Pelješac Bridge – Design and Maintenance

Pelješac bridge – design and maintenance Marjan Pipenbaher Gordana Hrelja Kovacevic Tatjana Peteršic Nijaz Mujkanovic Marjan Pipenbaher Ponting Consulting Engineers, Maribor, Slovenia Gordana Hrelja Kovacevic, Nijaz Mujkanovic University of Zagreb Faculty of Civil Engineering, Zagreb, Croatia Tatjana Peteršic Pipenbaher Consulting Engineers, Slovenska Bistrica, Slovenia Corresponding author: Gordana Hrelja Kovacevic, [email protected] https://doi.org/10.5592/CO/FTCE.2019.10 Scientific Symposium FUTURE TRENDS IN CIVIL ENGINEERING Zagreb, Croatia, 17 October 2019 Future Trends in Civil Engineering Pelješac bridge – design and maintenance Abstract The Pelješac Bridge is currently the largest bridge under construction in Croatia and in the wider region. The construction began in July 2018 (for the second time), and the bridge should be completed in 36 months. The design work for the bridge started in 2004. Various bridge alternatives were considered during preliminary studies, preliminary design and main design activities. The bridge is located in a high-activity seismic zone with strong winds, and is realised in extremely adverse foundation soil conditions, and in aggressive maritime environment. The multi-span extradosed semi-integral bridge with steel deck was finally chosen for the main design and construction. The history of design, final design and maintenance of the bridge is presented in the paper. Key words: cable stayed bridge, extradosed bridge, steel box girder, driven steel piles, durability, maintenance Most Pelješac – projektiranje i održavanje Sažetak Pelješki most trenutačno je najveći most koji se gradi u Hrvatskoj, ali i u široj regiji. Gradnja mosta započela je u srpnju 2018., po drugi put. Predviđa se da će gradnja trajati 36 mjeseci. Projektiranje mosta započelo je još 2004. godine. Kroz studije, idejna rješenja, idejne projekte i glavne projekte razmatrana su različita rješenja za most. Most je smješten na lokaciji s jakom seizmičkom aktivnošću i jakim vjetrovima. Također, na lokaciji mosta su izrazito nepovoljni uvjeti temeljenja i agresivni uvjeti okoliša. U završnoj fazi projektiranja, za razradu u glavnom projektu i za gradnju odabran je ekstrados most sa čeličnim rasponskim sklopom i poluintegralnom konstrukcijom. U radu je prikazana povijest projektiranja, glavni projekt za odabrano rješenje te preporuke za održavanje mosta. Ključne riječi: ovješeni most, ekstrados most, čelični rasponski sklop, čelični zabijeni piloti, trajnost, održavanje 220 Pelješac bridge – design and maintenance 1. Introduction The southern part of Croatia, including the city of Dubrovnik, is currently separated from the rest of Croatia by a small coastal stretch belonging to Bosnia and Herzegovina (Figure 1). The idea of fixed road link to connect the whole of Croatia, without having to cross state borders twice, has been studied for more than fifteen years, but has gained a new momentum after Croatia has become an EU member state. The new bridge crossing the sea strait between Croatian mainland and the Pelješac Peninsula shall fulfil this purpose, while also furthering development of the Pelješac Peninsula and nearby islands. Figure 1. Location of Pelješac Bridge 2. The history of design and construction 2.1. Bridge specifics The seabed at the bridge alignment is almost level at -27 m in elevation, with the stratigraphic pattern of a series of sub-horizontal layers and an irregular top of the rock along the bridge (Figure 2) [1]. The foundation soil alongside the planned location is of extremely poor quality, as has been confirmed by detailed geophysical surveys and extensive off-shore investigations, including continuous shear wave survey and geotechnical explorations from a specially equipped drill ship, which included drilling boreholes and sampling in 2004 and 2009 [1]. The depth to bedrock is variable along the bridge alignment, ranging from 75 m to 102 m in depth in the central portion of the crossing, and amounting to about 39 m close to the mainland coast. 221 Future Trends in Civil Engineering Figure 2. Stratigraphic section at bridge location The crossing is approximately 2140 m wide at the sea level and 2380 m at the gradient line level. The navigation clearance was originally specified as 150x30 m, but had to be changed to a navigation channel at least 200 m wide with minimum vertical clearance of 55 m at the specific request of the neighbouring country of Bosnia and Herzegovina, although there is no navigation channel currently chartered in this area and although the Mali Ston Bay is ecologically an extremely sensitive area, where any larger ship traffic might disturb delicate environmental balance of one of the last European natural habitats of oyster mussel and clam farms. The bridge site lies in a zone of high seismic activity in the vicinity of active seismic faults, where seven significant earthquakes of magnitude M>6 occurred in the passed century within 100 km of the site. The bridge site is also exposed to high winds with maximum average 10-minute wind speeds of 33,4 m/s and wind gust speeds of 47,1 m/s. Hence, the key challenges for the bridge design were high bridge alignment lying at approximately +90,0 m a.s.l., adverse soil conditions, high seismicity of the site, and stringent environmental requirements. 2.2. Preliminary studies (2004-2005) Because of the above mentioned constraints, it was evident that the total bridge dead weight, and especially the dead weight of the superstructure had to be reduced as much as possible, which was done by adopting a steel superstructure. Also, relatively long spans were utilized to limit the number of expensive foundations. At this stage, the navigation clearance was specified as 150x30 m. Over 10 bridge types such as girder bridge, truss bridge, arch bridge and cable- stayed bridge (Figure 3), were studied at the preliminary stage and, after a detailed evaluation, two alternative solutions were short-listed for the next design phase [2]. 222 Pelješac bridge – design and maintenance Figure 3. Alternative solutions (2005) 223 Future Trends in Civil Engineering The choice was based on aesthetical considerations and economic criteria comprising material quantity and complexity of erection technology, but also on predicted future maintenance costs of this large project. The Committee, appointed by the state of Croatia, finally selected the continuous steel box type superstructure as the most appropriate bridge for the Mainland- Pelješac Peninsula Crossing, finding this alternative the least imposing on the environment. Therefore, the next stage was the preliminary design for the continuous beam alternative solution. 2.3. Preliminary design (2005) The superstructure was a continuous trapezoidal steel box over 15 spans, with the overall length of L=94.0+122.0+142.-0+3x170.0+178.0+7x170.0+138.0=2,374.0 m and constant depth of 6.5 m (Figures 4 and 5) [3]. Figure 4. Longitudinal layout of the bridge Figure 5. Typical cross sections at midspan and at supports The total calculated weight of the steel superstructure was 20,000.0 tons or 556 kg/ m2 of the bridge deck area (using steel grade S355). All piers in the sea strait were founded on deep foundations, consisting of tubular steel driven piles with the pile cap at the sea level. This turned out to be the most cost-effective solution from the point of view of ease of construction and quantity 224 Pelješac bridge – design and maintenance of material used. The pile steel grade was S 355 with the tube thickness varying up to maximum 60.0 mm. The cathodic protection of piles against steel corrosion in an aggressive environment was to be applied. Reinforced concrete piles were 5.0 m deep and 20.0x20.0 m in ground plan. After the preliminary design was completed and the location permit obtained, new demands on the navigation clearance were defined. The navigation channel was to be at least 200 m wide with the vertical clearance of 55.0 m. New alternative solutions for the bridge were therefore analysed. All of them were cable stayed bridges and, finally, the cable stayed bridge with the main span of 568 m and the steel box type superstructure was chosen for the main design. 2.4. Main design (2009) The original main design of the bridge comprising a cable-stayed bridge with the main span of 568 m and a continuous aerodynamically shaped steel box deck structure (Figure 7), with an overall length of 2404,0 m (Figure 6) was completed in 2008 [4], [5]. The width of the four-lane roadway was 2x8,0=16,0 m, with two lanes in each direction, separated by a 3,0 m wide median strip, so that the total width between safety barriers amounted to 20,0 m. Figure 6. Original bridge design – longitudinal layout Figure 7. Original bridge design – deck cross-section of main cable-stayed bridge 225 Future Trends in Civil Engineering The depth of the deck was 3,0 m for the main cable stayed bridge and 5,0 m for the approach spans, with a smooth transition between these depths at cable stayed bridge ends. The longitudinal layout of the stay cables was of modified fan type with partial suspension, with cable stays in two inclined planes spaced at 20,0 m in the longitudinal direction. The streamlined deck cross section was designed to reduce wind action on the bridge and to provide for a high torsional rigidity. All vertical actions from the deck were to be assumed by structural bearings. Dampers were to be installed on all supports with stiff foundations to mitigate the seismic energy input and minimize structural damage. Pylons of the cable stayed bridge were designed as diamond shaped pylons with the total height of 176,0 m above the sea, with the lower parts in concrete, and with only the top 52,0 m parts, where cable stays are anchored, of composite type.

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