Pont d'Arcole

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Crosses the River Seine in Paris.

Built 1854-6.

Ironwork by J. F. Cail et Cie.

Span 80m. Width 20m. 14 arch ribs. The arch ribs are assembled from riveted wrought iron plates with angle iron and T-section stiffeners.

The bridge is impressive for the low rise/span ratio of the arch (5.70m/80m = 1/14), and for the shallow depth of the rib at the crown (0.40m at the crown, compared with 1.40m at the springing points at the abutments). 0.40m is remarkably shallow, as is evident in photos 2 & 8. The impetus for minimising the rise/span ratio was the need to minimise the gradient on the bridge and the approach roads, and to maximise the headroom for boats on the river. The gradient was particularly important in the days of horse-drawn traffic.

The designers, Nicolas Cadiat and Alphonse Oudry, adopted an unusual design. It appears that the upper longitudinal members which support the deck were pre-tensioned. It was a bold design. Frédéric Bourgeon helpfully explains that the bridge functioned both as an arch and a cantilever. Due to a great cold and after 30 some years, one of the anchors broke and the bridge sagged as it was then "only" an arch.

The bridge suddenly sagged during a cold spell in 1888, necessitating repairs and modifications. Comparison of photo 2 with an 1883 photograph here shows that one of the changes was the addition of two arch ribs. Originally there were 12 arch ribs, 10 ribs being grouped together under the roadway, and an arch rib either side of these, pitched further out (in the plane of the balustrades). The 2019 photo above shows that there are now 14 arch ribs, the extra ribs flanking the original central group under the roadway.

Gustave Eiffel wrote about the failure , and a translation of the relevant text follows (with original text in brackets where the precise English equivalent is unclear]:

'But we had the unfortunate idea of ​​tyeing the upper rails [longerons supérieurs] to the masonry, so that the construction worked both as an arch and as two [consoles] abutted at their ends. As the tie-rods [tirants d'amarrage] failed due to cold contraction and possibly rusting, the arches began to move in full and deformations occurred that would require a complete reconstruction of the bridge, especially in the vicinity of the [clef].' The word 'clef' is problematic here. It literally translates as 'key', but it probably refers to the crown of the arch, as will be explored shortly.

Photo 3 shows that the end of the beam, immediately below the new concrete deck, no longer connects with the masonry abutment.

Another source, Jean Resal, offers more information. It does seem to confirm (with apologies for imperfect translation!) that the top beams were tied to the masonry. The combination of arch, upper beams, and spandrels represented a stiff structure, and together with the lack of articulation of the arch, the structure was susceptible to the effects of thermal expansion and contraction. Deterioration progressively occurred as the the bridge expanded and contracted, and after 33 years failure (of a top beam?) occurred during a particularly cold morning when a line of carts was crossing the bridge. The failure did not lead to disaster, but the arch dropped appreciably due to the increased thrust imposed upon it. Repairs included fitting additional plates to reinforce the central portion of the arch. Photo 6 shows the relative thickness of the flanges, composed of numerous plates riveted together.

The foregoing descriptions do not paint a clear picture, but this 1855 picture does! It shows that embedded in the concrete abutments are tie bars which were originally connected to the ends of the longitudinal deck beams of the bridge. The tie bars passed just above the top of the pedestrian tunnel, sloping slightly downwards from the bridge structure. It appears that the ends of the tie bars were secured against the end of the abutment by tapered cotters, and the load from the tie bars was spread by large vertical plates. However it also appears from the drawing that the tie bars would have been bonded to the concrete, and also fixed by embedded vertical bars, greatly limiting the elasticity available to accommodate expansion and contraction of the deck. This raises the question of why the anchor point of the tie bars was taken so far back through the abutments. One possibility is that the deck was pre-tensioned before the tie bars were fully embedded in concrete.

The critical connection between the tie bars and the bridge upper beams is close to the junction between the bridge and the abutment, and it is easy to envisage that corrosion would take hold here.

One source states that Barlow Rails were used to support the original deck material. The 1855 drawing shows the chevron section charactersitic of the rails invented by William Henry Barlow. In the UK large quantities of unwanted Barlow Rails became available after they were withdrawn from railway service after the concept proved unsatisfactory, and were reused for a variety of purposes. One source states that Barlow rails, made at Decazeville, were réutilisé in the Pont d'Arcole.

It is interesting to compare this bridge with the Pont Alexandre III, further downstream. This has an even longer span and flatter arch (rise/span ratio of just 1/17). It was built over 50 years later, and the design avoided the thermal expansion problems of the Pont d'Arcole. The arches are hinged at both the springing points and at the crown. The deck is supported from the arch ribs by vertical struts, not by triangulated struts and ties as on Pont d'Arcole. There are appreciable differences aesthetically, the decoration of the Pont d'Arcole being tastefully restrained, whereas the Alexandre is festooned with festoons and other high Victorian excesses, which serve to hide the work of the engineers.

G. Humar noted that it was the first bridge to span the Seine in Paris without the use of intermediate piers, and, as constructed, the arch was extremely slender (38cm, 15 inches) at the crown, providing some flexibility to accommodate thermal expansion. He observed that the builders may have gone too far with the slenderness of the arch, since on 16 February 1888 the bridge suddenly sagged by 20 centimetres. Additional strengthening of the bridge was quickly carried out before more serious damage could occur. The author regarded this bridge as an important step towards understanding the role of hinges, which led directly to the introduction and use of the first hinge in the next generation of iron bridges, and states that the first bridge-builders to put the theory of hinges in bridge structures into practice were the French engineers Couche and Salle. In 1858 they built a wrought-iron railway bridge to carry the Paris–Creil line over the Saint-Denis Canal. It comprised a horizontal lattice truss structure supported by an iron arch.

The lack of hinges at the ends of the arch rib, evident from photo 4, followed long-established practice for large iron arch bridges. J. G. James observed that in 1818 problems due to thermal movement came to prominence on Rennie's Southwark Bridge, and that on some cast iron arch bridges in Yorkshire the ends of the ribs and their bearings were curved to facilitate movement. These included a number of bridges designed by George Leather in the 1830s and 1840s, including Crown Point Bridge (Leeds), constructed in 1842.

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