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Palmanova-Trieste motorway bridge (A4)
Silvia FURLAN
Luca VITTORI
Luca VITTORI
HISTORY
FROM PALMANOVA TO TRIESTE, CAPITAL OF MITTELEUROPA
The motorway crosses the Isonzo River via two parallel, straight, structurally independent bridges. Each bridge carries one roadway—the upstream bridge leads to Palmanova, while the downstream bridge leads to Trieste.The two structures were built at different times by different contractors. The engineering for both bridges was managed by Autovie Servizi S.p.A. of Trieste, with specialised structural engineering expertise provided by IN.CO. S.p.A of Milan.
The upstream bridge, completed in the late 1990s, was constructed by Toto S.p.A. Costruzioni Generali of Chieti. The downstream bridge followed in the early 2000s, built by CMB Cooperativa Muratori e Braccianti of Carpi (Modena). Construction supervision for both projects was handled by the concessionary company’s in-house staff.
BRIDGE CONSTRUCTION AND STRUCTURAL ENGINEERING
The A4 motorway crosses the Isonzo River via two distinct parallel bridges, one for each carriageway. Each bridge features a continuous deck with twenty-six 27-metre spans and two 22-metre shore spans, yielding a total length of 746 metres. The deck’s design integrates a composite structure, merging three engineered steel beams with a reinforced concrete slab measuring 30 centimetres in thickness. Nelson-type studs (22 mm in diameter) secure the structural connection between the slab and steel beams. The steel beams maintain a constant thickness with a double “T” shape, reaching 1.35 metres in height. Cross beams are positioned at 6.75-metre intervals, with three cross beams per typical span. The roadway features two 3.75-metre traffic lanes, a 3.00-metre right shoulder, and an additional 1.50-metre asphalted shoulder. A left-side asphalted shoulder serves as a safety buffer zone, enhancing drivers’ awareness of the metal safety barrier, which features a New Jersey profile and upper handrail.Each pier features two circular columns with a tapered cap and foundation block positioned beneath the gravel bed. The piers rest on two indirect foundation piles, each 1.50 metres in diameter, with varying lengths that never extend deeper than 4.50 metres below mean sea level.
BRIDGE CONSTRUCTION AND STRUCTURAL ENGINEERING
The A4 motorway crosses the Isonzo River via two distinct parallel bridges, one for each carriageway. Each bridge features a continuous deck with twenty-six 27-metre spans and two 22-metre shore spans, yielding a total length of 746 metres. The deck’s design integrates a composite structure, merging three engineered steel beams with a reinforced concrete slab measuring 30 centimetres in thickness. Nelson-type studs (22 mm in diameter) secure the structural connection between the slab and steel beams. The steel beams maintain a constant thickness with a double “T” shape, reaching 1.35 metres in height. Cross beams are positioned at 6.75-metre intervals, with three cross beams per typical span. The roadway features two 3.75-metre traffic lanes, a 3.00-metre right shoulder, and an additional 1.50-metre asphalted shoulder. A left-side asphalted shoulder serves as a safety buffer zone, enhancing drivers’ awareness of the metal safety barrier, which features a New Jersey profile and upper handrail.Each pier features two circular columns with a tapered cap and foundation block positioned beneath the gravel bed. The piers rest on two indirect foundation piles, each 1.50 metres in diameter, with varying lengths that never extend deeper than 4.50 metres below mean sea level.
CURRENT STATE
The Autostrada Alto Adriatico S.p.a., wholly owned by the Autonomous Region of Friuli Venezia Giulia, manages the A4 motorway and the two bridges over the Isonzo River. Their jurisdiction covers the A4 from Venice to Trieste, including branch routes: the A23 to Udine, the A28 to Conegliano, and the A34 to Gorizia and the state border.This company succeeded Autovie Venete S.p.a., which held the concession when the bridges were contracted.
