[Railway reconstruction Italy 1943-1946 published by Royal Engineers, 1946]
INDIVIDUAL PROJECTS OF SPECIAL INTEREST
1. - Introduction.
Drawings of the viaduct are attached at Plate N°. 42.
The method of launching the Roth Waagner Span is shown at Plate N°. 43.
The reconstruction of this viaduct was carried out by 150 Railway Construction Company, R.E. under command of N°. 1 Railway Construction and Maintenance Group R.E. between 9 June 1945 and the end of the year.
A certain amount of preliminary work was done by 538 Field Company, R.E. between 28 May and 19 June 1945.
The work of reconstruction of the bridge was carried out in three stages.
a) A quick repair to open the line to traffic, resulting in the bridge opening to traffic on 1 August 1945.
b) A second repair of more ambitious design to render the bridge proof against winter conditions. The concluding stages of this part of the work involved closing the line to traffic for 10 days. The bridge was reopened to traffic on completion of the occupation on 15 October 1945.
c) From 14 October, 1945 to the end of the year, work was in hand on the dismantling of the first repair, clearance of debris from the river bed etc.
2. - Description of Site.
About 61 kilometres north of Udine, Line 71 (Section Udine-Pontebba) which at this point is running through the gorge of the River Fella, crosses the River Dogna, a tributary of the Fella, just north of Dogna station. The line is located on a side cut some distance up the eastern slope of the gorge, and, at the actual point of the viaduct, is on a curve of 5° 7'. The viaduct had consisted of four steel girder deck spans supported on masonry piers. The length of the end spans was 128 ft. 4 ins. and of the centre spans 153 ft. The height of rail level above water level was 145 ft. There was a rising gradient across the bridge of 1.587% from South to North. Owing to the curvature of the alignment, the track on the viaduct had been given a superelevation of 5 ˝ inches.
The viaduct and the area immediately surrounding it had been extensively bombed. The principal damage consisted of the total destruction of the second span from the South abutment, but several bombs which narrowly missed the target had caused considerable shrapnel damage to the piers and the three remaining spans. The third span from the South had a severe distortion of a portion of the top flange of the West truss, caused by a direct hit by a bomb which failed to explode.
3. - The First Repair.
As the viaduct was on such a sharp curve, and as the piers were very slender, and set at an angle to each other, it was quite apparent that a Lattice Girder span could not be used to bridge the gap. An alternative was the use of two U.C.R.B. spans with a new central pier. This was perhaps not the soundest solution from an engineering point of view, but Eighth Army were pressing for the use of the line as a feeder from the ports of Trieste and Venice, to their troops in Austria, and as the latter solution promised to be the most rapid repair, it was adopted.
Figure 125. - Line 71. Dogna Bridge, Job N°. 71/8. View showing raising of one of the U.C.R.B. girders. Another girder is seen prepared for lifting.
As Eighth Army were pressing for the use of the line, and as there were at that time no Tn. troops available to start repairs, a meting was held with the Commander, 22 A.G.R.E. at which it was agreed that preliminary works should be put in hand forthwith by a Field Company R.E.
As a result, work was commenced on 28 May 1945, by 538 Field Company, R.E. on the construction of foundations for the new pier. This foundations which rested on the gravel of the river bed and was strengthened by 14 concrete pressure piles, 2 ˝ inches in diameter, driven to a depth of 10 feet, consisted of a reinforced concrete block 45 ft. long by 34 ft. wide and 64 ft. high. This work was completed on 19 June. On this concrete block, 150 Railway Construction Company R.E. erected a Standard Steel Trestle 4 bays by 2 bays by 72 ft. high.
Figure 126. - Line 71. Dogna Bridge, Job N°. 71/8. View of Bridge showing the U.C.R.B. girders being placed. Note the great height of the structure.
While this immense concrete block was being constructed and the trestle erected upon it, repairs to the three original spans had proceeded apace. Damaged diagonal web members were replaced by channels rivetted into position. Other members, less damaged, were repaired by welding patches over the damaged portions.
Owing to the track over the viaduct being on a sharp curve, it was impossible to launch the U.C.R.B. span. As there were no road accesses to the railway at this point and space was restricted, to prevent hampering the repairs to the original spans, it was decided not to erect «in situ», but to build the separate truses in the valley bottom and hoist them into position by means of S.W.R. tackles suspended from cantilevered gantries constructed in Light Steel Trestling. The first truss was hoisted on 18th. July 1945 and the fourth on 22nd. July 1945. Power was supplied by two 5 ton hand winches and the maximum rate of lifting was 40 ft. per hour. In the event of any similar work being undertaken, power hoists should be used. The four trusses were then skidded into alignment, seated and braced.
It was decided to reduce the superelevation of the track to 1 ˝", as a speed restriction of 8 K.P.H. was imposed. The new sleepers laid over the U.C.R.Bs. were adzed to effect this. In the case of the original spans, the sleepers were previously tilted by stools riveted to the stringers which carried the outside of the curve. The cant was reduced in this case, by fitting hardwood packs under the low leg, between the rail and the sleepers. This method, though, unsatisfactory, had to be adopted as the stools could not be removed. The packs could not be fitted between the stringers and the sleepers as, then, on the one side the sleepers would have had only a knife edge bearing on the upper lip of the inclined stools.
The bridge was opened to traffic on 1 Augusts 1945.
4. — The Second Repair.
When the line was opened, the situation was reviewed and it was decided that as the rapid construction of the new pier had not permitted construction of deep foundations, some further steps would have to be taken to ensure the safety of the bridge. Two courses were open to investigation, firstly, the protection of the new pier by sheet piling, or secondly, the replacement of the two U.C.R.B. spans by a single span and the retirement of the new pier.
It was found that there were many large boulders in the river bed and sheet piling would have been difficult, if not impossible. Certain stocks of Roth Waagner bridging material had recently been located in the country and investigation showed that this material could very conveniently be used to bridge the gap in a single span. It was therefore decided to use this material, in replacement of the U.C.R.B. spans and to dispense with the newly erected pier.
Two Roth Waagner assemblies were capable of spanning the gap, a single tier, triple chord section, and a double tier, double chord section. In view of the distance between rail level and bearings of the existing spans, the former assembly could be used as either a deck span or a through span, while the latter could be used only as a through span. Therefore since curved track could not be laid on a through span, the double tier, double chord assembly was rejected, although economical in material.
As the line was open to traffic and was then the only British rail link between Austria and Italy, it was necessary that any interruption of traffic should be reduced to a minimum. Since the only access to the site for material was by rail, this prohibited erection «in situ». In view of the weight of the bridge, (each girder weighed 84 tons when fully assembled), the bridge could not be built as separate girders in the valley and lifted, as no tackle of sufficient strength could be satisfactorily rigged, nor could cantilever gantries of sufficient strength be erected on the pier, owing to the confined space.
It was decided to build the bridge in the siding at Dogna station, as separate half trusses and to splice the halves together when they were in position on the bridge, using the newly erected pier as a centre prop. Dogna station is located in a cutting with a small loading dock only large enough to take a Byers crane. This meant that all material had to be reloaded at the adjacent station of Chiusaforte, in such a manner that each wagon contained materials for a section of a truss: i.e. top chord members in the bottom of the wagon, then vertical and diagonal members with the bottom chord members on top of the load. An engine was in continual use for shunting individual wagons up to the crane. A section of the half truss was built on trollies, and pushed along the track to leave erection space opposite the crane, another wagon shunted up to the crane and a second section built on the end of the first. The process was then repeated, until the half truss was completed. Each half truss took four days to erect.
Before erection commenced, the overhead electric cable was removed from the loop. Immediately prior to the launching of the four half trusses, the overhead cable across the bridge was raised to a height of 7 metres above rail level and electric trains were propelled across the bridge by a steam banker.
Six cantilevered gantries were erected, two on each pier and two on the trestle pier, to take the half trusses. Each half truss was trolleyed from the siding across the U.C.R.B. on erection trollies. Power was supplied by a rail mounted winch. On leaving the siding, each half truss was jacked up and a wedged shaped pack inserted on top of the trolleys, so that the trusses remained vertical when crossing the bridge, in spite of the superelevation. When opposite the respective gantries, each half truss was jacked up and skidding joists were introduced. The half trusses were then skidded to the centre line of the gantries, and were thus clear of structure gauge, allowing traffic to pass. Launching and skidding each half truss occupied eight hours.
Each half truss was then lowered, inside the cantilevered gantries, 19 ft. on to the skidding joists. Lowering was effected by four «FELCO» blocks, each of 10 ton capacity. In order to avoid any undue share of the load falling on any one block, two compensating arms were constructed from parts removed from a demolished electric locomotive. These were bolted to the top flange of each half truss and the «FELCO» blocks were coupled to the end of the arms. Lowering each half truss occupied 7 1/2 hours. Each «FELCO» block was overloaded by half a ton, and in point of fact, one did fail when the last half truss was luckily only 9" above the skidding joists. When the four half trusses were seated upon the skidding joists, each pair was spliced, forming two complete trusses. The gantries were then removed.
At this point, the line was closed to traffic for a period of 10 days. The track was removed from the U.C.R.Bs. which were traversed into alignment and lowered four feet. This allowed L.S.T. units which had supported the U.C.R.Bs. to be removed, thus permitting the two Roth Waagner trusses to be skidded into final alignment. The U.C.R.Bs. were then used as an erection platform for the erection of the top cross girders and stringers of the Roth Waagner span. The fitting of the stringers and stringer connectors was one of the major difficulties. As the track was on a curve, each stringer connector had to be offset on the cross girder. Although it was possible to calculate the theoretical position of each stringer connector, this did not permit prior drilling, owing to the great flexibility of the main trusses, which were not absolutely straight, even alter all the sway bracing was fitted. Accordingly, all cross girders and top seat bracings were first fitted, and a line was then set across the bridge enabling fresh holes to be drilled « in situ ». The U.C.R.Bs. were then seated upon the bottom cross girders, and split into sections at the chord splices, enabling five out of eighteen diagonal bracings to be fitted.
Track was then laid across the bridge which was reopened to traffic on 14 October 1945.
The U.C.R.Bs. were then dismantled inside the Roth-Waagner span, the components being lowered to the ground by means of a tackle connected to the Hyster winch of a D4 angledozer. As dismantling proceeded, the remaining diagonal bracings were fitted. The final stage of the task was the dismantling of the temporary trestle pier. Apart from the difficulty of setting the stringers to follow a curved track, the Roth-Waagner was found to be an easy bridge to erect. Almost all of the difficulties which arose on site, and there were many such, were due entirely to the confined working space on the originai piers, there being only a space four feet square on each side of the U.C.R.Bs. This area appears very small when situated 145 ft. above the ground.
It says much for the British soldier, that the men, under trying conditions and more often than not in bad weather, worked with a zeal and determination which enabled the task to be completed without mishap or serious casualties.
[Railway reconstruction Italy 1943-1946 published by Royal Engineers, 1946]