Eiffel Tower: Construction Features of the Iron Giant

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The Eiffel Tower was the first structure to reach the coveted height of 1000 feet and maintained its status as the world’s tallest structure until 1930. It was constructed as a landmark monument to commemorate the centenary of the 1789 French Revolution. Later on, the city officials recognized its significance as a radio tower, which proved instrumental during the first and second world wars.

Presently, the Eiffel Tower situated in Paris, France, is one of the world’s most iconic tourist destinations. Its architectural intricacies welcome more visitors than any other tourist attraction in the world.

In addition to its architectural value, the tower also marks significant improvements in engineering design with its four legs roughly representing an exponential curve, to resist wind loads in the best possible way. Due to the unique design of its legs, the tower could reach the height never attained before by any other structure.

The Eiffel Tower is a large wrought iron lattice structure comprising 7,300 tons of iron and 2.5 million rivets. It has three observation decks connected by multiple stairways and elevators. These observation decks attract an average of 30,000 visitors each day. This makes the tower’s observation decks a highly-visited area in the world. The tower also contains two restaurants that offer panoramic views of the surrounding city.

Eiffel Tower is standing tall even after so many years because of its remarkable engineering design. To explain this in detail, we’ll discuss the geotechnical, structural, and construction features that make it extraordinary.

1. Geology of Site

The following points describe the geology of the site of the Eiffel Tower:

1. The soil around the Eiffel Tower is a mix of both hard (dense sand) and soft soil (silt and clay).
2. The soft soil is predominantly located underneath the riverside legs, requiring deeper and broader foundations than the park side legs to reduce the settlement of the tower.
3. The lower bed of subsoil consists of plastic clay and is around 16 m deep, resting on the chalk rock.
4. The chalk rock bed is approximately 30 m from the ground layer.
5. The area around the bed of Eiffel Tower is characterized by an outcropping of conglomerates, silts and sands. Such soil conditions were not suitable for the foundation construction of such a tall tower.
6. Some of the present researches conclude that a clear resonance frequency of 2 Hz was observed at the tower site. The average shear velocity is around 250 m/s in alluvial layers of the tower site.

2. Structural Members of Eiffel Tower

The structural members of the Eiffel Tower consist of the floor system, beams, and legs. These structural members are discussed below:

2.1 Floor System

The floor of all the observation deck is made up of wrought iron. These decks are supported by the large grid of trusses that span between the legs.

2.2 Beams

The large grid of trusses that span between the legs acts as a beam for the structure. These trusses are made up of wrought iron. Trusses provide the support to the flooring system and also help in load distribution.

The lower and middle platforms are supported by a 16 m x 16 m and 6 m x 6 m square grid trusses with an area of 4200 m² and 1650 m², respectively. Beams in the upper half of the tower provide continuity between the four legs before they join to form a single leg. The spans of these beams vary from 1.8 m to 15.7 m.

2.3 Legs

The Eiffel Tower consists of four legs. These legs are of exponential curve shape, such that they meet to form a single point at the peak. The size of each leg is approximately 15 m x 15 m. Each leg consists of four columns. Each column is connected to the iron lattice. Thus, the columns of each leg behave as a truss member. The columns of each leg are made up of a hollow square cross-section, formed by a series of angles and flat sections.

The section properties of both the hollow square column sections and the diagonal bracing members vary with height, reflecting the increase in permanent loads of the tower at lower heights.

3. Foundation Details

The following are the important points about the foundation of the Eiffel Tower:

1. The lower bed of subsoil consists of plastic clay and is around 16 m deep, resting on the chalk rock.
2. Clayey soil is dry, compact, and capable of resisting 30 to 40 KN/m².
3. Each of the four legs rests on masonry blocks (10 m long and 6 m wide). These blocks are anchored into the foundation by a total of 16 anchor bolts. The bolts have a diameter of 10 cm and a length of 7.5 m.
4. The upper bed of masonry blocks was made up of Landon freestone. Such stones have a crushing strength capacity of 1235 kg/cm².
5. In addition to masonry blocks, each leg of the tower has a concrete pedestal, which is laid on the pier foundation through the walls.

Two different approaches were considered for the construction of the Eiffel Tower foundation on two different sides of the tower. i.e., park-side and riverside.

3.1 Park-Side Foundation

The following points describe the details of the park-side foundation of the Eiffel Tower:

1. Excavation and stabilization of park-side foundations were relatively easy.
2. The level of Seine River was around +27 m during the construction of the tower. The level of natural soil around the park side of the tower was at +34 m, making it easy to construct the foundation on such strata.
3. During the excavation, wooden formwork was used to protect the walls from collapsing. Gravel and concrete were poured to make piers for the foundation. Each foundation consists of two piers.

3.2 Riverside Foundation

The following points describe the details of the riverside foundation of the Eiffel Tower:

1. Excavation and stabilization of riverside foundations required complex engineering process. Extra care was taken because the foundations were located close to the Seine river.
2. The level of the Seine river was around +27 m during the construction of the tower. The bed of natural soil was not met until +22 m around the riverside of the tower. Therefore, it was not easy to construct the foundation on the riverside.
3. Each foundation consists of two piers. The laying of each pier foundation was done using compressed air. For each pier, four caissons, 15 m long and 6 m thick, were provided and each of the caissons was sunk 5 m below the water level of the Seine river. The caissons were filled with concrete to form an enormous mass of immovable solid.
4. Extra support in the form of reinforcement was provided. The reinforcements were driven deep into the riverbed to keep their foundation slabs from sliding or sinking in the softer ground.

4. Wind Load Consideration

Wind load is a critical factor for designing tall towers. It affects tall towers in two ways— statically and dynamically. The wind load conditions considered for the Eiffel Tower are discussed below:

1. The effect of wind load on the tower was considered in two configurations by Eiffel. First, a uniform load of 3 KN/m2 on the entire height. Second, a linearly varying load of 2 to 4 KN/m2 on the entire height.
2. During that time, Eiffel considered only the static effect of wind loads using the Culmann graphical method.
3. Current research studies revealed that although Eiffel did not consider the dynamic effect of wind load but still the static values were high enough to neglect the dynamic behavior.
4. The maximum displacement calculated by Eiffel was found at the top of the tower. The maximum reported value of displacement was 10 cm and 15 cm in case of a uniform wind load of 2 KN/m² and varying wind load of 2-4 KN/m², respectively.
5. Eiffel designed the tower as an open-air structure with minimum obstacles, allowing the strongest winds to easily pass through the structure.

5. Construction Process of Eiffel Tower

The following points describe the details of the construction process of the Eiffel Tower:

1. Casting and drilling of holes for rivets installation in the iron pieces was started in Levalloi-Perret factory with around 150 employees.
2. The iron pieces were riveted at the factory itself to get the desired shape of segments. After that, these segments were transported to the site.
3. The foundation work started on 23 January, 1887 and took around five months to complete. This involved the excavation for four legs of the tower. The foundation of the tower was about 16.3 m deep. More details of the foundation are discussed in the “Foundation” section of this article.
4. After completion of the foundation work, the masonry blocks were put on top of each foundation. The purpose of masonry blocks was to form shoes for the feet of the tower’s legs. Tower’s legs were then fit into the masonry blocks by gigantic bolts.

Further, the construction process details are discussed in three different segments for the first floor level, second floor level, and third floor level.

5.1 First Floor Level

In June 1887, the tower rising process started. Workers started assembling the huge iron pieces. Each section was designed carefully in a factory so that it could be easily reassembled and fit into another piece at the site.

The workers carried an iron beam on their shoulders. Lattice iron girders were constructed at the site to climb up and down to reach the desired height. Cranes were used to provide tools and building materials to the workers.

The weight of the legs of the towers was huge and hence to make the tiniest adjustment, great power was required. For this purpose, the Eiffel crew developed a jack hammer machine to provide power. Meanwhile, to keep the legs of the tower in place, sand-filled bags were used as a counterweight. By the end of March 1888, construction till the first floor was finished.

5.2 Second Floor Level

Since the cranes were only able to reach up to the first level and hence to reach a greater height, Eiffel developed the “Creepers”.

Creepers are steam-powered cranes which can run inside the leg of the tower. Later on, these Creepers were used as elevators for the public.

Scaffolds were also required to reach a greater height and to support the weight of the legs. However, after the construction of first-floor level, legs of the tower leaned at 54 degrees inwards. Huge gigantic bolts, which were installed in the foundation were only holding the legs. Further construction of the tower without support could have led to the collapse of the tower.

To support the tower further, Eiffel developed the wooden scaffolds around the legs of the tower. These scaffolds also helped in supporting all the tools and materials required for construction. Scaffolds were made on the pile foundation so that there was no chance of subsidence.

With the help of cranes and scaffolds, the construction of second floor was finished in June 1988.

5.3 Final Level

After reaching the second-floor level, the slope of the four legs of the tower varied and diminished as it rose. A pivot platform was made at the top of the tower where all the legs of the tower had resembled a flat platform.

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