A New Movable Steel Bridge at Viareggio Harbour M. Viviani*, F

A new movable steel bridge at Viareggio harbour

M. Viviani*, F. Morelli^ "Civil Engineer, Via S Nicolao 57, 55100 Lucca, Italy

** Tuscany Region - Lucca Civil Engineering Corps, Via della Quarquonm, 55100 Lucca, Italy

Abstract

The new drawbridge has a fixed counterweight and the rotation axis is in the high part of the structure so as to ensure a good arm to move the structure. The lifting mechanism, based on hydraulic pistons with safety valves, has permitted considerable reduction in the cost of construction as well as ensuring good span for transit vessels. The structure was entirely prefabricated in workshops, thus attaining excellent quality with regard to welded structural joints and protective systems in view of its use in a marine environment.

1 The new bridge within the context of the Viareggio harbour

The Viareggio port was created by the progressive enlargement of the sea outlet of nearby Lake Massaciuccoli. With the passing of time the development of the port has led to the construction of new docking quays and the enlargement of work and manoeuvre basins.

The planimetrical configuration of the Viareggio port features a central island connected to the exterior by a small swing footbridge over the Burlamacca canal and an old swing road-bridge, replaced by the new bridge being discussed here.

The reasons for the building of a mobile bridge lie in the need to provide, essentially for safety reasons, a link with the sea outlet alternative to the navigable Burlamacca canal, as well as to provide the original harbour basins with direct access to the outer harbour. The old swing-bridge had become unusable both because the moving apparatus was out of order and difficult to repair, and because the bearing structure had been designed for mobile loads greatly inferior to those at present foreseen by the regulations and those observable

272 Mobile and Rapidly Assembled Structures as normal port traffic. Furthermore, the width of the roadway, little more than 3.0 m, was no longer suitable for present-day traffic, partly owing to the presence of shipbuilding workshops within the island area, while the limited width of the lateral pedestrian walkways appeared in fact in contrast with regulations regarding architectural barriers. After a long period of limited traffic in access to the island, the Tuscany Region called for tenders for the construction of a new mobile bridge. The details specified in the call for tenders were particularly demanding as they required a draw bridge in contrast with the old swing type, the need to maintain the continuity of the link between the island and the external area during construction, and the expediency of obtaining a good lifting angle such as to ensure the transit of communications links between basins, for the entire width, without limiting the height of vessels.

2 Definition of the final plan

2.1 Analysis of the decisions made

The need to provide a mobile bridge of the lifting type limited the range of possiblities to three: - draw bridge with mobile counterweight and pit; - rackwork bridge;

- draw bridge with fixed counterweight. The first type was rejected mainly owing to the increased costs involved, together with a series of disadvantages in both the construction and management stages. In short, the mobile counterweight solution involves a considerable quantity of steel for the increased length of the deck; furthermore, the depth of the pit would have required the use of costly working techniques with the installation of spiles at considerable depth, as well as the danger of having to occupy, for work-safety precautions, the entire quayside with resultant interruption of traffic on the island. As well as increased costs during the construction phase, there was also the problem of managing the bridge movement stages as the opening to transit on the canal involved, as a direct consequence, the need to protect the pit to avoid accidental falls, a very real danger in view of the considerable amount of pedestrian and motor traffic and the depth of the pit itself (approx. 7.00 m). Further to these disadvantages it was impossible to obtain perfect hydraulic sealing of the joint in bridge-closed position, with foreseeable flooding of the pit and consequent damage to lifting apparatus, with added uncertainty regarding the exact nature of the existing quayside. The rackwork type of bridge was more carefully studied as it would

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allow a series of advantages of which the most important is the possibility of opening the canal with bridge inclination very close to vertical and the use of moving apparatus which is not particularly complex. However, a closer examination of the project indicated the need to provide crown gears with module 50-80 and radius of approx. 1800 mm, whose cost of realization greatly limited the

economic competitiveness of this solution. This type of construction would also have involved the use of the old thrust-block basement of the swing-bridge, thus eliminating the possibility of its future demolition, preventing an increase in the useful width of the canal.

The solution selected, a draw bridge with fixed counterweight, provides the possibility of obtaining an excellent cost-performance ratio, considering all the limitations stipulated in the proposed

contract in terms of operational functionality, installation and overall cost. The principal advantages are the following: - the counterweight block is built entirely above average sea-level; - the bridge opens to an inclination of approximately 75deg from the

horizontal; - the lifting apparatus consists of two standard-production single- outlet pistons of the non-telescopic type; - the weight of the steel for the entire structure is comparable with

currently-used types of decking in orthotropic plating. Considering these advantages it must however be noted that the planned aim to avoid telescopic pistons led to the use of increased power in the hydraulic drive and the need to provide devices for the

diffusion of the non-negligible load entity transmitted from the pistons to the structure.

2.2 Draw bridge with fixed counterweight

The structure realized has a net span between the two quayside basements of 14.85 metres. The draw bridge, with single bay, consists of an open-style steel section with a 4.20 metre roadway flanked by two walkways, each of 1.20 metres.The rotation axis is located above

and externally to the deck: this solution permitted an increased active movement arm together with the advantage of not needing a lifting pit considering that during manoeuvre operations the extremity of the bridge tends, for kinematic reasons, to move away

from the shoulder. Considering the strongly aggressive environment, particular care was given to construction details. The transversal section is compact, typical of metal bridges with

orthotropic decking and features high efficiency in terms of material use. The arrangement of the sheeting was studied with particular reference to structural fatigue together with the essential resistance

274 Mobile and Rapidly Assembled Structures and deformability characteristics necessary for works of this type. In view of expected heavy pedestrian traffic the two walkways are of such a height as to prevent the mounting of vehicles, but are without guardrail protection: this solution has the advantage of better utilization of the bridge in the case of heavy pedestrian traffic, even if it necessarily implies consideration of the presence of a deviation wheel with consequent increased weight on the overhang. The decking is in 12 mm sheeting stiffened longitudinally with bulb plates at a distance of 280 mm, and transvesally by ribs at intervals of

1500 mm. The deck structure has excellent rigidity characteristics, necessary during the lifting phase when the resisting structure has the static characteristics of cantilever. Considering the need for rigidity and durability, the structure was prepared using top-grade welded joints, entirely carried out in the workshop, which allowed high quality results even in view of the considerble thickness of the elements to be joined (50 mm for the leaves of the pin hinge). At the same time all the surfaces were protected with a cycle of epoxy paint, typical of those used in shipbuilding, and consisting of: sanding with specification SA2, Swedish code, a coat of Primer (60 micron) and two coats of paint using airless technology (200 micron). For the decking, protection consisted in roadway paving in epoxy resin typical of light metallic structures. The couterweight block consists of concrete reinforced both with normal steel bars for reinforced concrete and with prestressed

Dywidag bars, 32 mm in diameter. The decision to apply prestressing to the anchor block led to the reduction, as far as is possible, of cracking in the concrete during the various stages of manoeuvre, as well as the efficient anchorage of the metallic structure within the counterweight block.

3 Construction and Installation

Construction of the steel structure was effected entirely in the workshop, with complete prefabrication of the decking with the lifting arms. The welded joints were all of top-grade quality and checked both by radiography and with penetrating liquids. Particular attention was given to the realization of the piston-structure joints and the horizontal rotation axis. For this latter aspect, the need to limit as far as possible alignment errors between the axes of the two fixed hinge pins led to the boring of the two holes using techniques similar to those adopted by shipyards for propeller shafts. The entire prefabricated structure, already connected with the attachment plates to the counterweight block and to the pistons, was transported by sea on a pontoon towed from the workshops in Bocca di Magra (SP) where it was built to its destined location. Transfer from the

Mobile and Rapidly Assembled Structures 275

pontoon to its final position was carried out by three self-propelled cranes located on the quayside. The technique of total prefabrication allowed a notable reduction in cost together with considerable improvement in the geometric accuracy of the kinematic lifting system with a high degree of limitation of errors in parallelism and the misalignment of rotation pins.

4 Lifting and Safety Arrangements

The lifting arrangement features two double-acting pistons, of dimensions such as to support double the theoretical maximum working pressure. The cylinder liner is in Fe52.2 internally lapped, with rod of chromium plated steel C40 with double crossed chromium plating and stainless steel coating on the permanently exposed part. Each cylinder is fitted with blocking valve and descent control (over center valve) flanged directly to it. This valve guarantees blockage in position both by interrupting control and in the case of accidental breakage of the feed pipe. The arrangement applied is typical of hydraulic structures wherever it is required to guarantee non return, the irreversibility of movement.The classical technological case is represented by the articulated joints of the crane which must guarantee complete reliability in blocking movement under load conditions with considerable frequency. In short, the application of over center valves transforms the pistons to double- acting, and therefore the descent is controlled and no longer a gravity descent based on traditional parachute valves. In the case of breakdown in any part of the circuit or a drop in feeding, absolute safety is ensured: in fact the valves block, preventing any entry or exit of oil and only subsequent manual intervention allows the bridge to be manoeuvred.

5 Conclusions

Construction of the bridge described started in February 1994 and the bridge was put into use at the end of September in the same year. Its realization both respected prescribed time-limits and confirmed all project expectations regarding the assembly of the structural work and the lifting system. In economic terms there was no modification to the original agreed sum of £IT 428,500,000= inclusive of all expenses relative to link-up with existing quaysides and port roadways. Relative Data: - Purchasing Body: Tuscany Region - Lucca Civil Engineering Corps, via della Quarquonia, 55100 Lucca, Italy.

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- Executant Firms: Metalcost spa, (SP) - Toscana Costruzioni srl, (LU) - Design Engineer: Dr. Eng. Massimo Viviani - v. S; Nicolao 57, 55100 Lucca, Italy

- Works Manager: Dr. Eng. Fabrizio Morelli -c/o Purchasing Body.

Drawbridge with mobile counterweight

Drawbridge with fixed counterweight

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LONGITUDINAL SECTION O

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278 Mobile and Rapidly Assembled Structures

CROSS SECTION

ROTATION AXIS MIDSPAN

DETAIL OF THE OPENING ARRANGEMENT

DYWIDAG bars

COUNTER WEIGHT

Mobile and Rapidly Assembled Structures 279

Upwind I ^>

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i i i ACTION ON THE PIN (mobile part) LONGITUDINAL SECTION

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ACTION ON THE PIN (fixed part)

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stresses during opening — closing movement

Upwind ACTION ON THE PIN Lee (fixed part)