Main Viaducts 

The design and subsequent construction of long-span viaducts along the Messina-Palermo motorway belongs to the second  period of the works, after the removal of the block on financing to the motorway sector, which lasted from 1975 to 1983. 

In this period new and modern construction techniques were developed which greatly reduced the construction times for long-span viaducts and offered good guarantees in relation to the quality of the materials used. The original design was thus revised, introducing the solution of long-span viaducts (70-100 m) for crossing the more difficult valleys, instead of the traditional solution of supported girders and decks (maximum span 30-35 m), which had been widely used in the first construction period 1967 - 1975.

One important benefit derived from the adoption of the new type of long-span viaduct was the reduced visual impact, the so-called “barrier effect” - due to the apparently lighter, slimmer structures. It also proved an advantage in the numerous geologically difficult areas because of the  reduction in the number of supporting structures and consequent foundation works. The environmental impact was further mitigated by the extensive landscaping works to mask the retaining and supporting structures.   

The main elements comprising the viaduct are the following:

Deck: Continuous caisson structure built with pre-cast interlocking ashlars launched from the piers. The continuous deck structure eliminates the need for intermediate joints and improves the stability of the structure.

Piers: Two different types of piers are used according to their height:

• piers not exceeding 20-25 m in height have an octagonal section with maximum dimensions of 3.50x3.50 m, broadening at the top to form a square support (5.40x5.40 m)

• piers of more than 235 m in height have a square section diminishing from base to top to arrive at a square support of 5.40x5.40 m.

Both types are aesthetically pleasing and can be satisfactorily merged into the environment.

Longitudinal and transversal seismic checks: The longitudinal seismic forces in viaducts are concentrated in a well-defined area, usually one of the two abutments. Since the impact of  a longitudinal tremor on a rigid structure would be disastrous, oleo-dynamic shock absorbers are interposed between the deck and the abutment at one end while the other end is left free to move longitudinally without constraints or elastic parts.

Support systems: During construction the stability of the structure is guaranteed by hydraulic jacks which can protect against the effects of unequal loading due to the weight of the ashlars, the reaction of the launching cranes, or adverse weather conditions. The permanent bearings, two at each bearing point,  are made of steel and teflon with an encapsulated neoprene cushion and are multi-directional, transmitting the transversal and longitudinal stresses to the appropriate checks and shock absorbers.

Joints: The continuous deck system allows the elimination of intermediate joints and the concentration of expansion joints (designed to absorb seismic events) mainly at the abutments. These expansion joints which are of suitably impermeabilized bridge plate type, are therefore capable of resisting the movements transmitted by the lateral shock absorbers, due to thermal expansion, shrinkage and fluage, coherent with the bearings.

Inspectionability: Manholes located at the piers and abutments permit access inside the deck caissons and the inspection of their entire length. Special openings in ashlars on top of the piers give access from the caisson to the top of the piers for the inspection of the support systems. At the abutments it is possible to inspect both the supporting systems and the shock absorbers. By means of an opening at the base of each pier and an inside ladder, the piers can be fully inspected. Square section piers also have a manhole allowing the inspection of the bearings and transversal checks. These piers are therefore accessible from the pier base and from the abutments.  

Rainwater runoff: A system of gulleys and down-pipes located at each pier allow for surface runoff. The down-pipes convey the rainwater, inside each pier, to a trap at the pier base. As for the road formation, therefore, the rainwater can be channelled into the local streams.

 Viaduct construction techniques

In situ casting of the ashlars, their joining and storage: The ashlars comprising the deck are usually pre-cast close to the site of construction. A single metal caisson is used, with an average production of one ashlar per day. In the case of the Vallone Marzo viaduct, the pre-cast ashlars underwent a long seasoning period (about a year) as the tunnel connecting their manufacturing site and the construction site was unusable  Loss of tension and shrinking due to fluage as a result of this seasoning period was taken into consideration in the design.

Construction of the piers: The construction of the piers is perhaps the simplest part of the whole construction job. The piers are placed on the foundations in the shaft and have a box section which is built up from the base by means of rampant caissons.  

Assembly of the ashlars: The ashlars are assembled alternately, starting with the end one on the piers. The faces to be joined are painted with epoxy resin and are fastened temporarily to the ashlars already in place by means of p.r.c. steel bars. At the end of the assembly process the closing ashlar is placed and the lower pre-stressing cables are threaded in. 

Tensioning of the pre-stressing cables and their injection: This is the last operation to be carried out and is highly complex due to the fact that the hyperstatic pre-stressing reactions must be controlled as they are induced during the tensioning process.  A system of oleo-dynamic hydraulic jacks was used to guarantee the stability of the supports during these operations so that the necessary adjustments could be made through to an advanced construction phase. It was thus possible to remedy at least in part any construction inaccuracies. On completion the load is transferred to permanent steel-teflon supports.  

The following table bears witness to the number of major viaducts on the motorway alignment, with reference to the second construction period. 

Read more about the Messina - Palermo Motorway : 
·        Background     ·        First Construction period     ·        Second Construction period     ·       Main Tunnels