CONSTRUCTION OF ROAD BRIDGE OVER THE SAVA RIVER NEAR OSTRUŽNICA

Present road bridge over the Sava river near Ostružnica is located near Belgrade, in the Republic of Serbia. It is constructed within the first phase of the south-western bypass around Belgrade. The increase of traffic, especially transit, resulted in the necessity of increasing the capacities of the bypass around Belgrade, to relieve the traffic load on the highway that runs through the city centre.


INTRODUCTION
Present r oad bridge over the Sava river near Ostružnica is located near Belgrade, in the Republic of Serbia. It is constructed within the first phase of the south-western bypass around Belgrade. The increase of traffic, especially transit, resulted in the necessity of increasing the capacities of the bypass around Belgrade, to relieve the traffic load on the highway that runs through the city centre. Fig. 1
Construction of the bypass around Belgrade began in 1991, and the first phase included a semi-profile (right lane) of the highway from Dobanovci to Bubanj Potok, intended to be used for two-way traffic until the completion of the second phase. Due to known circumstances (international community sanctions, hyperinflation, warfare, NATO bombing, etc.), construction of the first phase was repeatedly interrupted, so that the first phase on the section from Dobanovci to Resnik (Sectors 1-5) was completed in May 2012. Currently, both phases are completed on the section from Dobanovci to Ostružnica (Sections 1 and 2), the works on the second phase of the bridge over the Sava River near Ostružnica (Sector 3) are in the final phase, and it is planned that the remaining works on the bypass (Sections 4, 5 and 6) will be completed by the end of 2021.
The construction of the existing (right lane) bridge had begun in 1991 and lasted until 1999 due to the above circumstances. Shortly before its opening for traffic, on April 28 th , 1999, the bridge was partially demolished during the NATO bombing, and by 2004 it had been completely rebuilt and put into service. The left lane bridge, whose construction is in progress, is constructively identical to the existing one, since the first phase of construction included river piers for both bridge structures. Construction of this structure began in July 2016.

BRIEF DESCRIPTION OF BRIDGE STRUCTURE
Total length of the bridge is 1.963 m and it comprises 4 sections: prestressed concrete approach structures on the left riverside (L=592,30 m) and right riverside (L=699,13 m), reinforced concrete structure over Belgrade-Obrenovac highway (L= 85,30 m) and steel structure over the Sava river (L=586,00 m). Steel structure over the Sava River is 5-span continuous steel girder, with box cross section of variable height, from 3,8 m to 7,9 m and with the longest span of 198 m. The total weight of the steel structure is 4250 tons. Fig. 2. Layout of the steel structure over the river

METHODOLOGY OF STEEL STRUCTURE INSTALLATION
The designed installation methodology envisaged a cantilevered construction using a floating crane and temporary supports -steel piles driven into the river bed. Therefore, the construction is divided into 56 mounting sections, 7.5 -11.0 m long and weighing 60-85 tons. After the high-resolution magnetic survey of the riverbed, it was discovered that there were a large number of unknown steel objects that could have been unexploded biti neeksplodirana ubojna sredstva (NUS). Zbog toga se projektovana tehnologija montaže pokazala visoko-ordnance (UXO). Therefore, the designed installation methodology turned out to be high risky since the rizičnom pošto bi moguća NUS mogla biti aktivirana tokom pobijanja čeličnih šipova u rečno korito.
In order to avoid the placement of temporary supports in the riverbed, various alternatives were considered, subject to condition that the already executed parts of the structure are used as far as possible. After considering possible alternatives, new methodology that includes (I) incremental launching over floating pier of the bridge (section previously preassembled on the riverside L=187 m) and (II) lifting of 3 structure sections (L=77 m, G=550 tons) pre-assembled in the shipyard 10 km far from the bridge location, launched into river and transported to the site were selected as optimal in current conditions.

(I) Incremental launching
Bridge section between piers S4 and S6 (L=187 m) was installed by incremental launching since the crosssection is of constant height. This section of the bridge is preassembled on the launching platform located on the riverside and then pushed over the river by hydraulic jacks until reaching final position. This operation is divided in four phases: Da bi se omogućilo podizanje segmenata konstrukcije koji su predmontirani u brodogradilištu i transportovani na gradilište rekom, potrebno je formirati osnovu sa koje će se raditi podizanje. Zbog toga je izvršena slobodna konzolna montaža najpre baznih (oslonačkih) segmenata iznad stubova S1, S2, S3 i S4, a nakon toga i susednih segmenata.
(II) Cantilever assembly of structure segments over river piers S1, S2, S3 and S4 In order to allow the lifting of segments of structure that are pre-assembled in the shipyard and transported to the site by the river, it was necessary to form the basis from which the lifting is to be carried out. Therefore, free cantilever assembly of the base (support) segments above the piers S1, S2, S3 and S4, and subsequently of adjacent segments was performed.
In order to ensure the stability of the steel structure elements at this stage of assembly, it was necessary to construct and install auxiliary tools -temporary supports above the piers S2, S3 and S4. In addition to providing stability, these temporary supports take over some of the impacts during the lift phase and transfer them to the piers.

(III) Lifting of 3 structure sections
The rest of the steel structure was installed by lifting of 3 structure sections (L=77/68 m, G=550/450 tons) previously assembled in the shipyard 10 km far from the bridge location. Additional elements for providing water tightness and navigability were installed before launching the segments into the river. Transport to the site was performed by tugboats and lifting was done by "Derrick" cranes and 4 hydraulic jacks, each capacity of 200 t. Slika 9. Rečni transport sekcija konstrukcije mosta Fig. 9. River transport of structure sections Slika 10. Podizanje segmenta S1-S2 Fig. 10. Lifting of segment S1 -S2

INFORMATION ABOUT PROJECT, EMPLOYER, DESIGNER, CONTRACTOR AND SUPERVISION
In addition to the construction of bridge structures, the scope of project also includes a road section -the left lane of the highway 955m long, as well as all supporting works (collection and purification of atmospheric waters, public lighting, traffic signs and road furniture, road reserve landscaping etc.) The employer is Public Enterprise "Roads of Serbia", and the works are funded by a loan from the European Investment Bank. The design documentation was prepared by the Belgrade "Highway Institute" and the part of the design related to bridges was made by the company "Mostprojekt AD" from Belgrade. The contractor is a consortium consisting of Strabag AG -Ed. Zublin AG -Dywidag Bau GmbH -PZP Zajecar A.D. (Strabag d.o.o.). The subcontractor for the production and installation of the steel bridge structure is company "Mostogradnja AD" from Belgrade, which also in the past built the existing right-line bridge. Supervision of the works was entrusted to the consortium comprising UTIBER Kozuti Beruhazo Kft. -Project Bureau UTIBER d.o.o. -Institute IMS AD.