Design and Construction of the My Thuan Bridge, Vietnam

Edwin J. Rogers, Director Infrastructure, Maunsell McIntyre Pty Ltd DipCE, BE, FIE Aust, CP Eng

SYNOPSIS

The My Thuan Bridge was opened to traffic on 21 May 2000, some 8 months ahead of the original schedule. It is the first crossing of the massive Mekong River in Vietnam the first cable stayed and the largest span bridge in the country.

The $90 million bridge was jointly funded by the Governments of Australia and Vietnam and designed and supervised by Maunsell McIntyre Pty Ltd.

The structure is located approximately 125km from Ho Chi Minh City on National Highway No. 1 and has an overall length of 1535m with a central 350m span cable stayed bridge providing a 37.5m high navigation clearance for shipping to Phnom Penh in Cambodia.

It features 100m deep large diameter bored pile foundations, load tested to 30MN, Super Tee beam approaches with spans of 40m and damping devices built in to the cable anchorage's to eliminate cable vibrations.

This paper describes the background to the project and provides an insight into its design and construction.

1 BACKGROUND TO PROJECT

The Mekong is the world's twelfth largest river both in terms of annual discharge and overall length. It flows from the Tibetan plateau in China for some 4,500km, passing through five countries before discharging into the South China Sea as shown in Figure 1.

The Mekong Delta extends over 500km upstream from its mouth as far as Kratie in Cambodia. Downstream from Phnom Penh, the river divides into two main branches, the Han Giang (Bassac) and Tien Giang. The Tien Giang flows past My Thuan before dividing into five separate channels. At the My Thuan bridge site the Tien Giang is confined within a single narrow channel some 800m in width and up to 25m in depth.

The Mekong Delta region supports 16 million people and provides 40% of Vietnam's agricultural produce equating to 27% of the countries GDP. It is linked to Ho Chi Minh City (HCMC) via National Highway No. 1, and at the township of My Thuan a ferry service has operated since 1936.

Figure 1 — Location of bridge site

The notion of a bridge crossing at this location had been discussed for many years but the Governments of Vietnam and Cambodia could not reach agreement on minimum navigation clearances. Cambodia relies on the Mekong River for international shipping access to Phnom Penh, with vessel sizes limited only by water depths at the mouth of the river.

After bilateral aid to Vietnam was resumed in 1993, the Australian Government was able to facilitate an agreement on river navigation clearances with the Government of Cambodia, thus clearing the way for a new bridge to be considered. In May 1993 the Governments of Australia and Vietnam concluded a bilateral agreement to jointly fund the construction of a crossing at My Thuan. The agreed funding split was on the basis of 66% of the total cost being met by Australia and 34% by Vietnam.

In June 1994, a Feasibility Study was undertaken by SMEC-MBK to investigate and report on the technical, economic and financial feasibility of the project, together with an initial assessment of the environmental, social and institutional impacts and alternative engineering solutions for the structure. This study recommended a large span cable stayed bridge as the preferred option.

In March 1996, Maunsell McIntyre Pty Ltd was awarded the detailed design, contract documentation and construction supervision of the project, which was also to include the largest engineering construction training and technology program ever undertaken by the Australian Government, involving over 60 trainees over a five year period.

In November 1996, tenders were called for the construction of the bridge and by June 1997 a contract was awarded to Baulderstone Hornibrook.

2 EVALUATION OF ALTERNATIVES

During the development of the preferred bridge arrangement, considerable effort went into the optimisation of the individual components of the bridge, in line with the objectives of the Terms of Reference, which called for:

• a cable stayed bridge for the main river crossing • a deck width able to accommodate two traffic lanes, two motorcycle lanes and two pedestrian

footways and capable of being extended in the future to four traffic lanes and two pedestrian footways

• maximum approach gradients of 5%

The primary focus of attention was the main bridge and particularly the river foundations, as together these make up over 55% of the cost of the overall structure. A secondary study was also undertaken to determine the optimum height of embankments by comparison with the incremental cost of extended approach structures. Amongst the alternative configurations considered were:

• main span lengths of 300m (both towers and tie-down piers in river), 350m (both towers in river), 450m (single tower in river) and 600m (both towers out of river)

• cast insitu, precast and composite decks for the main deck • 'A' frame versus 'H' frame configuration for the towers • vertical bored piles, raked bored piles and raked driven steel piles for the main bridge foundations • precast beam and slab, incrementally launched box girders and precast segmental box girders for

the approach decks.

2

437 600 660 000 437 600 4ORTHERN APPROACH STRUCTUR

CABLE STAYED BRIDGE

HE RN APPROACH STRUCTURE

OVERALL LENGTH OF DECK =1535 200

150 000

350 000 150 000 5000

NORTH TOWER SOUTH TOWER

5% AWL RL 20m

NORTHERN CAUSEWAY

5000

<FROM HO CHI MINH 110m 37.5rn —SOUTHERN 3000 z 30rn BANK

NAVIGATIONAL CLEARANCE PROTECTION

I TO VINH LONG ✓

Early in the design development, it was established that for an incremental cost of only A$1.0 million the bridge could be provided with the desired long-term lane configuration and this change was subsequently approved by both Governments and incorporated in the design.

A 350m main span was ultimately chosen as the optimum configuration for the river channel geometry at the bridge site. Although alternative design solutions were encouraged from the shortlisted tenderers, no significant alternatives were offered.

At the client's request, an alternative raked driven steel pile foundation system was also presented at tender for pricing, but the vertical bored pile design proved to be significantly more economic. The driven 1,800mm diameter steel tube alternative would have called for pile driving plant equivalent to the largest currently utilised in the offshore industry.

3 DESCRIPTION OF PROJECT

The My Thuan bridge provides a high level road crossing of the Tien Giang with a total of four traffic lanes and two pedestrian footways. The overall length of the bridge structure is 1,535m based on limiting approach gradients of 5% as shown in Figure 2.

Figure 2 — Longitudinal section of bridge

The main cable stayed structure over the river is 660 m long with a 350m central span and side spans of 150m. The cast insitu segmental concrete superstructure is supported by 64 stay cables from each tower, with anchorage points located at 10.4m centres along both edges of the deck.

The deck is cast monolithically with the tie-down piers and fixed against horizontal and longitudinal translation at the towers. The towers have a modified H frame configuration in order to maintain cables in vertical planes, and comprise hollow box section legs of varying section with crossbeams at two levels.

The approach structures each have 11 spans with an overall length between expansion joints of 442.6m. The deck section is a beam and slab configuration utilising 10 No. 1.8m deep 37.6m long precast prestressed Super Tee beams in each span. The 1.5m difference in width between the main deck (width 23.66m) and approach decks (width 22.06m) is facilitated by splaying the precast girders over the penultimate span. Girders are provided with halving joints to conceal the pier crossheads as part of the overall attention to the aesthetics of the bridge.

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The approach substructure consists of twin solid blade columns supported on driven 400mm square driven reinforced concrete piles. Abutments are conventional spill-through type with reinforced concrete crossheads supported on twin rows of vertical driven steel tubes to provide rotational stability to the abutment under the anticipated relatively high lateral displacements induced by ground consolidation effects.

On the basis of cost effectiveness analyses approach embankments were limited to a maximum height of 6 m because of the poor ground conditions encountered. Even at this height they required extensive ground treatment, staged construction and a comprehensive monitoring program to ensure a satisfactory long-term performance.

The scope of the bridge contract included roadworks back to existing ground level. All associated linkages into the National Highway system were undertaken by the Government of Vietnam.

The project also includes the following works in addition to the normal provisions of highway furniture and services:

• a rock faced causeway with sheet pile nosing for protection of river piers up to and including the northern tie-down pier within the shallow northern flank of the river

• localised bank scour protection in the vicinity of the southern tie-down pier • special lighting and other architectural enhancements as discussed in Section 7.8 • a fire main and associated pump station for the main deck, primarily to assist in the protection of

the cable stays • permanent maintenance access roads • provision of maintenance gantries for the main deck • landscaping of the entire bridge corridor

As shown on Figure 3, further riverbank protection works were identified on both banks upstream of the bridge, but these were not included in the bridge construction contract. A detailed risk analysis indicated that they should be constructed within five years of the end of the bridge construction, in order to ensure the long term stability of the river planform in the vicinity of the bridge site.

4 SITE CONDITIONS

The bridge is located some 11cm downstream of the existing ferry crossing at My Thuan. The alignment for the bridge was chosen after an extensive investigation of alternative crossing locations along a 20km stretch of the river during the feasibility study.

The topography is generally flat with the riverbanks being slightly higher than the surrounding farmland, which is characterised by small rice paddy holdings interspersed with natural vegetation.

The river is approximately 800m wide between banks with a 600m wide main channel located towards the southern bank and a 200m wide shallow zone adjacent the northern bank.

4

PERMEABLE GROYNES

NATIONAL HIGHWAY No I TO HCMC

DIRECTION OF RIVER BANK EROSION

EXISTING FERRY CROSSING

TO VINH LONG

FUTURE SOUTH BANK lAW PROTECTION WORKS

NEW APPROACH ROAD

Figure 3 — Site plan

Flow measurements indicated that the river is tidal with flow velocities varying between 2m/sec downstream and 0.7m/sec upstream. The river exhibits a complex flow pattern throughout the channel section with considerable turbulence associated with flow reversals and vertical flow stratification, particularly in the vicinity of a deep scour hole that exists some 600m upstream of the bridge site.

Due to the proximity to the sea, flood level variations are relatively minor with a maximum predicted flood level (with a return period of 100 years) equivalent to the top of the existing banks.

Ground conditions along the bridge alignment exhibit a complex stratigraphy characterised by six major sediment deposition sequences as shown in Figure 4.

The variable levels and thickness of each sequence is the consequence of localised river erosion and deposition processes over time.

The southern bank is primarily comprised of a very stiff to hard clay with thin sand lenses making it less susceptible to erosion than the northern bank which consists of an upper strata of very soft to firm cohesive sediments overlying a relatively extensive strata of granular sediments.

Climatic conditions are typical for a monsoon region with the wet season coinciding with the SW monsoon between May and October and the dry season between November and March associated with the NE monsoon.

5

SOUTH TO VINH LONG

APPROACH SPANS

NORTH TO HO CHI MINH CITY

APPROACH SPANS MAIN SPAN

411 EMBANKMENT EMBANKMENT

SEQUENCE Cl OFT SILTY CLAYS SAND-CLAY SAND LAYERS

• MAINLY SILTY BANDS AND CLAYEY SANDS

QUENCE C2 SILTY CLAYS AND CLAYEY SANDS

0 100 200 300 400 500 I 1 1 1 1 1

HORIZONTAL SCALE (m)

SEQUENCE S3

S

SILTY SAND CLAYEY SANDS

MAINLY CLAYS SEQUENCE C3 S SILTY CLAYS

• 0

-20

1L Om

-10

-20

-30

-40

-50

-60

-70

430

430

-100

-110

-120

The bridge is located in a low seismic risk area and the wind climate is relatively benign. However, historic records show that six typhoons have nevertheless passed close to the bridge site over the past 50 years with a maximum recorded velocity of 26m/sec.

Figure 4 — Inferred ground stratigraphy

5 RIVER STUDIES

From the outset of the detailed design, it was recognised that a thorough understanding of the river's behavior was paramount to providing a cost effective design solution, and investigations were therefore undertaken to determine:

• the depth of local scour which could be expected at the river piers • the long term stability of the overall river platform and in particular, the likelihood of changes in

the location of the large scour hole upstream of the bridge site • protection measures required to stabilise the riverbanks upstream of the bridge site.

A hydraulic flume study was undertaken at Monash University to ascertain local scour depths. The study used a 1 to 100 scale model of the tower foundation and concluded that scour depths of 13m should be adopted for the design.

A large scale hydraulic model was undertaken at the Vietnam Institute for Water Resources Research in Hanoi. The model was used to develop the physical parameters of the groyne field, which is proposed for the future protection of the north bank upstream of the existing ferry crossing. This led to a preferred design consisting of a series of 10 groynes each extending into the river up to 110m over a bank length of 2,500m upstream of the ferry. The groynes are made up of 450mm square section driven reinforced concrete piles spaced at 1,125mm, as shown in Figure 5.

6

-10

-15

93000 22000

ORIGINAL HANK SLOPE

NEW BANK SLOPE 2 TO

RIVER LEVEL

R.C. CAPPING BEAM 1

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1111 NPAIR OF RAKED PILES

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-35

-40 — ROCK REVETMENT (0.8m THICK)

Figure 5 — Typical groyne configuration for north bank protection

The presence of a 48m deep scour hole (relative to river level) some 600m upstream of the bridge site was recognised as a potential risk to the bridge foundations and was therefore the focus of considerable attention. Historic records show that the position of the scour hole is shifting downstream, due primarily to the reverse bend in the river, the confluence of the Rach Sa Dec and Tien Giang and the severe narrowing of the channel at this location

Extensive field investigations with an Acoustic Doppler current profiler were undertaken to study the flow patterns associated with the scour hole. However, it was concluded that any attempt to model the future behavior of the river would be inconclusive given the complex interaction between the many physical parameters involved.

Ultimately, after consideration by an international panel of river training specialists, it was agreed that the only way to ensure the long term stability of the bridge site was to install additional upstream bank protection works within five years of the construction of the bridge.

6 DESIGN CRITERIA

The bridge is designed in accordance with the requirements of the AUSTROADS Bridge Design Code, with the following particular specifications:

• Traffic Loading is taken as T44, L44 (constant regardless of span) and HLP200 (10 No. axles at 1.8m centres, each 200 kN). The HLP vehicle is assumed to act at a lm offset from the centreline of one carriageway with two lanes of 50% L44 loading in the adjacent carriageway.

• The design ship impact force for the river piers is in accordance with the provisions of the AASHTO Guide Specification and Commentary For Vessel Collision Design on Highway Bridges (1991), based on a 3610 DWT design vessel at an impact velocity of 4.5m/sec. The design impact velocities reflect the fact that all vessels in excess of 1900 DWT require a pilot under Vietnam Regulations.

• Design Wind Speed Vg,10 (gust wind speed at 10m above ground level) of 52m/sec for inservice ULS, 41m/sec for construction ULS and 38 tn/sec for inservice SLS. Gust turbulence effects in accordance with classical stochastic theory.

• Thermal effects based on a mean of 27°C, range 45°C to 10°C, gradient per Austroads region 3, and a temperature differential 15°C between stays and deck.

7

• Hydrodynamic effects based on ULS stream velocities of 3.2m/sec downstream and 0.6m/sec upstream with an assumed high water level of +2.1m.

• Seismic loading based on 0.05g ground accelerations in accordance with Vietnamese standards. A dynamic analysis of the main structure was also required on the basis of a conservative design spectrum appropriate to the bridge site.

• Sudden loss of any stay cable considered only at ULS coinciding with four lanes of L44 live load with a load factor of 1.05.

• Main tower foundations are designed for a scour depth of 22m (local scour 13m plus 9m general bed scour) under design ship impact force and scour to RL-56m (to the level of the third stiff clay layer) under wind loading (50 year return period) and normal stream flow forces.

• The design utilises concrete grades up to Grade 50 for the main deck, towers, precast girders and driven piles. Bored piles and pile caps utilise Grade 40 concrete

7 DESIGN OF CABLE STAY BRIDGE

7.1 Analysis

The cable stay bridge was analysed with a 3D model of the entire structure, including the foundations, using RM-SPACEFRAME. This program has been specifically developed for the design of bridge structures and uses a step-by-step analysis with allowances for concrete creep and shrinkage. The resulting linear elastic analysis, including second order effects was utilised for the design of the deck beams, stay cables and tie-down piers, after consideration of both construction stage and inservice loading effects.

A separate 3D SPACEGASS model was used to confirm this analysis and to investigate the effects of asymmetric loads such as live loads and loss of a stay cable.

The effects of wind on the structure were investigated for both construction and inservice stages using the well known stochastic method developed by Professor Davenport. The dynamic analysis utilised PCROM, a general purpose finite element program developed by Aas-Jackobsen for the Norwegian Government. The results of this analysis were used for the design of the towers and to nominate temporary bracing requirements. The key finding of the construction stage analysis indicated that the free-standing towers require stabilising after the construction of the fifth segment, in order to limit out of balance forces in the tower legs.

Seismic effects were investigated using the same PCROM model utilising a conservative response spectrum based on a bedrock acceleration of 0.1 g and a ground magnification factor of 3.0. This analysis concluded that seismic loading is less critical than wind loading for this location.

Ship impact loading was also investigated using the SPACEGASS model for the assumed loadings developed from AASHTO.

7.2 Wind Tunnel Study

Wind tunnel tests were carried out on a 1/125 scale two-dimensional model of the deck section to: • measure the aerodynamics derivatives of the deck section • establish the critical onset wind speed for rotational response as a function of critical damping

ratios (down to 1%) at angles of attack from 0° to 3° for different levels of turbulence intensity • measure the response to vortex shedding at low velocities with a critical damping ratio at or

below 1%. 8

FINAL POUR

INRIAL POUR 1000 THICK

H.W.L. RL 2.000

L.W.L. RL -1500

02.5m BORED PILE

TOWER LEG

PRECAST SKIRTING PANELS

BRIDGE AND CONTROL LINE

61000

TOWER

Because of the relatively mild wind climate at the bridge site it was decided that testing of a full aeroelastic bridge model was unnecessary. Instead, the dynamic behaviour of the structure could be investigated by using a stochastic 3D computer analysis.

The results of the wind tunnel studies showed that the deck section is aerodynamically stable for wind speeds up to 80m/sec.

7.3 Foundations

The main bridge foundations utilise 2.5m diameter bored piles extending to a depth of approximately 95m below river level. The tie-down foundations comprise a single bored pile beneath each column, with a common pile cap.

The towers are supported on 16 No. 2.5m diameter vertical bored piles in two clusters beneath each tower leg, as shown in Figure 6. The pile caps have overall dimensions of 15m x 61m x 4m and are designed to be constructed above the normal river level with a combination of precast fascia panels and insitu infill concrete.

SFCTION PLAN ON PILE CAP

Figure 6 — Tower foundations

The vertical pile group was designed to resist lateral loads in flexure. Pile loads due to ship impact effects were determined using the equivalent static force method based on AASHTO guidelines and confirmed by force-time spectrum dynamic analysis. Peak pile loads under the towers are limited to 28MN at ULS under ship impact loading.

Vertical loading was assumed to be carried by skin friction alone at SLS and compaction grouting is specified at the base of the piles to minimise settlements under ULS loadings. In accordance with AUSTROADS requirements, the characteristic geotechnical resistance of the piles was determined by load testing five working piles. The Osterberg load cell testing method was employed for this purpose as shown in Figure 7

9

TREMIE PIPE FOR CONCRETING

OSTERBERG LOAD CELLS

DETAIL A STEEL TUBE FOR SONIC LOGGING AND COMPACTION GROUTING

BEARING SUPPORT FRAME

TOE OF PILE

1 DETAIL ink

7.4 Towers

The towers have a splayed H frame configuration in order to keep the stay cables in a vertical plane, thereby simplifying the geometry of the anchorages. As shown in Figure 8, the transverse spacing of the legs varies from 28.9m at the pile cap level to a constant 16.0m from the upper crossbeam to the top of the architectural finials.

In elevation the tower legs taper from 6 m at their base to 4m at the top, 129.5m above pile cap level. In the other direction, the legs have a constant width of 2.5m. Wall thickness is maintained at a uniform thickness of 500mm except at crossbeam and stay cable anchorage locations.

The design of the towers is generally governed by wind loading in combination with permanent effects and although the legs are relatively slender, under frame action they are particularly stable in the transverse direction.

F TOWER & .4000 BRIDGE RL 127.500 IS

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Figure 8 — General arrangement of towers Figure 7 — Bored pile load testing

10

I

STEEL CASIN

XIS • 11 R.L -2s R.L

LOCAL SCOUR DEPTH R.L -38

DESIGN SCOUR DEPTH R.L -47

MAX. SCOUR DEPTH R.L. -56

CLAY (C3)

SAND (53)

R.L -38

R.L -47

R.L -58

TUBE.A.MANCHETTE

OSTERBERG LOAD CELL

TREMIE PIP

STEEL TUBE FOR SONIC LOGGING AND COMPACTION GROUTING

ANNULAR LOAD CELL BEARING AND SUPPORT FRAMES

SECTION CD

STEEL TUB

SPARE STEEL TUBE & TUBE-A-MANCHETTE

TUBE-A-MANCHETTE

SECTION

COLUMN COLUMN

5500 I 5500 /BACK STAY CABLES

3% 3%

TIE DOWN PIER

MINIMUM LOOP RADIUS 1000mm INITIAL POUR

F S L

PRESTRESSING TENDONS

02.5m BORED PILES

APPROACI GIRDER BEARINGS D.J.

SOLID DECK SECTION MONOLOTHIG

CONNECTION DUCTS FOR ELECTRICAL SERVICES

COLUMN 1500 x 3500

Cross beams, located below deck level and at the base of the cable stayed anchorage zone, are designed as insitu prestressed concrete box sections. The prestressing tendons are confined to the webs and anchored in recesses on the outside face of the tower legs.

The tower anchorage zones are designed using strut-tie principles with a combination of prestressing tendons and bars to accommodate the high radial and splitting forces generated by the stay reactions.

Each tower leg is provided with an internal steelwork ladder/stairway system accessed through security doors at deck level.

7.5 Tie —Down Piers

The tie-down piers serve an important role in providing resistance to the main backstay cable reactions. For this reason, as shown in Figure 9 the twin column tie-down piers are designed to be connected monolithically with the deck section and vertically prestressed to the pile cap, in order to activate the full mass of the substructure.

Figure 9 — Tie down pier

Because of the flexibility of the columns and pile foundations the tie-down piers are able to accommodate the large deformations due to creep, shrinkage and thermal deformations in the main superstructure.

7.6 Deck

The main bridge deck has an overall width of 23.6m and depth of 2.0m and consists of a grillage of external longitudinal girders with transverse crossbeams at 5.2m centres, supporting a 0.2m thick reinforced concrete deck slab.

As shown in Figure 10, the longitudinal beams are offset 2m from the outer edge of the deck to accommodate precast anchorage pods for the cable stays at typically 10.4m centres. Precast anchorage pods were chosen to ensure accurate positioning of stay cable anchorage assemblies, to simplify the placement of reinforcement in the longitudinal girders, and to simplify the use of temporary compression struts for the construction of the deck segments.

11

- DRAINAGE SCUPPER

12m HIGH LIGHT MAST

23860

8600 8600

BRIDGE 1200 1200

2100 1000 8000 300 300 8000 1000 2100

_.... 500 2 LANES AT 3500 2 LANES AT 3500 500

65 ASPHALTIC - CONCRETE

3% L.....ii ELECTRICAL SERVICES DU

3%

3

Ai 1 li

- ARE MAIN

430 430

Figure 10 — Main bridge typical section

The longitudinal girders are generally designed as reinforced concrete sections with reinforcement requirements determined for strength and then checked for serviceability compliance. Prestressing strands are incorporated over the central 120m of the main span to maintain a net compressive stress in the deck after long term creep and shrinkage deformations, as well as to improve the ultimate flexural capacity of the girders, particularly under the critical load case of a sudden loss of one stay cable.

Vehicular live loading is transferred to the longitudinal beams by 350mm thick, partially prestressed concrete crossbeams, each with a single multistrand tendon. At each stay cable anchorage pod the prestressing tendon from the crossbeam is brought to the outside of the anchorage pod to assist in its connection to the longitudinal girder.

The main deck section changes to a solid section above the tie-down piers in order to accommodate:

• a transition in section depth for support of the deeper approach girders • the concentrated loads from the backstay cable anchorage's • a monolithic connection to the tie-down pier • an expansion joint between main and approach structures • additional weight to help balance the backstay reactions

This complex deck section is designed as an insitu concrete slab utilising bi-directional prestressing tendons, and is connected to the main deck by a 2.5m wide closure pour accomplished by use of the traveller.

The main deck is restrained at the tower crossheads in all directions, as shown in Figure 11. Ship impact forces are designed to be transferred to the deck in a controlled manner in order to limit peak moments in the tower legs. Because of the relatively flexible foundation system, it was possible to use special shear keys incorporating 73mm diameter prestressing bars with an ultimate capacity of

12

VERTICAL SUPPORT BEARING

VERTICAL SUPPOR BEARING

LATERAL RESTRAINT

BRIDGE

5,000kN, in lieu of the more expensive dynamic dampers commonly employed on structures of this type. Under ship impact loading perpendicular to the bridge centre line, the in plane stiffness of the deck is utilised to transfer some of the lateral loading to the other foundations via the lateral restraint bearings. Under ship impact loadings at oblique angles to the bridge centreline, the pair of shear keys acts as a couple to resist the twisting of the pile cap relative to the deck. To avoid overstressing the tower legs, the prestressing bars are designed to yield and fail to allow the pile cap to rotate and therefore transfer more of the lateral forces down into the foundations rather than through the tower legs into the deck.

TOWER BEARING LAYOUT

VOID AROUND STRESS BAR GROLTED PRIOR TO STRESSING

VERTICAL SUPPORT LONGITUDINAL BEARING RESTRAINT

BEARING

440 11111.1 111

ANCHORAGE RECESS FILLED WITH GREASE

ANCHORAGE CAP- FILLED WITH GREASE

MAIN SPAN SIDE

2 No. 73 DIA. STRESS BARS WRAPPED IN DENSO TAPE

LONGITUDINAL RESTRAINT BEARING

Figure 11 — Main bridge deck restraint at towers

Vertical pot type bearings are provided directly beneath the edge beams to minimise stay cable requirements. Controlled adjustments of the level of the longitudinal girders is necessary during the construction phase to minimise induced hogging moments

The deck has a transverse crossfall of 3% with scuppers located adjacent to the outer pedestrian barriers. A continuous central concrete barrier is provided to divide the carriageways and to support the overhead light masts. The outer barriers are a composite steel and concrete type to minimise wind loads and maximise the peripheral visibility of motorists.

7.7 Stay Cable System

Stay cables are critical elements of the structure and particular attention was therefore given to ensure that the bridge would incorporate the latest technology. The key technical specifications for the stay cables are as follows:

13

TOWER

CAP

SENR)NG PLATE

COMPRESSION SEAL (STUFFING BOX)

FORMWORK TUBE

WATERPROOFING SEALANT

HOPE DUCT

BOLTED CONNECTION

GUIDE DEVATOR AND INTERNAL DAMPING DEVICE

STRANDS

STAY CABLES • TOWER ANCHORAGE

CAP

STRANDS

HDPE DUCT

ATERPROOFING SEALANT

BOLTED CONNECTION

ANTI-VANDALISM TUBE

GUIDE DEVIATOR AND INTERNAL DAMPING DEVICE

- FORMWORK TUBE

PRECAST STAY POD

COMPRESSION SEAL (STUFFING BOX)

BEARING PLATE

STAY CABLES • DECK ANCHORAGE

15.7mm DIA. SEVEN

HDPE PIPE WIRE STRANDS

CABLE CROSS SECTION

Figure 12 — Stay cable details

• They must provide a minimum of three levels of corrosion protection

• Individual strands must be capable of being inspected and replaced

• They must comply with the requirements of the PTI Recommendations for Stay Cable Design, Testing and Installation (1993)

• The cable system must be free of wind induced vibrations.

The cable anchorage's comprise standard cast-in anchorage assemblies complete with steel guide pipes, as shown in Figure 12, with the spatial arrangements at the tower head based on the use of monostrand jacks. The stays are provided with passive anchorages at the tower head and active anchorages at deck level.

Stay cables vary in size from 27 No. 15.7mm diameter strands adjacent to the towers up to 68 No. strands for the backspan cables. Corresponding outer HDPE sheath diameters vary from 150mm to 250mm respectively.

7.8 Architectural Considerations

The My Thuan bridge is destined to become a showpiece of Australia's goodwill and engineering technology.

The architectural appearance of the bridge was therefore given particular emphasis during the design development stage and the end result (refer Figures 13 and 14) is a bridge of classic proportions and simple structural form expressed in the following basic design and detailing features:

14

Figure 13 — Aerial view of completed bridge

• The overall general arrangement of the bridge is symmetrical within the river channel with span lengths chosen to provide a well proportioned span to pier height configuration throughout.

• The towers rise from massive pile caps at river level in the form of a tapered H frame with each leg gradually curving upwards to a vertical section from which the cable stays emerge. The tower legs are rectangular in section with rounded corners and are joined at two levels by crossbeams with an arched soffit profile. They are capped with 6m high precast finial units finished in polished concrete.

• The overall depth and width of the main Figure 14 — View from north bank deck and approach decks have been maintained as close as possible (given the fundamentally different structural forms involved), with differences accommodated by tapering the deck near the tie-down piers.

• The entire deck has a common edge parapet detail which features an inclined pedestrian railing mounted upon precast concrete edge panels. The panels also conceal the deck drainage and fire main systems.

• The approach piers have been provided with twin slender columns similar to the tie-down piers, for uniformity of structural form.

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• The main deck features external anchorage pods on the outer faces of the longitudinal deck beams, with cable anchorage's located above soffit level to ensure a clean line to the main deck.

• The stay cables are coloured a light shade of blue (instead of the normal black) in order to provide a more dynamic and elegant backdrop for motorists. In overcast conditions the cables take on the grey tones of their background and chameleon like, with a clear blue sky they tend to disappear from view. The towers and cables are also illuminated at night with floodlights mounted on the tower pile caps.

• Roadway lighting masts are mounted on the central median and feature specially elongated masts with dual luminaires as shown in Figure 15.

Figure 15 View from tower crossbeam

8 CONSTRUCTION OF CABLE STAY BRIDGE

8.1 Bored Piling

The technical specification called for the piles to be test loaded by Osterberg Load Cell method as a means of confirming final founding depths, together with the following test and measurement requirements:

• Bentonite mix control. • Pile excavation alignment, vertically and diameter. • Base grout pressure and volume take. • Concrete placement volume and strength • Cross hole sonic integrity logging.

The piles were constructed using the following methodology:

• steel casings (up to 43m long) were fabricated full length in HCMC and barged to site where they were vibrated into position with the use of a 250tonne crane and a special hydraulic casing guide.

• pile excavation was undertaken with the use of a purpose designed 5m long 23.5tonne excavator grab and separate 1 1 m long 26.5tonne calibration grab supported by a 150tonne crane as shown in Figure 16.

• stabilization of the open excavation was achieved with a bentonite and water slurry. Spoil was removed from site by barge to an approved dumping location

• reinforcement cages were prefabricated in 36m lengths (weighing up to 30tonne) and spliced at the pile platform level with special suspension clips

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• concrete was batched on site and transported to the piles by truck mixer along a temporary access jetty. A concrete pump discharged directly to a conventional 250mm diameter tremie tube system.

Base grouting was achieved with the installation of a tube-a-manchette system prefabricated together with the reinforcement cage, as shown in Figure 17.

Figure 16 — Excavation of bored pile Figure 17 — Tube-a-manchette installation

Measurement of Pile Excavation

Alignment, cross section and depth of pile excavation was measured by the use of a Koden DM-682 monitoring unit. This unit is a sonic wave speed measurement device which provides a continuous record in two orthogonal directions as it is lowered and raised over the pile length.

The results showed that the uncased bored piles were constructed within specification with the verticality of most piles not exceeding 1 in 200.

Pile Base Grouting

The base of each pile was grouted a minimum of 3 days after concrete placement to stiffen the base load deflection response. Grouting pressures of 10Mpa at the top of the pile were maintained for a minimum of 10 minutes, with grout take normally in the range form 2 to 3m3 per pile

Osterberg Load Cells

A series of 5 Osterberg load cell tests were carried out starting with two at the southern tie down pier, two at the southern tower and one at the northern tower. This was the deepest and largest capacity application of the testing method ever used in soils and proved to be extremely successful.

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Northern Tower Pier - Pile p2/12

90

ea

oo

Load cell at R.L. 81.5 He Base R.L. -90.5

1 —

1 I I

J. 60

— T

60—

2 30 - I I 121211 I

7 20 — — — —I— — — — — —

4 1 -- I _L

•'3 0 — H IBAEfem SWIFT I I

200 5 IO 15 21 25

TOTAL LOAD IMNI

Figure 18 — Osterberg load cell test — for shaft & base

The load cells, each with a capacity of 30MN were installed into the reinforcement cage and instrumentation and monitoring was undertaken by Loadtest Inc (Florida). The first two piles comprised two levels of load cells in order to allow the base of the pile and two sections of the pile shaft to be isolated and tested to failure

An example of the Osterberg Load test base load and shaft load versus movement at the southern tie down pier is shown in Figure 18

Sonic Logging

All piles were subjected to integrity testing along the four 75mm diameter steel tubes provided for the base grouting.

Figure 19 - Pile cap showing soffit form

pour, with the bottom lm designed to support the

The three section pour helped to overcome restrained shrinkage effects. Concrete heat of hydration was monitored using cast-in thermocouples and controlled with the use of 50mm think polystyrene insulation panels and chilled mix water.

8.3 Towers

The tower legs were constructed with a purpose built jump form designed to accommodate the curved profile with varying sections and blockout requirements. Construction of the legs was done in 4m lengths with the use of temporary bracing falsework as shown in Figure 20.

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An Olson Instruments Unit CSL-1 was successfully used to provide up to 6 cross hole traverses along the length of each pile. In general, zones of integrity loss were only detected in a few piles and these were rectified with pressure grouting.

8.2 Tower Pile Caps

The tower pile caps were constructed in three sections, commencing abovc the pile groups at each end followed by the central voided infill.

The construction sequence was as follows:

• Steel falsework system welded to pile casings within tidal zone

• Precast soffit slabs installed followed by perimeter skirting panels Figure 19

• Concrete placed by pump in a two stage second 3m thick pour .

The lower crossbeam was cast at pile cap level and raised into position together with deck falsework for the initial deck segments. The 920t mule heavy 1;n ar.himeril with 1 strand jacks supported off the lower strut. The upper crossbeam was cast in place off the upper strut with conventional formwnrk Both crossbeams are postensioned to the tower legs

Cable stay anchorage assemblies were pre-fabricated at ground level within a steel frame to ensure geometric compliance in the tower form prior to concreting. Concrete was Grade 50 and supplied to the jump form by tower crane using a kibble bucket.

8.4 Deck

The cable stayed deck was constructed in balanced cantilever with deck segments cast insitu using a purpose built underslung form traveller as shown in Figure 21, with a total weight of 240tonne, inclusive of the timber form system.

The first three segments at the tower were east VII a separate falsework system raised together with the lower tower crossbeam.

Individual 10.4m long deck segments were cast symmetrically about the tower with a 2 day lag between the main span and side span segments. At its peak two segments were constructed over a 5 day cycle, with an average cycle rate of"/ days achieved over the full deck.

Figure 20 — Tower construction showing temporary bracing

Figure 21 — Main deck construction

Stability of the balanced cantilever and structural protection of the tower legs was achieved with preloaded inclined buffeting cables connecting the deck to the tower pile cap, together with a temporary tie down system midway along the side span. The tie down system utilised a 1000tonne temporary pile supported concrete counterweight, which was required to be in place until the insitu connection of the deck with the tie-down pier.

Closure pours at the tie-down piers and midspan were achieved with the use of the form traveller with additional external stiffening beams to control deformations in the fresh concrete.

The typical construction sequence for each segment was as follows:

• Secure form traveller in position to predetermined spatial geometry 19

Figure 22 — Stay erection showing prefabrication of HDPE casing

• Install precast stay pods and crossbeams • Install prefabricated edge beam reinforcement cage • Install stay cable at permanent anchorage location (with corbel postensioned to the traveller) • Pump concrete using placing boom, finish deck surface and treat with ant-evaporation sealant. • Adjust cable force to predetermined load after concrete has reached 30mpa.

The Contract required a Construction Consultant to be engaged to control the geometry and stresses within the deck during the stage by stage construction sequence.

Prior to placement of the final asphaltic wearing course the entire deck slab under the roadway was treated with a SAIVII waterproofing layer similar to RTA specifications.

8.5 Stay Cables

The stay cables were supplied and installed by Austress Freyssinet in line with the deck casting program.

Each stay HDPE casing was prefabricated at deck level as shown in Figure 22 and lifted into position with the tower crane using a single reference strand.

All stays are fitted with the latest anti vibration technology developed by Freyssinet, which has only been used on two other bridges to date. The so called Internal Hydraulic Dampers are located at the end of each steel guide tube, as shown in Figure 12. They were calibrated by site testing to damp vibrations in the cables and limit amplitudes to acceptable levels under wind velocities up to 42m/sec.

9 DESIGN OF BRIDGE APPROACHES

9.1 Approach Bridge

The approach structures on either side of the main bridge are each 442.6m long and comprise 9 No. 40m intermediate spans with a penultimate span of 43.8m and end span of 38.8m.

The overall width of the deck is 22.06m as shown in Figure 23 but the penultimate span tapers to 23.66m at the interface with the main deck, by a simple splaying of the beams. The deck is made up of ten precast Super Tee Beams and an insitu composite reinforced concrete deck slab, made continuous between expansion joints at the abutments and tie-down piers. The deck slab is fixed to each support pier crosshead by the use of 60mm diameter steel pins with neoprene caps, as shown in Figure 24.

20

12m HIGH LIGHT MAST AT 40m CTS.

A

3% 3%

1800

DECK DRAINAGE - PIPE

• — FcrillAR°

//4/x5Y/.9%.6///

Ill 41\ DRIVEN RC PILES

PIER

STEEL DOWELS WITH ASPHALTIC CONCRETE NEOPRENE CAPS.

OVER SAMI WATERPROFING

20 THICK RIGID FOAMED PLASTIC SHEET 0300 HOLE FOR

DECK DRAINAGE TYP.

TEMPORARY PACKING

ELASTOMERIC BEARING

Figure 23 — Approach structure typical section

Figure 24 — Approach structure pier fixity

The pretensioned Super Tee Beams were adopted because of their cost effectiveness and aesthetic qualities. The beams have a clear span between bearings of 37.6m and an overall depth of 1.9m and are partially prestressed using 20 No. straight 15.2mm diameter strands. Stub diaphragms have been provided to stiffen the ends of the deck and also provide space for jacking in order to facilitate future bearing replacement.

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3500 9500 3500 3500

WALK !RAMO WAY LANE

LLE IL 39__L_

All internal girders are provided with simple elastomeric bearings set on the longitudinal grade of 5% for ease of installation. Bearings at the abutments and tie-down piers are pot type sliding bearings able to accommodate the larger displacements associated with the expansion joints. A modular type expansion joint is required to accommodate the design movement range of 650mm at the main joint, with the abutment joint required to accommodate a movement of only 190mm. Support piers feature a concealed crosshead supported on twin slender column blades varying in height between 6m and 24m. The majority of all longitudinal loading is carried by the shorter columns with the longer columns designed effectively as pin-ended struts.

The approach structure piers are supported on ten 400mm square section driven reinforced concrete piles founded in medium to dense sands between 33m and 40m below ground level. The piles are raked at 1 in 10 and designed with a maximum ultimate penetration resistance of 3,100IcN, verified by insitu dynamic testing methods.

Abutments are a simple bank seat configuration supported on a twin row of driven 600mm diameter vertical steel tubes. The steel piles were chosen for their greater flexibility and will be required to deform in double curvature under long term lateral displacements of the embankment fill, without excessive rotation of the abutment. The abutments also incorporate 8m long settlement compensation slabs to minimise the impact of differential pavement displacement.

9.2 Approach Embankments

Considerable effort was put into the optimisation of the embankment lengths, which ultimately resulted in a decision to limit the maximum height of embankments to approximately 6m. The construction costs associated with embankments in excess of this height exceeded the incremental cost of extending the approach deck. The underlying ground conditions are complex and variable but are characterised by a relatively deep stratum of low strength cohesive sediments with intermediate sand lenses.

As shown on Figure 25, the embankments have a formation width of 21.8m, with batter slopes of 2 to 1 and extensive side berms to ensure overall slope stability.

Figure 25 — Embankment section

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Because of the poor ground conditions and relatively short construction period available, the embankment design incorporated the following features:

• wick drains on a 1.5 m grid • geotextile fabric as a working platform • staged placement of fill • berms up to 24m in width • surcharging by up to I .5m • extensive monitoring of settlements, lateral movements and pore pressures.

Total consolidation settlements of up to 2,000mm were measured with residual creep settlements of the order of 100mm to be accommodated post construction. Embankments are constructed using sand fill with a 500mm surface capping of clay to support conventional landscaping. Flexible roadway pavements are provided in line with normal Vietnamese highway standards.

10 CONSTRUCTION OF BRIDGE APPROACHES

10.1 Super Tee Beams

The Super Tee beams were cast on site adjacent a loadout facility on the southern riverbank as shown on Figure 26. A total of three single beam casting beds were established and serviced by purpose made 50tonne capacity motorised gantries.

Each girder weighed close to 70tonne and the yard was set up to produce one unit per day with a concrete demand of 30m3 . Girders were treated with a wax emulsion curing compound and stripped after 24 hours before being taken to storage.

10.2 Deck Structure

The approach deck structure was constructed span by span using a simple lifting truss mounted on the pier head stocks.

Girders were transported by road into location for erection using a prime mover with a steerable rear bogie. This system worked well even though half of the girders had to be transported to the other side of the river by barge as shown in Figure 27.

Figure 26 — Casting yard & erection truss Figure 27 — Transporting girders by barge

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Once lifted, the girders were moved laterally into final position and placed directly onto the permanent elastomeric bearings without the need for temporary support, by virtue of the inherent stability of the halving joint configuration. Up to six girders were erected each day, depending on pier height.

Deck concrete was placed following girder erection, with the infill section over each pier left until all spans were completed to control shrinkage movements.

11 PROJECT IMPLEMENTATION

The project was implemented over a period of 6 years from the beginning of the Feasibility Study to the completion of the Construction Contract, as shown in Figure 28

1994 I 1995

1996

1997 ; 1998 1999

Feasibility Study Detailed Design Construction

Towers Cable Stay Bridge ApplaaCh &fate Approach Enira Art e nts

Smith

Nrih

South

Nath

South rth

Figure 28 — Project implementation timeframe

The entire project was undertaken within the requirements of the Memorandum of Understanding between the two Governments which called for regular liaison meetings at both Project Coordination and Joint Steering Committee levels. Both meetings involved representatives from the two Governments, the Engineer and Contractor, and was instrumental in achieving a successful project implementation.

12 CONCLUSION

The My Thuan Bridge is Vietnam's first cable stayed structure and is the largest span crossing in the country. It is also the Australian Governments single biggest international aid project.

The bridge design embodies the latest in cable stay technology and was constructed ahead of schedule and under budget.

It is a showpiece of Australian engineering expertise which has established a new benchmark in cost effective innovative bridge design and construction in Asia, and is set to transform the lives of the many people who inhabit the vast delta region of Southern Vietnam.

13 ACKNOWLEDGEMENTS

The author gratefully acknowledges permission to publish this paper by Mr Laurie Engel, Assistant Director General, AusAID and the helpful assistance given by Baulderstone Hornibrook.

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