12. Conclusions and Recommendations 12.1 Cost Comparison 12.2 Sustainability Comparison 12.3 Recommended Options for Further Development

12. Conclusions and Recommendations

12.1 Cost Comparison

The conceptual designs proposals have been developed to scheme design stage to allow assessment of different scheme options. Structural quantities were derived in order to make a comparison of the relative cost of the different options. The results of the cost comparison are given below:


Three Corridor Option

Double Level Option


Orthotropic Deck

Composite Deck

2 Plane Warren Truss

4 Plane Warren Truss

2 Plane Vierendeel Truss






Relative Cost








The relative cost is expressed as a proportion of the base option. The orthotropic deck with concrete approaches was selected as the base option (with a relative cost of 1.00) because it is the closest in configuration to the FRCS design.

The above costs are for the Main Crossing from abutment to abutment exclusive of ITS systems. The relative costs do not include the transition structures required to stream the HOV lanes from the main carriageways to the multi-modal corridor.

The 2 Plane Vierendeel Truss option is shown to be significantly more expensive than all of the other options due to higher structural steel quantities.

12.2 Sustainability Comparison

Sustainability appraisal of the project has been included at the earliest planning stages. The FRCS studies identified 5 key indicators related to supporting sustainable development and economic growth. The main focus on meeting these key indicators was the promotion of sustainable transport modes which led to the inclusion of a multi-modal corridor on the bridge for future provision of trams or LRT together with designing the hard shoulders for peak hour bus operation.

Sustainability could also be considered as a differentiator in the later planning stages of the project to assist with appraising the relative merits of different options.

A sustainability appraisal should focus on the key elements of environmental protection, social equity, economic viability and efficient use of natural resources. However, many of these aspects have already been fixed as a result of the FRCS studies which have determined:

  • The need for a bridge, the alignment and the basic form (double main span cable stay)
  • The functional cross section in terms of number and usage of lanes


It is intended to carry out a Main Crossing Sustainability Appraisal at DMRB Stage 3 to confirm the suitability of the option carried forward to Specimen Design.

Considering the Main Crossing in isolation this assessment will be mainly focussed on the environmental impacts and depletion of natural resources associated with the construction methods and materials. One of the most challenging aspects will be to differentiate between steel and concrete as a more sustainable material in terms of future availability of resources and impact on climate change. Previous life cycle comparisons have not shown a clear and consistent preference. In some cases lower energy costs and emissions have been indicated for concrete bridges if new-won materials are considered. However, the relative ease of recycling steel compared to concrete changes the picture with lower energy costs and emissions associated with steel if recycled products are used.

At this stage several points are noted:

  • The surface area of steel requiring painting is similar for all of the cable stayed bridge options
  • The quantities of materials to be used in the cable stayed bridge are similar between the composite deck and the warren truss options. Therefore the sustainability of these solutions will be similar.
  • The quantities of materials to be used in the orthotropic deck option are different with larger quantities of some items and reduced quantities of others. Therefore the sustainability of this solution may be better or worse than the alternatives.

12.3 Recommended Options for Further Development

A Main Crossing Options Selection Workshop was held on July 28th 2008 where the findings of the Scheme Assessment as documented in this report were presented and discussed. The issues discussed during the workshop and the conclusions are described below:

12.3.1 Comparison between Three Corridor and Double Level Options

The two different functional cross sections proposed at the conceptual design stage are both considered feasible options. However, a review of the relative merits of each option indicates the Three Corridor Option to be more favourable and it is recommended that this be carried forwards. The relevant criteria are:

(a) Multi Modal – Relative cost of transition structures in Phase 1 (HOV’s)

Although a quantified cost comparison has not been made it is clear that the cost of transition structures to stream the HOV lanes from the main carriageways to the multi-modal corridor will be higher for the Double Level option. This is due to the grade separation between the main carriageways and the multi-modal corridor.

(b) Multi Modal – Driver Perception in Phase 1 (HOV’s)

The grade separation is also likely to result in poor driver perception of the road layout for the Double Level Option since the lane transitions will involve curved ramp structures and minimum sight lines. In contrast the lane transitions for the Three Corridor Option will be on straight level sections of road with open sight lines.

(c) Multi Modal – Cost / Disruption to Modify to Phase 2 (Tram / LRT)

As noted in Section 4.5 the cost and disruption associated with modifying the approach transitions from the Phase 1 to the Phase 2 usage is anticipated to be significantly higher for the Double Level Option.

(d) Multi Modal – Phase 2 Operation (Tram / LRT)

Both configurations offer equally good operability of the trams or LRT in Phase 2 with the tracks located adjacent to each other.

(e) Relative Cost Excluding Transition Structures

The vierendeel truss option is excluded from the comparison since it can be rejected on cost grounds alone.

The work done to date indicates that if the composite approach were adopted for the Three Corridor Option then this would have a similar cost (1.06 to 1.08) to the warren truss options (1.09 to 1.11). Both the composite approach and the truss decks provide good visual continuity between the cable stayed bridges and the approaches.

However, the concrete deck offers the possibility of a cost saving for the Three Corridor Option, albeit at a slight aesthetic penalty.

(f) Tower Aesthetics

Any comparison on aesthetic grounds is somewhat subjective but a broad consensus has been built within the team that the towers of the Three Corridor Option are superior to the Double Level Option. Whilst the Double Level Option tower could be further developed into a reasonable solution it does not offer the instant appeal and excitement of the Needle Tower which is the preferred shape for the Three Corridor Option.

(g) Construction Programme

As described in Section 9.4 an initial assessment has been made of the construction programme which results in comparable total durations for both options assuming the truss units of the Double Level Option will be lifted in 24 m double length units.

(h) Environmental Impact

Environmental impact assessment of the two options has not been undertaken. However, given that the options comprise broadly similar structures, it is envisaged that their environmental impacts will be generally comparable.

(i) Sustainability

As noted above, a sustainability assessment has not yet been carried out. Although the material quantities are similar between the truss decks and the composite decks, the relative sustainability of the orthotropic and composite deck options has not yet been assessed.

(j) Maintenance

In broad terms the maintenance requirements of the different cable stayed bridge options will be similar. All of the options have the same basic elements of steel deck, proprietary stay cables and concrete towers. However, there are some minor differences.

Whilst the exposed surface area of steel is almost the same for the trusses and box girders, the large flat surfaces of the box girders will be easier to maintain than the multi-faceted small surfaces of the trusses. Furthermore, maintenance of some of the parts of the truss decks will require mobile elevated working platforms to operate on the lower deck which will be disruptive to the operation of the multi-modal corridor.

(k) Summary

A comparison table is given below. Where differentiation between the options is qualitative a three point scale is used: (Good – Fair – Poor). Reference should be made to the written explanations given above.


Three Corridor

Double Level

MM - Phase 1 Relative Cost of Transition Structures

Lower Cost

Higher Cost

MM – Phase 1 Driver Perception



MM – Cost / disruption to modify to Phase 2



MM – Phase 2 Operation



Relative Cost (excluding transition structures)

0.98 to 1.09

1.09 to 1.11 (excluding vierendeel)

Tower Aesthetics



Construction Programme


Environmental Impact



Assessment to be made between Orthotropic and Composite Deck Options

Comparable to Composite Deck Option

Ability to maintain structure with minimal disruption to operation



On the basis of this comparison it is recommended that the Double Level Option is not carried forward. The primary reason for this recommendation is the poor configuration of the multi-modal corridor for HOV usage in Phase 1. If the Double Level Option showed major advantages in other areas then further investigation could perhaps be justified but on all the other points of comparison the Three Corridor Option is shown to be comparable or superior.

12.3.2 Deck Type

As discussed in Section 5.1.4 it is recommended that both orthotropic and composite box girder decks be progressed with a view to offering both options to tendering Design & Build contractors. Further comparison between the deck types will include a sustainability assessment and more detailed proposals for the surfacing options on the orthotropic deck.

12.3.3 Tower Form

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

12.3.4 Approach Bridge Type

The cost comparison showed the Concrete Box Girder approach to be more economical than the wide Composite Box Girder approach. However, the Composite Box Girder approach 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 risk assessments will allow the proposed foundations to be further developed including more detailed 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 represent the consensus of the Options Selection Workshop attendees:

Functional Cross Section

Three Corridor Option

Deck Type

Orthotropic and
Composite Box Girder

Tower Form

Needle Tower

Approach Bridge Type

Composite Box Girder

Concrete Box Girder

Foundation Type (Towers)

Flanking Towers: Piled
Central Tower: Pad Footing