Bridge works in LTA Contract N107
A Refresher in Bridge Engineering
1. Topics covered
1. 1. Introduction
2. 2. Overview of Bridge works in LTA Contract N107
3. 3. Pedestrian Overhead Bridge (temporary) over Marymount Road
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1. Introduction
Bridges are ‘raw’ engineered structures and are also a form of functional art.
Cavenagh bridge (Anderson Bridge in b/g) Elgin bridge The Helix bridge
Some of the iconic pedestrian bridges in Singapore.
1. Iconic Bridges
Benjamin Sheares bridge Esplanade bridge MRT viaduct at Jurong East
Some of the iconic Road & Rail Bridges in Singapore
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1. Bridge engineering
Bridge engineering encompass functional planning, geotechnical &
structural engineering, designing with concrete, steel & composite materials,
inspection, assessment, long-term structural health monitoring.
Bridge building is a magnificent example of the practical and everyday use
of science. Unfortunately, there are always gaps between what we know,
what we do, and why things go wrong. Bridge engineers must manage risks
carefully.
Bridges in LTA Contract N107 provide an opportunity from planning, design,
construction, inspection to demolition for reconstruction.
1. Overview of Bridge works in LTA Contract N107
1. 1. Pedestrian Overhead Bridge (POB) over Marymount Road
1. a) Design & construction of New temporary POB (designated as POB16T);
2. b) Demolition of existing POB; and
3. c) Design & construction of New Permanent POB
2. 2. Lornie Viaduct & MacRitchie Viaduct – Pre-construction condition survey
including recording condition / measurement of Bearings & MJ
3. 3. Design & construction of Temporary traffic decks
4. 4. Design & construction of New Temporary Marymount Flyover
5. 5. Demolition of existing Marymount Flyover
6. 6. Design & Construction of New permanent Marymount Flyover
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Location of the Bridge structures involved
Existing & new
temporary POB
1. Perspective view
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1. Bridge works in LTA Contract N107
1. Topics covered today
1. 1. Pedestrian Overhead Bridge (POB) over Marymount Road
1. a) Design & construction of New temporary POB (designated as POB 16T);
2. b) Demolition of existing POB; and
3. c) Design & construction of New Permanent POB
2. 2. Lornie Viaduct & MacRitchie Viaduct – Pre-construction condition survey
including recording condition / measurement of Bearings
3. 3. Design & construction of Temporary traffic decks
4. 4. Design & construction of New Temporary Marymount Flyover
5. 5. Demolition of existing Marymount Flyover
6. 6. Design & construction of New permanent Marymount Flyover
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1. New Temporary POB
1. Topics covered
1. 1. Site details & Functional requirements
2. 2. Design basis: Design working life & Loading
3. 3. Geotechnical conditions
4. 4. Construction materials & construction / erection tolerances
5. 5. Conceptual design, aesthetics & Details of structure adopted
6. 6. Structural & geotechnical design aspects
7. 7. Construction & supervision
Note: Though construction of this POB was completed in April 2020, it is yet to
be opened to the public for use since TOP has not yet been received from BCA
1. Location map & site details
Located adjacent to Marymount Convent School
Utilities found at site:
a) a) 300dia sewer line; b) Telecommunication (SingTel) cable line;
b) c) Power (6.6kV) cable line; & d) Power (LV) & optical cable lines
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1. Functional requirements
1. Facilitate pedestrian movement across the Marymount Road with an overhead covered
bridge of about 49m span during the construction underground of the North South Corridor
tunnels. Minimum clear width: 2m & clear height: 2.5m
2. Provide minimum headroom of 5.4m (§ 8.10.1 of Civil Design Criteria CDC, providing a safety margin
over the displayed value of 4.5m) between the soffit of the POB to the at grade carriageway
3. Foundations (micro-piles) of the supporting trestles of the POB & foundation (pad footings)
of stairs on either side to clear / cross the utilities requiring no or minimum diversion
4. Resist any accidental collision load from vehicles plying on the Marymount Road (vehicle
speed with 70km/h limit)
5. Pedestrians should not feel undue discomfort of vibration while walking, jogging or
overcrowding
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1. Design Basis: Design working life & Reliability
Referring to SS EN 1990 Basis of Structural Design,
Design working life for temporary structures (durability): 10year (§ 2.3)
Consequence class: CC2 (§ B3.1)
Medium consequence for loss of human life, economic, social or environmental consequences considerable
Reliability class: RC2 (§ B3.2)
Design supervision level: DSL3 (Extended supervision) (§ B4)
Inspection level: IL3 (Extended inspection) (§ B5)
Minimum value of reliability index, β ≈ 4 & Probability of failure, Pf ≈ 10-5
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1. Design Basis: Loading
Variable actions from: (§ 3.4 CDC)
a) a) Pedestrians on POB & stairs: udl of 5kPa;
b) b) Maintenance load of 0.5kPa on roof
c) c) Provision for future installation of roof structure: nil
Environmental actions: Wind (§ NA 2.4 to SS EN 1991-1-4)
fundamental (10-minute) wind velocity, Vb,o= 20m/s
Thermal actions (for 50-year return period, § NA 2.4 to SS EN 1991-1-4)
shade air temperature: max = 35.7oC & min = 19.5oC
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Accidental actions on structures adjacent to roadway
1. (§ 3.3.4 CDC)
Road restraint system: Vehicle impact guard rail for support 1.2m to 4m from road kerb
Type P1 for supports located < 1.2m from road kerb
a) Impact force on substructure: 250kN ║ to CW & 250kN ┬ to CW acting at 1.25m above CW
165kN ║ to CW & 165kN ┬ to CW acting at 2m above CW
b) Impact force on superstructure: 85kN ║ to CW & 85kN ┬ to CW
Symbols:║ parallel ┬ perpendicular CW carriageway
Note: CDC specify values higher than that of NA to SS EN 1991-1-7 but are not adopted for bridge projects of HDB / URA
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Pedestrian comfort - Vibration serviceability requirement
London’s Wobbly Millennium Bridge
Designed by British Architect Lord Foster & Arup
Opened to public in 2000 but after 2 days of random
swaying, swinging & oscillating wildly, the bridge was closed
down by embarrassed engineers.
Reopened in 2002 after repairs!
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Pedestrian comfort - Vibration serviceability requirement
(NA to SS EN 1991-2)
Walking, jogging & crowding of pedestrians on the bridge generate vibration
Movement Specific features Frequency range (Hz)
Walking Continuous contact with the ground 1.6 to 2.4
Running Discontinuous contact 2 to 3.5
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Dynamic analysis to evaluate response under pedestrian movement
Dynamic analysis is to be carried out to check
a) maximum vertical acceleration of the deck;
b) likelihood of large synchronized lateral responses
Vertical pulsating force moving across the span of the bridge at a constant speed, vb
F = F0.k(fv). √ (1 + γ.(N – 1)).sin(2π.fv.t)
Crowd densities for design
Bridge class: C
Bridge usage: Urban routes subject to significant variation in daily
usage (e.g. structures serving access to offices or schools).
Group size (walking): N = 8
Crowd density, ρ (persons/m2) walking: 0.8
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Vibration serviceability to be complied
Check if Fundamental frequency of the deck is less than:
5 Hz for vertical vibrations,
2.5 Hz for horizontal (lateral) and torsional vibrations.
Maximum acceptable acceleration(m/s2) of any part of the deck:
0.7 for vertical vibrations,
0.2 for horizontal vibrations due to normal use,
0.4 for exceptional crowd conditions.
Natural frequency of the new temporary POB over Marymount Road:
Lateral & torsional vibration: 1.92 & 2.56Hz
Vertical vibration: 2.54Hz < 5Hz hence comfort criteria was checked & found to be OK
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Measures to achieve Vibration serviceability
Damping measures such as Tuned Mass Damper (TMD) must be used
when vibration serviceability checks for the bridge are not satisfied
Walking test done with 150
pedestrians circulating
around the bridge
Mezzanine bridge at Changi airport
TMD with 500kg pendular mass
fixed on the skeletal frame
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Standards & Guidelines for design & construction of POB
EN 1990 Basis of structural design
EN 1991-2 Actions on structures Part 2 Traffic loads on bridges
EN 14199:2015 Execution of special geotechnical works. Micropiles
EN 1090 Execution of steel structures & aluminium structures
Part 1: Requirements for conformity assessment of structural components
European design guide for footbridge vibration, Proceedings of Footbridge 2008 –
Third International Conference, Portugal 2008
Technical guide - Footbridges –Assessment of vibrational behavior of footbridges
under pedestrian loading, SETA, Oct 2006
Guidelines for the design of footbridges, fib bulletin 32, 2005
Civil design handbook: Pedestrian overhead bridges, Civil Design Department, LTA,
2002
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1. Geotechnical conditions
Borehole No. ESN/401, PZS
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1. Borelogs
Borehole No.
ESN/B01/PRM
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1. Geotechnical parameters
Skin friction & end bearing values in different types of soil / rock used for design of micro-piles
soil / rock type f s(kPa)/N limiting max.value(kPa) qb/N max (kPa)
F1 0.466*σ'v NA NA NA
GV/GVI 2.5 50 120 6,000
GIV 500 7,500
GIII 800 7,500
GII 1000 120 7,500
Geotechnical capacity of 350mm diameter micro-piles: compression 2.1MN & tension 1.3MN
Safe bearing pressure adopted for pad footings supporting stairs: 135 & 160kPa verified by
Plate Load Tests
Safe bearing pressure used for road pavement for mobile crane movement lifting fabricated
POB = 240kPa verified by Plate Load Tests
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Plate load test
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Maximum allowable settlement & horizontal deflection
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Construction materials & construction tolerances
Construction materials
• Substructure in structural concrete grade: C32/40
• Reinforcing steel: Deformed bars of grade 500
• Superstructure in Structure steel grade S275 / BC1
• Threaded fasteners: Grade 8.8
• Surface protection of steel structures: System 2 of M&W spec
• Flooring: Chequered plate with anti-slip coating
• Roofing: 0.48mm thick Colorbond (corrugated coated steel)
Construction / erection tolerances
• Fabrication & erection tolerances as per EN 1090-1
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Details of micropile
Cement grout mix proportion
Required grout strength(MPa)40 proposed w/c ratio0.45
min. weight density(kg/m3)1800 pile dia(m)0.35
Grout: volume(m3/m)0.0962 weight(kg/m)173.2
cement water
specific gravity 3150 1000
proposed mix for grout 50 22.5 72.5
volume(m3) check 0.0159 0.0225 0.0384
density (kg/m3) of grout 1303 586 1889
volume(m3) check 0.41 0.59
weight(m) of material reqd 125 56 181
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1. Conceptual design & aesthetics
1. (applicable for permanent POBs)
Footbridges offer the designer a wide variety of choices:
• Less restriction of the shape & curvature in plan & inclination of the
deck;
• More options for the form & shape to suit the landscape & adjacent
environment;
• Scope for use of variety of materials for the main structure as well as
for the deck surface & finishes (subjected to Contract specifications);
• Possibility of using minimum space & multiple entrances to connect
the footbridge to adjacent buildings, natural parks & trails
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Cross-sections of POB in Reinforced concrete
1. (of PWD era)
Precast pretensioned concrete bridges
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Cross-sections of POB in Prestressed concrete
1. (using planter boxes)
Section used for existing PO B
Cross-sections of Precast pretensioned concrete bridges
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1. Details of structures adopted
Duo-pitch roof changed to mono-pitch roof
during construction
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1. Steel truss of POB
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1. Micro-pile foundations of POB
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1. Pad Footings of POB stairs
Duo-pitch roof changed to mono-pitch roof
during construction
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Structural & geotechnical design aspects
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1. Structural arrangement of the POB & stair
• Steel trestles erected on either side supported on 350mm diameter micro-pile
foundation
• Trestles: 6mx2.4mx6m height, weight 100kN/ea
• Steel trusses (2.4m wide, 2.75m high, 18 panels, weight: 870kN) of the POB
prefabricated & brought to site as 2 segments (24m+29m), spliced at site, launched
in position and bolted to the trestles at site forming a portal frame
• Only 2 steel sections (grade S275) have been used for the truss & trestles:
UC 305x305x158kg/m and UC203x203x60kg/m
• Chequered plates span (one way) over the floor beams of the POB truss
• Stairs: Stringer beams C300x100x45.5kg/m
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Construction & supervision
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Transport of truss & stair segments with Cometto
Logistics by M/s Hup Hin Transport Co Pte Ltd
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1. Construction of the POB trestles
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Unloading of POB truss segments
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1. Laying of POB truss segments on temporary
supports at site & splicing them together
Plate load tests carried out to
verify safe bearing pressure
under crane loading
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Launching of POB using 2 mobile cranes Demag AC350
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Erection of POB truss by tandem lifting with mobile cranes
Erection of POB truss using mobile cranes Demag AC 350
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Construction of stairs
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1. Erection of the staircase tower
• Lifting of the staircase
tower using slings
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1. Fabrication of POB truss segments
• Fit-up inspection
• Welding NDT: UT
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1. Painting of POB truss
System 2 of M&W specs adopted
1. Blast clean the surfaces;
2. 1 coat of shop applied 2-pack epoxy zinc
rich primer. Min DFT 40μ;
3. 1 coat of shop applied 2-pack epoxy
micaceous iron oxide high build primer.
Min DFT 100μ;
4. 1 coat of shop applied 2-pack epoxy fire
resistant chemical resistant pigmented
paint. Min DFT 125μ;
5. Solvent clean;
6. 1 coat site applied 2-pack PU fire
resistant chemical resistant pigmented
finish paint. Min DFT 50μ;
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Traffic lane closure for erection of POB & stairs
Lane closure before unloading & connecting steel bridge segments using
mobile cranes Demag AC 350
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1. Construction supervision
• Micropile: Tensile test for mechanical couplers H25 & H40, PDA
• Plate Load Tests at Pad footings of stairs & mobile crane support location for
launching trestles & POB truss
• Material inspection, Fit-up inspection, Welding inspection & witness NDT: UT for
POB trestles & truss, stairs
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Misalignment of POB truss over trestle at north end
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1. Thank you
Coming up in the next presentation:
Demolition of existing POB
Design & construction of Temporary traffic decks
….
QA
&
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