Phu My Bridge

Structural Engineering International 3/2010 Technical Report 331

Construction Engineering of Phu My Cable-Stayed Bridge, VietnamGeorge Moir, Design Manager, Bilfinger Berger Civil; Ho Chi Minh City, Vietnam; Colin Edmonds, Principal, Cardno, Sydney,

Australia; Peter Walser, Project Manager and Martin Romberg, Structural Engineer; LAP, Stuttgart, Germany.

Contact: [email protected]

further downstream within the next 10 years, generates a considerable amount of heavy-goods traffic. In addition, a significant amount of goods from manufacturing industries in the Mekong Delta also travel up through HCMC. This traffic currently crosses the Saigon River at the most southerly bridge, the Saigon Bridge, located in the central district of HCMC. A reduc-tion of the traffic congestion in the centre of the city is necessary to ensure efficient transportation conditions and will be realized by the creation of an inner ring road, including the large Phu My Bridge over the Saigon River, linking the Southeast quadrant to the rest of the city.

An Engineering, Procurement and Construction (EPC) contract was let for the construction of the Phu My Bridge. The project involved the design and construction of a 2101 m long viaduct including a 705 m long cable-stayed main bridge with a clear span of 380 m, as well as approach viaduct structures on either side of the river, being 758 m on District 7 and 638 m on District 2.

Design of the permanent works was split into two packages, the main bridge design and that of the approach structures. A separate design pack-age was then created for the design of the main bridge - temporary work and construction engineering for the main bridge. This paper focuses on the construction engineering aspects, particularly the work carried out prior to construction and the ongoing role of the construction engineer during construction.

Description of the Main Bridge

The main bridge is a three span cable-stayed bridge. The spans are 380 m for central span and 162,50 m for each back span (Fig. 2). The total height of the pylons is 134,50 m above pile caps’ levels and 93 m above deck level. Clearance to river traffic is provided with a minimum 45 m vertical clear-ance at high tide, over a 250 m wide zone at the centre of the bridge.

Substructure

Each main bridge pylon is founded on two groups of 14 piles with 2,05 m diameter, and up to 80 m depth (Fig. 3). The underlying geology is of alluvial deposits of alternating clay and sand layers. The pile toe levels are within a layer of dense sand.

Each pile cap is composed of two sec-tions of reinforced mass concrete cap-ping – each group of 14 piles. These are locked together by means of a two-cell box girder.

The pile cap construction was carried out in phases within a shell of precast concrete permanent formwork ele-ments. These elements were fixed to the steel pile casings and allowed con-struction work to be carried out within a dry working area. The lateral form-work of the pile cap is made of precast skirting panels, which extend below water level.

Towers

The concrete pylons are H shaped. Each leg is a box section, with outer dimension varying from 5,50 m × 7 m to 3 m × 5 m. The stay anchorages are located inside the upper legs and are in a vertical plane. The legs are linked by two cross-beams, the lower of which was precast before being lifted into its final position (Fig. 4).

Tie-Down Piers

The tie-down piers are located at the end of each cable-stayed back span. They are twin rectangular concrete columns – with vertical prestress, supported on large-diameter bored piles. The last three stay cables on the back span are connected at the tie-down pier location. The deck slab is of solid concrete and is prestressed in both the longitudinal and transverse directions.

Superstructure

The total width of the main span deck is 27 m; this incorporates three lanes of traffic in each direction, two car and truck lanes, a separated motorcycle

Abstract

The Phu My Bridge spans the Saigon River between Districts 7 and 2 of Ho Chi Minh City (HCMC). The bridge forms part of a new ring road currently under construction around HCMC. The ring road will be an important transport link from the southern Mekong Delta region to the central and northern parts of Vietnam.

The Phu My Bridge contract includes the design and construction of a 705 m long cable-stayed main bridge with a clear span of 380 m, as well as the approach structures on either side being approximately 758 m on District 7 and 638 m on District 2.

The main span deck is 27 m wide. Clearance to river traffic is provided with a 45 m vertical clearance in a 250 mwide zone. The pylons carrying the deck are designed as an H frame and are approximately 140 m high.

The bridge was constructed by a joint venture between two companies and was completed in September 2009. This paper describes the construction engineering used during the stage by stage analysis of the bridge and the on-site role to control the cable forces and bridge geometry during construction of the bridge.

Keywords: cable-stayed bridge; con-struction engineering analysis; pylon construction; deck erection.

Introduction

The Phu My Bridge is a newly com-pleted, cable-stayed bridge construc-tion, in Ho Chi Minh City (HCMC), in Vietnam. The bridge spans the Saigon River between Districts 7 and 2 and forms part of a new ring road currently under construction around HCMC (Fig. 1). The ring road will be an impor-tant transport link from the southern Mekong Delta region to the central and northern parts of Vietnam.

The main port, which is located slightly downstream (or south) of the city cen-tre and is forecasted to be relocated

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332 Technical Report Structural Engineering International 3/2010

lane and footways for pedestrians (Fig. 5).

There are two vertical planes of stay cables. The structure of the deck is composed of two longitudinal con-crete girders linked by transverse prestressed concrete cross girders at 5 m centres. The stay cables are con-nected to the longitudinal edge beams by precast anchor pods located at 10 m intervals.

Construction Method

Tower Construction

The towers were built in 4 m lifts using an enclosed jump form and by adopting the following sequence. The

controlling load cases on the tower were caused by high-wind events and the forces created by the inclined geometry (Fig. 6):

– Constructing a 56 m tower leg and installing the temporary lower strut/lifting beam 48 m above the pile cap. The lower strut was used to control the forces and defl ections in the tower leg and also for heavy lifting the lower cross-beam.

– Constructing the lower cross-beam on the pile cap by attaching the pier table falsework.

– Lifting the lower cross-beam and pier table falsework using the lower strut. This heavy lift involved lift-ing the 1200 t assembly 30 m into

position. With the cross-beam in its correct position, casting the in situ concrete stitch thereby securing the lower cross-beam to the tower leg.

– Once the lower cross-beam was con-nected, the pier table had to be cast. This was done in three 10 m stages to complete a 30 m section of deck. Deck construction was started as early as possible in parallel with ongoing tower construction.

– Continuation of building the tower legs and installing the temporary intermediate strut 70 m above the pile cap. The lower strut was removed. The intermediate strut is a needle beam used to control the forces in the tower legs. It is also used to support a catch platform for

110°00'EPEOPLE’S REPUBLIC

OF CHINA

HANOI

Da Nang

CAMBODIA

PROJECT AREA

20°00'N

10°00'N

110°00'E105°00'E

10°00'N

Vinh

Quy Nhon

PR 16NH 13

NH 1ANH 1A

District 9

Phu My Bridge

Huynh Tan PhatHung Vuong

Tan Son NhatAirport

Ho Chi Minh City

VIETNAMPhu my bridge approach roads project

Rach ChiecBridge

District 2

District 7

Kilometers

NH 1K

PR 25

PR 15

PR 34

NH 50NH 1A

NH 10

National capitalCity/Town

Ring road no. 2 (missing sections)

Ring road No. 2 (existing)

Primary road

Flyover (to be constructed)

Flyover (existing)

Western approach roadW

Eastern approach roadE

National highwayNH

Provincial road

River

International boundary

Boundaries are not necessarilyauthoritative.

PR

E

Vung Tau

Nha TrangDa Lat

Ho Chi Minh City

HaiphongGulf ofTonkin

Gulf ofThailand

South China Sea

Dong Nai

Rive

r

LAOPEOPLE’S

DEMOCRATICREPUBLIC

105°00'E

20°00'N

Sei

gon

Riv

e

0 1 2 3 4 5PHU MY BRIDGE

Nguyen

Van

Linh

Fig. 1: Phu My Bridge forms a critical element of the new ring road

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Z = +51,25

Z = +38,375Z = +46,50

Z = +38,375

District 2District 7

Western approachstructure

Navigational clearance

Eastern approachstructure

162,50

−65,00

162,50

190,00 190,00

45,0

0

705,00380,00

250,00

5,20

5 %

Wes

t bea

ring

Wes

t tie

-dow

n pi

er

Eas

t tie

-dow

n pi

er

Tow

er 7

Mid

spa

n

Tow

er 2

−50,00

5,20

−70,00−80,00

5 %

Z = +46,50

Refer. el +0,00 Sai gon river

Fig. 2: Main bridge profile (Units: m)

Fig. 3: Pile group layout and precast form-work

safety during the construction of the upper cross-beam.

– Installation of the temporary upper strut. The upper strut acts as both a brace for the tower legs and as support for the formwork of the upper cross-beam. Construction of the upper cross-beam followed by removal of the intermediate and upper struts and completion of the tower leg construction.

– Installation and stressing the fi rst stays to the pier table section. Releasing and lowering the pier table formwork then lifting of the form travellers into position. At this stage, the geometry of the deck is controlled by the length of the stays more than the force as the deck is free to pivot on the bearing. The stays were installed up to 75% of their installation force. At this stage

a survey was carried out, prior to making a fi nal adjustment to the cable length.

Deck Construction

The following overall construction sequence was used for the main bridge (Fig. 7).

Build Deck Out from D7 Tower

The deck is constructed one segment at a time starting on the river side, by fol-lowing the sequence described in the next section. The sequence between the two sides was partially offset to reduce the out of balance moments and speed up the construction, that is, the travel-ler on the river side was launched to the next segment prior to casting the land side segment.

Install Buffeting Cables

After seven segments are cast on both sides the buffeting cables are installed. Only the land side is tensioned. The river side is kept slack unless high winds occur (greater than 20 m/s gust at 10 m elevation). This limits the bending moments in the tower at the deck level.

Install Temporary Tie-Down Cables

After 11 segments, the temporary tie-down cables are installed on the land side. The land side buffeting cables are released and the river side is kept under tension. This limits the bending moments in the whole tower.

D7 Side Span Closure

After 15 segments are cast, the 1,5 m long side span closure is cast. A survey is carried out prior to the closure and the deck is positioned by releasing the temporary tie-down cables. The pour is carried out in the evening to limit the effects of temperature on the new concrete.

Move Travellers and Repeat for D2 Deck

After the side span closure is complete, the three remaining river side segments are cast and the travellers are moved to the D2 side to repeat the process.

Main Span Closure

The 10 m main span closure is cast using one of the travellers. A full sur-vey and adjustment of the stay cables are carried out prior to casting. After casting, the SDL loads are placed and any further stay adjustments if neces-sary are carried out.

Deck Segment Construction Cycle

The deck is constructed in bal-anced cantilever starting from each tower. The 10 m long segments are cast using a form traveller (Fig. 8). The typical erection sequence is as follows:

– Casting of new segment. On the morning after casting, carrying out an “as-built” survey.

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– Moving the rolling beam and stress-ing of front cross girder. Required concrete strength is 30 MPa.

– Destressing and removing stress bars at anchor pods and restressing the stay cable. Required concrete strength is 30 MPa.

– Removing the longitudinal strut. Lowering the traveller and stressing of the rear cross girder. Required concrete strength is 35 MPa.

– Launching the traveller halfway towards the new position.

– Installation of precast anchor pods during the launch.

– Completing the launch and raising the traveller. Surveying the position

of the traveller and adjusting align-ment if required.

– Placing the precast cross girders and edge beam reinforcement cages. Required concrete strength is 40 MPa.

– Starting installation of deck rein-forcement. Up to 25 t of rebar could be placed before stay installation.

– Installation of longitudinal struts between anchor pods and the previ-ous segment.

– Installation and stressing of stay cable. The stay cable is attached to the anchor pod which is in turn con-nected to the traveller using stress bars. The horizontal component of

7,000

5,50

3,00

3,0003,000

3,00

Lower crossbeam

Bridge deck

Cables

Upper cross beam

5,50 33,921

44,921

28,95734,957

20,300

26,300

∇ Referencelevel +0,00

5,000

CL

of t

ower

Stay N-1Stay N-1

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ower

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,715

(Upp

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gs)

37,4

00(T

ower

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Stay N-18 Stay N-18

10,00 10,00

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10,00 10,00 10,00 10,00

Fig. 4: Pylon layout and lower cross-beam heavy lift

3,00 0,55

11,65

2 Lanes at 3,75 2 Lanes at 3,75

11,65

7,50 7,50 0,55 3,00 2,150,30 0,302,15

Fig. 5: Deck cross section (Units: m)

Fig. 6: Tower construction

the load is transferred to the pre-vious segment via the longitudinal strut. The vertical load component

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Fig. 7: Deck sequence

Phu my cable stayed bridgedeck construction sequence

Late October 08′

District 7

Stay

7105

Stay

710

2

Stay 7202

Stay 7205

District 2

+1.49

Completion of stage WM05 at 29 Oct. 08′based on August 08′ programme


Mid December 08′

District 7

Stay 7113

Stay 7110

Stay 7210

Stay 7213

ST22

01 ST2101

District 2

+1.49

Completion of stage WM13 at 13 Dec. 08′based on August 08′ programme


Mid March 09′

District 7

Stay

220

5 Stay 2105ST22

02

Stay2102

District 2

+1.49

Completion of stage EM05 at 21 Mar. 09′based on August 08′ programme


Late November 08′

District 7

Stay 71

09

Stay

7106

Stay 7206

Stay 7209

District 2

+1.49

Completion of stage WM09 at 20 Nov. 08′based on August 08 programme


Late January 09′District 7

Stay 7214

Stay 7218

ST22

01

Stay2101

District 2

+1.49

Completion of stage WM18 at 23 Jan. 09′based on August 08′ programme


Early July 09′District 7 District 2

+1.49

Completion of closure at 01 July. 09′based on August 08′ programme

of this fi rst stay force is kept con-stant at about 100 t for each stay, which helps to support the travel-ler during casting of the concrete. Required concrete strength was 45 MPa.

– Completion of deck reinforcement and carrying out pre-pour survey and casting new segment.

After the initial learning phase, this cycle was regularly achieved in five days with the side span cycle lag-ging two days behind the main span cycle.

Construction Engineer Role

Construction Engineer Role—Design Phase

The contractor engaged a design con-sultant to carry out the construction engineering and the design of some of the major temporary works. Prior to commencing on site construction, the construction engineer and the contrac-tor agreed on the following:

– Weights and requirements for major temporary works items.

– The best estimate of the time required for construction, as it was important to accurately determine the time-dependant effects.

– When to install the stay cable.– When to install the temporary tie-

down cables and when and how often to restress.

– When to dismantle the forms and supports at the pier tables for both the tower and the tie-down pier.

– Exact local sequences, for typical segment construction as well as for side span and main span closure procedures.

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Once this was done, the construc-tion engineer modelled the sequence including consideration of the time-dependant effects. To do this, a sophis-ticated stage by stage analysis of the bridge was carried out.

The software program also featured an efficient solution provider that deter-mined the forces to be applied at each stay installation such that the forces at the end of construction were the same as those calculated by the permanent works designer. This was done by the program by solving a set of simulta-neous equations with the constraints being the final stay forces, tower forces and so on. The input variables were the stay stressing forces and other adjust-ments such as jacking forces prior to the closure pours.

Once the stay forces were calculated, the camber curves and support reac-tions could be calculated. The purpose of the analysis was to achieve a clear understanding of the following:

– Exact forces and defl ections of all parts during all erection stages.

– Forces in the buffeting cables and temporary tie-down cables.

– Forces in the temporary falsework at the hold-down pier.

The analysis also calculated adjustment values to account for variations in:

– temperature conditions;– construction live loads.

The erection analysis was based on the results of the permanent works design and took into account the most accu-rate actual parameters, dimensions and weights. While doing this, special care was taken to address the following:

– dead weight, actual concrete density and reinforcement content;

– section properties;– P/T forces;– shrinkage and creep values.

The object of the analysis was to esti-mate as closely as possible both the position of the bridge and the built-in forces provided by the permanent works design, in order to be consist-ent with the final design assumptions. In addition, an erection stress analysis was carried out to ensure that—the structural capacity of all the bridge ele-ments would not be exceeded at any of the individual erection stages.

On the basis of their previous experi-ence in cable-stayed bridges and work-ing in Vietnam, the contractor had a clear view of how to construct the bridge with speed and cost-effectiveness.

Construction Engineer Role—Construction Phase

During construction, the role of the construction engineer included the fol-lowing responsibilities:

– Monitor the survey results.

Fig. 8: Formwork traveller

Rolling beam

Strength45 MPa

Strut

P/T in place

First stressing

(cable sag)

Fig. 9: Balanced cantilever deck construction

– Adjust traveller alignment to create the correct pre-camber.

– Adjust restress force to account for deck loading, temperature and stay forces.

– Advise contractor on possible changes to construction sequence.

Following casting of a deck segment, the following information was obtained the next morning:

– A survey was carried out on the last four segments—on the leading edge plus the tower.

– In addition, a load map was pre-pared based on the major loading on the deck such as reinforcement stock, strand, mobile crane and so on (Fig. 9).

– A lift-off test was carried out on the cable in the new segment to deter-mine the actual stay force.

The survey was compared with the pre-dicted position of the deck and tower for that stage. An adjustment was made for the actual loads on the deck, cable-stayed force and temperature. From this analysis, the construction engineer determined the restress length of the stay cable and the appropriate align-ment values for the next traveller posi-tion. This information was required by the site team within a few hours in order to avoid delay in the construc-tion cycle.

Adjustments to the traveller alignment shims affect the local curvature while changes to the stay force have a greater effect on the overall geometry of the bridge. During construction, the results from the survey were studied to check the accuracy of the analysis. Some of the variables for which sensitivity anal-ysis was carried out included:

– Variation in traveller weight: The design had an assumed traveller weight of 210 t. When it was lifted, the weight of the traveller was found to be 235 t. An update of the analysis was therefore carried out.

– Deck stiffness: The defl ections of the deck depend on the modulus of

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elasticity of the concrete used and also the effective width of the deck section.

– Stay stiffness: The strand used was provided with test certifi cates giving the area and modulus of elasticity.

– Deck weight: The deck weight included in the design allowed for an assumed reinforcement rate. A comparison was carried out with the density of the actual concrete and reinforcement used and the original assumption.

– Traveller stiffness: The traveller was modelled separately using a three dimensional (3D) program with all the members to calculate its stiff-ness. The computed stiffness of the traveller was compared with the true stiffness and analysed from measurements made on-site under actual loading conditions.

During the construction of the bridge deck, it was noted that the deflections of the bridge were greater than those predicted by the model. As noted there are several possible reasons for this occurance; however, one of the more likely reason seemed to be an increase in the traveller and deck weights.

During the erection of the bridge, the stay forces were slightly increased to account for these deflections.

Conclusion

In summary, this paper highlights some of the features used for the speedy con-struction of a cable-stayed bridge.

The project required significant planning and design to agree with and optimize the construction meth-ods. This involved considerable col-laboration between the contractor and the construction engineer and resulted in relatively few issues occurring during the construction process and a rapid achievement of the five day construction cycle with a short “learning process”. The role of the construction engineer both dur-ing the preconstruction and the con-struction phase formed an important part of this project.

A contract duration of 34 months was originally anticipated with the project scheduled to be opened to traffic by the end of 2009. Construction was how-ever completed in 30 months, allowing

the bridge to be opened for traffic in September 2009, (Fig. 10).

Fig. 10: Finished bridge

SEI Data Block

Owner:Phu My Bridge Corporation

Contractor:Bilfinger Berger Baulderstone Hornibrook (BBBH Consortium)

Designers:Arcadis (Permanent Works Main Bridge)Cardno/Golders (Permanent Works Approach Structures)Cardno/Leonhardt, Andrä und Partners (Temp. Works and Const. Eng.)

Employers representative:Maunsell (AECOM) Vietnam

Steel (t): 17 000

Concrete (m³): 100 000

Prestressing steel and stay cables (t): 2000

Estimated cost (USD million): approx. 105

Service date: 2 September 2009

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Phu My Bridge

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