MAINLAND – ISLAND OF KRK BRIDGE
Krk, the largest Adriatic island, is connected to the mainland by a solid connection across the islet of Sveti Marko, and consists of two magnificent arch bridges, the larger of which held for a long time the title of record holder for the largest span in its category. The island has significant transport, industrial and tourist facilities (airport, oil industry and LG terminal, a large number of hotels, etc.) as well as autochthon agriculture, which gives this bridge a special significance. The idea of building a bridge appeared as early as the 1920s. In the period from 1960 to 1975, various solutions were considered and analyzed (V. Draganić – IPZ, and K. Šavor – IPB suspension bridge, V. Čandrlić – IGH reinforced concrete girder bridge). Finally, the solution proposed by the engineer Ilija Stojadinović from Mostogradnja was chosen and accepted, according to which the existing bridge was built in the period from 1976 to 1980. The bridge is located on the D102 road, and in addition to road traffic, it is used for crossing oil pipelines, chemical industry lines, water supply lines, electricity lines, etc.
The crossing from the mainland to the island of Krk, with a total length of 1,429.5 meters, consists of the following: an access viaduct on the mainland side (10x13 = 130 m), an access to the bridge (5 + 3 x 28.5 + 5 x 33.5 = 258 m, a large arch of 390 m; the crossing to the island of Sv. Marko 100.5 meters, the road to Sv. Marko 96 meters; the crossing to the small arch 38.5 meters; the small arch 244 meters; the crossing from the small arch to the island of Krk 5 + 5 x 33.5 = 172.5 meters. The width of the bridge is 11.4 meters, of which the roadway is 7.5 meters, the pedestrian paths are 2 x 1.3 meters and the bridge clear zones are 2 x 0.65 meters. The highest point of the bridge’s vertical level, in the middle of the large arch, is 67 meters above sea level. The pavement structure of the access viaduct consists of a 0.8-meter-thick reinforced concrete slab with relief cavities, while the access elements and bridges each have three precast girders 1.86 (2.02) meters high, with a span of 33 (28) meters, and transverse girders made of prestressed concrete and a 13-cm-thick reinforced concrete pavement slab.
The cross-section of the large arch is 13.0 x 6.5 meters, and that of the small arch is 8.0 x 4.0 meters, while both arches have three chambers in their cross-section. The bridge arches were constructed using the cantilever method from larger (base) and smaller prefabricated pieces (slabs) connected into a complete cross-section by concreting the joints in situ. The installation was carried out using a crane with a span of 670 meters, a load capacity of 2 x 10 tons for the large arch, and a span of 450 meters and a load capacity of 10 tons for the small arch. The central chambers of the arches, as well as the joints made in situ, are made of MB50 concrete, whereas the prefabricated elements are made of MB 45 concrete. The thickness of the arch walls ranges from 15 to 40 cm.
The columns on the access parts and above the arches have a double T section with oval openings in the ridge walls and are made of the MB35 concrete by using the slip forming method. The cross-sectional outline is 6.35 x 2 meters, and the wall thickness is 25 cm.
The construction of a concrete arch bridge, which will have exceeded the previous record (Gladesville Bridge, 305 meters) by 85 meters or 28%, was a major challenge for both the company and the designer, who had already designed and built two large concrete arch bridges (Šibenik L=246.4 meters and Pag L=193.2 meters). Since the width of the water surface at the site of the large arch is 460 meters, the arch is divided at the spring into a horizontal bracing structure at water level, supported by the coastal block and an immersed strut supported on a caisson foundation at a depth of 19 meters. This original solution enabled the bridging of the Silent Channel (Tihi kanal) with a span of 390 meters. Individual parts weighing up to 600 tons were designed as prefabricated on barges and were mounted with the help of floating cranes and ingenious underwater crossing by using buoyancy. Parts of the foundation are made of MB30 concrete.
Amount of work: non-reinforced concrete 3,655 m3; reinforced concrete 20,515 m3; reinforcement 4,415 tons; cables 12x7 mm, 130 tons. Steel arch scaffolding, steel ties and connections 542 tons, 23-ton 12x7 mm cables. The average number of workers employed was 350 (500 by the end of construction). Total construction costs 633,900,000 dinars.
The contractors were GP Mostogradnja Belgrade and Hidroelektra Zagreb. Designer Ilija Stojadinović; construction manager Stanko Šram, B.Sc.C.E.
DREŽNIK VIADUCT
The Drežnik Viaduct was built in 2001 on the Karlovac–Novigrad–Bosiljevo section of the A1 motorway, with a total length of 2,485 meters, currently the longest one in the Republic of Croatia. It is actually two identical structures two meters apart, each for one traffic direction. Since the height of the roadway above the terrain is up to 20 meters, and on the viaduct stretch there is a need for only one larger span over the Kupa River, spans between 25 and 35 meters turned out to be a rational choice in the aesthetic, economic and performance terms. The viaduct span assembly consists of standard precast T-section girders with a span size of 35 meters with the exception of 28 meters for the end spans. The precast girders, 32.5 meters long, 1.82 meters high and weighing 90 tons, are made of prestressed concrete, and were monolithically cast after assembly by concreting the bearing parts, cross girders and pavement slab. A total of 616 pieces of girders, each weighing 90 tons, were manufactured and installed. The entire length of the viaduct is divided into seven sections separated by transition devices. The dimensions of these sections are 225 + 270 + 4 x 408 + 358 meters. In the middle of the sixth section is the center of the 70-meter bridge span over the Kupa River. This span is bridged by prefabricated cantilever elements with a span of 17.5 meters, with a variable section height from 1.8 to 3.5 meters, mounted above the columns and by inserting standard girders in the central part of the span. The total width of each structure consists of two 3.75-meter-wide lanes, two one-meter-wide shoulder lanes, and New Jersey-type safety barriers on both sides. Due to the length of the structure, a stop lane was omitted on both structures.
The viaduct pillars are single, with a cross-section of 4.0 x 1.7 meters, hollow with 0.3-meter-thick walls. They are finished with a cap and are based on bored piles with a diameter of 1.2 meters and a length of 25 to 30 meters. During construction, MB50 concrete was used for the longitudinal and transverse beams, and MB 30 for the pavement slab, with the non-presstresed reinforcement RA400/500-2.
The design documentation was prepared at the Croatian Institute of Civil Engineering, and the authors of the design were the late prof. Zvonimir Marić, PhD, B.Sc.C.E., Petar Sesar, MSc.Eng., and Damir Tkalčić, B.Sc.C.E.
The viaduct was built in 2001 by the companies Viadukt Zagreb, Hidroelektra Zagreb and Konstruktor Split. Total construction costs are 259 million kuna.
Zvonimir Zdenko Šimunjak, B.Sc.C.E.
