Highways Agency

• 1 Introduction
• 2 Highway Design and Layout
• 3 Bridge Design and General Arrangement
• 4 Environment and Land
• 5 Scheme Costs
• 6 Traffic and Economics
• 7 Project Appraisal Report
• 8 Conclusions and Recommendations

A19 Dishforth to Tyne Tunnel

Feasibility Study

3 Bridge Design and General Arrangement

3.1 Introduction

3.1.1 Alternative forms of construction are discussed that consider both a single and tow-lanecarriageway with accommodation for pedestrians. A summary comparison table of theoptions is included at the end of this section of the report.

3.1.2 The disruption effects of construction are considered and an estimated price for thecapital works for the structures only are provided. This review is based upon a desktopstudy with limited design work to merely verify the feasibility of the structural optionsthat are discussed in this report. No specific survey work or geotechnical investigationhas been carried out.

3.2 Description of Site and Geometric Constraints

3.2.1 The existing at grade junction is not signalled and originally provided bothnorthbound/southbound access from Trenholme Lane onto the A19 andeastbound/westbound access from the A19 onto Trenholme Lane. The central reservegap is currently closed by an experimental order that expires in January 2006.

3.2.2 The existing terrain is reasonably flat. To construct the compact grade separatedjunction, new earth embankments would be required to provide approaches to the siteof the proposed structure. Of the two highway alignment options, Option 1 is a distanceof 103 metres north of the junction and Option 2 is a distance of 86 metres north.Within these two options the provision of both a single and two-lane carriageway bridgefor the road crossing has been considered. With the highway face of the proposedabutments set back at 3.5 metres from the edge of the A19 carriageway a minimumtotal structure span of approximately 40 metres is required.

3.2.3 Although minimum headroom of 5.7 metres above the A19 would be desirable,minimum headroom of 5.3 metres, as permitted by the Highways Agency's DesignManual for Roads and Bridges (DMRB), has been assumed for this study. This hasbeen chosen in order to improve the vertical alignment and to reduce the extent of theapproach embankment works that are required. This has the consequence that theproposed bridge deck will be required to accommodate vehicle collision loads inaccordance with current standards. This may have the effect of increasing thesuperstructure costs; dependant upon the chosen structural form but this will be morethan offset by the cost savings associated with the reduced approach works. Thechosen highway vertical alignments result in an allowable superstructure constructiondepth, not including surfacing of 1.35 metres.

3.2.4 The proposed replacement of the existing junction removes the requirement of thesouthbound right turning lane off the A19 and therefore allows the option to widen thecentral reserve. As a result a central bridge support could be provided and this is alsoconsidered as an option.

3.3 Aspects Considered within the Design

3.3.1 Design and construction needs to take into account the following:-

• Provide solutions that minimise disruption to the traffic flow on the A19 during construction.
• Construction periods.
• Collision loads impacting on super and substructures.
• Aesthetics.
• Construction cost.
• Be easily maintainable.
• Buildability
• Heath and Safety
• Environmental aspects

The above are not stated in any order of priority.

3.3 Forms of Construction Considered

3.3.1 Superstructure

• Steel/Concrete Composite Deck.
• Pre-cast Pre-stressed Concrete Deck.
• Insitu Reinforced Concrete Deck.

3.3.2 Substructure

• Reinforced Concrete Bank Seat Abutments (Open Aspect).
• Reinforced Concrete Cantilever Abutments (Closed Aspect).
• Reinforced Concrete Cantilever Wingwalls.
• Reinforced Earth, Concrete Panel Clad, Wing Walls.

3.3.3 Foundations

• At this stage, we are unable to consider the most appropriate form of foundations with the absence of geotechnical information.

3.4 Discussion of Structural Forms

3.4.1 Superstructure

a. Insitu Reinforced Concrete Deck

With a central support providing a 2 span continuous structure, with equal spans ofapproximately 20 metres each, this type of deck is structurally viable option. Theminimum construction depth could be in the region of 1.1 metres with a reasonably highreinforcement percentage. With this construction depth the vertical highway alignmentcould be lowered resulting in cost savings with the approaches. Alternatively the fullallowable construction depth could be utilised resulting in a less heavily reinforced deck.

This type of deck could also be voided to significantly reduce the self-weight and hencethe reinforcement quantity and would also reduce loads on the substructure. Howeverit must be noted that contractors do not generally favour the provision of voids in areinforced concrete deck slab.

To construct the deck slab the required falsework and temporary works would result inlong closures of both carriageways of the A19.

Advantages

• Robust with regard to impact
• Alleviates the need for heavy lifting plant
• Lower construction costs than other options considered (this may be marginal)
• Lower maintenance costs than steel option.

Disadvantages

• Longer Construction period.
• Major disruption to traffic.
• High traffic management costs.
• Numerous concrete deliveries to site.
• Requires central support.

The above disadvantages make this an unfavourable option. Therefore this option hasnot been taken further.

b. Steel/Reinforced Concrete Composite Deck

This form of construction allows the provision of a single 40 metre span structure,resulting in an option with the lowest substructure costs. Due to the vertical alignmentconstraints the resulting span to depth ratio (30) lies at the very top of the acceptedrange. The construction depth will be equal to the desired maximum depth of 1.35metres. This will result in heavy lower flanges making this particular deck structure notas economical as this type of structure can be. However, the heavier lower flangeswould assist in resisting vehicular collision loads.

A two span structure of this type of construction could also be provided but is a lessfavourable option when compared to a similar structure of pre-cast pre-stressedconcrete. This option is therefore not considered further.

Refer to drawing numbers 31315/S/SK21 AND 31315/S/SK23 for general arrangementdetails of the single span option.

Advantages

• Least disruption to traffic than other options considered.
• Lower traffic management costs than other options considered.
• Aesthetically pleasing.
• Does not require central support.

Disadvantages

• Most susceptible to impact damage.
• Higher maintenance costs than other options considered.
• Requires heavy lifting plant but slightly less than the pre-cast concrete option.
• Total road closure required during beam lift.
• May require fabrication area adjacent to site.

c. Pre-cast Pre-stressed Concrete Deck.

A single 40 metre span structure is achievable with the largest of the SY type bridgebeams but the required construction depth would be approximately 2.2 metres, fargreater than the desired 1.35 metres. However, a two span option is viable using thesmaller Y type beams. This results in a construction depth of 1.15 metres thus allowingthe vertical alignment to be lowered producing possible cost savings with theapproaches.

Refer to drawing numbers 31315/S/SK20 and 31315/S/SK22 for general arrangementdetails of the two span option.

Advantages

• Reduced level of disruption to traffic when compared to the insitu r/c option.
• Lower maintenance costs than steel option.
• Does not require a total road closure.
• Robust with regard to impact.
• Shorter procurement period than steel.

Disadvantages

• Requires central support increasing disruption to traffic.
• Requires heavy lifting plant, slightly greater than the steel option.

3.4.2 Substructure

a. Reinforced Concrete Bank Seat Abutments (Open Aspect).

Considered on the basis of aesthetics, this method of superstructure support within thegeometrical constraints is only structurally viable for a two span structure. Two equalspans, of approximately 29 metres, could be provided in either steel/concrete compositeor pre-cast pre-stressed concrete construction. Bank seat abutments could bepositioned at the top of approximately 1 in 2 sloping revetments giving cost savings dueto the reduced approach works, when compared with a 'closed' abutment structure.Without pre-loading of the new earth embankment structures unacceptable settlementof the bank seats, designed as spread footings, are almost certain to occur. This willtherefore necessitate the use of piled foundations, that detract from any cost savingsthat the bank seats would have had when compared to vertical cantilever abutments.Another disadvantage is that due to the overall increase of the total deck span, the costof the superstructure would increase in the order of 50%, which would not make thisoption economically viable.

b. Reinforced Concrete Vertical Cantilever Abutments (Closed Aspect).

These comprise reinforced concrete retaining walls to the approach embankments thatgive end support to the deck structure. Positioned 3.5 metres from the edge of the A19carriageway they provide a 'closed' aspect, keeping the superstructure spans to aminimum. The faces of these could be clad in various materials with the aim ofaesthetically complimenting the local environment. However, a far less expensiveoption, which is generally acceptable is to provide a profiled patterned finish to thesurface of the concrete.

c. Reinforced Concrete Cantilever Wingwalls.

Using the same type of construction as the vertical cantilever abutments, thesestructures retain the sloping sides of the earth embankments. Finishes to these wallswould be as that applied to the abutment walls.

d. Reinforced Earth Concrete Panel Clad Wing Walls.

This form of construction is only deemed appropriate for the wing walls and not theabutment structures due to reinforced earth not being suitable to accommodatesignificant vertical loading. They are therefore considered as an alternative to thereinforced concrete option to be used in conjunction with reinforced concreteabutments. It seems generally accepted that they are more cost effective structureswhen compared to reinforced concrete retaining walls. However, considering theirrelatively short lengths, the unknown ground conditions and the absence of detaildesign it is difficult to judge at this stage what the most cost effective solution will be.

3.5 Traffic Management Considerations

3.5.1 The extent of traffic management during the construction period of this project will havea significant influence on the chosen structural option. The influence being in terms ofboth cost and disruption to the traffic on the A19.

3.5.2 During the abutment construction, due to the existing width of the carriageway, it maybe possible to accommodate two-lane running in both directions, assuming closure ofthe existing merge/diverge splays. With phased construction this may also be possibleduring the construction of a bridge support in the central reserve.

3.5.3 The construction of an in situ reinforced concrete deck will require closures to each ofthe northbound and southbound carriageways, in turn and for considerable period. Thismay be the overriding factor that forces the dismissal of this option.

3.5.4 During the 'lift in' of beams for the single span steel composite option then a total roadclosure of both carriageways will be required. This will probably be accommodated witha single overnight closure that would keep traffic management and disruption to aminimum.

3.5.5 The 'lift in' of pre-cast beams will require closures to each of the northbound andsouthbound carriageways in turn but for relatively short periods.

3.6 Costing Summary

3.6.1 A costing exercise has been carried out to determine the capital cost of the structurealone and has identified that there is only a small difference (just under 3%) for highwayOption 1 and Option 2. The costs in the table below are to be taken as representing thefirst quarter of 2005 and do not take into account the following:

• Temporary works.
• Approach works.
• Traffic management.
• Reinstatement works to the A19.
• Traffic signalling works associated with the single lane option.
• Preliminaries.
• Contingencies
• VAT.

Span	Deck Construction	Carriageway	Cost (£1000)
			Option 1	Option
1	Steel/Reinforced Concrete Composite	Two Lane	614	632
1	Steel/Reinforced Concrete Composite	Single Lane	416	428
2	Pre-Cast Pre-Stressed Concrete	Two Lane	640	658
2	Pre-Cast Pre-Stressed Concrete	Single Lane	443	456

3.7 Structural Options Summary

3.8.1 The structural options that have been considered have been summarised in thefollowing table. An attempt has been made to score each viable option by weightingeach of the design and construction considerations from 1 to 3, with 3 being of thehighest importance. Then each option has been given a rating with regards as to how itcompares with the other options within each category. Again using a scale of 1 to 3,where 3 mean it fares best.

Structural Options Summary Table (28KB PDF)