positions necessary to maintain the high-value maximum floor-to-ceiling height required by the client’s brief.

Figure 5 shows the pre-set shape for the cantilever X-frames; thenorth and south façade trusses were similarly pre-cambered withelement lengths adjusted to account for axial deformations.

Pre-cambering full façade height structures in buildings isrelatively unusual. Typically maximum deflections are calculated andchecked to see that they lie within allowable tolerances. In this case,

not only were the displacements during construction sufficientlylarge that a pre-set shape was required, it was equally important that

displacements were neither over- nor underestimated.Initially, a construction-stage analysis was carried out for the

design of the permanent works structure. This staged analysisincluded ten key construction stages that were identified as beingfundamental to the final stress and strain states in the structural frameand foundations.

Later, a separate, more detailed construction-stage analysiswas developed that included 38 construction stages. This analysismore accurately reflected the planned construction methodologyand included stages where temporary forces were imposed andsubsequently removed from the structure. This analysis was usedto determine whether the critical condition for individual elementsoccurred during construction or subsequently in service.

The results from the ten-stage and 38-stage analysis models of thestructure were compared as one of the collaboration exercises withinthe design and construct team, resulting in an increased level of

confidence in both permanent and temporary stage designs.Some of the construction stages are illustrated in Figure 6.

Figure 5. The four cantilever X-frames were built to a pre-cambered shapeto allow for around 60 mm deflection under normal working load

Figure 6. Three of the 38 construction stages that were structurally analysed

COS 50 .60021 000 mm

The black outline (exagerated forclarity) illustrates the fabricated

and erected pre-set shapeThe red outline showsthe theoretical position

after loading

The pre-set shape isformed by vertical and

horizontal adjustment ofnode positions allowing for

element length changesand rotations

COS 38 .900

COS 35 .000

COS 31 .100

COS 19 .400

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5. Demolition and construction

5.1 Concourse and viaduct – passenger segregation‘Islands’ surrounded by fire-rated hoardings were created in the

station to allow demolition or construction activities to take placein phases within exclusion zones. The islands prevented pedestriansfrom walking below demolition or lifting operations. The aim ofthis approach was to maximise the amount of work that could becarried out during the daytime and minimise track possessions ornight-time working.

The exclusion zones displaced the passenger waiting areas androutes between the surrounding streets and the station platforms.A series of phasing drawings were produced to coordinate the

demolition and construction with passenger movement within thestation. Drawings of the eight main phases were used to carry outpedestrian flow analysis and to demonstrate that the station crowd-management strategy was not compromised by the temporarychanges (Figure 7).

Phasing of the demolition prevented foundations for all fourcores starting at the same time. The north-east core was particularlyaffected and was the least advanced. As a result, the constructionsequence for the deck-level slab commenced with the three availablecores and only the innermost pair of the north-east core columns.Stability analysis had to be carried out to ensure it was acceptable toconcrete the deck-level slab without the benefit of this core.

The islands were a visible sign of construction activities on andabove the station, while below the existing Victorian arches werecut, carved and modi ed in preparation for the structure to emergefrom beneath and pass through the station. At the same time, low-headroom piling was also underway within the arches, of whichstation users were generally unaware. Extensive temporary workswere required, peaking at in excess of 60 live temporary schemes.

Figure 8 shows the island created in the station for construction ofthe north-west core through the existing station roof.

5.2 Deck construction – steel erection within stationThe first level of the new development (the ‘deck’) over the main-

line platforms and running tracks occurs just below the existingstation roof. All of the steelwork had to be fed from an opening onone side of the station, transported by overhead gantry beams and

on bogeys along the platforms and lifted into position during nightpossessions (Figure 9).

A thorough survey of the existing station roof structure wasundertaken and plotted on the project three-dimensional model tocheck for potential clashes with the lifting eyes and with the newdeck-level structure in its final position. Additionally, existingservices, signage, closed-circuit television and other featureshanging below the existing roof had to be surveyed and relocatedto allow the deck-level elements to be raised up without hindrance.

The north and south cantilever areas at deck level had to betemporarily propped. To the north, propping comprised a systemof beams and columns founded on the underground platform roofstructure. To the south, it was not possible to prop off the main-line station concourse so the south cantilever Fabsec beams weretemporarily suspended from hangers, supported by a 1·5 m deepplate girder above roof level, spanning between existing stationcolumns (Figure 10).

Extensive analysis and safe load assessments of both theLondon Underground and Network Rail structures were carried

Figure 7. Examples of construction phase exclusion zones

Figure 9. A 21 m long Fabsec deck beam being inserted across themain-line platforms during a night possession

Figure 8. The north-west core under construction within an exclusion‘island’ created in the station – foundations are some 20 m below

Exclusion zone forconstruction anddemolitionUnpaid publicaccess areaPaid public accessareaTemporary worksfrom previousphaseLondonUnderground entryand exit

Cannon Street Cannon Street

D o w g a t e

H i l l

D o w g a t e

H i l l

C a b R o a d

C a b R o a d

B u s h L a

n e

B u s h L a

n e

Phase 3

10/08/09Phase 5

31/08/09

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out. Relevant drawings of the existing structures had to be foundout of the thousands available from the archives. The analysisdemonstrated that the capacity was limited to the self-weight ofthe deck cantilever structure plus a small allowance for live loads.

Each activity, including the major temporary works schemes,had been planned in detail at the very early stages (2007) of theproject before the existing structure could be fully opened up andinvestigated. The sequence and methods of work were re ned asmore was learnt during enabling works; however, the phasing ingeneral remained consistent with the original strategy throughoutthe works on site (2008–2010).

5.3 Central deck to roof – shielding the station

The cores and intermediate parts of the vertical structure emergedthrough the station from within the individual islands and wereconnected by the horizontal deck-level structure above the station.Future activities involved demolition and lifting operations overthe footprint of the station during normal station working hours.Large steel elements, if accidentally dropped, could not be allowedto penetrate into the live station. The strategy was to utilise the newdeck level slab as a shield, and the station roof was not thereforedemolished until the deck-level slab was complete just below it.

Risk to passengers and staff in the operating station was examinedby combining the element-by-element craneage and erection planwith data on lifting equipment failure from accident records in thestructural steelwork industry. This risk analysis identified operationswhere further risk mitigation was required.

Analysis of impacts from falling elements of varying weightsand shapes from varying heights established the level of load thatwould penetrate the existing roof structure or new deck-level slab.To prevent penetration through the slab, lifting height limits for eachelement were identified and impact-absorbing layers designed forcertain areas.

5.4 Movement prediction and monitoringMovement monitoring of the building during construction had

been identified as desirable to corroborate the structural analysiswith actual behaviour during construction. Monitoring buildingmovement should promptly identify any unexpected buildingbehaviour including

n foundation settlement outside the expected rangen horizontal movement of piles outside the expected rangen any unexpected behaviour of main structure of the building.

Considerable time and effort had been put into the structuralanalysis before construction so, while unexpected movement wasconsidered unlikely, should it arise the unexpected behaviour couldbe promptly identified, understood and changes made if necessary.

Fifty target points were identified and placed on the main externalstructure of the building. Their locations were chosen to enablethe structural behaviour of the building to be interpreted from themovement data.

Variables that would influence the building movement duringconstruction included

n a range of possible foundation stiffnessesn the level of loading applied to the building at each stage

during construction including structural dead loads, fit-out loads,

cladding loads and construction loadsn minor variations in the sequence of construction.

An envelope of upper and lower bound building movement wasconstructed from parametric modelling using the construction-stagemodel and a range of input values taking account of the variablesdiscussed above.

All structural movement survey data were recorded in a movement-monitoring spreadsheet that had been prepared previously. Assurveyed values were entered into the spreadsheet, these values wereautomatically compared to the upper and lower bound expectedmovement limits. Any values outside the limits were immediatelyand easily identified. Charts plotting the measured movement against

the expected range were available for each point on the building,graphically illustrating any emerging trends (Figure 11).

Surveys were typically carried out at 2-week intervals, with tighterintervals during critical periods. Measured displacements were allwithin their predicted range. The strongest trend to emerge was thatvertical movements of the building were generally at the lower endof the expected range, which indicated that the foundations weregenerally at the upper end of their stiffness range.

Vertical – measured valueVertical – upper/lower bound

0

–5

–10

–15

–20

Construction progress1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

M o v e m e n

t f r o m

i n i t i a l p o s i t i o n : m m

Figure 11. Typical plot of the new structure’s actual against predictedmovement – all were within the predicted upper and lower bounds

Figure 10. A 1·5 m deep plate girder above the station roof was used toprovide initial temporary support to the south cantilever beams below

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Once the structure was complete and all pin connections made,the transfer of load from strand jacks to permanent works was made(Figure 15).

5.7 Strand jack release and concreting – meticulous planningThroughout the 17 weeks taken to erect the cantilever steelwork,

the behaviour of the structure was closely monitored and comparedwith predictions.

An intensive series of checks on the structure commenced severalweeks in advance of the load-transfer date, another major milestone.These included element-by-element and connection-by-connectionchecks along the extensive load paths of the façade trusses andX-frame cantilevers.

The load transfer was carried out in stages. All the jacks wereactivated and strands were released, initially in 20 mm increments,subsequently increased to 50 mm. A round of movement measurementswas carried out after every release, which were checked against a setof predicted movement ranges.

Load transfer was achieved after release of up to 185 mm ofstrand (at mid-span of the north and south façade trusses), equatingto approximately 60 mm of vertical deflection of the structure. Thetotal strand release included allowance for strand extension due toload and catenary effects in the strand, which cause the load-releaserate to tail off asymptotically at the end of the process. A final set ofmovement readings was taken at this point.

The operation took about 3·5 h and started in the early hours of themorning so as to be completed by 07:30, just before the monitoringstations were submerged by the usual rush-hour crowds.

Strands were kept engaged with the jacks for another 12 h, whenanother set of readings was taken. This confirmed that there had notbeen any further movement and the structure had reached a steadystate. The strands and the jacks were then dismantled and concretingof the cantilever slabs commenced.

Movement monitoring continued during the concreting of the cantileverslabs. The final readings taken some 12 weeks after the strand jack releaseshowed that the bottom boom level was only millimetres away from itspredicted final position and within the set tolerance.

6. Conclusion

The £214 million Cannon Place project was carried out under aJCT Major Projects Design and Build contract. Work started onsite on 3 September 2007 and was completed on 5 September 2011(Figure 16).

It was accepted from day one that only a truly collaborativeapproach between the designers and builders would allow theimmensely complex construction project to succeed. The temporaryworks design was carefully integrated with the permanent works toensure that the systematically planned construction methodologycould be carried out seamlessly. Often this meant several partiestaking ownership and working on solutions to the same problembefore bringing them together at the end – that is, working beyondthe interfaces. The whole process needed careful thought, meticulousplanning and effective management to ensure there were no gaps orunnecessary overlaps between the parties.

The result is that the project has been delivered in line with theoriginal construction strategy and without major disruption to theworking station below. Furthermore, and possibly more importantly,the product sought by the client has been achieved. The success

of this was fundamentally sensitive to the interaction of design,fabrication and construction, by virtue of the need to achieve a tightlyprescribed line and level.

Acknowledgements

The project team for the structure was as follows: Hines – clientand development manager; Laing O’Rourke – main contractor, designleader and temporary works design; Foggo Associates – architect,structural and building services engineer; Robert Bird Group –temporary (construction) stage analysis and temporary works design;Watson Steel Structures Ltd – steelwork fabricator and erector andtemporary works design; Expanded Piling & Expanded Structures –

piling, concrete works and temporary works design; McGee Group –demolition, enabling works and temporary works design; Tony Gee andPartners – enabling works development and temporary works design;Faggiolli – strand jack supply and operation.

Figure 15. When the cantilevers were complete, the strand jacks werereleased in 20–50 mm increments over 3·5 h and the load transferredinto the pre-cambered X-frames

Figure 16. The £214 million project was completed in September 2011

What do you think?If you would like to comment on this paper, please email up to 200 wordsto the editor at [email protected] you would like to write a paper of 2000 to 3500 words about your ownexperience in this or any related area of civil engineering, the editor will behappy to provide any help or advice you need.

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