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JKR Seminar, Kuala Lumpur, 27 August 02 Design And Construction Of Pre-stressed Concrete Cable Stayed Bridges

RAMA VIII Bridge in Bangkok: Construction and Stay Cable System

Roy LENGWEILER Eric KAUFMANN Stay Cable Division Research & Development Project Manager JV-BBR Senior Stay Cable Engineer BBR Systems Ltd. BBR Systems Ltd. Switzerland Switzerland

SUMMARY The recently completed Rama VIII Cable Stayed Bridge with its 300m main-span is among the worlds largest asymmetric cable stayed structures with a single pylon. The stay cables, as one of the key structural elements, are certainly the most delicate members of the structure since highly stressed. At the same time, they are the most exposed to climatic conditions such as temperature, wind and rain and, by nature, the least accessible of all the elements. During the free cantilever erection of the deck structure the installation of stays is closely interfacing with each single step of the deck erection. Therefore all these requirements have to be reflected in and accounted for in the stay cable technology. Optimized assembly and installation methods are the deciding parameters with regard to achieved quality and have to be in perfect coordination with the deck erection method and in line with the progress expectations and the overall project scheduling targets. The paper describes the stay cable system used on Rama VIII bridge and presents the successfully applied working methods for the assembly, installation and stressing. The installation of the stay cables was carried out strand by strand and for tensioning an innovative single strand stressing method was applied, ensuring that all the strands are stressed to identical force. The vibration problem caused by wind-rain and parametric excitations is addressed by equipping the stay cables with state of the art damping devices. The dampers had been subjected to tests in the laboratory as well as under site conditions and the results are herein presented. Keywords: stay cable system, full scale testing, deck erection sequence, installation and tensioning of

cable stays, anti-vibration damping devices 1. INTRODUCTION The Rama VIII Cable Stayed bridge is the most recent crossing over the Chao Praya River into Central Bangkok and was needed to alleviate the daily traffic congestion at the nearby Pinklao bridge, which was to date narrowing the 10 lanes of traffic on the Thonburi side to 6 lanes. The project was initiated on His Majesty the Kings initiative in commemoration of his elder brother.

Figure 1 : View of the Completed Bridge


The structure of the cable stayed main bridge and the associated viaducts is being constructed under a design-build contract between the Bangkok Metropolitan Administration and the contractors. Despite the recent economic slowdown which Thailand experienced, the project was awarded in 1998 to the CSCEC-PPD-BBR Joint Venture. The Rama VIII bridge project was constructed within 3 years and was opened to traffic at the end of March 2002. The key element of the project is the 475m long cable stayed bridge which is crossing the Chao Praya River with a 300m main span. The longitudinal arrangement is asymmetric and with only one single pylon, which is located on the river bank at Thonburi side, hence placing the bridge into the group of the longest single pylon cable stayed structures which have been realized to date. 2. GENERAL PROJECT DESCRIPTION The 300m main span is supported by means of 28 pairs of stay cables in a semi fan configuration, which are anchored in the single 160m tall tower. The tower is stayed back to the anchor-span by 28 cables in a vertical centre plane and arranged in harp configuration.

The 29m wide deck is a hybrid structure, which utilises a composite steel concrete deck for the main-span, and a multiple span cast in-situ box girder in the back span. The main span stays are anchored in the deck edge beams at typical 10m spacings. The back stays are terminated at the deck level in the longitudinal anchor beam, which is tied down to the anchor span cell structure. 2.1 Tower The inverted Y shape tower consists of two inclined legs and the pylon, which all have hollow box cross-sections. The outer dimensions at the top of pylon are 5m by 7.0m and at the junction of the legs 7.5m by 7.0m, the width in the longitudinal direction being constant with 7.0m. The designer has chosen a quite untypical anchorage arrangement whereby the anchor span and main span stays are anchored in the 2.0m thick riverside wall of the pylon. With this design only nominal transverse post-tensioning is required to resist the moderate bending in the main span wall, whereas the box section provides the required vertical stiffness for the tall tower.

Figure 2: General Arrangement


2.2 Deck Structure The composite main span consists of a steel grid and a concrete deck slab. The edge girders, floor beams and sidewalk elements form the 10m long steel grid units, which weigh approx. 50mt each. The deck slab is composed of pre-cast deck panels, each weighing 30mt. The panels are interconnected and connected with the steel grid for composite action by means of in-situ stitching concrete along the floor- and edge-beams. For aerodynamic and also esthetical reasons the underside of the main span deck section is enclosed by a system of fiber reinforced lightweight panels. The back spans consist of two fifty meter cast in place concrete spans and the anchor span multi-cell structure, where the back stays are anchored. The 75m long solid anchor beam runs longitudinally in the centerline of the roadway, anchoring the 28 back stays. The anchor beam is tied down by post-tensioning to the multiple-cell caisson, which is partly filled with ballast, in order to resist the uplifting forces from the stay cables. 2.3 Stay Cable Arrangement The 28 pairs of main span cable stays are arranged in semi harp shape, with each stay consisting of between 15 to 29 nos. of dia. 0.6 high strength steel strands. The lengths vary from 65m up to 325m. The 28 back stays are in a vertical center plane harp configuration, with a maximum size of 65 strands for the largest stay, which is also the longest back stay with 230m length.

Figures 3 & 4: Cross-Section Pylon, Completed Tower

Figure 7: Stay Cable Arrangement

Figures 5 & 6: Main Span Deck Structure, Back Spans with Anchor Beam


3. STAY CABLE SYSTEM 3.1 Description The chosen stay cable technology is the BBR Cona Stay, which is a multiple strand system and most suited for site assembly and for the strand by the strand installation method. The high-strength 0.6 steel strands are waxed and coated with a protective and tightly extruded PE layer. Each strand is individually

secured in the anchor-head by high quality three-piece stay cable wedges. The anchor head is threaded and equipped with an adjustable lock-nut, both manufactured of quenched and tempered carbon steel. The lock-nut transfers the stay force to the bearing plate. The so-called guide pipes consisting of bearing plate and steel pipe accommodate the stay cable anchorages, and are either pre-installed into the concrete structure or integrated into the steel deck structure respectively. The stay cable anchorage is composed of the anchor-head and anchorage transition.

The latter having the function to bundle the strands to a tight pattern at the exit of the transition part. This is achieved by having each individual strand pass through a guide tube. The space between the guide tubes and the transition is filled with grout. This technology allows to assemble the anchorage parts off site in a controlled environment and to pre-install on site into the already installed guide pipes. Between the pylon and deck anchorage all the strands are running in compact bundle, perfectly parallel within the encapsulating HDPE stay cable sheath. On Rama VIII the HDPE sheath has a yellow HDPE layer which was co-extruded with the black inner HDPE. The stay sheath will remain non-grouted, which allows replacement of

individual, strands if required during the service stage of the bridge. Compared to other projects small sheath diameters were used, which will reduce the risk of the undesirable and noisy sheath vibrations under wind. The helical ribs on the outer surface of the sheath in combination with the specially designed SYLO dampers in

Figure 8 : BBR CONA STAY System

Figure 9: Anchorage Assembly Figure 10: HDPE Stay Sheath


the vicinity of the anchorages address the wind-rain vibration problem and eliminate the need for secondary ropes, which was an architectural requirement of the client. 3.2 Corrosion Protection The stay cables are protected by four barriers against corrosion: Galvanization, wax and tightly extruded high density polyethylene coating of each individual strand plus the encapsulating HDPE stay sheath of the entire strand bundle. In the area of the anchorage zone, where the strand coating is removed to allow the gripping of wedges, special lithium grease is injected at each strand to fill the space between strand and guide tubes. The anchor head, wedges and strand stick-outs are covered with a protection cap which is filled with the same aforementioned grease. 3.3 Dedicated Anchorage Test The full-scale test, which was dedicated to the Rama VIII project, was carried out in accordance with the PTI Recommendations for Stay Cable Design, Testing and Installation, dated August 1993, clause 6 Test Criteria. The strand specified for the project is seven-wire-strand with a nominal tensile strength of 1770MPa. The dedicated full-scale test of a 73strand specimen took place at the CTL Laboratory in Chicago, with the following parameters: Upper stress level 45% UTS 797MPa 119.5 kN/strand Fatigue stress amplitude 160MPa 24.0 kN/strand Load Cyles / Frequency: 2 Million cycles with 1.0Hz Acceptance criteria: Not more than 2% of the numbers of individual wires may fail, i.e. 2% of 73strands x 7wires = 10 nos. No failure shall occur in the anchor material or any component of the anchorage. In the subsequent tensile test the specimen shall develop a minimum tensile force equal to 95% of

the guaranteed tensile strength of the cable, i.e. 73strands x 95% of 265.5kN/strand= 18412kN. Test Results: The results were extremely satisfactory and above the international standard requirements for stay cable systems. Some of the key parameters are the robust design and the high quality of the BBR Cona stay wedges.

Fatigue Test Result: no wire breakage recorded compared to the allowable number of 10 breakages

Ultimate Tensile Test Result: 19004kN ( = 98% of UTS) achieved compared to minimum required of 18412kN (=95% of UTS). No failure in the anchor material or any of its component was observed.

Figures 11&12: Vertical Test Setup at CTL, Graph Elongation vs. Force


4. STAY CABLE DAMPING DEVICES 4.1 Description Dampers were installed in all stay cables of the RAMA VIII bridge. These included 2 types:

A. Standard neoprene doughnut type for the main span stays between M1 and M10, as well as for the back stays A1 to A28, and

B. Specially developed SYLO dampers for the main span stays between M11 and M28 The length of the respective stay cables varies from 153m (M11) to 325m (M28). Due to the fact that the stay cable arrangement for the main span is, contrary to the back stays, in 2 planes, it was required to install a total of 72 dampers of the above mentioned special development. The SYLO dampers are made of a steel casing consisting of two (2) half shells (see view below), which were bolted around the HDPE pipe of the stay cable inside the guide pipe both in the pylon and in the deck anchorage. The elastomer damping material, consisting of slices, was also cut in half shells and placed between the 2 longitudinal steel webs, thus filling completely the available free space between (inner) front and rear flange. In order to: a. Compensate possible creep of the elastomer

material, and b. Create the maximum possible contact pressure

(in transverse direction) between the elastomer material and the inner area of the guide pipe,

longitudinal pre-stressing was applied by a movable flange (seen just behind the fixed front flange) upon which the end of the bolts were welded. The springs placed between the 2 front flanges were tightened before the elastomer material was placed around the steel casing. Then the elastomer material was placed between the inner front flange and the rear flange. Afterwards the completed assembly was placed inside the guide pipe. The next step was to release the bolts, upon which the stored energy in the springs was transferred by the inner front flange to the elastomer, thus compressing it. Due to the reduced longitudinal space, it reacted by deforming transversally until it made contact with the inner face of the guide pipe. Thus when transverse vibration occurs, energy can be dissipated in the contact area. It was decided to consider the M23 stay cable, with a length of 269m, as typical for calculating the damping properties. According to the designers requirement taking into account the local project conditions, the required equivalent viscous damping ratio for this stay cable shall be 0.34%, which corresponds to a logarithmic damping ratio of 2%.

Figure 13: SYLO Damper Assembly

Figure 14: Installed SYLO Damper


4.2 Laboratory Testing The above described assembly was successfully fatigue tested in the laboratory of the manufacturer of the elastomer material for up to 2.08 Million cycles. Testing was executed in 5 stages, due to the fact that the testing machine was not available for a continuous operation of 2 X106 cycles.

It was calculated that in order to achieve the above mentioned damping ratio, a dissipated energy of 34Nm was necessary. The peaks in the above presentation indicate the start of each new loading cycle. Noteworthy is the recovering capacity of the material during periods of rest, which corresponds very well to the expected behavior of the stay cable in reality. Under standard conditions, i.e. constant amplitude and frequency, a high equivalent viscous damping ratio of 10% was achieved. 4.3 Site Tests In order to verify the actual performance of the SYLO dampers, on-site tests were conducted on stay cables with the dampers installed. These tests included: a. Determination of the stay cable internal damping, i.e, with

no damper installed, b. With only one SYLO damper installed, and finally c. With both SYLO dampers installed at top and bottom. These tests were undertaken by the EMPA (Swiss Federal Laboratories for Materials Testing and Research) and included the installation of 3 pairs of accelerometers in various positions along the chosen Stay Cables. These were M13 (171m), M23 (269m) and M26 (300m) A predetermined vertical deflection, varying between 7.5cm and 20cm at around 40m away from the lower anchorage point, was statically induced by means of tirfors. The respective stay cable was suddenly released and the resulting acceleration was electronically recorded. The results obtained showed a scatter depending on the fact that the internal strands and the HDPE pipe were excited in phase for only a short period of time. Thus the values obtained for the viscous damping ratio were in the range of 0.25% to 0.55%.

Linear Presentation Dissipated Energy versus Cycles

0

1020

30

4050

60

0 500000 1000000 1500000 2000000 2500000Cycles

Dis

sipa

ted

Ener

gy [N

m]

Test 1 from 21 - 22.12.00 Test 2 from 08 - 12.02.01 Test 3 from 15 - 19.02.01Test 4 from 02 - 05.03.01 Test 5 from 09 - 12.03.01

Figure 15: Graph dissipated Energy versus Cycles

Figure 16: Views of the Installed 3 Accelerometer pairs


5. STAY CABLE INSTALLATION 5.1 General Deck Erection Sequence The main span was constructed by the free cantilever method and the erection work was subdivided in typical erection cycles. One erection cycle is resulting in a cantilever progress of 10m completed deck section and entails the installation of one deck segment, one back stay and one pair of main span stay cables. The latter support the newly added deck portion. Each typical cycle was split into more than 30 working operations, of which 15 are directly related to the stay cable works. All these operations are inseparably interrelated, either in strict sequence

or as parallel activities. In order to optimize the cycle duration, the deck erection contractor, the stay cable contractor and the construction engineer worked out an optimized sequence by maximizing the number of parallel operations and minimizing the number of sequential activities. In addition there were hold-points in the cycle related to day time, such as the early morning survey for the global geometry control and the late afternoon hours for placing the in-situ stitching concrete. Heavy rainfalls during the monsoon season had to be considered as some activities could not be carried out under rain. The good cooperation and coordination of the involved parties allowed for 3 day cycles, which resulted in a cantilever progress of 90m during the peak month. The complete 300m main span deck with the 84 stay cables was installed in 5 months only. The attached schematic shows the five main construction stages of a typical deck erection cycle, and highlights the main operations on the stay cables.

STAGE 1: 1.1 Installation and Stressing to

Final Force of Anchor Span Stay A(i+1)

STAGE 2: 2.1 Main Span : Lifting and

Bolting of Steel Girder G(i)

STAGE 3: 3.1 Installation of Main Span

Stays M(i), Strands Part1 (Stop strand installation when force drops below 20kN per strand)

STAGE 4: 4.1 Lifting of 4nos Deck Panels 4.2 Stays M(i), Install Strands Part2 4.3 Adjust Stays M(i) to Force

STAGE 5: 5.1 Stitching Pour between the

Deck Panels 5.2 Curing of Stitching Concrete

to 15MPa 5.3 Adjust Stays M(i) to Final Length

back to stage 1 for next cycle (i+1)

Figure 17: Typical Deck General Erection Cycle

Figures 18, 19, 20 & 21, from top down: - Lifting of Girder (Stage 2) - Install. of Main Stays (Stage 3) - Lifting of Deck Panels (Stage 4) - Stitching Pour (Stage 5)


5.2 Installation and Stressing of Stay Cables The stay cables are site assembled, maximising the local content of materials and employment of local working force, without compromising the quality of the end product.

By using an innovative stressing method, all the strands for a particular stay cable were cut to identical length. The accuracy to cut to an absolute length was 1/10000 of the total length, whereby and more importantly the length difference between the individual strands of the same stay was less than 4mm, resulting in a variation of 1/50000 on a 200m long strand.

The strand installation into the pre-erected and rather snug HDPE pipes is carried out strand by strand, utilising custom made high speed pulling equipment. The strands are tensioned exactly to predetermined stick-out length using single strand stressing jacks. Experience showed that with the adopted stressing method the stays could be tensioned to the exact predicted load, and it was further demonstrated that the strand forces are equalized within a range of 1.5%, compared to the PTI standards which allow up to max. 2.5%. This confirmed that for the strand by strand installation the stressing to length is accurate and superior to the commonly used method of stressing to equal force by means of a load cell on a master-strand. The reasons for it had been examined in a theoretical simulation, which was in agreement with the findings from site.

H W L.

P 40LLP 41P 42LP 45L

12

34

5

6

7

BB

M11

A11

INSTALLATION STEPS FOR STAY CABLES

1) Install pre-assembled anchor bodies into top and bottom guide pipes.

2) Lift HDPE incl. installation strands with tower crane.

3) Insert installation strands into pylon anchorage and secure by wedges, secure HDPE to the pylon face.

4) Connect deck winch at bottom of HDPE and pull towards guide pipe.

5) Insert installation strands into deck anchorage and secure by wedges.

6) Pull installation strand by mono-jack at pylon anchorage. Sag will come out.

7) Installation and stressing of strand by strand.

Figure 22: Stay Cable Installation Sequence

Figures 23, 24 & 25: Strand Pre-Cutting, Lifting of Stay Sheath, Installed Stay Pipe

Figure 26: Strand by Strand Installation


Despite initial concerns from various parties regarding the stay installation duration, it was proven that the stay cable works were never critical for the progress. The erection cycle for a 10m long deck segment was reduced to 3 days only, which could only be achieved due to the optimized working methods, custom made temporary works such as pylon platforms, selected equipment, the detailed co-ordination between the parties and the knowledge/experience available for the management of the works. 5.3 Final Tuning The final tuning operation was limited to only 34 numbers of main span stays. Thanks to the close cooperation during deck erection with the party responsible for the construction analysis and geometry control, release operations using large multi strand jacks could be avoided and only positive adjustments had to be made. Although such equipment with all the accessories was readily available on site, the good accuracy of the single strand tensioning proven during the erection stage resulted in the Engineers and Designers approval of the Contractors request to carry out the final adjustment by mono-jacks. 6. CONCLUSION The 300m long main span was constructed in free cantilever erection within only 5 months. This achievement has proven the suitability of the structural design, the construction methods and the chosen stay cable technology, which has been adopted for the realization of the prestigious Rama VIII bridge. The project is an excellent example that a carefully evaluated stay cable system in combination with the use of advanced methods for the stay cable installation are inseparable and essential for a rapid deck erection progress.

The experience gleaned from Rama VIII bridge with the very long stay cables (max. length 325m) encapsulated in a complete and small diameter HDPE pipe sheath, confirms that the described BBR Cona Stay cable system is particularly suitable for long span bridges. The requirements on stay cables with regard to quality and erection speed for future record breaking bridges with 1000m main-spans and above can be realised with the herein presented technology.

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