Feasibility Report Brent Swing Bridge (KCC No 1277) · Feasibility Report March 2012 Brent Swing...

Feasibility Report Brent Swing Bridge (KCC No 1277)

April 2012 30 March 2012

Feasibility Report March 2012 Brent Swing Bridge KCC No 1277 B1629200

Brent Feasibility 2011 non tech rev.doc

Document Control Sheet BPP 04 F8 Version 7 April 2011

Client: Kent County Council Project No: B1629200 Project: PBIs 2011 2012 Document Title: Feasibility Study Ref. No: B1629200/1277/02

Originated by Checked by Reviewed by Approved by

NAME NAME NAME NAME ORIGINAL A J Muir S R Biggs S R Biggs J Denness

DATE INITIALS AJM INITIALS SRB INITIALS SRB INITIALS JD Mar 2012 Document Status

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NAME NAME NAME NAME REVISION A A Muir S Biggs S Biggs P Heath

DATE INITIALS AJM INITIALS SRB INITIALS SRB INITIALS PJH April 2012 Document Status

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NAME NAME NAME NAME REVISION

DATE INITIALS INITIALS INITIALS INITIALS Document Status





Jacobs Engineering U.K. Limited This document has been prepared by a division, subsidiary or affiliate of Jacobs Engineering U.K. Limited (“Jacobs”) in its professional capacity as consultants in accordance with the terms and conditions of Jacobs’ contract with the commissioning party (the “Client”). Regard should be had to those terms and conditions when considering and/or placing any reliance on this document. No part of this document may be copied or reproduced by any means without prior written permission from Jacobs. If you have received this document in error, please destroy all copies in your possession or control and notify Jacobs. Any advice, opinions, or recommendations within this document (a) should be read and relied upon only in the context of the document as a whole; (b) do not, in any way, purport to include any manner of legal advice or opinion; (c) are based upon the information made available to Jacobs at the date of this document and on current UK standards, codes, technology and construction practices as at the date of this document. It should be noted and it is expressly stated that no independent verification of any of the documents or information supplied to Jacobs has been made. No liability is accepted by Jacobs for any use of this document, other than for the purposes for which it was originally prepared and provided. Following final delivery of this document to the Client, Jacobs will have no further obligations or duty to advise the Client on any matters, including development affecting the information or advice provided in this document. This document has been prepared for the exclusive use of the Client and unless otherwise agreed in writing by Jacobs, no other party may use, make use of or rely on the contents of this document. Should the Client wish to release this document to a third party, Jacobs may, at its discretion, agree to such release provided that (a) Jacobs’ written agreement is obtained prior to such release; and (b) by release of the document to the third party, that third party does not acquire any rights, contractual or otherwise, whatsoever against Jacobs and Jacobs, accordingly, assume no duties, liabilities or obligations to that third party; and (c) Jacobs accepts no responsibility for any loss or damage incurred by the Client or for any conflict of Jacobs’ interests arising out of the Client's release of this document to the third party.



Executive summary

1. The steel deck of Brent Swing Bridge has suffered from corrosion for many years and it is in the final stages of its serviceable life.

2. The extent of the visible deterioration and the assumed extent of deterioration of the concealed elements mean that it would be unsafe to lift the bridge without considerable repairs and additional access arrangements.

3. A decision on the timing of repairs or replacement of the existing deck should be taken within the next one or two years and certainly not later than five years.

4. The estimated cost to KCC of carrying out a trial lift would be of the order of £127,000 excluding any requirements from Medway Ports. A one-off trial lift is not recommended.

5. If a trial lift were successful to accommodate regular opening of the bridge a more robust programme of strengthening and repairs would be required. The estimated cost to KCC of this would be of the order of £192,000 with a considerable input also needed from Medway Ports.

6. If an opening bridge is still required, the most cost effective long term solution would be a complete replacement structure at some £706,000 to KCC and a large commitment from Medway Ports.

7. If the bridge were to be closed to navigation a cost effective solution would be to construct a replacement fixed crossing costing some £386,000. Adapting the existing structure to be a fixed crossing at some £302,000 would be possible and be environmentally sustainable.

8. The estimated residual life of the existing bridge is some 10 to 12 years, but it must be emphasised that the additional access and inspection recommended in the Principal Bridge Inspection Report could vary this figure.



Contents

Section Page Number

1 Introduction 1 1.1 Brief 1 1.2 Scope of Report 1

2 Description & History of the Bridge 3 2.1 Description 3 2.2 Brief History 3 2.3 Background 3 2.4 Current Situation 3 2.5 Strengthening 2007 4 2.6 Legal Position 4 2.7 Costs 4

3 Strength Assessment 5 3.1 Bridge Loads - General 5 3.2 Background to Assessment of this Bridge 5 3.3 Bridge Lifting Loads 6 3.4 Section Loss 6 3.5 North Abutment Settlement 7

4 Opening the Bridge 8 4.1 Processes 8 4.2 Structural Implications 8 4.3 Traffic Implications 9 4.4 Mechanical Implications 9 4.5 Conclusion 9

5 Analysis 10 5.1 Analysis 10 5.2 Analysis Results 11 5.3 Conclusion of Re-Assessment 12 5.4 Expected Life of the Bridge 12

6 Remedial Work Options 13 6.1 Work if Bridge is to be opened to Navigation 13 6.2 Work if Bridge is to be not to be opened to Navigation 13

7 Budget Estimates 14 7.1 For the bridge to be opened to navigation 14 7.2 Work if the bridge is closed to navigation 15 7.3 Discussion of Estimates 15 7.4 Value for Money 16



8 Recommendations 17 8.1 Recommendations if Bridge is to be Opened to Navigation 17 8.2 Recommendations if Bridge is to Remain Closed to Navigation 17 8.3 Risks 17 Appendix A Bridge Drawings


Brent Feasibility 2011 non tech rev.doc page 1

1 Introduction

1.1 Brief Jacobs Engineering UK Ltd. working as part of Kent County Council was commissioned to carry out a study into the feasibility of opening Brent Swing Bridge KCC No 1277 to marine navigation.

The site plan has been reproduced from an Ordnance Survey map, with the permission of Her Majesty’s Stationery Office, Crown copyright reserved licence No. 100019238:2011.

Brent Swing Bridge

1.2 Scope of Report This report is an updated and revised version of a previous one produced by Jacobs for KCC and dated October 2004 1. The 2004 report contained much background information and historical information some of which is reproduced in an edited format.



The brief received in September 2011 from the KCC client following an earlier request is summarised as follows:-

Main objective - determine the structural condition and requirements for an all encompassing but realistic estimate of cost to KCC to attempt a single trial opening of the bridge and, separately, the additional costs to KCC for regular opening.

• The cost estimate of any additional strengthening or works/equipment requirements to the deck to attempt an actual lifting.

• The cost estimate of any physical issues which may present a problem for opening, e.g. nosing getting snagged.

• The cost estimate for all activities to deliver the trial bridge opening, including general project management, any design, supervision, liaison (e.g. Medway Ports, KCC Highways) and construction.

• The additional requirements and cost estimate for the bridge to be used for more regular opening, including all of the above.

Following discussions the brief was extended by the client to include examining the possibility of the adequacy of the existing deck should it be needed as a single span fixed crossing at the site.

A Principal Inspection of the bridge was carried out in November 2011 2 to determine the condition of the bridge from a structural point of view. The following report is based on the findings of the 2011 inspection.

The condition of the bridge machinery is excluded from this report.

Concerns about the strength of the deck under highway loads stem from its condition following that Inspection, so strength under highway loading was checked also.



2 Description & History of the Bridge

2.1 Description The structure is a two span bridge carrying Bridge Road over Faversham Creek in Faversham Kent. OS Grid coordinates are 601521E 161622N.

The bridge is described more fully in the Principal Inspection Report carried out in November 2011.

Drawings of the bridge are in Appendix A.

2.2 Brief History

Examination of existing records indicated that there was a crossing over Faversham Creek at Bridge Road as far back as the early 1800s. The history of the bridge appears to be intimately tied to the sluice gates on the upstream side of the bridge which are attached to the abutments.

2.3 Background A bridge was built on this site originally in approximately 1880. At that time it was a swing bridge which when closed was a two span structure, supported at each end on abutments with a centre pier support. A single navigation channel only was provided, as the “back span” contained the kentledge counterweight.

Sketches on the following page illustrate the span arrangements.

A large hydraulic jack was mounted on the pier so that the deck could be raised clear of the permanent bearings as required. It would then be able to be rotated open to navigation by a hand operated winch.

Opening the bridge allowed boats to move in and out of the inner basin of the Creek. Lock gates were used to ensure the water level in the inner basin to the west of the bridge was maintained irrespective of the state of the tide.

2.4 Current Situation The form of the current bridge is the same as that originally conceived in 1880, and the substructure and machinery are original features. The steel deck is a more recent replacement and dates from 1977.

The pier which supports the hydraulic jack, pivot and centre bearings has separate foundations from those of the south abutment, although both are approached at ground level on the concrete slab installed during maintenance carried out in 1988. Records show that the south abutment is founded on piles and the pier on spread footings.

The deck consists of two longitudinal steel edge girders with 9 no transverse beams between. The deck top plate comprises steel plate stiffened by longitudinal Tee section stringers. The road surface utilises blacktop surfacing.



2.5 Strengthening 2007 The bridge was strengthened by welding additional steelwork to transverse beams 6 and 7 which had been found to be corroded and distorted.

2.6 Legal Position The bridge ownership is uncertain but it is believed that the hydraulic machinery and opening mechanism is owned by Medway Ports Authority (now owned by Peel Ports) which is responsible for navigation in the Creek. Kent County Council has an interest in the deck as it supports a Highway crossing.

2.7 Costs

Note that reference in this report to “costs to KCC” relate to the costs for structural aspects of the road crossing, as opposed to the mechanical and operating aspects and is not an acceptance by KCC of all structural costs.

SKETCH ELEVATION OF BRIDGE When Closed to Navigation

SKETCH ELEVATION OF BRIDGE When Opened to Navigation

North Abutment South Abutment

Pivot Bearing

Back Span Navigation Span



3 Strength Assessment

3.1 Bridge Loads - General Any structure is to be capable of supporting the Dead Loads (usually the self weight of the beams and girders), the Superimposed Dead Loads (permanent fixtures and fittings such as surfacing) and Live loads (transient loads such as the weight of vehicles and pedestrians).

Broadly speaking, a bridge which is strong enough to support the effects of permanent loads and the effects of un-restricted ‘normal’ highway traffic load is given a 40 tonne assessment rating.

A structure which is unable to support such loads can still be allowed to be open to traffic if the vehicles using it are weight restricted. These restrictions run in steps from the 40 tonne (un-restricted) rating to 3 tonne rating which is the minimum allowable restriction. Any restriction can be disruptive to hauliers, and the general public, but a rating of less than 18 tonnes would be seen as particularly disruptive because larger commercial vehicles would not be permitted to use the bridge.

3.2 Background to Assessment of this Bridge An assessment of the bridge to standard highway loading was carried out in 1993 in accordance with Highways Agency Standard BD 21/93.

The method used in this type of assessment was to calculate the strength of the critical elements and components of the structure. Following this the effect of dead load was subtracted from this ‘total’ capacity leaving the live load capacity.

It is often the case for small span bridges that the critical loading is a single axle rather than the uniform “HA” load (the Highway vehicle area load) for the deck as a whole.

Thus if any transverse beam could support an 11.5 tonne axle load, the maximum ‘normal’ axle load permitted, it would be able to carry unrestricted vehicle load. If it could not support the 11.5 tonne axle, then the next lower axle load would be 6.0 tonnes which corresponds to a 7.5 tonne vehicle rating.

In 1993, the bridge beams were found to be capable of carrying 40 tonnes of assessment live loading (highway loading). However, the pair of cross beams closest to the pivot point were found not to be able to carry the weight of the deck in the open position.

Although other accessible members were found to be capable of supporting the dead load of the bridge in the open position, the conclusion was that the bridge could not safely be opened without strengthening.

In 2007 repair work as mentioned in section 2.5 was carried out. This work included repairing the transverse steel beams which were visibly damaged by corrosion, but excluded the inaccessible beams or minor elements such as nosings. A full programme of repairs was not undertaken because of the inaccessibility of some elements, so although the reliability of the bridge in the closed position was improved, the repairs are not now considered sufficient to allow the bridge to be opened.



3.3 Bridge Lifting Loads

The bridge was designed to be jacked up from the pivot point and balanced by the kentledge weights, due to the back span being shorter (and lighter) than the navigation span, as indicated on the sketches.

The bearer beams connected to transverse beams 6 and 7 and, in turn, the edge girders would have to support all the dead plus superimposed dead load of the bridge. As this load includes the kentledge the uncertainty of the weight of kentledge adds a degree of risk to the calculations.

Records show that the kentledge weighed 12 tonnes but the 1993 assessment report highlighted discrepancies resulting in the kentledge being calculated as 16.7 tonnes. The effect of both figures has been considered in the calculation of the Highway load capacity of the bridge.

An additional consideration is the out of balance effects between spans. Previous investigation concluded the kentledge to be the original pig iron and this together with loss of section from many members means it is unlikely that the deck would be in balance about the pivot. As the only way to balance the girders is to add weights, it would be unwise to underestimate these loads.

It has been assumed also that temporary jacking points may be needed.

3.4 Section Loss

As a steel structure with visibly corroded beams, without substantial maintenance and repairs the bridge deck has a limited life. The steel plate deck and beams have corroded in places.

An assessment of the probable rates of section loss has been made based on observations of the actual beams and guidance in the relevant codes of practice.

3.4.1 Typical Section Loss

The section properties for beams 8 and 9 have been calculated assuming a loss of steel thickness of 0.10 mm/year based on the assumption that significant loss of section did not start until 10 years after the deck was installed. This corresponds to the section thicknesses measured during the Principal Inspection of 2011 and enables us to calculate likely steel thicknesses years into the future.

By comparison, BS6349 the Code of Practice for Maritime Structures suggests an upper limit for corrosion of such structures in the splash zone as having a mean value of 0.08mm/year with a maximum value of 0.17 mm/year. This implies that an assumed rate of 0.10mm/year is not unreasonable.

3.4.2 Conclusion

The loss of section means that several beams and girders have been assessed as being weaker than when previously analysed.



3.5 North Abutment Settlement

As noted in the Principal Inspection the north abutment had settled unevenly to such an extent that the north end of the east main girder did not, under permanent loads, contact the bearing. Vehicular movement across the bridge results in contact being transiently restored.

It has been assumed that the bearings have settled by some 7mm. This figure was based on the visual inspection of the end of the deck. The value is approximate only because level data referenced to a constant datum is not available.



4 Opening the Bridge

4.1 Processes The following processes are required to carry out a trial opening only of the bridge to navigation:-

• Closure of the road to vehicular traffic

• Jacking up of the deck clear of the permanent bearings

• Rotation of the deck

Closing the bridge to navigation requires a reversal of the above sequence.

4.2 Structural Implications

Jacking up of the deck requires the following to be checked or assessed:-

• That the strength of the bearer beams is sufficient to support the entire dead weight of the deck.

• That cross beams 6 and 7 supporting the bearer beams have sufficient capacity to take the dead load from the deck and the bending moments induced by the bridge having only one support.

• That the edge girders, as back to back cantilevers, have sufficient capacity to support the dead load of the deck in hogging instead of as two span continuous beams.

• That the deck balances about the pivot when it is lifted.

• That the corroded bridge nosings do not snag on abutment nosings. This is a construction/maintenance matter rather than an analysis load case, but nevertheless poses a significant physical constraint.

• The three pairs of bearings are corroded so need to be overhauled to allow them to separate as the deck is lifted. The condition of each pair of bearings is different and, in the case of the south abutment bearings, completely unknown as they are inaccessible behind the kentledge.

• The cross beams and their connections must be checked for jacking loads when the centre pier bearings are maintained as discussed below. These checks must be carried out because it is not known whether or not these beams were designed for bearing maintenance/replacement.

• The pier bearings are accessible but it will be necessary to take the load off these bearings by jacking the deck from the pier gallery.

• The north abutment bearings might be accessed by jacking at the abutment but it is not clear that there is sufficient room on the bearing shelf for this to be done. The abutment cross beam has been checked for jacking loads should it be possible to jack up the north end of the deck only.



4.3 Traffic Implications No traffic study was carried out as this subject was covered in the Feasibility Report by Jacobs dated October 2004. If the bridge is to be opened regularly a further report is recommended to assess the effects of the resulting interruption to traffic.

4.4 Mechanical Implications In addition to the above implications if the bridge is to be jacked up by the existing machinery there are requirements for safe testing and operation of the mechanical system. It is believed that the responsibility for this system lies with Medway Ports.

4.5 Conclusion

Given the issues raised it is not possible to open the bridge in its current condition without the unacceptable risk of causing significant structural damage to the bridge. Also the responsibility for the condition of the mechanical equipment is unclear and the Ports Authority cannot confirm whether or not this machinery is capable of effecting an opening.



5 Analysis

5.1 Analysis

5.1.1 Analysis – Computer Model

The deck was analysed using a grillage model on program SuperStress© for both the open and closed positions taking into account the following: section properties as measured, variations in kentledge load and the loss of contact of the north east bearing.

5.1.2 Bridge capacity Analysis

The section capacity required to resist the applied loads took into account the repair work to the transverse beams to the south span carried out in 2007. It also took account of section dimensions as measured during the 2011 inspection.

As part of this study an analysis of the bridge was carried out to establish when the live load capacity would reach a level that would render the structure unfit for purpose.

This prediction of residual life included a fatigue life assessment.

An essential requirement of the assessment analysis was to consider how the structure may deteriorate and which members would be critical to the overall strength and thus the assessment rating which can be provided.

5.1.3 Single Span Analysis

Referring to the modified commissioning brief Jacobs were instructed to examine the strength of the existing deck as a simply supported structure comprising a single span over the mouth of the basin. This configuration could occur as a result of collapse of the girders between beams 6 to 9 or, in a more controlled way, by deliberately cutting the deck girders between beams 5 & 6. The back span complete with kentledge could then be removed and concrete fill placed with road surfacing to meet the new top of abutment level.

The analysis carried out was of the simply supported deck without any inaccessible members so that results were able to be calculated with some certainty.

SKETCH ELEVATION OF BRIDGE Reduced to Single Span

Concrete fill

New South Abutment

Navigation Span



5.2 Analysis Results 5.2.1 Basic Strength Results

(a) Bearer Beams

The analysis highlighted that because of section loss from corrosion the bearer beams were overstressed under lifting loads. They are not loaded by highway loads.

(b) Transverse Beams 1 to 5

The analysis showed that the typical transverse beams were adequate under the “lifting” load case and under “normal” highway loading.

(c) Transverse Beams 6 to 7

As in the previous assessments the elements which were shown to be deficient were transverse beams 6 and 7 under lifting loads. Transverse beams 6 to 7 were adequate under highway loads.

(d) Main edge girders

The analysis showed the edge girders to be adequate under lifting loads and under highway loads.

5.2.2 Corroded Section Results

The analysis showed that these transverse beams 1 to 5 could become overstressed under highway loading in approximately 10 to 12 years.

5.2.3 Fatigue Analysis Results

The fatigue analysis did not show a reduced life for the deck.

5.2.4 Single Span Analysis Results

The analysis showed that the edge girders were adequate if the deck was adapted to become a single span bridge with bearings on the river pier and provided with proper bearing stiffeners. Other elements were unchanged by this load arrangement.

This shows that a collapse of the “back span” would be unlikely directly to cause a collapse of the whole deck.



5.3 Conclusion of Re-Assessment 5.3.1 Access

Several recommendations listed in the 2011 Jacobs Principal Inspection Report centred on the lack of access to many members. These recommendations are reiterated.

5.3.2 Strength

The conclusion of the strength re-assessment was that in its present condition the bridge was not capable of being lifted & swung.

The re-assessment essentially confirmed that neither the transverse members either side of the pivot point nor the bearer beams would be capable of supporting the bridge in the open position. The repair work carried out in 2006/7 has since been deemed suitable only to strengthen accessible transverse beams for highway loading.

Re-assessment of the cross members (which also supported the kentledge) at the southern end of the deck showed that they were probably adequate under the opening loads and highway loads.

5.4 Expected Life of the Bridge 5.4.1 Definition of Expected Life

The expected life of this particular deck has been defined as the minimum time taken to reach a level when the vehicle rating goes below 18 tonnes of Assessment Live Loading. This level has been set recognising that a weight restriction below 18 tonnes would become too disruptive to hauliers and the current users of the bridge in the area.

It has been assumed that no significant routine maintenance would take place during this period so that corrosion due to exposed steel would worsen as the damaged paint layers peel off. This applies in particular to the river span which was not painted during previous maintenance activity.

5.4.2 Assessment of Expected Life

Based on section loss only it is expected that the bridge would reach the end of its useful life within 10 to 12 years, but it must be emphasised that this is an approximate assessment.

When access to cross beams 7 to 9 (the beams forming the kentledge box) is installed as recommended it is quite possible that more strengthening work may be needed to reach the life indicated above.



6 Remedial Work Options

6.1 Work if Bridge is to be opened to Navigation 6.1.1 Option One - work for a Trial Opening

The following repairs are required as a minimum:-

• Carry out the additional access arrangements and beam repairs as listed in the Principal Inspection report dated November 2011.

• Repair corroded nosings sufficient to avoid the deck becoming stuck. Such a temporary repair might be possible without lifting the deck.

• A full survey and test of both lifting and swinging mechanisms would be required by the owners including suitable repairs. It may be possible to replace the jack by demolishing the plinth and removing it from beneath; then replacing the jack and rebuilding the plinth.

Included in the cost estimate is the provision of a stand-by crane to allow repairs to the deck should it get stuck whilst open.

A trial opening is not recommended to be attempted before the access arrangements and repairs are carried out due to the risk of multiple failure of the deck elements.

6.1.2 Option Two - work for Routine Opening

Further to the works listed in 6.1.1 additional maintenance work to the deck would be required as follows

• Re-paint parapets and beams.

• Re-surface the deck.

Medway Ports as the assumed owner would also be required to undertake the following:

• Review of the manually operated lifting and opening system with a view to its complete replacement.

• The current barriers and safety control systems should be reviewed by the owners and renewed if the bridge is to be opened to navigation. Remote control of the whole system is required for reasons of safety and economy.

6.1.3 Option Three - Major Overhaul

It is possible that a more economical long term way of providing an opening bridge would be to carry out a major overhaul to the existing deck. This would entail lifting the deck out, removing the ballast, repairing the beams and repainting all steelwork. A temporary bridge would need to be provided while the work was in progress.

6.2 Work if Bridge is not to be opened to Navigation

The minimum work required for this option is removal of the back span, and converting the bridge to a single span structure as discussed in section 7.3.2. This would be a low risk option as the resulting structure would have no inaccessible beams. It would also be environmentally sustainable as the greatest use is made of existing materials and the deck would be much more accessible for maintenance.



7 Budget Estimates

7.1 For the bridge to be opened to navigation The budget estimates of costs to KCC for all six options are in section 7.4.

7.1.1 Option One - Trial Opening

The cost estimate is described in Section 6.1.1. Also required would be a commitment from Medway Ports to carry out testing and maintenance on the lifting machinery and swing gates. It is understood that the lock gates will also need to be repaired.

7.1.2 Option Two – Routine Opening

The estimated minimum cost to KCC of refurbishment to give some confidence of safe operation on a routine basis is much the same as option one with additional beam re-painting and re-surfacing work.

No allowance has been made for removing the bridge in this option.

In addition a significant amount may need to be spent by Medway Ports on refurbishing and upgrading the lifting and swing machinery and the safety control systems.

7.1.3 Option Three - Major Overhaul

It is estimated that the cost to KCC of a major overhaul is significantly greater than the first two options as heavy craneage is required to carry out the deck lift. A temporary bridge has been allowed for due to the length of time for which the crossing would otherwise be closed.

A temporary bridge is needed to remove the deck for repairs and painting, and allow access for Medway Ports. The result of this work should be a more durable solution to which a design life of 40 years has been assigned as the working conditions can be more conducive to good workmanship.

7.1.4 Option Four - Replacement Bridge

Given the age and poor condition of the deck the alternative of a bridge replacement must be examined.

The advantage of a replacement deck is that it would have a design life of some 120 years as required for new structures.

This level of cost is higher than a fixed crossing because such a bridge is considerably more expensive given the rigours of regular movement. A long closure of the site will be needed to carry out refurbishment of the substructure, and the lifting mechanism. A large crane will be needed to lift out the old deck, lift in a temporary bridge, remove the temporary bridge and finally lift in a replacement deck.

• It should be noted that the machinery elements of any opening bridge are likely to attract considerable initial and maintenance costs neither of which have been included in the estimates as they are considered the responsibility of Medway Ports.



7.2 Work if the bridge is closed to navigation 7.2.1 Option Five – Existing Bridge Adapted

The cost of adapting the existing bridge as a single span fixed crossing is less than that of complete deck replacement. As discussed, this is the most sustainable solution, but the design life would be of the order of 40 years, so the option is unlikely to represent best value.

7.2.2 Option Six - Replacement Fixed Crossing

As mentioned, a new structure would have a design life of some 120 years and be designed to allow easy maintenance.

The level of cost is partly because a single span replacement can be lifted in over a much reduced period compared with the opening bridge deck and is therefore likely to represent best value.

A new deck would also have a more predictable design life than adaptation of the existing.

7.3 Discussion of Estimates The indications are that a decision to instigate repairs or replacement of the bridge deck should be taken within the next 1 to 2 years and not more than 5 years hence, as there is a potential risk of it becoming unserviceable soon after that time. The rate of steel corrosion is variable, depending upon the exposure to airborne salt spray, and the effectiveness of painting, so all estimates have a considerable margin of uncertainty.

7.3.1 For the bridge to be opened to navigation

If the bridge is to be opened to navigation, it is recommended that consideration should be given to replacing the deck with a new deck of advanced composite materials, because they do not corrode as steel does.

7.3.2 For the bridge to be closed to navigation

The kentledge box beams have little residual life. A least cost course of action would be to wait until this residual life has expired and then remove the kentledge box and replace it with a solid concrete infill. There are risks in this action because it is not possible to predict with sufficient accuracy the end of the residual life, its exact failure mode or the possible damage done by the failure.

A more planned variation on the above course of action would be to assess when the bridge deck will cease to be serviceable and to programme the deck removal and replacement before that time.

If the bridge were to remain closed to navigation it would be better value to both KCC and Medway Ports to adapt the deck to become a single span fixed crossing. A single span bridge would be shorter than the comparable moveable bridge and a fixed crossing would also be considerably cheaper to maintain.



7.4 Value for Money

The various options of repair & replacement are tabulated below:-

Option Budget Estimate cost to KCC

Design Life

(years)

Bridge Open to navigation

Option One (trial opening) £127,000 10

Option Two (routine opening) £192,000 20

Option Three (major overhaul) £402,000 40

Option Four (replacement opening bridge) £706,000 120

Bridge Closed to navigation

Option Five (bridge adapted to be single span )

£302,000 40

Option Six Replacement fixed crossing £386,000 120

From this table an indication of value for money can be inferred and option six (the replacement fixed crossing) is seen to be the best with option four second best value. It is clear also that option one is the worst value for money, as it would lead to very little extension of the bridge life.



8 Recommendations

8.1 Recommendations if Bridge is to be Opened to Navigation It is recommended that if the bridge is to be opened this cannot be done without a significant amount of work and cost, even for a trial. For this reason, a one-off trial is not recommended.

From section 7.4 it can be seen that the trial opening option is not the best value for money and is significantly worse value than the overhaul options. It is also clear from the extent of work required that it would be disruptive and costly to repair and upgrade the existing bridge without serious interruption to vehicular traffic.

The comparison in section 7.4 indicates also that replacement of the deck could give the best long term value for an opening bridge, although the initial cost would be high. There is also the possibility of designing a replacement using advanced composites which have very low maintenance requirements compared to steelwork.

It is not possible to comment in detail on the mechanical equipment owned by Medway Ports, however, their input and costs are likely to be significant.

A repair scheme to allow the bridge to be opened using the existing manually operated machinery (repaired as necessary) may be cheaper than upgrading the controls, but it may not be acceptable because it is slower and more costly to operate than an automated system would be.

8.2 Recommendations if Bridge is to Remain Closed to Navigation If the bridge is not required to be opened to navigation the recommendation is that a replacement deck of advanced composites as outlined in option six could be used to minimise maintenance and represents the best value for money of all options.

Such a single span deck would be shorter than the moveable bridge deck and require less maintenance.

8.3 Risks The Principal Inspection report recommended work to enable access for maintenance and inspection. If this work is not carried out the risk of premature failure of elements of the bridge increases. If, however, adequate maintenance is carried out the life of the deck could be extended by many years.

It has been assumed that some of the repair schemes would be carried out with a temporary bridge on the line of the existing bridge. If this is not possible the cost of temporary bridging would be significantly increased, as the only suitable alternative location for a bridge over the creek is from within the Shepherd Neame Yard west of the bridge. This would require a longer crossing, some approach road work and the agreement of the Company.

References 1. Brent Swing Bridge Feasibility Study dated October 2004, Babtie Maidstone

office. 2. Brent Swing Bridge Principal Inspection Report dated November 2011, Jacobs

Maidstone office.



Appendix A Bridge Drawings



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