hybrid wrought iron and steel connections. the example of the …131.111.147.69/chs-conf/papers/7....

Stéphane Sire and Muriel Ragueneau

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Hybrid wrought iron and steel connections. The example of the electric arc welding reinforcement of the swing bridge in Brest (France)

Stéphane Sire1 and Muriel Ragueneau2 1. University of Brest, Brest, France; 2. SNCF Réseau, La Plaine Saint-Denis, France

Introduction

The arsenal of Brest (western Brittany, France) is notable for a high level and acknowledged technological innovation. During its development and expansion, the townscape of Brest, particularly the port landscape, has evolved significantly. New communication lines were opened to enable industrial and economic development outside the constricted boundaries of the city. Among these, a mobile, wrought iron bridge was built over the River Penfeld between the two cities of Brest and Recouvrance and inaugurated in 1861. At that time, the hot riveting process was the preferred joining method in metal constructions. In addition, wrought iron was the material used in these constructions, as steels were not yet produced. Due to the ageing of the bridge, particularly explained by the increase in traffic, it was decided to repair and reinforce the structure at the end of the 1920s. But how can a structure be repaired and reinforced when its constitutive material is no longer manufactured? During the 1930s, another joining process replaced progressively the hot riveted technique: the electric arc welding. It was the chosen process for the repairs and reinforcement of the Brest bridge. This was therefore a major scientific and industrial challenge: the welded metal structures were still recent and the welding of the hybrid wrought iron and low carbon strengthening steel assemblies had been studied very little.

The swing bridge over the River Penfeld

The city of Brest and its arsenal developed historically around the crossing of the Penfeld, a short river separating the city centre from the Recouvrance district. Until the 19th century, the passage from one side of the river to the other was made either by making a deviation of about ten kilometres to go up the Penfeld, or on a small boat designed to receive passengers, or later by the installation of a floating bridge. The economic expansion of Brest required the construction of a permanent and large bridge.

A remarkable swing bridge spanning the River Penfeld was built between 1856 and 1861 to link the two cities of Brest and Recouvrance. The choice of the swing bridge as a route over the Penfeld was indeed the result of more than ten years of projects and conflicts within Brest society [1]. On November 3, 1843 the municipal councillor Joseph Victor Edouard Tritschler (1815-1879) submitted his project for a suspension bridge. This project was endorsed and defended for nearly 10 years by the municipality of Brest. Indeed, despite the rejection of the Tritschler project by the Conseil Général des Ponts et Chaussées, the situation remained unchanged until April 1852. Following an intervention by President Louis-Napoléon Bonaparte (who became Emperor Napoléon III on 2 December 1852) to build a bridge over the Penfeld, projects by Ponts et Chaussées engineers competed with the


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local project in 1852. Finally, the project of the two Parisian engineers, Nicolas Cadiat (1805-1856) and Alphonse Oudry (1819-1869), was chosen; the Emperor decided in favour of the adoption of this bridge and promulgated a decree on 1 May 1854. This bridge was first designed following the same model used for the Pont d’Arcole over the River Seine in Paris, by the same two engineers [2].

The Brest bridge includes two identical wrought iron rotating spans. Each span is supported on a masonry pier by a ring of fifty conical rollers which facilitates its rotation. It consists of a 58.325 metre long part over the river and a smaller 28.6 metre one, on the docks side. The latter includes a cast iron counterweight to ensure the balance of the span. It consists of two main beams with latticework in the form of a St. Andrew's cross and vertical members. These beams are spaced 6.2 metres apart and connected by horizontal and vertical bracings (Fig. 1) [3]. Three mechanisms enable the rotation and positioning of the two spans: a first mechanism ensuring the rotation of each arm, a second one allowing the hanging to the abutments and a locking system ensuring the continuity of the two mobile parts [4].

Fig. 1. The swing bridge opening for the cruiser Montcalm (City of Brest archives).

The structure was calculated taking into account an overload of 200 kilograms per square metre and considering the gross cross-sections of the different parts. Under these conditions, the calculated stresses did not exceed 60 megapascals. Calculations made in 1913 showed that the crossing of 8 metric ton axles and an overload of 400 kilograms per square metre could be allowed without exceeding a stress of 100 megapascals. During the First World War, the bridge had to support excessive loads; in 1918, a prefectural decree thus increased the maximum weight of vehicles allowed to cross the bridge to 8 metric tons and the maximum speed of crossing to 5 kilometres per hour. The problems resulting from this load limit became more and more sensitive as the traffic increased. A detailed inspection of the structure in 1929 showed moreover that it was also weakened by some corrosion; it was therefore necessary to carry out repair work. The public works administration considered that it


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was appropriate to take advantage of the repairs to reinforce the metal structure so as to allow the passage of overloads provided for by the ministerial regulation of 10 May 1927. In order to meet this application, the best studied proposal included excessive strengthening and unacceptable execution procedures requiring very important riveting and unriveting operations. On 26 March 1932, after a study of the mechanical characteristics of the wrought iron of the bridge, the Ministry of Public Works recommended the use of welding for repairs and reinforcement [5].

The electric arc welding strengthening

The first full-welded road bridge was completed during the 1928-1929 winter at Lowicz, Poland [6]. Few months earlier in 1927, a full welded railway bridge was built in Turtle Creek (Pennsylvania, USA) and was the first welded bridge in the world. It should be noted, however, that a welded footbridge was built in Zurich, Switzerland in 1926 [7]. The requirements of the Lowicz bridge constitute the first official regulations relating to arc welded metal construction. These regulations included conditions of acceptance of the welding process. Moreover, mandatory conditions of the mechanical characterization had to be respected [8]. First of all, tensile tests of welded specimens were specified and the fracture load had to reach at least 80 per cent of the ultimate tensile load of the base material; the total elongation had to reach at least 15 per cent. Second, bending tests had to be carried out and dedicated specimens had to be bended to 180 degrees (the two extremities of the specimen had to be parallel) without any fracture or visible cracks. Finally, shearing tests were required: the ultimate shearing load had to be 11 to 12 times smaller than the ultimate tensile load of the considered tested material (depending on the thickness). These tests therefore required prior determination of the mechanical characteristics of the base material.

In the quarterly publication Acier of 1936 (n°2), the OTUA (Technical Office for the Use of Steel) gave a summary table of the various fully welded bridges up to 1934 [9]. This review shows a significant increase in the number of welded bridges (footbridges, railway and roadway bridges) since 1926 (Table 1). Note that the large majority of welded bridges from 1934 are indicated as Vierendeel-type bridges and were built in Belgium. Espion indicates that between 1933 and 1937, some 50 Vierendeel-type welded road bridges were erected in Belgium [10]. Let us add that until 1934 this Table does not mention any welded bridge built in France, even though welding is recommended to reinforce the structure of Brest from 1932.

Table1: Number of fully welded bridges and type of bridges according to OTUA.


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Indeed, in France the first fully welded structure was built in 1928, it is a hangar of a factory located in Pont-Sainte-Maxence (Oise department) [11]. The public authorities founded the Ecole Supérieure de la Soudure Autogène in 1930, and although numerous tests proved the possible replacement of riveting by welding, the first circular related to steel constructions with welded assemblies is only issued in 1934.

Two kinds of welding were considered on riveted existing structures: repairing and strengthening. Repairing works consisted in replacing damaged or corroded parts by new arc welded ones in order to build up again full sections. Strengthening works were, on the other hand, done by arc-welding new sections to existing parts of the structure. Consequently, riveted bridges reinforced by arc welding have riveted joints and welded joints on the same structure. To calculate the reinforcement, A. Goelzer suggested proceeding in three steps [12]. First, stresses in the existing parts of the structure in its original configuration for dead loads must be determined. Then additional stresses due to live loads and increased dead loads must be calculated. Lastly, final stresses after strengthening are determined.

The Brest swing bridge is made of wrought iron; this material includes non-metallic inclusions and is non-isotropic because of the lamination induced by the puddling process [13]. The welding of this kind of metal can therefore be complex and few studies had been carried out by 1932 (when the solution of arc welding was recommended by the Minister of Public Works for the reinforcement of the bridge in Brest) on the weldability of wrought iron used in bridges. All the studies and the reinforcement work by welding of this large structure therefore constituted an experimental, technological and industrial challenge by aiming at the same time at the success of the welding of a complex material, the dimensioning and the execution of hybrid assemblies with the iron of the original structure and the mild steel chosen for the reinforcement. Indeed, wrought iron was no longer produced industrially and had been progressively replaced by steel in metal constructions. Lecomte also points out that the reinforcement of a wrought iron bridge had never been carried out in France; only a Danish bridge built in 1867 was reinforced in 1933 and two German bridges built in 1884-1888 were also reinforced by welding in 1933. The knowledge acquired during this reinforcement operation was used by the SNCF (French National Railways), a few years later, for repairing and reinforcing the railway bridges damaged during the Second World War [14]. Vallette and Goelzer, SNCF engineers, also specified that the repair of the Oissel bridge, validated after the tests of June 1947, was carried out thanks to the experience acquired by SNCF from works such as those conducted in Brest [15].

Strengthening the swing bridge in Brest

The strengthening weld, a complex assembly of three materials

Mechanical tests (tensile tests) conducted on wrought iron samples taken from the bridge structure revealed scattered characteristics. Indeed, on all the tests carried out, the elastic limit was evaluated between 183 and 302 megapascals, the stress at failure between 280 and 343 megapascals and finally, the elongation at failure between 4.2 and 7.6 per cent. These results highlight principally that the characteristics of the iron are related to the part from which they were extracted; for instance, the specimens taken from a floor beam had much higher characteristics than those of a plate taken from a transverse bracing. Since wrought iron includes non-metallic inclusions, the scatter of these results is also not surprising and is in accordance with the numerous observations reported in the dedicated literature. The composition of the iron, showing a high phosphorus content (from 0.37 to 0.41 per cent depending on the samples), is also in good agreement with the data from the dedicated literature. Given the very low elongation measured, the iron from Brest is to be classified as a brittle metal.


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Because of inclusions, the welding operation is more challenging because the induced thermal cycle can change the metallographic structure of the iron and in particular the distribution and shape of the inclusions in the heated zones. Welding tests carried out by the Institut de Soudure Autogène (Autogenous Welding Institute) have shown the weldability of Brest iron and suggested that this base material should be melted as little as possible to weld it to the reinforcing steel. There is little information on the latter in the various documents relating to the reinforcement of the Brest bridge. We know that it is a mild steel from the Schneider companies, therefore a low carbon steel. This type of steel is known to have good weldability [16] and mechanical characteristics typical of a ductile metal. Article 1 of the circular of 19 July 1934 on the construction of steel structures and bridges with electric arc-welded assemblies states that the requirements of the Regulation of 10 May 1927 (calculation and testing of steel bridges) and those of the circular of 7 February 1933 (use of high-strength steels [17]) are applicable [18]. Thus, in accordance with these two requirements, the reinforcing steel used has a minimum yield stress of 235 megapascals, a minimum stress at failure of 412 megapascals and a minimum elongation at failure of 25 per cent.

On the other hand, the steel constituting the electrodes of type Safer special C40, has been characterized by the Institut de Soudure Autogène. On the Brest bridge, only electrodes with a diameter of less than 5 millimetres were used, in successive passes, in order to limit the heated zone of the wrought iron. Tensile tests carried out on the filler metal revealed a ductile metal with a yield stress between 275 and 322 megapascals, a stress at failure between 412 and 510 megapascals and an elongation of 20 to 29 per cent [19]. These characteristics are similar to those of the reinforcing steel.

As the reinforcement of the Brest bridge involved three different metals, it required original characterization tests; different welded specimens were then designed, manufactured and tested. It was indeed necessary to ensure the performance of the technological solutions proposed, as different types of welds were actually performed on the structure. First of all, the reinforcement procedure on wrought iron was conducted on iron-steel butt welds; a weld overlay on the edge of the iron piece was previously carried out. This overlay has the effect of joining the different sheets of wrought iron together and enables a steel-on-steel weld to be carried out under better conditions. The same principle with overlay was used for thicker parts with a double-V butt joint. In accordance with the circular of 1934, tensile tests were carried out; for this purpose iron-steel specimens of 350 square millimetres section were used. The specimens always broke in the iron. Bending tests showed the first crack to occur between 20 and 40 degrees in the zone of attachment on the iron side. This value is lower than the recommendations of the circular specifying a minimum angle before cracking of 45 degrees. Finally, original cross-shaped specimens were fabricated and tested (Fig. 2). Depending on the group of test specimens, the tensile stress was applied on the iron or on the reinforcing steel. In the most cases, the fracture occurred at the limit of the weld bead on the iron side.


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Fig. 2. Macrograph example of an iron-steel cross assembly, from A. Lecomte, Annales des Ponts et Chaussées (1938).

Strengthening calculation and design

The calculation of the reinforcement was done first by determining the stresses in the different parts under the permanent load before reinforcement and then with the increase in the permanent load applied to the reinforced cross section. Then, the stresses were obtained, considering the reinforced section, under overload and wind. The overloads were calculated at 586.7 kilograms per square metre for the large part of the rotating span, 707 kilograms per square metre for the small one and 400 kilograms per square metre for the sidewalks. The maximum stress in the iron elements was thereby calculated at 12 kilograms per square millimetre i.e. 118 megapascals, two-thirds of the minimum elastic limit identified by tensile tests. For reinforcing steel, the stress did not exceed 69 megapascals at any point, well within the limits set by the Regulation of 10 May 1927. Stresses in the welds also never exceeded the limits imposed by the circular of 19 July 1934.

The reinforcement required the use of 283 metric tons of steel for steel structure and 120 metric tons of cast iron to increase the counterweight. Taking into account all the materials added, the mass of a span increased from 742 to 980 metric tons. In particular, the steel reinforced the deck. Indeed, in its initial version, the deck did not have any stringers. Sixteen stringers made of IPN-160 profiles were added and welded at their ends to the floor beams. The bottom chords were reinforced with two reinforcing webs symmetrically on each side and welded all around to the iron original chord. The top chords (Fig. 3) were reinforced by an additional web, the addition of a flange in the lower part and the welding of three rectangular pieces (up to 120 x 120 square millimetre) of steel in the upper part.


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Fig. 3. Reinforcement of the top chord, from A. Lecomte (1938).

Welding work progress, control and validation

On both sides of each span, a 64 metric tons scaffolding was set up for the reinforcement of the main beams. It consisted of two floors; the first one below the top chords and the second, connected by tension rods to the first one, below the bottom chords. The requirement to have the bridge open during the repairs imposed that the work had to be carried out in a certain time order so that longitudinal and transverse balance could be ensured all the time. Transverse balance was easily achieved by carrying out the reinforcement work simultaneously and with the same progress on both main girders. Longitudinal balance was achieved by carrying out the work simultaneously on both sides of the rotating span and by adjusting the counterweight at the end of its small part. The work on the steel structure itself began at the end of March 1935 with the reinforcement of the bottom frames. During the execution of the work to strengthen the deck, from the beginning of January to the end of May 1936, traffic had to pass over half the width of the bridge. Traffic was carried during the day alternately in one direction then the other. At night, traffic was closed to vehicles and free for pedestrians (Fig. 4) [20]. Trams operated to each end of the bridge and passengers walked across the bridge.


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Fig. 4. Crossing the bridge during reinforcement work (La Dépêche de Brest, 1936).

All the welders employed on the construction site had undergone, before being hired, the tests prescribed by the 1934 circular under the control of the Autogenous Welding Institute. These tests were repeated several times during the work. Regardless of this control, all welding operations were carried out under the supervision of an engineer from the Autogenous Welding Institute. X-ray examination of the quality of the welds was considered, only tested but not conducted because the non-metallic inclusions included in the wrought iron made the results delicate and uncertain to evaluate.

The tests of the structure were carried out on June 21, 1936. The overload of the roadway was composed of fourteen trucks with an average mass of 11.9 metric tons and six tramcars with an average mass of 13 metric tons, which represents an overload of approximately 575 kilograms per square metre. The stresses were measured using Huggenberger, Manet-Rabut and Bourdon instruments [21]. The deflections were also measured at the end


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of the span on the Brest side (Fig. 5). In all cases, the measured stresses were lower than the stresses calculated for these tests. Also, the measured deflections were less than those obtained during the 1861 tests, when the bridge was subjected to an overload of only 200 kilograms per square metre. The strengthening was thus shown to be successful and the Brest Bridge met all the requirements of the 1936 traffic regarding overloads.

Fig. 5. Measurements during the bridge tests (La Dépêche de Brest, 1936).

Conclusions

Following the increase in traffic and the passage of the tramway over the swing bridge of Brest since 1898, it was decided to reinforce it in 1927 and the solution of a strengthening by arc welding was chosen in 1932. The metallurgical weldability of wrought iron with low carbon strengthening steel was first studied. The electrodes were chosen in accordance with the French Circulaire of 25 July 1935. The stresses in the various bridge components, including the added ones were calculated. Dedicated specimens were also manufactured and tests were performed in order to assess the mechanical strength of the iron and steel welded connections. In addition, the welding operations were controlled by the Institut de Soudure Autogène. The result was a 32 per cent increase in the weight of the structure without modifying its general aspect. The swing bridge over the Penfeld was thus the first wrought iron bridge of the French National roadway network to be reinforced by arc welding. Having met the scientific and industrial challenges related to the large-scale welding of assemblies of two different materials, the strengthening of the Brest bridge is part of the history of metal construction, the history of bridges and their regulations.


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References [1] P. Levot, Le passage et divers droits ou coutumes de Brest avant 1789. Le Pont Impérial en 1861. Brest (FR) :

Librairie de J-B. et A. Lefournier, 1862, pp.1-59. [2] F. Bosman, M. Mille and G. Piernas, L’art du vide : ponts d’ici et d’ailleurs, trois siècles de génie français,

XVIIIe-XXe. Paris (FR) : Somogy éditions d’art, 2010, pp. 64-75. [3] City of Brest archives, postal card 3Fi083-067, Ed. Masson, 1937. [4] E. Aumaitre, ‘Note relative au pont tournant construit sur la Penfeld pour la traverse de la route impériale

n°12, dans la ville de Brest’, Annales des Ponts et Chaussées, Tome XIV, 4ème série, 1867, pp. 265-276. [5] A. Lecomte, ‘Réparation et renforcement du pont tournant de Brest’, Annales des Ponts et Chaussées, May

1938, pp. 629-673. [6] S. Bryla, ‘Le pont-route métallique soudé à l’arc électrique de Lowicz (Pologne)’, Le Génie Civil (14

September 1929), pp. 250-255. [7] OTUA, La soudure à l’arc électrique, Paris (FR) : Edition OTUA, 1936, p.18. [8] S. Sire, J-F. Douroux, ‘The electric arc welding reinforcement of steel bridges from the Paris Metro in the

1930s: the case of the Austerlitz viaduct over the Seine’, pp. 345-353 in B. Bowen, D. Friedman, T. Leslie, J. Ochsendorf, (Eds.), Proceedings of the Fifth International Congress on Construction History, Vol.3, Chicago 2015, Chicago: Palmer House Hilton Hotel, 2015.

[9] OTUA, (Note 6), pp. 18-20. [10] B. Espion, ‘The Vierendeel bridges over the Albert Canal, Belgium – their significance in the story of brittle

failures’, Steel Construction, Vol.5, no.4, 2012, pp. 238-243. [11] A. d’Angio, Schneider et Cie et les travaux publics (1895-1949), Paris (FR) : Ecole des chartes, 1995. [12] A. Goelzer, ‘Strengthening of steel bridges by electric arc welding’, IABSE publications, Vol.4, 1938, pp.

305-318. [13] L. Gallegos Mayorga, S. Sire, M. Ragueneau, B.Plu, ‘Understanding the behaviour of wrought-iron riveted

assemblies: manufacture and testing in France’, Proceedings of the Institution of Civil Engineers - Engineering History and Heritage, Vol.170, no.2, 2017, pp. 67-79.

[14] SNCF, La reconstruction des ouvrages d’art de la SNCF, juillet 1940 – juillet 1942, 1942. [15] R. Vallette, A. Goelzer, ‘Welding applied to the reconstruction of the Oissel bridge over the Seine’, IABSE

congress report, Vol.3, 1948, pp. 91-104. [16] R. Granjon, R. Salelles, Manuel pratique de soudure électrique à l’arc. Paris (FR) : Soudure Autogène,

1939. [17] b54 grade steels are allowed for the construction of steel bridges. Their minimum limit at failure is 54

kilograms per square millimetre, i.e. 530 megapascals. [18] ‘Instruction provisoire du Ministère des Travaux Publics, pour l’exécution des charpentes en bois et en acier

avec assemblages soudés à l‘arc électrique. Circulaire ministérielle du 19 juillet 1934’, Le Génie Civil (16 March 1935), p. 259.

[19] A. Lecomte, (Note 4), p. 654. [20] ‘La circulation normale va être rétablie sur le Pont National’, La Dépêche de Brest (11 June 1936), p. 3. [21] ‘Les épreuves d’essai du Pont National. Les résultats obtenus paraissent très satisfaisants’, La Dépêche de

Brest (22 June 1936), p. 3.

hybrid wrought iron and steel connections. the example of the …131.111.147.69/chs-conf/papers/7....

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