Committee(s): Date(s): Item no. Public For Decision …democracy.cityoflondon.gov.uk/Data/Finance Committee/20110726... · A feasibility study for strengthening or ... allowing the

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Committee(s): Date(s): Item no.

Planning and Transportation 19th July 2011

Finance 26th July 2011

Subject:

Farringdon Street Bridge Refurbishment – Design

Report

Public

Report of:

City Surveyor

For Decision

Summary

Farringdon Street Bridge was opened in 1869 as part of the Holborn

Valley Improvement Scheme.

The bridge parapets and granite columns have been assessed as not

able to withstand vehicle impact and have been protected by concrete

barriers for a number of years.

In September 2008 a large section of cast iron weighing

approximately 16kg fell from the bridge. A scaffolding protection

system was erected on both fascias of the structure and will remain in

place until remedial works on the cast iron elements have been

undertaken to ensure public safety.

This report takes a holistic approach to the bridge. It looks at the

condition of each element and the remedial works required to bring

the bridge back to the required operational condition, whilst also

enhancing the historic appearance of the structure. The total project

cost (at current prices) is estimated at £2.190m, £0.369m less than the

cost reported at evaluation stage.

There is currently a provision of £2.484m for this project in the

Capital budget, to be funded from the On Street Parking Reserve. A

contribution of £600k has also been secured from LoBEG for these

works to be spent this financial year, reducing the City‟s contribution

to some £1.5m. Should there be any delays to the project due to the

Olympics this contribution would be at risk, unless renegotiated with

LoBEG, for which there is no guarantee of future funding.

Recommendations

It is recommended that, subject to Listed Building consent,

(i) a complete refurbishment of Farringdon Street Bridge be

undertaken at an estimated cost of £2.190m, with £1.590m


being met from the City‟s On Street Parking Reserve and

£0.600m from LoBEG;

(ii) the project proceed with the aim of completing the works prior

to the Olympics.

Main Report

Background

1. Farringdon Street Bridge is a Grade II Listed Structure and was opened in

1869 as an integral part of Holborn Viaduct over Farringdon Street. The

bridge was constructed of cast iron and supported on granite columns.

2. In the early 1990s, the original deck was replaced with a new composite

deck supported on steel girders. Following these works unacceptable

lateral deflections occurred in the original cast iron girders, and remedial

works were undertaken to stabilise the movement in 2000.

3. During works to the bridge, cracks in the parapets were reported and a risk

assessment was undertaken. In order to reduce the risk of failure of the

parapets, temporary vertical concrete barriers were installed adjacent to

the parapets along Holborn Viaduct in order to protect the cast iron

parapets from vehicle impacts.

4. In 2003 inspections revealed that the intermediate granite columns were

cracked and were assessed as being at risk of failure if struck by vehicles.

In 2004 temporary higher vertical concrete barriers were installed adjacent

to the granite columns to provide protection.

5. In June 2006 a bid report for the parapet and column strengthening was

submitted and approved. A feasibility study for strengthening or

protecting the granite piers and cast iron parapets was then completed by

Mouchel Group, the City‟s Term Consultants, in 2008/9. The feasibility

study identified a number of options that could be implemented to

strengthen or protect the bridge columns and parapets.

6. On 22nd

September 2008 a decorative section of cast iron, weighing

approximately 16kg, fell from the bridge narrowly missing a pedestrian

walking below. An immediate inspection of the bridge at the location of

the broken section of cast iron was undertaken.

7. Mouchel Group were commissioned to undertake the emergency

inspection of all the decorative cast iron elements of the bridge and to


investigate the cause of the failure. During this inspection a number of

small sections of cast iron were identified as not being securely attached

and were removed from the bridge for safety reasons. The emergency

inspection was unable to determine the cause of the failures; as a result the

scaffold protection system and inspection platforms were installed on both

parapet girders to mitigate further risks.

8. Investigation and testing work has been undertaken to establish the cause

and ascertain the risk of further failures occurring on the remaining pieces

of cast iron. These investigations were inconclusive as to the exact cause

of the failure to the cast iron corbel. A number of factors could have

contributed to the failure, including water ingress from the joints/

waterproofing, movement in the bridge, fault in the casting; however no

link was established between the failure and the possible contributory

factors. The most likely cause is the corrosion due to water ingress

through the bridge joints and waterproofing.

9. During these investigations preliminary discussions were undertaken with

English Heritage and the City Planning Officer, initially to inform and

include them in the emergency situation regarding the failure to the cast

iron corbel and then to include them in further discussions regarding

suitable options for refurbishment of this Grade II Listed Structure.

10. Recent inspection reports have indicated that the current waterproofing

membrane is not providing adequate protection to the structure.

Investigations have revealed that there is no overlap between the parapets

and the bridge deck, which allows water ingress into the structural

elements of the bridge, the most likely causative effect for failure. The

inspections have also indicated that significant water ingress has occurred

in the vicinity of the bridge joints, resulting in staining and damage to the

abutments.

11. The existing lighting columns and pedestals are structurally sound, but

would benefit from a replacement of the electrical elements and internal

wiring system, which is aged, and a clean of the existing spheres.

12. Having reviewed the reports and assessed the merits of each option, an

Evaluation Report for the refurbishment of the bridge was submitted and

approved in November 2009, allowing the project to progress to Design

Report stage and for possible funding options to be considered.

Current Position

13. Following a successful tender, Halcrow Group Ltd (HGL) was

commissioned, in June 2010, for the provision of consultancy services for


the refurbishment of the bridge. The brief was to develop the design from

the proposals as set out within the Feasibility Study, produced by

Mouchel.

14. A holistic approach was taken, looking at all elements of the bridge. The

following paragraphs identify the current conditions of the individual

elements and the associated issues.

15. Parapets - The existing ornate bridge parapets are constructed of cast iron

and are weakly connected to the underlying fascia girders. The parapets

have been assessed based on the containment levels set out within

TD19/06 „Requirement for Road Restraint Systems‟. An assessment of

the parapet has been undertaken, which showed that the parapets are

capable of meeting the pedestrian criteria but unsurprisingly do not meet

the criteria required for vehicle parapets.

16. Statuary – The statuary was last inspected in March 2009. The surface

condition is generally good, however the iron dowels that fix the statues to

the granite plinth are not up to current standard, are in poor condition and

represent a health and safety hazard.

17. Cast Iron Elements – HGL carried out a survey and investigation works to

the cast iron elements. Where visible, the castings appear to be of good

quality with relatively little distortion. The ironwork was inspected

without the removal of the coatings, and as the bridge is generally coated

with multiple layers of paint it is anticipated that further defects may be

hidden. Cast iron is a brittle material which may contain historic cracks

and original material defects which are not visible.

18. From the visual inspections of the bridge it has been noted that a number

of smaller cast iron elements are secured to the bridge with a mastic type

adhesive.

19. When the bridge was constructed the faces of the decorative features that

are not on display are not protected from the elements, therefore where

water has seeped between the interfaces rusting has occurred.

20. On the parapets the visible defects are in two main areas, the upstand

below the decorative fill panel and the top rail capping detail. At some

locations this cracking has resulted in triangular section of the upstand

breaking away from the parapet.

21. Paint Coating – An assessment was undertaken of the painting system,

which identified that the current system is in fair condition with some

corrosion. The paint system has poor levels of adhesion is some areas, it


is therefore not advisable to over paint. At the joints/gaps the sealant and

filler is failing which is leaving them exposed to the elements causing

further deterioration. In the past patch painting has been undertaken in

several locations resulting in an aesthetically poor finish. Analysis of the

paint system has been undertaken which confirms that lead is present,

therefore a full encapsulation of the bridge will be required in order to

remove the paint system; the cost for this has been allowed for in the

budget.

22. Expansion Joints and Waterproofing – The existing joints and

waterproofing were installed in 1990 as part of the deck strengthening

works. The joints are showing sign of failure as water seepage can be

seen on the abutment faces. The waterproofing has not been lapped onto

the parapet which has caused water ingress into the structural elements.

23. Steel Girders – The steel girders were installed in the 1990‟s as part of the

strengthening works, they are in overall fair condition.

24. Lighting – The wiring to the existing globes and the internal lighting

fixtures are in a poor condition and require replacement.

25. Statuary – The winged lions and statues are dirty and tarnished in places,

however they are showing no signs of movement or distress.

26. Masonry Abutments – The abutments are heavily stained due to water

seepage through the joints and there are areas of localised spalling.

27. Columns –The columns are in a fair condition; however they are dirty,

with some hairline cracks on the polished faces and areas of localised

spalling.

28. In mid-2010 trial holes were excavated on Farringdon Street, these

investigations revealed that there was insufficient depth in the footpath to

provide the foundation for the bollard system proposed in the Feasibility

Report.

29. HGL undertook a review of the column impact assessment previously

undertaken, which confirmed that the columns did not have the required

capacity for vehicle impact loading in accordance with Highway Standard,

BD48/93, The Assessment and Strengthening of Highway Bridge

Supports. The original impact assessment states that should one of the

columns be removed as a result of a collision, then a significant section of

the bridge would collapse.


30. Reviewing the impact assessment HGL were able to reduce the impact

loading using a risk based method, however the outer columns still failed

assessment for a head on collision and still require protection. Using this

risk based method the inner columns would be capable of withstanding

collisions up to 40mph.

Proposals

31. The key project objectives are:

Maintain public safety

Ensure the integrity of the cast iron elements of the bridge

Enable the removal of the temporary protective concrete barriers

from above and below the bridge

Improve the appearance of the bridge

Maintain the Grade II Listed Structure in accordance with English

Heritage requirements.

32. In proposing the measures to achieve these objectives, a holistic view of

the structure has been taken rather than dealing with the individual

elements. A summary of the proposed works for each element is set out

below.

33. Parapets - It is proposed to remove the temporary concrete barriers that

are currently protecting the parapets on Holborn Viaduct from impact

loadings. In order to retain the original parapets it is proposed to install

high security bollards, set back approximately 600mm from the edge of

the carriageway, in order to provide the necessary protection.

34. As the construction depth across the bridge is limited the bollards will use

a shallow mount foundation. It is also proposed to use small diameter

bollards as these will provide the minimal visual impact. See Appendix A

for the General Arrangement Drawings

35. Statuary – It is proposed to remove the statuary from the bridge, clean it,

remove any active corrosion, repatinate and cover it with three layers of

microcrystalline wax, filling the porosity in the surface with tinted wax,

before remounting the statuary on the bridge using new security fixings.

36. Cast Iron Elements – It is proposed to remove all the paint from the cast

iron elements of the parapets and outside edge girders - this will require

full encapsulation as the historic paint system contains lead.


37. Although the cracks in the cast iron that are currently visible have been

identified and the remedial works required to these have been specified,

there is a risk that further minor cracking could become apparent once the

paint system has been removed and a visual inspection undertaken. A

contingent provision of £30,000 has therefore been made in the estimates

to cover the risk of further repair work being required.

38. In order to protect the internal faces of the cast iron elements from further

corrosion it is intended to remove each casting (cast iron cappings, fleur-

de-lys nosings, scroll castings and leaf castings - see appendix B for an

annotated photograph), clean and paint the concealed surfaces and

reattach with new fixings. It is also proposed to install additional bolts for

security. The cast iron corbels are bolted to the star panels from behind

and are not easily removed from the bridge. It is proposed to clean,

inspect and paint these in situ.

39. Where any of these castings are cracked the following repair options will

be used depending on the extent of the cracks identified and type of

casting:

Additional fastenings – it may be possible to bolt broken sections

back in place with additional fixings.

Adhesives – small castings may be secured with high-spec

adhesives, however mechanical fixings would also be used where

failure could cause injury.

Plating – where the broken castings are of large enough section

they may be bolted to a steel repair-plate on the back of the

castings and bedded on a flexible mastic sealant to ensure that

water ingress is avoided.

Slender castings – where the castings are thin they may be repaired

by gas-welding.

Stitching – castings that are over 8mm thick can be stitched by

drilling a chain of holes at right angles to the crack installing a

specially shaped lock/stitch. The spaces between the stitch are then

filled with steel studs tapped in and finished flush with the surface.

40. The above methods will also be used for the parapet sections, but in some

instances new castings and fixings may be required, made from patterns

prepared from the existing components.


41. Coating System – Where the structure is blast cleaned a modern Highway

Agency approved coating system is proposed to match the existing

colours of the bridge. The system includes the use of a primer coat and a

high performance coating system appropriate for use on cast iron

structures. It is also proposed to undertake patch repairs to the paintwork

on the exposed beams within the deck soffit and the longitudinal arch

girders.

42. Expansion Joints and Waterproofing – The expansion joints will be

replaced in accordance with the Specification for Highway Works. It is

proposed to use a asphaltic plug joint to the east abutment and a

elastomeric joint to the west abutment.

43. Due to its age and the likely disruption during the joint replacement it is

proposed to replace the waterproofing as part of the works. There is a risk

that the odours from sprayed systems can taint food and, therefore, due to

the location of food wholesalers in one of the abutments, it is proposed to

use a protective sheet membrane type system in accordance with the

Specification for Highways works.

44. The carriageway will then be resurfaced to the existing camber and levels

and all road markings will be reinstated. Although Holborn Viaduct is not

currently programmed to be resurfaced in the immediate future,

consideration will been given for this section of the Viaduct to be

resurfaced next financial year 2012/13 as part of the bridge refurbishment

project.

45. Lighting – Installation of feature lighting is proposed to the winged lions

at the corners of the structure and to the City crests utilising LED strip

lights and LED spot lights. Where the fixings will be visible they will be

painted to match the final colour of the bridge. The existing globes and

internal lighting will be refurbished and improved together with provision

of lighting to enhance the Statuary. The upper level lighting will be

rewired from the existing supply point.

46. Masonry – It is proposed to clean the column piers and abutment faces

and to repair areas of significant weathering.

47. Columns – It is proposed to remove the concrete structures that are

currently protecting the columns and install a Trief Cadet kerb, allowing a

protected cycle-lane between the columns and the rear of the Trief Cadet

kerb. As this will not fully protect the columns from a head on collision it

is proposed to install high security bollards on the approach to the outer

columns. The proposed carriageway will comprise 2 traffic lanes and a


northbound bus lane. See the General Arrangement Drawing in appendix

A.

Programme

48. The expected programme for the works is as follows:

Activity Start Date Completion Date

Invite works tenders and award

contract

August 2011 November 2011

Construction period December 2011 June 2012

49. Should there be any delay to this programme the works may need to be

suspended during the Olympics; this has been identified as a risk and

monies included in the risk budget.

50. However, should the works be postponed until after the Olympics the

expected programme for the works would be as follows:

Activity Start Date Completion Date

Invite works tenders and award

contract

June 2012 September 2012

Construction period October 2012 April 2013

51. If the programme is delayed until after the Olympics the £600k

contributions from LoBEG will be lost, unless it can be renegotiated.

There is no guarantee of this and the project is unlikely to get future

funding due to the prioritisation of other works across London.

52. The above programme has taken into account the expected environmental

and traffic restrictions that are likely to be imposed on these works by the

appropriate statutory authorities and the restrictions during the Olympics.


Financial and Risk Implications

The estimated cost of the works as covered in this report equates to

£2.160m, a reduction of £399k compared to the evaluation costs. Further

information on the financial implications can be found in Appendix D –

Financial and Risk Implications in the non-public Annex.

Strategic Implications

53. The proposals are consistent with the objectives set out in the City of

London UDP, the Mayor‟s Strategy and the City of London‟s Community

Strategy. A breakdown of the relevant points are provided in Appendix C.

54. Neighbouring properties are keen to see the works completed on the

bridge, including the removal of the scaffolding and concrete barriers.

These works together with the construction of the fourth Gate House as

part of the Bath House development works will improve the area.

Corporate Property Implications

55. This structure is one of a number of City of London bridges that are

included on the List of Buildings of Special Architectural or Historic

Interest. It represents a major historical investment on the creation of the

modern City and is part of a set-piece sequence of urban spaces at one of

the main entrances to the City. Along with other improvements and

developments in this area, the proposed comprehensive and efficient

approach to the refurbishment of Farringdon Street Bridge should bring

about significant improvements to this part of the City and its property

values. Should the works not go ahead at this time the unsightly concrete

barriers above and below the bridge and the scaffolding shrouding the

bridge will need to remain in place until the works to the associated

elements are undertaken.

Consultees

56. The Director of Environmental Services, City Planning Officer,

Comptroller and City Solicitor, and Chamberlain have been consulted in

the preparation of this report.

Conclusion

57. The works to the cast iron elements of the bridge are required as a matter

of urgency to ensure the safety of the structure. The waterproofing, joint

replacement, secondary protection system, abutment repairs and lighting

maintenance should also be undertaken concurrently to ensure a holistic

approach and to minimise the necessity for future works on the structure.


58. The structure is Grade II Listed, failure to undertake these works will

eventually result in the addition of the bridge to the Heritage at Risk

Register by English Heritage. Over recent years the City of London has

reduced the number of buildings it has on the Heritage at Risk Register.

Background Papers:

59. Planning and Transportation 27/06/06 Item no. 8a – Farringdon Street

Bridge Parapet and Column Strengthening: Bid Report

60. Planning and Transportation 11/11/09 Item no. CS:340-09 Farringdon

Street Bridge Refurbishment – Evaluation Report

Appendices

Appendix A: General Arrangement Drawings

Appendix B: Cast Iron Identification

Appendix C: Strategic Implications

Contact:

Frances Mickleburgh

020 7332 3913

[email protected]

mailto:[email protected]


Appendix A: General Arrangement Drawings




Appendix B: Cast Iron Identification

Arched Capping

Fleur-de-lys nosing

Scroll Casting

Leaf casting


Cast Iron Corbel

Star Panel


Appendix C: Strategic Implications

This proposal is in Line with the Community Strategy in the following areas:

Good Transport for a Thriving City

Improve the „pedestrian experience‟

Encourage improvements to the safety of all modes of transport

Encourage cycling

A Safer City

Improve road safety

The City of London Community Strategy: The City Together

Relevant objectives:

To improve the pedestrian experience by enhancing street scene, road safety and

transport interchanges (GT4)

To encourage improvements to the safety of all modes of transport (GT5)

To encourage cycling (GT6)

To improve the health and fitness of City workers and residents (HC1)

To improve road safety (SC9)

This proposal is in Line with the Unitary Development Policies:

ENV 8 “To promote and ensure high standards in the layout, design, surface

treatment and landscaping of open spaces and streets, and to seek the retention

of existing surfaces which contribute positively to the character and appearance

of the City.”

TRANS 6 “To improve the environment for pedestrians, particularly at street

level, by:

providing facilities to enhance safety and convenience etc…….

TRANS 9 “To ensure that the highway hierarchy functions as planned to assist

in the management and improvement of traffic circulation and the

environment…”


TRANS 11 “Appropriate traffic management measures will be introduced to

ensure that the highway hierarchy operates as effectively and safely as possible,

in accordance with the needs of all user groups and functions associated with

each street…”.

Documents

Committee(s): Date(s): Item no. Public For Decision …democracy.cityoflondon.gov.uk/Data/Finance Committee/20110726... · A feasibility study for strengthening or ... allowing the