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Forth Replacement Crossing

Main Crossing (Bridge)

Scheme Assessment ReportDevelopment ofOptionsReport on Scheme DevelopmentJanuaryto August2008

No part of this report may be copied or reproduced by any means without prior written permission from Jacobs Arup Jacobs UK Limited and Ove Arup & Partners International Limited Consortium. If you have received this report in error, pleasedestroy all copies in your possession or control and notify Jacobs Arup.

This report has been prepared for the exclusive use of the commissioning party and unless otherwise agreed in writing by Jacobs Arup, no other party may use, make use of or rely on the contents of this report. No liability is accepted by JacobsArup for any use of this report, other than for the purposes for which it was originally prepared and provided.

Opinions and information provided in the report are on the basis of Jacobs Arup using due skill, care and diligence in the preparation of the same and no warranty is provided as to their accuracy.

It should be noted and it is expressly stated that no independent verification of any of the documents or information supplied to Jacobs Arup has been made.


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Appendix D - Central Tower Stability

Appendix E - Schematic Multi-Modal Alignments

Appendix F - Main Crossing Construction Programmes


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Orthotropic Box Girder Section

Composite Box Girder Section

2.3.2 Double Level option

The double level options utilise a deep stiffening truss that assists with stabilising thecentral tower. The key driver for the overall behaviour of the suspended decks is therelative stiffness of the tower and the deck. The deck must be stiff enough toaccommodate the deformations at mid-span under asymmetric live loading and reducebending effects in the tower such that the tower can be kept relatively slender andelegant. Three alternatives are considered which are illustrated in Drawings

FRC/C/076/D/111 to FRC/C/076/D/113 in Appendix B:

2 Plane Warren Truss 4 Plane Warren Truss 2 Plane Vierendeel Truss

The logical truss arrangement is a Warren truss which is more elegant than a Pratt truss.The shear forces are also reversible in most parts of the deck which means thatstructurally the Pratt truss is not particularly relevant since its defining feature is that thebracing arrangements relate to the direction of the shear force. Providing two planes oftrusses, one beneath each plane of cables, creates a torsionally stiff and robust structure.However an alternative with four truss planes is also considered in order to triangulate

the transverse span to reduce the cross beam depth and also reduce the section size ofthe bracing members.

The Vierendeel truss alternative is proposed in order to create a visually less complexstructure since the bracing members of the Warren truss are inclined in two differentdirections and result in possible visual interference effects when viewed from certainangles. However, it is well established that Vierendeel trusses are less efficient thantriangulated trusses and the feasibility of this proposal has been carefully studied.

4 Plane Warren Truss

2 Plane Vierendeel Truss

2.4 Tower Forms

Three alternative tower forms have been developed, in each case the tower is areinforced concrete hollow structure with a fabricated steel anchor box to house theupper stay anchors. Provision is made within the towers for access during constructionand for inspection and maintenance.

Tower Forms

Needle Inverted Y H Shape


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Additional windshields may be required along short lengths close to the towers, wheresudden changes in cross wind can occur due to the shielding nature of the towerstructure.

The windshields along the edges of the deck will be designed to be difficult to climb over.

Possible layout of anti-climb windshields


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anchored in a fabricated steel anchor box structure which will act compositely with theupper tower. This is a common arrangement which is adopted on Pont de Normandieand Stonecutters Bridge amongst many others. An exercise was carried out to determinethe preferred anchor box height considering a balance between ease of fabrication andmaintenance of the anchor box versus reduction in structural demands on the tower. Theresult was a height difference of 60m between the highest and lowest stay anchor points.

6.2.2 Needle

A great many variations on the Needle Tower were considered during the conceptualdesign development which resulted in the development of an N1 concept and an N2concept. The N1 concept is that the tower is developed from an initially circular form,modified to suit the structural and practical requirements. The N2 concept is that thetower is developed from an initially rectangular form, modified to provide improvedaesthetics. Drawings FRC/C/076/S/201 and FRC/C/076/S/202 in Appendix B showcurrent versions of the N1 and N2 concepts.

N1 Tower N2 Tower

The N2 tower is recommended because it better achieves the aesthetic concept of asingle vertical element centrally located with a hole punched through. Whilst the lowerpart of the N1 tower is attractive in that a circular shape can be achieved it is difficultvisually to resolve that shape in the upper tower without an appearance of two distinctlegs separated by the anchor box.

There is no visible cross beam below deck. This is important aesthetically in achieving a

simplicity of the deck/tower connection and achieves the further benefit that an under-deck inspection gantry may pass between the tower legs. A crossbeam is requiredstructurally for the flanking towers although it is only required to act as a tie between thetwo legs so is slim enough to fit within the depth of the deck. The crossbeam could beeither steel or prestressed concrete. At the central tower the monolithic connection to thedeck acts as the crossbeam.

6.2.3 Inverted Y

Y1 Tower Y2 Tower


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options have been developed to avoid this; the Needle and the Inverted Y. The slimtowers which can be achieved would be in scale with the towers of the existing roadbridge. Moreover the shallow depth of the deck will be like a blade across the water.Comparing between these two options, whilst the Inverted Y could be developed into agood aesthetic solution, there is no doubt that the Needle emphasises the aesthetic idealof a single element piercing the blade-like deck.

For the Double Level Option, the development of the H-Shape tower is an exercise in selfrestraint. Simple slender elements are arranged so as to complement the more complextruss form of the deck. With the tower having two vertical elements, it is even morecritical that the tower should be simple in form to avoid dominating the towers of theexisting road bridge. This is achieved with a simple conical form, sliced through with aplane on the inner face to create a shadow line and encase the deck. The crossbeamswhich are required structurally have been developed as slim minimalistic tubes.

6.4.3 Summary

All three tower options are technically feasible with little to differentiate them apart fromthe footprint which is expected to lead to higher foundation costs for the H-Shapecompared to the other towers. Similarly, all three options are believed to be goodaesthetic solutions which can be developed into a final tower form worthy of theprominent site and in sympathy with the existing bridges. However, for the ThreeCorridor Option, the Needle Tower is believed to be aesthetically superior to theInverted Y.

It is therefore recommended to develop the N2 and H1 options for the Stage 3Assessment.

N2 Tower Y2 Tower H1 Tower


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Both the Needle and the Inverted Y towers have common features of curved faces,inclined legs and a connection between the legs at water level. These features areadopted in the approach piers. The small variation in pier height over water due to thevertical profile of the bridge is planned to be accommodated by cropping the top of thepier so that the shape of the pier remains constant. The large variation in pier height overland is achieved by varying the inclination of the legs which results in a small number ofunique pier shapes.

7.3.2 Double Level Option

The approach span piers for the Warren and Vierendeel Truss options consist of simplerectangular sections tapered in both elevations. In transverse elevation the inclination ofthe tapered edge matches the inclination of the main tower legs. The pier cross sectionwill be of hollow reinforced concrete construction.

Approach Pier Form


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12.3.3 Tower Form

As described in Section 6 4 2 the Needle Tower better emphasises the aesthetic ideal of


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As described in Section 6.4.2 the Needle Tower better emphasises the aesthetic ideal ofthe single element piercing the blade like deck of the bridge. The recommendation of theOptions Selection Workshop was that the Needle Tower should be developed inpreference to the Inverted Y.

12.3.4 Approach Bridge Type

The cost comparison showed the Concrete Box Girder approach to be more economicalthan the wide Composite Box Girder approach. However, the Composite Box Girderapproach gives a cleaner visual continuity of the cable stayed bridge into the approaches.It is recommended that both options are investigated further.

12.3.5 Foundation Type

Inclusion of the results of the marine ground investigation and the ship impact riskassessments will allow the proposed foundations to be further developed including moredetailed assessment of the constructability of the precast pile caps and footings.

12.3.6 Conclusion

The following recommendations on which scheme options to develop further representthe consensus of the Options Selection Workshop attendees:

Functional Cross Section Three Corridor Option

Deck Type Orthotropic andComposite Box Girder

Tower Form Needle Tower

Approach Bridge Type Composite Box Girder Concrete Box Girder

Foundation Type(Towers)

Flanking Towers: PiledCentral Tower: Pad Footing

Appendix A - Reference Design Drawings


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FRC MM Scheme Assessment Feb 09 Issue.doc

the forth replacement crossing analysis of costs

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