4 th International Conference A BRIDGE TO THE CAVE THE NEW MARINIČ BRIDGE IN THE ŠKOCJAN CAVES PARK IN SLOVENIA Rok MLAKAR Structural Engineer Ponting d.o.o. (Ltd.) Maribor, Slovenia rok.mlakar@ponting.si Viktor MARKELJ Design Manager Ponting d.o.o. (Ltd.), University of Maribor Maribor, Slovenia viktor.markelj@ponting.si Summary The paper presents reconstruction of the new Marinič Bridge in the Škocjan Caves Park in Slovenia. The bridge, together with belonging footpaths and other cave infrastructure, was rebuilt in 2010 on initiative of the Public Service Agency Škocjan Caves Park to expand their tourist activity and enable visits to the eastern parts of the caves, which has been closed to the public since 1965, when much of the existing cave infrastructure was destroyed by devastating floods. The new Marinič Bridge is designed as curved, lightweight, rock anchored and transparent steel structure. The footbridge is almost 30 m long and overcomes a height difference of 4.50 m between both end supports. Structural design and shape of the new bridge answer to all conditions given by the distinctive natural karst environment. With its curvature, towards the cave entrance, the bridge avoids falling debris and unstable support conditions, and with its lightness preserves the fragile, natural, UNESCO protective ambience. Due to inaccessibility of the bridge location, high environmental terms and exact and strict national legislations, the construction of the bridge demanded a great deal of engineering know-how, patience and inventiveness of all project participants the result of all efforts is a unique, well engineered, transparent and extraordinary pedestrian bridge. Keywords: footbridge; reconstruction; structural concepts; sensitive natural environment; sophisticated design approach; difficult boundary and construction conditions; steel structure; aesthetic. 1. Introduction The new Marinič Bridge, named after one of the earliest cave explorer Joseph Marinitsch, is situated in the heart of Škocjan Caves Park in Slovenia. The bridge, originally built as Bridge Concordia (1891), then rebuilt as Bertarelli Bridge (between wars), connects the footpath at the edge of the Mala dolina Gorge and entrance to the Mariničeva jama Cave, underneath, over 100 m deep, impressive, vertical rock wall of the distinctive Škocjan karst depression. The new bridge replaced the existing bridge of two steel I-sections and wooden deck, warren down, feared unstable supported and partially heavily damaged by falling icicles and rocks from vertical, over 100 m deep, rock wall above the cave entrance. Due to bad shape of the existing bridge, given environment conditions and Owners ambitions, the best and almost only choice was to give up the old bridge and build a completely new one for the future. In this aspect the Owner engaged the Engineer to design and build a new, lightweight, transparent and attractive footbridge which would fulfil its basic function of assuring a safe passage over the wild Reka River Gorge, fit into the sensitive natural cave environment, offer the visitor a closer, intensive, exciting almost adventurous cave experience and all in all give the whole Škocjan Caves Park a new, additional value and become one of the high points of the renewed eastern cave tour the new Marinič Bridge is all that. 479
Footbridges and Environment The Engineer faced a difficult, challenging yet very interesting and pleasant task, since bridge design had to meet Owners high expectations and at the same time fulfil harsh geological and hydrological site conditions, answer all durability terms and meet all extensive and strict law regulations, high environmental terms and exact national legislations in forced for this very sensitive and highly protected natural environment, protected also as the UNESCO World Heritage all these within budget of app. 300.000 EUR. Fig. 1 Existing, damaged and unsuitable bridge (- 2010) Fig. 2 Finished new Marinič footbridge (October 2010) 480
4 th International Conference Fig. 3 Škocjan Caves system with marked Marinič Bridge location 2. Bridge Design 2.1 Design Approach Due to pretentious natural boundary conditions (natural irregular cave formations) and difficult site access, a new sophisticated design approach was chosen, using 3d-SCAN for detailed geodetic site definition which then was used to create a real digital space model as basis for the actual bridge design, for the Engineer. From the digital 3d-Model a plaster model of the location, using 3d-Print technology, was produced and used for principal structure design considerations, which then, over several debates, resulted in a original shape and structural scheme of the new Marinič Bridge. The design was then presented and with great deal of enthusiasm accepted by the Owner. Fig. 4 Site 3d-SCAN 3d Solid-model 3d Mesh-model of the site 481
Footbridges and Environment Fig. 5 Plaster-model and principal design idea 2.2 Structural Concept and Design The new Marinič Bridge is designed as curved, lightweight, rock anchored and transparent steel structure. The footbridge is almost 30 m long, with clear span over the gorge of 27.00 m, and overcomes a height difference of 4.50 m between both end supports. Fig. 6 3D computer model of the bridge 482
th 4 International Conference 28.00 m long, main steel structure of the bridge has a curve-shaped geometry, consisting of straight and radius R = 4.00 m sections, tangential connected into a S-shaped structure, assuring adequate deviations from all natural obstacles and withdrawing the footpath towards the cave entrance, avoiding falling debris and icicles. The S-shape of the structure also adds a great deal of rigidness to the otherwise very slender bridge structure and gives the bridge it s unique and distinctive character. In longitudinal view, the main bearing structure of the bridge lies in a constant inclination of 15.5%, while the bridge deck is designed as a stair-landing system uniformly distributed over the whole length of the bridge, enabling the pedestrians a smooth and pleasant walk. The main bearing structure is a steel tube 457.1/20 mm, consisting of three separate manufactured sections, connected over round steel plates and pre-stressed bolts. In layout the structure is divided into 12 sections between cross beams positioned 2.25 m apart. The cross-beams are T-shaped steel sections of varying depths, manufactured of 10-25 mm thick plates, directly welded onto the main tube, supporting the bridge deck. The cross-beams are longitudinally connected via 400/10 mm steel band, which increases the overall bridge rigidity, supports the bridge deck lattice and serves as outer fascia beam, concluding the smooth curved bridge geometry. Bridge bearing structure is made of structural steel S 355, while bridge deck, railings and other equipment are manufactured of prefabricated stainless steel elements. Fig. 7 Bridge longitudinal section and layout 483
Footbridges and Environment End supports of the bridge are designed as small concrete-stone abutments, integrated into the natural rock wall and cave footpath scarps which are also being reconstructed. On the north side the abutment forms also a small belvedere plateau, overlooking the Mala dolina Gorge, raising the footpath above reoccurring floods level which continuously threatened and damaged the existing cave infrastructure especially in this area. The bridge main bearing structure is rigidly fixed into the reinforced and rock-anchored concrete abutments base slabs at both end supports. Beside both end-supports, fixed into the ground rock, the bridge is additionally supported by two elastic M 28 tie bars in the bridge span, allowing the lightweight and transparent bridge appearance and assuring overall bridge stability. The tie bars are hinge-connected to the bridge over specially designed and strengthened cross beams and hinge-anchored into the south cave wall by two permanent, pre-stressed, 13.0 m deep, ground anchors, following the tie bar geometry into the firm rock wall. The bridge deck stair-landing system is designed as lightweight, custom tailed, 33 mm deep stainless steel lattice, mounted to the longitudinal fascia steel sheets and equipped with anti sliding stripes, enabling safe and easy way for the pedestrian. The bridge deck consists of ten characteristic deck section of two stairs B/H = 33/15 cm and a 159 cm long landing, which originate from the designed bridge end supports height difference and general architectural design criteria. Fig. 8 Bridge characteristic cross-sections The bridge railings consist of vertical railing columns, longitudinal handle and outer framed net cover. The railings are architecturally designed, full of common yet fine stainless steel details in shape of authentic wrought iron Škocjan fence, one of the characteristic and protected historical trademarks of the Škocjan Caves Region, stretching along most of the cave footpaths. The safety and pedestrian s welfare are additionally guaranteed by 1.10 m 1.38 m high, framed, fine net cover, reaching beneath the main bridge structure, giving the bridge an extra aesthetic value and character, accenting its curved shape. 484
th 4 International Conference Fig. 9 Rock anchorage and tie bar details 3. Construction Construction of the bridge is an engineering story of its own. The bridge site lies over 100 m beneath vertical walls of the Škocjan Karst depression and is accessible only by foot along a narrow footpath, therefore the construction demanded a lot of engineering know-how, patience and inventiveness. 3.1 Steel Structure Fig. 10 Steel structure assembly and test-mounting Almost 10 tons heavy welded steel structure was manufactured in a workshop and consists of three separate sections weighting up to 3.5 t each. The structure was also completely checked for structure and weld failure and test-mounted in the workshop. After test completion and geometry conformation, the structure was dissembled again for corrosion protection implementation, under strict controlled conditions, and transport to the construction site. 485
Footbridges and Environment 3.2 Construction Site The most efficient way to reach the bridge location was to lend the whole steel structure from a small plateau, over a vertical overhang of the karst depression into the over 100 m deep abyss to the cave entrance. The steel structure was transported to the plateau in the middle of the Matavun Village, reached only by a steep and narrow local road that had to be widened and a bit adjusted a few times to enable the transport vehicle to pass and deliver the steel structure to the site. The structure was then mounted together just over the bridge location and placed into its final position, 107 m beneath the plateau, using a mobile crane. Fig. 11 Construction site of the New Marinič Bridge 486
th 4 International Conference Other works footpath reconstruction, old bridge demolition, bridge abutments and rock bearing construction, were carried out entirely prior to steel structure assembly, using existing footpaths to reach the bridge location and a smaller, fixed crane for the vertical material transport. Extremely demanding preliminary anchoring works in the south rock wall have been performed by professional climbers. The whole bridge construction and lending of the new steel structure was carried out without any problems and delay, according to the exact construction scheme, prepared by Contractor and bridge Designer. The new bridge assembly was very important to the Owner and the locals and was accompanied and documented by the local and national media. Fig. 12 Raging waters of the Reka River during construction 4. Materials and Bridge Basics 4.1 Materials bridge main bearing structure: S 355 J2 corrosion protection coating min. 250 m in silver metal colour tie bars: S 460 (Macalloy), stainless steel version anchorages: stainless steel 1.4301 anchors: CATI-ZRMK tipa RCP/D permanent pre-stressed geotechnical anchors bridge deck and railings: stainless steel 1.4301 (inox) concrete abutments: C 25/30 XD1, XF1 BSt 500 S (B) steel reinforcement 487
Footbridges and Environment 4.2 Bridge Basics length / span: 30.0 m / 27.0 m height difference: 4.50 m width / clear width: 1.50 m / 1.25 m railings height: min 1.10 m 1.38 m deck area: 45 m 2 structure weight: cca. 11.300 kg overall cost: 298.800 EUR 5. Conclusion A unique, well engineered and extraordinary new Marinič Footbridge blends engineering know-how, inventiveness; patience and efforts of all project participants and meets all of the Owners expectations and requests as well. Fig. 13 New Marinič Bridge 488