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Le Pont des Arts

Le Pont des Arts

Structure Information

The Pont des Arts is an iconic bridge spanning across the Seine River in Paris, France. Construction of the current bridge began in 1981 and finished in 1984. Figure 1 below is a photo of the bridge today.

Figure 1: Le Pont des Arts, Paris, France

In english, “Le Pont des Arts” translates to “The Bridge of the Arts.” The name of this bridge is very fitting for its function because it serves as a pedestrian bridge that links the Institut de France and the central square of the Palais du Louvre. The Institut de France is a French learned society that houses French Academies such as the Academies of Music, Humanities, and Sciences. The Palais du Louvre is a former royal palace which is now the largest art museum in the world. Figure 2 below shows the bridge name carved in to the abutment closest to the Institut de France.

Figure 2: Bridge name carved in to the stone of one abutment

The Pont des Arts was designed by Architect Louis Arretche, and the structural engineering was done by Enterprise Morillon Corvol Courbot (EMCC) [1]. The bridge was built as a replacement for the former bridge built under Napolean Bonaparte. This bridge is a structure paid for by French Public Works.

Historical Significance

As previously stated, the Pont des Arts was built as a replacement for a bridge that was built in the same place across the Seine in 1804. The current bridge is almost identical in design to the original bridge, so by modern standards, the current bridge cannot be considered an innovative structural engineering design. However, the original bridge, completed in 1804 was the first metal bridge to be constructed in Paris, 19 years after the building of Iron Bridge in England. Napolean Bonaparte asked engineers to design a bridge that resembled a garden that was suspended over the Seine [2]. The original bridge was elegant, lightweight and constructed from cast iron, placing it on the cutting edge of engineering in its day. The piers of the original bridge were constructed in masonry as were all the piers of bridges along the Seine, but the use of cast iron was a very new construction technique. The construction of the present bridge did not involve any new construction techniques.

The current Pont des Arts is a steel arch bridge. Its historical connection and consequently almost identical design to the original bridge in the same location makes it unimpressive structurally by modern standards. The best existing example of a steel arch bridge is the Syndey Harbour Bridge in Sydney, Australia. It is the largest steel arch bridge in the world. Figure 3 below shows an image of the Sydney Harbour Bridge.

Figure 3: Sydney Harbour Bridge

Cultural Significance

The current Pont des Arts bridge is internationally known and is one of the most famous bridges in Paris. It is first iconic because of its location–the link between two of Paris’ most iconic buildings. The Palais du Louvre on the bridges right end has housed French kings since its construction in the 1200’s, and is now the largest and arguably most famous art museum in the world. On the left end of the bridge, the Institut de France houses the agency that manages over 1000 foundations, museums and chateaux that are open to the public, making it a major cultural landmark.

In addition to its locale, the Pont des Arts is a cultural landmark because of its connection to history. The bridge that was originally in its place was ordered to be built at the beginning of the rule of Napolean Bonaparte. Napolean’s empire dominated the French Revolutionary wars and facilitated the development of Paris with structures such as the Arc de Triomphe and the Pont des Arts.

The original concept for a metal bridge in Paris in the beginning of the 19th century was largely rejected by famous Parisian architects of the day. These experts thought that it would lack monument because of its lightness and metal form. The aesthetic of metal was vastly different from the monumental stone bridges between which the Pont des Arts was to be built. However, when construction was completed, the Parisians “took the bridge to heart” [3]. As with most bridges of the time, the Pont des Arts was a toll bridge which cost one cent to cross. On the day the bridge was opened, 65,000 Parisians paid their penny to cross the new bridge [3]. The permanence of the original design that continues in the Pont des Arts today describes the iconic and loved nature of this bridge.

There is no record of injured workers in the construction of the original Pont des Arts built in 1804 or the new Pont des Arts built in 1984. However, the original Pont des Arts was demolished in 1980 because of structural weakening and damage from barges. There were barge collisions throughout the life of the original bridge, considered to be the human cost of the bridge.

Today the Pont des Arts is an internationally known landmark. The bridge is a favorite for artists, musicians and people in love. In 2008, a tradition began which gave the Pont des Arts the unofficial name of the “Love Lock Bridge.” Couples would write their initials on a lock, attach it to the side rails of the bridge and throw the key in to the Seine River as a symbol of eternal love. The tradition became so popular that there was an additional 45 tons of weight added to the bridge from the locks [4]. This loading caused structural weakening and eventual collapse of one railing section. In 2014 the railing sections were removed and replaced with plexiglass sheets. Figure 4 below shows a railing section with the love locks in tact.

Figure 4: Pont des Arts railing with love locks attached [4]

The Pont des Arts is still used as a pedestrian bridge and remains an iconic part of Paris.

Structural Art

The design of the current Pont des Arts was dictated by the original iron design in 1804. The structure has been described as “light” since its original construction, which has been considered a good and a bad thing depending on the critic. I think that the original design boldly rejected the heavy monument of stone that was the norm for Parisian bridges at the time. The structure was able to be made light because of the new material of iron. In this sense, form was dictated by function. I think this is a major requirment of structural art, and the Pont des Arts embodies this requirement. David Billington states that aesthetics should be the final judgement when deciding if something is structural art. I was immediately drawn to this bridge. It stood out to me while strolling along the Seine because of its elegance and lightness when compared to the countless bridges of heavy stone that span the Seine. I think that the only thing that subtracts from the status of the Pont des Arts as structural art is the fact that the modern bridge was designed by an architect and not an engineer. Similar to this, the piers of the modern bridge are made of reinforced concrete, but faced in stone to pay tribute to the design of the original bridge. Hiding the true material that takes load is a way that the structure does not demonstrate structural art.

Structural Analysis

The modern Pont des Arts is designed to replicate the original bridge that was built in 1804 and demolished in 1980. The original bridge was a cast iron arch bridge with nine iron arches spanning a total of 509 feet. The form of the bridge was modeled after the British metal arch bridges that preceded it. The supporting bridge piers were constructed of stone masonry using cofferdam systems to block the water around the pier and pump it dry with buckets. The deck was contructed using wooden planks.

The design of the modern Pont des Arts bridge is almost identical to that of the original bridge. The differences lie mostly in construction materials used. The current Pont des Arts superstructure is constructed in steel. Steel is lighter, stronger and more ductile than iron, making it the clear modern choice after the failure of the original iron bridge. The piers are constructed in reinforced concrete but faced with masonry to pay tribute to the original bridge. The deck is constructed in timber, another tribute to the original bridge design. Another departure of the modern bridge from the original bridge design is the number of arches. The current bridge has seven arches instead of nine, a design choice that was made to be consistent with the number of arches of the adjacent Pont du Neuf, and made possible because of the ability to make longer arch spans with modern construction materials and technology.

The structural system employed in this bridge is repeating three-hinged arches. There are seven arches from bank to bank and there are five repeating arch systems that span from edge of deck to edge of deck as shown in Figure 5 below.

Figure 5: Five arches which span from edge of deck to edge of deck meeting at one pier

Figure 6 below shows an elevation view of contiguous repeated arches meeting at one pier.

Figure 6: Arches meeting at one pier

The repeated arches are supported by reinforced concrete piers and abutments on either end of the bridge.

Load from pedestrian traffic and the timber deck is transmitted from the deck to the spandrels connecting the arches to the deck. The load moves through the spandrels to the arches. The arches are in compression. Since the structural system consists of repeated arches, the horizontal thrust generated by each arch at the arch connection to the pier is cancelled out by the horizontal thrust generated in the opposite direction from the contiguous arch. The vertical load at eah arch end is transmitted through the bridge pier to the ground. The only horizontal thrust that is realized is at each end of the bridge and is taken by the abutments. The load path is shown in Figure 7 below.

Figure 7: Load path of one repeated arch

Using this load path and assumptions about the dimensions of the bridge, one of the total 35 arches can be analyzed. The dead load of this bridge is calculated using the density of the timber decking which is assumed to be 41.8 pcf [6]. Assuming the deck is 1 ft thick, the area dead load is equal to 41.8 lb/ft^2.

European building codes specify that the live loading associated with pedestrian footbridges is typically 5 kN/m^2 [5] which is equal to 104.4 lb/ft^2. The deck has 7 spans totaling 155 m or 508.5 ft and a deck width of 10 m or 32.8 ft. By the principle of superposition, to get total linear loading, the dead and live area loads are added and multiplied by deck width as shown below.

(41.8+104.4) lb/ft^2*(32.8 ft)= 4795.4 lb/ft

The length of one arch span can be found by dividing total span by number of arches as shown below.

(508.5 ft)/(7 arches) = 72.6 ft/arch

Height of arch is assumed to be 24.2 ft based on the visual proportion to arch length.

From these data and assumptions, and assuming the the load will be transferred completely to the arch by the spandrels, the following model shown in Figure 8 can be used to perform the analysis of one arch.

Figure 8: Simplified model of one arch

To analyze find the reaction forces the arch will be cut at the center hinge. A model of the left side of the cut is shown below in Figure 9.

Figure 9: Left side of arch cut at center hinge

By symmetry using the global structure, reaction force By is equal to zero. Ay can be found using sum of forces in the y-direction as shown below.

Ay – (4795.4 lb/ft)*(36.3 ft) = 0

Ay=174073.0 lb

Using the sum of moments about the top hinge, reaction force Ax can be found as shown below.

Ay(36.3 ft) – Ax(24.2 ft)-((4795.4 lb/ft)(36.3 ft)(1/2)(36.3 ft))=0

Ax = 130552.6 lb

From these equations we know that 174.1 kips of force is being transmitted vertically from one end of the arch to the pier and 130.6 kips of horizontal thrust is generated, which is counteracted by the contiguous arch. Since there are 5 pin connections at each pier with two arches connected at each pin, the total vertical force exerted on the pier can be calculated using the following equation.

(174073.0 lbs)*(5 pins)*(2 arches) = 1740730 lbs

We can use this force and assumptions about the geometry of the cross section of the piers to calculate compressive stress in each pier. It is assumed that the piers are rectangular in cross section, 32.8 ft in length, and 2 feet in width. The area of the pier can be calculated using the following equation.

Area = (32.8 ft)*(2 feet) = 65.6 ft^2

Assuming the cross-section is constant, the stress in the pier is found using the following equation.

Stress in pier = (1740730 lbs/ 65.6 ft^2)*(1 ft^2/144 in^2)= 184.3 psi

This value can be compared to typical strength of reinforced concrete, equal to 4000 psi.

4000 psi >> 184.3 psi

Based on these calculations, the piers are designed with a safety factor of 21. This is very high for a bridge, and should be considered higher than actual design because of assumptions made.

Sum of forces in the x-direction can be performed to find the horizontal reaction force Bx as shown below.

Ax – Bx = 0

Bx=130552.6 lbs (in the negative x-direction)

It is assumed that Bx represents the compressive force in the arch. We can use this force and assumptions about the geometry of the arch cross-section to find compressive stress in the arches. It is assumed that the cross-section of the steel arches are rectangular and the area of the cross-section is equal to 10 in^2.

Stress in arch = 130552.6 lb/10 in^2 = 13055.3 psi

Steel has a compressive strength of about 25000 psi. Comparing the design stress to material properties of steel,

25000 psi > 13055.3 psi

This indicates that the steel superstructure is designed with a safety factor of about 1.9. This value is close to what would be used in the design of a bridge.

It is assumed that since this bridge was built as a structure of French public works, drawings or plans were made to the specifications of French bridge building codes and communicated to the the owner (French government).

Personal Reaction

I saw this bridge while strolling along the Seine River from Notre Dame Cathedral to The Eiffel Tower. As I previously stated, I was immediately drawn to this bridge because of its lightness compared to the heavily ornamented stone bridges around it. Standing on the bridge with two huge French monuments on either side of me, it was amazing to me how much history and culture could be built in to a structure as simple as a foot bridge.

 

 

[1] https://structurae.net/structures/pont-des-arts-1984

[2] https://www.cometoparis.com/paris-guide/paris-monuments/pont-des-arts-s959

[3] https://www.napoleon.org/en/magazine/places/pont-des-arts-bridge/

[4] https://www.citymetric.com/horizons/paris-has-replaced-padlocks-pont-des-arts-padlock-themed-graffiti-1113

[5] http://www.cbdg.org.uk/tech2.asp

[6] https://www.forza-doors.com/performance-guides/general-guidance/timber-density-chart.aspx

Comments

  1. rclayton30 says

    Since the “Lock Bridge” railing has been replaced, I seen some new locks on the railing. Do you think they will every put up a sign stating that it is a hazard to add the locks to the bridge, and ultimately fine people if they are caught?