REVIEW OF FOOTBRIDGES IN SINGAPORE

Footbridge 2014 5

th International Conference

Footbridges: Past, present & future

REVIEW OF FOOTBRIDGES IN SINGAPORE

Edmund S.K. TING Er Chelliah MURUGAMOORTHY Er Dr Ravi PRASAD Jayaraman RATHNAM

Project Manager, Project Director Associate Director Technical Director

Rail Group DTL3 Rail Group DTL3 Meinhardt Infrastrcture Pte Ltd Meinhardt Infrastrcture Pte Ltd

LTA, Singapore LTA, Singapore Singapore Singapore

[email protected] [email protected] [email protected] [email protected]

Summary

Stations of Mass Rapid Transit Systems, malls and housing estates extending across roads, natural parks, waterways and marinas provide excellent opportunity for a number of remarkable footbridges to be constructed over the years in Singapore. This article examines in detail a few of the footbridges for the concepts, materials and construction techniques used, vibration serviceability verification, aesthetics and conservation, sustainable supporting role in the public transport system.

Keywords: footbridge; heritage structures; steel bridge; concrete bridges; timber decking; vibration serviceability; tuned mass damper; bridge lighting; sustainable development.

1. Introduction

Footbridges offer the designer a wide variety of design choices due to

a) less restriction of the shape and curvature in plan and inclination of the deck, b) more options for the form and shape to suit the landscape and adjacent environment, c) scope for use of variety of materials for the main structure as well as for the deck surface and finishes, and d) possibility of using minimum space and multiple entrances to connect the footbridge to adjacent buildings,

natural parks and trails. Stations of Mass Rapid Transit (MRT) Systems, malls and housing estates extending across roads, natural parks, waterways and marinas provide excellent opportunity for a number of remarkable footbridges to be constructed over the years in Singapore.

There are over 500 pedestrian bridges in Singapore commencing from those built with wrought iron during the 19th century across Singapore River serving the community living along the river banks shaping the development of the city in the Straits Settlements. This article briefly describes some of the early pedestrian bridges in Singapore Planning Area that are being conserved by the Urban Redevelopment Authority (URA) and proceeds to some of the outstanding bridges with which the authors are associated individually or together in designing, independent checking or construction management.

Planning, development and maintenance of the pedestrian bridges used to come under the purview of the former Public Works Department (PWD) until Land Transport Authority (LTA) was formed in 1995 [1]. Handbook on Pedestrian Overhead Bridges (POB) [2] from LTA summarizes the design requirements and provides guidance on the design of reinforced and prestressed concrete POBs.

Among the 15 steel bridges, Changi Airport Mezzanine Bridge, King Albert Park (KAP) MRT Station Entrance Bridge, the Helix Bridge in Marina Bay, Alexandra Arch Bridge, and Henderson Waves Bridge are reviewed in terms of the form and shape, site constraints, aesthetics and vibration serviceability.

Among the 500 concrete bridges, Tan Kah Kee (TKK) MRT Station Entrance Bridge, and POB at Bedok Town Park (BDT) MRT Station are reviewed in terms of design, construction, durability and maintenance and economy.

Efforts in promoting sustainable solutions with the use of duplex stainless steel, energy saving lighting, intelligent lighting detection system and upgrading with ramps, lifts or escalators to enhance overall accessibility of the public transport network for commuters with diverse needs, which have been implemented on a number of pedestrian bridge projects in Singapore are described.

Footbridge 2014 – 5th

International Conference - Footbridges: Past, present & future

2. Heritage structures

The bridges erected between 1869 and 1929 for trade along 6km long Singapore River, reflect the prosperity brought about by the establishments of the Straits Settlements in 1867 and the opening of the Suez Canal in Egypt in 1869. Those bridges were named after governors and officials during the island’s early years as a crown colony and are of high heritage value. These bridges continue to serve the needs of Singaporeans and visitors alike connecting residential, commercial and entertainment area, and adding history, high technology and even colours to the skyline of Singapore.

Cavenagh Bridge is the oldest of the bridges to cross Singapore River in the Central Area with a single span of 60.96m (200 feet). Originally known as Edinburgh Bridge, it was designed by the colonial PWD’s John Turnbull Thomson and constructed by the P&W McLellan, Glasgow, shipped to Singapore in parts and reassembled in 1869 by convict labour and load tested by a party of 120 sepoy soldiers marching over it on completion. Though the wrought iron rigid cable structure using steel casting and rivets was designed as a drawbridge to carry rickshaws and ox carts (laden weight not exceeding 1.5kN), it was converted later to a pedestrian bridge due to its inability to cope with increasing vehicular traffic volume of the flourishing trade and unsuitability to function as a drawbridge to enable the passage of barges at high tide.

Fig. 1 A heritage structure - Cavenagh Bridge across Singapore River

Other bridges creating a promenade along both banks of Singapore River are: Ord Bridge (through type steel truss bridge built in 1886), Read Bridge (also known as Malacca Bridge, 3-span beam type built in 1931), Alkaff Bridge (also known as Singapore’s ArtBridge, built in 1999, a through-type steel truss bridge of 55m length shaped like a tongkang, a light weight boat), Jiak Kim Bridge (inclined arch rib type built in 1999).

3. Steel structures

3.1 Singapore Changi Airport Mezzanine Bridge

Long span steel pedestrian bridge at Changi Airport MRT Station [3] is a deck type steel truss structure with central span of 140m flanked on either side of anchor spans of 30m each with width of bridge deck varying from 8m at centre to 14m at both end and depth varying from 1.2m at centre and 5.8m at support. The bridge supports structural glass floor deck (with side cladding) for movement of passengers between Terminal 2 and Terminal 3 of the Singapore’s Changi Airport. The cross-section of the bridge varies along the length to accommodate two travelators over the central span and two escalators over each intermediate support. The soffit of the main span is parabolic shaped rising about 6.75m. The structural members mainly consist of circular hollow sections of grade S355 and are straight between the welded joints. In the central region of the central span, web plates have been adopted to provide required flexural resistance under patch loading. Bearings consist of steel pins at the internal supports and pot bearings with sliding arrangement and uplift resistance at the two ends.

The total deck area available for pedestrians is about 840m2 and the total mass of the bridge is about 260t. Non-structural cladding added mass and reduced the natural frequencies of the structure while also increasing the modal damping from 0.2% to 0.4% for critical vibration modes. Detailed experimental and analytical studies [4] indicated that the first symmetric lateral vibration mode (LS1) at about 0.9Hz and the first symmetric torsional vibration mode (TS1) at



about 1.64Hz could be excited by pedestrian movement. To mitigate the potential for strong and unsafe lateral oscillations in the unlikely event of larger number of pedestrians, a tuned mass damping (TMD) system [4] with mass of 2x500kg tuned to 0.9Hz has accordingly been installed at the centre span [5]. The bridge was commissioned in 2002.

Fig. 2 Structural model of the Changi Airport Mezzanine Bridge

3.2 KAP Station Entrance Link way Bridge

The overhead link way bridge spanning in a skew alignment across Bukit Timah Road, Dunearn Road and Bukit Timah Canal below enables movement of the pedestrians between the entrances A and B of the KAP MRT Station on the Downtown Line 2 (under construction). It is a through type structure using 4m deep Warren type steel trusses and reinforced concrete floor slab spanning 52m across the roads and the canal. The 4m wide walkway is covered with steel purlins and stringers supporting aluminium composite panels on top and glazing on sides. Structural hollow sections in Grade S355 have been used for the steel truss. The loads from superstructure are transferred through elastomeric bearings to the reinforced concrete pier cap and piers to the roof slab of the station structure below.

Fig. 3 Elevation of the KAP MRT Station Entrance Linkway Bridge without cladding

To facilitate erection of the steel trusses, two or three intermediate supporting trestles (on the median between the service road and Bukit Timah Road and near top edge of the canal) are envisaged adopting suitably designed splicing to be done on the field. Concrete floor slab is laid over permanent shuttering installed over the floor beams between the main steel trusses. Planter boxes are installed after the construction of the diaphragms and deck slab, composite action and continuity are considered in the analysis and design.

Natural frequency of vibration of the bridge in the vertical vibration mode is 1.89Hz and lateral vibration mode is 1.97Hz. The maximum vertical acceleration of 0.04m/s2 is less than 0.69m/s2, the limiting value as per BD 37/01[6].



3.3 Helix Bridge in Marina Bay

The 280m long bridge curved in plan and joining the foreshore promenades on either side is made of 3 intermediate spans of 64m each and end spans of 44m each. The design was inspired by the geometric helicoidal arrangement of DNA, a symbol of continuity and renewal. The major and minor helices which spiral in opposite directions having an overall diameter of 10.8m and 9.4m respectively have been fabricated from about 650t of Duplex Stainless Steel grade 1.4462 (S31803). Tubes of 273mm diameter have been used, six number placed equidistantly for the outer helix and fiver number for the inner helix. Oval shaped cantilevered viewing pods with capacity for 100 people extend out on the bay side to enable ring-side viewing of events. The bridge was completed in 2010.

Fig. 4 Section through the Helix Bridge at Viewing Pod & view from outside

The helix tubes touch each other in one position under the bridge deck. The two spiralling members are held apart by a series of struts and rods as well as stiffening rings. The piers are of inverted tripod shape using stainless columns filled with concrete. Erection of the helices was done by on-site welding of prefabricated segments (with struts and tension rods that hold and join the helices together using pins) making use of a temporary structural steel over the navigational channel. Non-linear analysis of the tubular truss comprising two helix structures acting together was carried out for the various load cases. Checks were also made for accidental or deliberate removal of a helix or supporting member.

3.4 Alexandra Arch Bridge

The 80m long bridge over Alexandra Road (a 6-lane highway) with 4m wide granite-surfaced steel box deck curved in plan is supported by raking reinforced concrete piers founded on piled foundations. It is an asymmetric structure comprising a single steel arch (leaning outward by 70o) rising 17m above ground level with 60m between the springing points and supporting the deck profiled in cross section by means of hanger steel fins. With colour-changing LED lighting, the bridge resembles an open fig leaf. The bridge enables barrier-free public access into the natural heritage of the Southern Ridges.

Fig.5 Views of Alexandra Arch Bridge

The gently curving approach frame had to be provided with TMD to satisfy the vibration serviceability requirements under



pedestrian loading. The bridge was completed in 2008.

3.5 Henderson Wave Bridge

The 274m long and 8m wide bridge over Henderson Road (a 6-lane freeway) connects Mount Faber at a slight gradient to Telok Blangah Hill forming key linkage in the Southern Ridges. The bridge with main span of 57m (6m high) and six spans of 24m each (3.5m high) was completed in 2008.

Its distinctive wave-like appearance comes from seven undulating curved steel ribs that alternately go above and below the bridge deck resting on reinforced concrete pylons (the tallest of which reaches 38m). When these ribs rise over the deck, they form alcoves that provide shelter and refuge niches seats for leisure walking and scenic viewing. Indigenous Yellow Balau timber was used extensively as modular tapestry surface and as curved balustrades and backrests connected to the supporting steel spans and wave structures. TMD has to be used to satisfy the vibration serviceability.

Fig. 6 Visual experience on the Henderson Waves Bridge similar to that while riding on Lombard Street in San Francisco

The visual experience pedestrians and cyclists get on this wave bridge would be similar to the thrill one gets on the slowest ride negotiating the eight sharp turns through blooming flower beds on Lombard Street (aka Crookedest Street) in Russian Hill of San Francisco.

4. Concrete structures

4.1 TKK Station Entrance Link way Bridge

The overhead linkway bridge at TKK MRT Station of the Downtown Line 2 (under construction) is a three span (27m-18.8m-29.4m) structure. The 4m wide concrete deck walkway over precast prestressed concrete beams is covered with steel portal frames supporting translucent sheeting on top and glazing on the sides. The two intermediate piers are in structural concrete resting on pile foundations.

Four number 1m deep precast pre-tensioned concrete girders launched in position and connected by cast in-situ cross-girders and 150mm thick deck slab form the superstructure. Planter boxes on either side are in precast reinforced concrete simply supported over the diaphragms extended beyond the outer precast girders. Circular reinforced concrete pier columns with piled foundations with wide pier caps at top support the superstructure. Accidental collision loads from the vehicles on the adjacent roadway is considered in the design.

Natural frequency of vibration of the bridge in the vertical vibration mode is 2.47Hz. The maximum vertical acceleration of 0.054m/s2 is less than 0.786m/s2, the limiting value as per BD 37/01.



Fig. 7 Cross-section of the TKK MRT Station Entrance Link way Bridge

4.2 POB at BDT MRT Station

The POB at BDT MRT Station of Downtown Line 3 (under construction) is single span simply supported deck type structure of about 43m length across Bedok North Road providing a deck width of 3m with 750mm wide planters on either side and covered with steel roof. The superstructure is made of two precast pretensioned concrete girders weighing 1.5MN each brought to the site from the precasting yard using long-bed trailers and launched in position using two mobile cranes of 500t capacity. Elastomeric bearings have been used to transfer the loads to substructure and foundation.

Natural frequency of vibration of the bridge in the vertical vibration mode is 1.33Hz. The maximum vertical acceleration of 0.064m/s2 is less than 0.577m/s2, the limiting value as per BD 37/01.

The project involved, on completion of the new POB, dismantling of the existing POB including staircases to make way for execution of the C928 contract.

Fig. 8 Aerial view & cross-section of the new POB at BDT MRT Station before construction of the ramps

5. Sustainable developments

Some of the developments in greening and sustainable developments in pedestrian bridge projects in Singapore are briefly described here below.



a) Adoption of duplex stainless steel as structural material

Duplex stainless steel with full recyclability possess high strength and corrosion resistance and assure longer life span and significantly lower maintenance and repair costs.

b) Switching to energy efficient lighting and use of intelligent sensors

The use of LED lighting reduces the energy consumption by up to 50% compared to the conventional florescent lights.

To further cut down the energy consumptions, if no one is walking on the pedestrian bridge between midnight and 6am, an Intelligent Lighting Detection System can “switch off” the lights except for those at the entrance and staircase for security and safety reasons.

c) Provision of planter boxes and rainwater harvesting

Most of the pedestrian bridges across roads in Singapore come equipped with planters on both sides of the bridge with sprinklers, for growing shrubs. The new bridge projects implement rainwater harvesting with underground sump on site.

d) Energy efficient façade material

For the pedestrian bridges linking malls and air-conditioned, adoption of façade with low value of thermal transmittance and shading coefficient

e) Enhance accessibility for commuters with different needs by installing lifts or escalators

Fig. 9 Upgrading of existing pedestrian bridge with lift

To enhance the overall accessibility of the public transport network for commuters with different needs (elderly, wheelchair-bound commuters, parents with prams), existing pedestrian overhead bridges satisfying the following criteria are upgraded by fitting with lifts:

a) They are located within 200m of MRT stations, bus interchanges/integrated transport hubs, 100m of health institutions or welfare homes for the aged;

b) Provision of a new at-grade pedestrian crossing is technically not feasible; c) There is sufficiently high usage of the bridge with no barrier-free alternative nearby d) The retrofitting of lifts at the POB is cost-effective and technically feasible

6. Discussions

Steel bridges are generally found to be quite suitable for longer spans with lighter weight to handle, negligible effect of construction sequence on the final completed structure, but are prone to vibration serviceability issues and need regular visual inspection for corrosion. Duplex stainless steel is an option for the planners demanding sustainable, low maintenance and aesthetically pleasing material.

Prestressed concrete bridges are generally found to have less maintenance issues, less vibration serviceability problems and more economical in a number of cases.



Stress ribbon type bridges [5] are amenable for adoption in a number of situations but have not yet been used in Singapore.

Solar energy is renewable, clean and abundant in the tropical climate of Singapore. Solar panels can be used over the roof of the pedestrian bridge to supply the lighting & pumping needs with surplus electricity fed into the national power grid as has been done on the Kurilpa Pedestrian Bridge over the Brisbane River in Australia and the Blackfriars Railway Bridge Project in UK.

Current practice of evaluating response of pedestrian induced vibration as per BD 37/01 has a number of shortcomings. Adoption of European design methodology [8],[9] is suggested.

7. Acknowledgements

The authors are grateful for the permission of LTA to publish this paper and would also like to thank Kee Chee Hiong and Seetho Kok Fong of LTA Road Asset Management Department and McConnell Dowell, SK engineering & construction, and Sato Kogyo (S) Pte Ltd for providing details of bridges in the preparation of this paper. The authors would like to thank the management of Meinhardt Infrastructure Private Ltd for the support.

8. References

[1] LEE K. W., and NG, C.Y. “Development, trends and improvements of pedestrian bridges and covered link ways in Singapore”, The Structural Engineer, 3 May 2005, pp. 32-35.

[2] Civil Design Department, LTA “Civil Design Handbook: Pedestrian Overhead Bridges”, 1st Edition, December 2002

[3] CHAN K.S., TSE T.K.D., MIRZA SHAFIQ IBRAHIM, and RATHNAM, J. “New Singapore Airport MRT Station”, PB network, Volume XVII, Issue 54, No 4, October 2002, pp. 64-65

[4] BROWNJOHN, J.M.W., FOK, P., ROCHE, M., and MOYO, P. “Long span steel pedestrian bridge at Singapore Changi Airport – part 1: Prediction of vibration serviceability problems”, The Structural Engineer, Vol.82, No.16, 17 August 2004, pp. 21-27.

[5] BROWNJOHN, J.M.W., FOK, P., ROCHE, M., and OMENZETTER, P. “Long span steel pedestrian bridge at Singapore Changi Airport – part 2: Crowd loading tests and vibration mitigation measures”, The Structural Engineer, Vol.82, No.16, 17 August 2004, pp. 28-34.

[6] “Design Manual for Roads and Bridges: Volume 1: Approval procedures and general design: Section 3: General design: Part 14: BD 37/01: Loads for Highway Bridges”, UK Highways Agency, 2001

[7] RATHNAM J., and ALIMCHANDANI C.R. “Stress ribbon bridges”, Bridge and Structural Engineer, Volume 18, No 3, September 1988, pp.

[8] Guidelines for the design of footbridges. fib bulletin 32, November 2005

[9] HEINEMEYER C., and FELDMANN M., ”European design guide for footbridge vibration”, Proceedings of Footbridge 2008 – Third International Conference, Portugal 2008

REVIEW OF FOOTBRIDGES IN SINGAPORE

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