CESB 07 PRAGUE Conference

Session XXX: xxxxxxxxxxxxxxxxxxxx

Construction of the bridge over Rybny creek

(passport photograph of an author) / (passport photograph of an author) / (passport photograph of an author) / (passport photograph of an author) /
Jan L. Vítek / Alexandr Tvrz / Jiří Bešta / Pavel Smíšek
Summary

A wide freeway bridge was erected by the incremental launching method. The original design was changed in order to satisfy the requirements for a short construction time. Many innovative technologies were tested and applied to satisfy the conditions of the client. Although the structure is rather thin-walled, the complete more than 30m wide bridge deck belongs to the heaviest launched bridges in the CzechRepublic and possibly inEurope.

Keywords:Bridge, concrete, hydraulic units, incremental launching, concrete, hydraulic units, incremental launching, prestressing

1Introduction

The bridge across the Rybny Creek is being built on the Freeway D8 from Prague to Dresdenthat crosses the mountains forming a border between the CzechRepublic and Germany. The bridge that is situated up to 52m above the terrain is formed by anincrementally launched continuous box girder of seven spans of length from 34 to 58m. The bridge of the width of 31m is being built by Metrostav, a.s., a subcontractor of the Joint Venture 7/II H, that is formed by firms STRABAG, a.s., Beroun and SKANSKA DS, a.s., Brno, Czech Republic. VSL Systémy (CZ) s.r.o. participates on the project as asupplier of post-tensioning and incremental launching technology concept and realization.

In a bid project the crossing was formed by a traditional twin box girder bridge. Since each of the bridges was assumed to be erected by incremental launching technique, the box girder had relatively large depth – 4.20m.

Due to the severe winter, the construction seasons are very short at the bridge site. Since the bridge has to be built within two years, the both bridges would have to be built simultaneously. That means that it would be necessary to use two forms and to procure two launching equipments and two launching noses. This would be rather uneconomical. Therefore finally after a close co-operation of Metrostav, a.s., and design office Strasky, Husty and Partners, the client agreed a proposed solution, when two bridges were replaced with one wide superstructure. The span lengths and the depth of the cross-section remained unchanged.

2Structural design

The bridge is curved horizontally and vertically. In order to make the launching technology possible, the curve was replaced by a circle segment lying in incline plane. The bridge is formed by acontinuous box girder of seven spans with span lengths of 34+48+54+58+58+58+44m (Fig. 1). The width of the bridge is 31.10m; the width of the deck is 30.50m. The deck is formed by a relatively narrow box girder with large overhangs (Fig. 2) that was incrementally launched uphill from the abutment No.80.

Fig. 1Elevation of the bridge

The beam is supported by narrow piers of the Ishape cross section. On intermediate supports No.30, 40, 50 and 60 the girder is supported by concrete hinges, on the piers No.20 and 70 the beam is supported by a couple of the unidirectional pot bearings. Since the portion of the torsional moment that originates from the live load situated on one longitudinal half of the bridge is partially transferred into the end abutments, the box girder is indirectly supported by wide end cross beams.

Several categories of tendons, corresponding to individual stages of the construction process, were used (Fig. 4):

▪Tendons located at the top surface, applied during cantilever erection stage; part of these tendons (located nearby supports) is anchored at the bottom surface of the beam

▪Tendons located at the bottom surface of the middle (main) span

▪Tendons located at the bottom surface of the first and third spans

▪Tendons located at the top surface of the first and third spans

▪Tendons located at the top surface over internal supports, applied at the time when box girder cantilevers are joined continuously at their ends to form the final structural system.

3Construction of the bridge

Construction of the bridge was influenced by many external phenomena. The time schedule was seriously delayed due to many objections in the stage of the approval of the project. Finally the construction started two years later, however, the completion was moved only by one year. It happened that the time available was only 2 years instead of 3years. The design of the bridges was modified, so that the requirement of the shorter construction time could be satisfied. The two separate bridges, each for two lanes were replaced by a single wide superstructure for four traffic lanes. The bridge superstructure, more than 30m wide was rather heavy (almost 20,000 t) and very demanding in terms of the accuracy of execution. The only main issues are mentioned in this paper due to the lack of the space. The more detailed information could be found in [1] or in [2].

3.1Piers

After completion of the foundations, the formwork for the first pier was assembled. The high speed of construction led to the decision to apply a slip form, which allows for acontinuous casting of the pier. Such formwork was not used in bridge construction for years, since the quality of concrete did not satisfy the hard requirements of the highway administration. The slip form was finally delivered by the Czech company Omega Teplotechna a.s. in co-operation with Austrian company Gleitbau Salzburg, GmbH. Prior to casting of piers, it was necessary to verify concrete composition, function of the formwork and quality of the surface, so that any possible poor quality was avoided.

Steel reinforcement of the piers became a major problem. Dense longitudinal and transversal reinforcement was necessary due to the slenderness of the piers. Individual bars had to be installed during the continuous movement of the formwork. Excellent co-operation of consultant, supplier of the formwork and contractor resulted in a problem free execution of piers with the speed of casting 5 m in 24 hours.

3.2Bridge deck

When using an incremental launching technology, the bridge deck is usually cast in aformwork located behind the abutment. In case of the bridge over Rybny Creek, various alternatives were considered. At some bridges the only box girder were launched and the cantilevers were cast after completion of launching using a casting carriage. After evaluation of the speed of construction, costs and local climatic conditions, it was decided to cast a complete bridge superstructure in the casting yard behind the northern abutment, however, in two phases. The phase 1 included casting of the bottom slab and of the walls of the cross-section in the yard 1. After this part was longitudinally prestressed, it moved into the yard 2 (closer to the abutment), where the precast struts supporting long cantilevers were installed and then the complete top slab of the cross section was cast (phase 2). Such process was rather advantageous. It was possible to work contemporarily at both parts of the cross-section which allowed for more people to work and shorten the time necessary mainly for installation of reinforcement and for manipulation with formwork. The reinforcement of both parts was rather extensive, involving both, the mild steel reinforcement and the prestressing tendons. The standard segment of the bridge was 30 m long. All the formwork was delivered by PERI.

Fig. 2Average pulse velocity in m/s

3.2.1Launching nose and auxiliary elements

During launching of the bridge superstructure the high negative bending moments have to be reduced. There are different possibilities; the most often a steel nose is used. In this case the steel nose is formed by a couple of welded I shape beams. When the nose approaches the next support, large tensile forces are induced in the bottom flange at the location where the nose is attached to the concrete superstructure. These forces are transferred from the nose to the concrete girder by means of prestressing tendons and prestressing bars. When the forces are too high, as it is in our case, the number of tendons and bars is increasing and they cannot be located in the web of the concrete girder. It was necessary to transfer the tensile force also into the bottom slab of the box girder. Therefore a steel membrane (Fig. 3) was designed in the bottom level of the main beams of the nose, which is anchored to the bottom slab of the concrete box girder with prestressing tendons.

A number of additional steel elements had to be produced like the hydraulic sliding supports at the casting yard, sliding bearings at the piers and abutments, steel structure of lateral guiding, steel pin transferring the launching force from the launching device to the concrete girder, and others. These elements were designed and produced for this bridge, although it is expected that they will be applied in the future for the next bridge. Their design was based on the technical data specified by the Technical board, which was responsible for the preparation and realization of the bridge.

3.2.2Launching of the bridge

The launching equipment was designed so that the safety and versatility during all stages of the launching was guaranteed. The hydraulic units with towing cables and anchoring device on the bridge deck were considered as the most suitable solution in the site conditions. It might be even more convenient to launch the bridge from the opposite higher end of the bridge, but it was not possible due to the lack of space behind that abutment. Initially it was thought to launch the bridge completely with the support diaphragms and deviators. Then it was decided to simplify the casting procedure and first to launch the bridge deck and to cast the support diaphragms and deviators later, after the launching is finished.

Ultrasonic pulse velocity test is a long established, non-destructive test method. The principle of this test is that velocity of sound in a solid material (V) is a function of square root of the ratio of its modulus of elasticity (E) to its density (ρ). According to the Polish standards [2] the equation is as follows:

(1)

The launching device was composed of four hydraulic jacks VSL SLU 330 (Fig. 4). Each of them was equipped with 31 strands attached by means of vertical steel pins into the bridge deck. The launching units were attached to the lower abutment of the bridge using steel cantilevers, which were anchored by prestressed bars. Until about 60% of the bridge deck was launched, two units were sufficient and then all four units had to be used. Hydraulic pumps were designed for the speed of launching 6 m/h, the complete time necessary for launching of 1 segment 30 m long was in the range from 6 to 8 hours. All activities on the site were organised efficiently, which allowed for a production of one segment in 10 days, i.e. 3 m of the complete bridge deck were produced every day.

Fig. 3Sliding bearings on the pier / Fig. 4Launching hydraulic units

3.3Support diaphragms and deviators

Support diaphragms and deviators were cast after completion of the launching. There were two reasons for this procedure: i) The savings of the weight in the stage of launching and ii) the simpler formwork in the casting yard 1. A limited space in the bridge box required to apply a self-compacting concrete for these internal structures. A special concrete mix was developed to satisfy the requirements of the client in terms of the durability and resistance to the environmental effects. The casting procedure was verified on the model of a half of the diaphragm. The long distance (more than 180 m) for concrete pumps induced additional requirements for the concrete mix.

Tab. 1Tendon arrangement

Tendon category / Total number of tendons / Unfavorable tendons
Number / [%]
A / 80 / 14 / 18
B / 12 / 1 / 8
C / 8 / 8 / 100
D / 4 / 0 / 0
Total / 104 / 23 / 22

4Conclusions

The bridge represents one of the successful projects built recently in the Czech Republic. The favourable aesthetical appearance and the speed of construction proved that the selected design was correct. There were many innovative solutions applied within the construction process. Renaissance of the application of slip formwork for bridge piers, using a two stage casting yard for rapid production of the bridge deck, launching of acomplete wide box girder and application of self-compacting concrete for important bridge structure belong to major achievements in bridge industry during last few years.

The project became an example of a successful co-operation among the client, consultant, and a number of contractors and subcontractors. It has been shown that a good understanding of all participating companies resulted in construction of a bridge of the excellent quality in a very short time and at reasonable costs.

During the construction of the bridge some results of the research project No. 103/06/1627 supported by the GrantAgency of the Czech Republicwere applied. This support is gratefully appreciated.

References

[1]Vítek, J.L., Stráský, J., Brož, R., Tvrz, A., Smíšek, P., Ševčík, P.Bridge over Rybny Creek. Czech national report at the 2nd fib Congress, Naples 2006, ČBS June 2006, pp.12-19.

[2]Vítek, J.L., Stráský, J., Brož, R.Bridge over Rybny Creek. Proc. of the 2nd fib Congress, Naples June 2006.

Prof. Jan L. Vítek, Ph.D., C.Eng.
Metrostav, a. s.
Koželužská 2246
180 00 Prague 8, CzechRepublic
+420266709 317
+420266709 193

URL / Pavel Smíšek, C.Eng.
VSL Systémy (CZ), s. r. o.
Kříženeckého nám. 322
152 53 Prague 5, CzechRepublic
+420267072 427
+420267072 406

URL
Alexandr Tvrz, Bc.
Metrostav, a. s., Division 5
Na Zatlance 13,
150 00 Prague 5, CzechRepublic
+420296246 404
+420296246 405

URL

1