PSI - Issue 72

Boris Folić et al. / Procedia Structural Integrity 72 (2025) 278 – 285

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5. Prefabricated scaffold, long arch. A similar structural system was applied for the mainland-Krk bridge, where the arch is three-cell, and only the inner cell is prefabricated, while the neighboring cells are cast on site. Instead of piles at the junction of the large arch and the prop, a caisson was constructed here, founded on the rock.

Figure 6. Prefabricated scaffold, long arch. With details of joint.

According to the lecture of Boško Petrović (BP), held during the period 1991 -2004 in Novi Sad (NS): the first critical moment during the construction of the bridge was the phase of introducing pressure forces into the semi arches and specifically the moment of lifting the structure from the formwork- scaffolding. "Žeželj had an inconvenient custom of sending the one who designed something new, to carry it out." Since I (BP) designed the propping of the arch, that "ungrateful" honor in this case fell to me. While the workers were preparing the presses in the crown for the propping of first arch stages, I expected them to tell me when the force reached 90% of the value and then slow down the pumping - which they did. Specifically, the pressure was measured with manometers, but they had pressure-force indicated in the tables as well. I was told what the force was, but nothing was happening. It should have show some movement, at least a squeak, but nothing was happening, and the force was already at 100% 102-103, I stopped pumping force before 105%, because if the semi-arches suddenly jump and detach from the formwork-scaffolding, the bridge could break, and then I'm afraid to think what would happen to the project and to me. It was already night, I told the weight lifters to release the tension on the presses, check the hoses again and start again tomorrow. The next day I came to the construction site and they told me: you were right, one part of the hose got tangled inside the scaffolding, so it was almost invisible in the dark, that's why it didn't transmit the pressure to the press. Now when it was approaching 100% of the force, there was a screeching sound, the concrete of the arch worked and then over time as if on command the future arch (currently only two 1/2) rose from the formwork. The presses are located at the lower end of the lower part of the arch, Fig. 7. The resultant of the lower press system is e p =25+45/2=47.5 cm from the lower edge of the arch. When adopting the cross-section of the lower and upper part of the arch, several sections were analyzed. For the adopted section, the center of gravity e1 is at 51.2 cm. The weight of the lower part of the arch is approximately 40% of the total weight of the arch, so the scaffolding is dimensioned accordingly, which significantly reduces its price. When the first, lower part of the arch is poured, and after hardening, it is spread by the presses at the top, the scaffolding is freed from the burden of the lower part. After that, the formwork of the upper part of the arch is formed, which now rests directly on the scaffolding, and partially on the spread lower part. When the upper part hardens, it is also spread with presses. The upper and lower parts of the arch are almost identical, the difference is that the upper arch has a transversely curved upper edge, due to the drainage of storm water.