6. Assessment of Tower Options
The tower options have been developed considering aesthetics, structural capacity and the space requirements for stay cable anchorages as well as inspection and maintenance facilities.
The towers are hollow reinforced concrete structures with the stay cables connected to a fabricated steel anchor box embedded in the upper part of the tower. The use of an anchor box maximises off-site fabrication allowing rapid construction progress and accurate geometry controlled in factory cinditions.
Structural demands on the central tower are particularly significant due to the double main span arrangement. At this stage of design the structural sizing of the tower is based upon grade C50/60 concrete and grade 500B 40mm diameter bars which is considered standard practice in the UK. However, concrete grades up to C70/85 are permitted by the relevant UK National Annexes to the Eurocodes and 50mm diameter bars are reasonably common within international practice. Some tendering contractors may opt for higher strength concrete and/or larger diameter bars which will allow reduction in concrete quantities and/or reinforcement densities. However, a more competitive tender should be possible if the tower dimensions allow for "standard" grades and diameters.
For aesthetic reasons the external dimensions of the flanking towers are kept the same as for the governing central tower although thinner wall sizes and/or lower reinforcement densities will be possible. The towers extend 145m above deck level and for all three towers the external geometry of each upper tower is identical. Due to the vertical profile of the bridge the height of the lower tower below deck varies. The vertical alignment of the bridge has been made symmetrical over the two main spans so that the height of the two flanking towers is identical but there is a difference between these and the central tower. The geometry of the lower tower is therefore slightly different but the variation is achieved in a subtle manner to suit the bridge profile and the aesthetics of the towers is not compromised.
The space required for stay anchorages as well as inspection and maintenance access facilities govern the dimensions required at the tower top, particularly in the transverse direction. For each tower a rack and pinion inspection and maintenance lift will be provided from deck level to the tower top. For the H-Tower the lift will also descend to the base level. Sizing is based on a minimum car size of 0.78 x 1.30 m which is sufficient for 5 persons. Intermediate lift stops together with the required access platforms will be provided to give access to the stay anchors which are spread out over a vertical height of up to 60m. In addition, emergency ladders must be accommodated to provide access/escape in the event of mechanical failure of the lift. The stay anchor box itself is sized to allow stressing of the stays at the tower top and provision is made for bringing large tension adjustment jacks to the anchor head,
The tower foundations are submerged below low-tide level to allow the towers to emerge uninterrupted from the water at all states of tide and avoid a bulky waterline object interfering with the aesthetic form. A consequence of this is that at high tide the tower legs could potentially be exposed to relatively significant ship impact forces. A connecting element is required between the tower legs to provide the necessary robustness.
Aesthetic studies of the towers and development of the tower forms has been carried out continuously from the concept to scheme stage. The aesthetic development and approach to the aesthetic design has been described in the Jacobs-Arup report "Approach to Aesthetic Design and Procurement" May 2008. The initial development focussed on overall conceptual forms based on rectangular cross sections with later refinement studying different cross sections to enhance the forms.
6.2 Three Corridor Options
The towers developed for the Three Corridor Option are applicable for either the Orthotropic or the Composite Deck. As described in Section 2.3.1 above, the external dimensions of the tower are kept the same for both deck types in order that the visual appearance of the bridge will not be affected by the deck type selection. However, the wall thickness and reinforcement ratios will differ depending on the deck type since the structural demands are not the same.
(b) Structural behaviour
The high structural demand in the central tower occurs both in the upper tower approximately 105m above deck level and in the lower tower immediately below deck. In both cases the critical loading is traffic load on one main span only as illustrated below:
Stay forces and tower moments for traffic on one span
In the span being loaded the shorter, steeper stays deliver load to the central tower whereas in the opposite main span it is the longer, shallower stays which transfer the load to the crossing stay region. The result is a large bending moment in the upper tower due to the vertical spread of the stay cable anchorages.
The relative flexibility of the structure results in the central tower being pulled towards the loaded span which results in moment in the tower. This moment is increased due to framing action with the deck provided by the monolithic joint. The result is a large bending moment immediately below deck level.
The behaviour is similar for both deck types but the axial forces due to permanent loads in the tower are higher for the Composite Deck whereas the longitudinal flexural moments due to live load described above are slightly higher for the Orthotropic Deck. Transverse flexural forces due to wind are similar for both deck types. The increased axial load for the Composite Deck is dominant and this deck type results in higher structural quantities in the tower.
(c) Stay anchorages
As described above, the vertical distribution of the stay anchorages is important in determining the bending moment in the upper tower. It is proposed that the stays be anchored in a fabricated steel anchor box structure which will act compositely with the upper tower. This is a common arrangement which is adopted on Pont de Normandie and Stonecutters Bridge amongst many others. An exercise was carried out to determine the preferred anchor box height considering a balance between ease of fabrication and maintenance of the anchor box versus reduction in structural demands on the tower. The result was a height difference of 60m between the highest and lowest stay anchor points.
A great many variations on the Needle Tower were considered during the conceptual design development which resulted in the development of an N1 concept and an N2 concept. 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 the tower is developed from an initially rectangular form, modified to provide improved aesthetics. Drawings FRC/C/076/S/201 and FRC/C/076/S/202 in Appendix B show current versions of the N1 and N2 concepts.
The N2 tower is recommended because it better achieves the aesthetic concept of a single vertical element centrally located with a hole punched through. Whilst the lower part of the N1 tower is attractive in that a circular shape can be achieved it is difficult visually to resolve that shape in the upper tower without an appearance of two distinct legs 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 required structurally for the flanking towers although it is only required to act as a tie between the two legs so is slim enough to fit within the depth of the deck. The crossbeam could be either steel or prestressed concrete. At the central tower the monolithic connection to the deck acts as the crossbeam.
6.2.3 Inverted Y
Two cross section shapes were also considered for the Inverted Y tower, which are shown in drawings FRC/C/076/S/221 and FRC/C/076/S/222 in Appendix B. The Y2 concept is similar in form to the N2 concept with an initially rectangular form being modified by introduction of a curved surface on the front and rear faces. The Y1 concept is based around a pentagonal cross section in order to achieve greater shadow definition to the section. The Y2 concept is preferred because of the greater simplicity achieved at the base with a single object rather than two legs and a crossbeam. As with the Needle tower there is no visible cross beam below deck.
6.3 Double Level Options
The tower developed for the Double Level Option is applicable for any of the three truss type decks considered. In principle the external tower dimensions could vary slightly according the to deck type since it is not intended that multiple truss alternatives will be offered to the tendering contractor. In other words if the Double Level Option is selected as the Specimen Design then one of the candidate deck types will have been selected and the others rejected. However, in practice, the aesthetic and structural requirements for the different deck types are similar and the external dimensions of the tower are kept the same for all types. However, the wall thickness and reinforcement ratios will differ depending on the deck type since the structural demands are not the same.
(b) Structural behaviour
As described in Section 6.1 the central tower forces are governing which is illustrated by the bending moment diagrams shown below.
Longitudinal flexure of the tower legs in the governing central tower increases linearly from the bottom stay anchorage towards deck level. However, as a consequence of the longitudinal restraint offered to the deck by the central tower, the rate of increase in longitudinal flexure of the latter is significantly reduced below deck level. This in turn leads to a critical tower leg section not at foundation level, but at deck level where the overall cross section dimensions are smaller yet applied forces are not significantly less than at foundation level.
The towers are restrained laterally at deck level only. As a result, lateral stability of the tower legs is a further major factor driving the design.
(c) Stay anchorages
The tapered tower leg cross section was selected as a natural and elegant solution to reflect the reduction in structural demand with height. However, longitudinal bending of the tower legs over the height of the stay anchorage zone does not follow this trend and requires a significant moment to be carried by a cross section of modest size.
The design proposed incorporates a fabricated steel stay anchorage box acting compositely with the reinforced concrete tower leg whilst maintaining the provision required for access during construction and for inspection and maintenance throughout the life of the structure. As for the Three Corridor Option, the height of this anchor box is a balance between ease of fabrication and maintenance of the anchor box which would be provided by a tall box versus reduction in structural demands on the tower which would be obtained from a short box.
Longitudinal Bending Moments in Tower Legs due to Live Load (Warren Truss Options)
A large number of different configurations were studied for the double level towers. This included variations on "A" shaped towers and "H" shaped towers. Within these a number of variations were considered. The final preferred option is a modified "H" shaped configuration shown in Drawing FRC/C/076/D/241 in Appendix B. Each tower consists of two reinforced concrete legs inclined towards each other and connected together by a series of steel struts in the stay anchorage zone. Thus the crossbeam of the "H" has become, in effect, a series of thin crossbeams moved up towards the top of the legs over the stay anchorage zone.
The selected tower design utilises a "D"-shaped cross section with the curved side of the legs facing away from each another. The legs are based geometrically upon a simple truncated cone with a section removed from the inner face as a result of a planar cut. Currently the plane of the cut passes through the apex of the cone. Further development and refinement of the tower form may include varying the plane of the cut relative to the leg axis to modify the tower proportions.
6.4 Assessment of Options
6.4.1 Technical Comparison
Comparison on structural performance of the towers is not particularly revealing since the performance of the Needle and the Inverted Y is almost identical whereas the differences in the performance of the H Shape are inextricably linked to the different deck type and articulation arrangement proposed for the Double Level Option. However, one major area of technical difference that can be compared is the footprint of the towers and the significance of that to foundations and ship impact.
The estimated minimum foundation sizes required are approximately 45m by 35m for the flanking towers and 35m by 25m for the central tower. Both the Needle and the Inverted Y tower can be accommodated within this footprint and therefore the foundations for these two options will be very similar. Because the Needle delivers a more concentrated load than the Inverted Y then a slightly thicker pilecap/footing is required but otherwise the foundations are the same. On the other hand the tower quantities for the Needle are slightly reduced compared to the Inverted Y and in particular the quantity of stainless steel reinforcement to be provided in the inter-tidal and splash zone. Therefore the net cost difference is expected to be marginal.
However the width of the H-Shape tower at foundation level is approximately 65m which is much larger than the required size for a single pilecap / footing. Thus, for the flanking towers two independent pilecaps are proposed connected together by a structural beam at waterline which also acts to strengthen the tower legs against ship impact. The overall length of the foundation is greater than for the Needle / Inverted Y options which potentially increases the vulnerability to ship impacts since impacts with the corner of the pilecap may act at a greater eccentricity to the overall pile group and produce a larger twisting effect on the foundation. This, combined with the heavier loads from the double level deck options, results in the need for greater numbers of piles. For the central tower, the greater width results in the tower straddling the high point of Beamer Rock with the pad footing foundations needing to be constructed at a lower elevation on the flanks of the rock. This will require more complex temporary works and greater quantities of rock excavation. The larger footprint also results in greater stainless steel reinforcement quantities in the tower.
6.4.2 Aesthetic Comparison
The objective of the aesthetic design of the main crossing is to build a bridge that will be elegant, unique and instantly recognisable as the Forth Replacement Crossing. At the same time it will fulfil all the functional requirements in a way that delivers value for money in whole life terms, having full regard to buildability and maintainability. The setting for the crossing is a world-famous landscape and the required standard of aesthetics is high.
It is fitting that the overall form of the new bridge heralds the 21st Century, just as the rail bridge is a memorial to19th Century engineering and the suspension bridge relates to the 20th Century.
It is also important that, in addition to being an aesthetically pleasing and iconic structure, the scale of the new bridge is sympathetic to the surrounding landscape and complementary to the form of the existing road and rail bridges. In particular, the towers must not dominate the slender towers of the existing road bridge.
The new bridge will be seen from many locations, both locally and at a distance from settlements, roads and hills in the landscape around the Forth estuary.
Historically, viewpoints at North and South Queensferry have enabled the closest and most dramatic views of the existing bridges.
However, from the north, the new bridge will be viewed most closely from Queensferry Hotel and Admiralty House (also known as St Margaret’s Hope), west of North Queensferry, while the majority of North Queensferry will have more distant views beyond one or both of the existing crossings.
From the south, the new crossing would be most visible from the north-west of South Queensferry, Port Edgar marina, Linn Mill, and Inchgarvie House.
Travellers using the new bridge, the existing road bridge or sailing on the Forth close to these bridges will also be able to view the new bridge in close proximity.
For the Three Corridor Option, the penetration of the tower through the deck is the key to achieving this. The alternative options where the legs straddle the deck would look squat and ungainly with the wide deck required for the Forth Replacement Crossing. Two options have been developed to avoid this; the Needle and the Inverted Y. The slim towers which can be achieved would be in scale with the towers of the existing road bridge. 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 a good aesthetic solution, there is no doubt that the Needle emphasises the aesthetic ideal of a single element piercing the blade-like deck.
For the Double Level Option, the development of the H-Shape tower is an exercise in self restraint. Simple slender elements are arranged so as to complement the more complex truss form of the deck. With the tower having two vertical elements, it is even more critical that the tower should be simple in form to avoid dominating the towers of the existing road bridge. This is achieved with a simple conical form, sliced through with a plane on the inner face to create a shadow line and encase the deck. The crossbeams which are required structurally have been developed as slim minimalistic tubes.
All three tower options are technically feasible with little to differentiate them apart from the footprint which is expected to lead to higher foundation costs for the H-Shape compared to the other towers. Similarly, all three options are believed to be good aesthetic solutions which can be developed into a final tower form worthy of the prominent site and in sympathy with the existing bridges. However, for the Three Corridor Option, the Needle Tower is believed to be aesthetically superior to the Inverted Y.
It is therefore recommended to develop the N2 and H1 options for the Stage 3 Assessment.