Ponte Luís I: the double-decker iron arch that joined Porto and Gaia

On Sunday 31 October 1886, Porto inaugurated the upper deck of its new iron bridge over the Douro. The deck ran high above the river from the cathedral quarter on the Porto bank to the heights of Vila Nova de Gaia opposite, and it rested on a single iron arch 172 metres across.3 sources

The date was also King Luís I's 48th birthday. He had been born at the Necessidades palace in Lisbon on 31 October 1838, and as a young prince he had been the first Duke of Porto. The bridge took his name. The plaque over the entrance to the lower deck still spells it the old way, Ponte Luiz I, although almost everyone in Porto calls it the Ponte D. Luís.2 sources

Beside it hung the bridge it was built to replace. The Ponte Pênsil, a suspension bridge opened in 1843, carried a wooden deck six metres wide about ten metres above the water, slung from cables passing over stone obelisks on each bank. Next to the new arch it looked tiny.2 sources

The new bridge did something the old one could not. It had two decks: the upper one at the level of the streets on the hilltops, and a lower one at the level of the quays, which was not yet finished in 1886. Infraestruturas de Portugal (IP), the state company that looks after the bridge today, says the lower deck was only completed in 1888.2 sources

For most of its history Porto crossed the Douro by boat. The river runs through a steep gorge between Porto and Gaia and floods hard. As late as 1787 the priest Agostinho Rebelo da Costa, describing the city, wrote that its heavy current did not allow a bridge. A regular ferry between the two banks had run since 1744.1 source

Bridges of boats were probably laid across when an army had to pass or a great festival called for one, and then taken up again. In 1806 Porto opened one meant to stay: 20 boats moored side by side and anchored upstream and downstream, with a roadway on top for people, animals and carts. It rose and fell with the tide, could be opened to let larger ships through and was taken apart when the river was in flood. It is attributed to the military engineer Carlos Amarante.1 source

On 29 March 1809, during the second French invasion of Portugal, Marshal Soult's army stormed the lines that defended Porto. Soldiers and townspeople ran down the steep streets to the bridge, the only way out across the river. Part of the roadway in the middle was missing, and the crowd behind, unable to see the gap, kept pushing the people in front into the water.2 sources

Accounts differ on how the gap opened. Oman followed Portuguese witnesses who said the first fugitives across had raised a drawbridge between the central boats to stop the French. The earlier historians Le Noble and Napier, and a witness named Hennegan who was in Porto that day, said the central boats sank under the weight. Nobody counted the dead. Contemporary figures for all the Portuguese killed and drowned in the storming of the city ran from 4,000 to 20,000. Oman put the true total at perhaps 7,000 or 8,000, thousands of them drowned in the river.1 source

The bridge was repaired, and on 12 May 1809 the French burned it to stop the Anglo-Portuguese army under Arthur Wellesley from crossing. Two improved bridges of boats followed before Porto got its first fixed crossing.1 source

That was the Ponte Pênsil, officially named after Queen Maria II, though nobody called it that. Work began in February 1841 and it opened on 17 February 1843, the first fixed bridge anywhere on the Portuguese stretch of the Douro. It was designed by the engineer Stanislas Bigot and built by the company of Claranges-Lucotte. Eight cables of parallel iron wires, four on each side, held a deck over a span of about 170 metres, and travellers paid a toll at a small booth at the Porto end.3 sources

It was light and graceful, and from the start people did not trust it. Some writers said it shook underfoot like a green twig. After suspension bridges of the same type failed at Angers in 1850, where about 200 people died, and at La Roche-Bernard in 1852, the government had the Porto bridge load-tested in March 1853 and ordered an inspection every year. When the builders' concession ended on 17 January 1877 it was tested again and passed, but the doubts remained.2 sources

Porto is built on a slope. Its old streets run steeply down to a broad quay on the river, and the hill of Serra do Pilar rises just as steeply on the Gaia side. A bridge low enough to land on the quays passed far below the upper town, and a high one left the riverside behind.2 sources

The idea of serving both levels at once came before the competition. In 1876 Bento Fortunato de Almeida d'Eça, an army engineer, drew a preliminary design with two decks on roughly the present site. Manuel de Azeredo, the Porto engineering professor who wrote the standard account of the city's bridges, thought it was probably the first. In 1878 or 1879 Gustave Eiffel proposed something else: a drawbridge modelled on his Maria Pia railway bridge, with a footway running along the bottom of the arch. That footway sat only a little above the roadway, so it did nothing for the upper town around the cathedral.1 source

When the government held an international competition in 1880, its programme required a link at two different heights. According to the engineer Almeida e Sousa, the programme was written by João Joaquim de Matos, then director of the Minho railway. Ten firms sent in twelve designs.2 sources

The winner was one of two similar variants entered by the Société de Willebroeck, a Belgian construction company from Brussels, both signed by Théophile Seyrig. It was one of the most expensive entries, at 369 contos (369 million réis, the currency of the day) against 219 for the cheapest, and the jury chose it on technical grounds. Eiffel's entry was much cheaper. The jury praised it but found it did not fit the programme, while it treated the points on which Seyrig's design strayed from the programme as minor and asked him to change them, which he did.5 sources

Seyrig was born in Berlin on 19 February 1843 and studied engineering at the École Centrale in Paris, where he lived most of his life. At the end of the 1860s, newly qualified and wealthy, he went into partnership with Gustave Eiffel, more than ten years older and already established. Eiffel's share of G. Eiffel et Cie was mostly equipment and know-how; Seyrig put up the larger part of the capital in cash, and he ran the firm's design office.3 sources

The firm's great success was the Maria Pia railway bridge over the same gorge, inaugurated by King Luís and Queen Maria Pia on 4 November 1877. It completed the Northern Line, the main railway from Lisbon to Porto, and its iron arch spanned 160 metres. Azeredo and José Manuel Lopes Cordeiro, a historian at the University of Minho, both argue that the arch was Seyrig's idea. In 1878 a booklet the Eiffel firm itself published, to go with a model of the bridge at the Paris Universal Exhibition, credited its design to him.4 sources

The partners fell out. Seyrig felt his work was hidden behind the name of the firm, and that the partnership treated him unfairly: Eiffel paid himself three times Seyrig's salary. The quarrel came into the open in 1879, and both historians say Seyrig stopped working with Eiffel that year. Azeredo has him as managing director of the Société de Willebroeck from 1879, in time to enter the Porto competition. Lopes Cordeiro dates his formal exit from the partnership to June 1881, after which Seyrig sued his former partner and lost.3 sources

So the competition of 1880 set the two former partners against each other, and Seyrig won. Eiffel, beaten in Porto, won a competition for a bridge at Szeged in Hungary in the same years. Seyrig went on to build bridges in Belgium, Spain and elsewhere, but he treated the Porto bridge as his masterpiece and used the closing of its arch as a worked example in a textbook on graphic statics.1 source

Work began in 1881. The iron arch spans 172 metres between its supports and rises about 45 metres; Azeredo gives 44.6 metres in one place and 45.1 in another. At each end it meets a large masonry portal, which makes the arch look fixed solid to the banks; the abutments are in fact hollow.4 sources

The two decks work in different ways. The upper deck runs over the top of the arch on iron piers and continues on both banks to the higher ground. The lower deck crosses where the arch meets the abutments: it hangs from the arch and simply rests on the abutments at each end, free to move. Like the Maria Pia, the arch is hinged at both ends, and Seyrig shaped it so that the lower deck could pass through it.2 sources

Seyrig did much of his calculation by graphic methods, working out the forces by drawing rather than by long equations. More than a century later, when engineers in Porto analysed the bridge by computer to see whether it could carry a light metro, they found that his figures held and that only a fairly small number of bars needed strengthening.1 source

The upper deck opened first, on 31 October 1886. IP dates the completion of the lower deck to 1888. The old suspension bridge was taken down in 1887. On the Gaia bank its obelisks were demolished with the cables, but on the Porto bank only their capitals were removed, to save the guardhouse built between the two towers. The capitals were put back much later, and the two obelisks still stand.3 sources

Until the early 2000s road traffic used the upper deck as well as the lower one. Electric trams reached Porto in 1895, and the city's tram museum says the power station built for them at Massarelos, in full operation from 1915, also electrified the Ponte Luiz I.2 sources

Edgar Cardoso, the engineer who designed the Arrábida bridge of 1963 and the São João railway bridge of 1991, later redesigned the upper deck. A plan of his to widen the bridge was never carried out.2 sources

In the late 1990s, as Porto planned its light metro, engineers studied whether the Luís I bridge could carry it. The answer was yes. Because the upper deck would be given over to the metro alone, Metro do Porto built a new road bridge, the Ponte do Infante. It opened on 30 March 2003, and the company called it the sixth crossing between Porto and Gaia.3 sources

On 17 September 2005 Metro do Porto opened Line D, the yellow line, from Câmara Gaia to Pólo Universitário. Its trains cross the upper deck between the cathedral quarter and the Jardim do Morro on the Gaia side.2 sources

The lower deck needed work too. In 2023 IP finished rebuilding it: corroded iron parts and rivets were replaced, bent plates straightened, and new bearings and expansion joints fitted. The new deck can take vehicles of up to 60 tonnes, where the old one had been limited to 30, and it is heavier and stiffer so that it sways less when big crowds cross on festival nights or after football matches. It is now open to pedestrians, public transport, taxis and bicycles.1 source

In 1996 UNESCO added Porto's historic centre to its World Heritage list, under its criterion (iv). The site is now named the Historic Centre of Porto, Luiz I Bridge and Monastery of Serra do Pilar. Portugal's national UNESCO commission describes its value partly as a view: the old town's streets fit together with the river and with Vila Nova de Gaia across it.1 source

That view is about wine as much as iron. For centuries wine from the Douro valley came down the river to be stored and traded in Gaia, which became the warehouse between the vineyards and markets abroad. In March 1809, Oman notes, nearly thirty merchant ships, most of them English and loaded with port wine, were stuck at the Porto quay by contrary winds when Soult's army arrived. The lower deck of the Luís I bridge now joins the Ribeira quays to the Gaia waterfront, and thousands of people walk across it every day.3 sources

The bridge has also kept a man's name half hidden. Azeredo, writing in the 1990s, argued that Seyrig had been left out of the history of engineering, even in Porto, and suggested the city name the avenue by the bridge after him. Lopes Cordeiro made the same case about the Maria Pia bridge in 2009. Seyrig's arch over the Douro has now carried walkers, cars, buses and metro trains for 140 years.4 sources