Eiffel Tower: The Structural Engineering Behind an Iron Landmark

When the Eiffel Tower opened on 31 March 1889 for the Paris Exposition Universelle, it stood 300 metres tall, more than twice the height of the Washington Monument and the tallest man-made structure in the world by a wide margin. It held that record for 41 years until the Chrysler Building surpassed it in 1930. The achievement was not merely symbolic. The Eiffel Tower represented the first time engineers had systematically analysed and designed a structure of that height for wind loads, producing a form that responded directly to the governing structural forces rather than to aesthetic convention. The result was a structure that has stood for over 135 years with no significant structural distress.

Historical Context and the Design Challenge

The 1889 Exposition Universelle commemorated the centennial of the French Revolution, and the French government sought a dramatic centrepiece. The competition brief called for an iron tower 300 metres tall on the Champ de Mars. The height was unprecedented: at that scale, wind loading dominated all other structural concerns. Self-weight could be managed by any competent engineer of the era, but accurately predicting and resisting the lateral forces from wind at 300 metres was entirely uncharted territory.

The winning proposal came from the firm of Gustave Eiffel et Compagnie. The structural conception is most accurately attributed to two of Eiffel's senior engineers: Maurice Koechlin, the firm's chief structural designer, and Emile Nouguier. Koechlin produced the original sketches and stress calculations in 1884, five years before construction began. Architect Stephen Sauvestre added the decorative arches at the base and other aesthetic elements. Eiffel himself championed the project, secured the financing and contract, and managed construction. The engineering authorship is a matter of historical record: Koechlin's original calculations survive and show a systematic approach to wind load analysis that was exceptional for its time.

Why Wrought Iron, Not Steel

By 1889, Bessemer and open-hearth steelmaking processes were producing structural steel in commercial quantities, and steel had already been used in bridge construction including Eiffel's own Garabit Viaduct. Yet Eiffel chose puddled iron, a refined form of wrought iron with very low carbon content, for the tower. The decision was deliberate and technically sound for the era. Steel production was not yet sufficiently standardised for Eiffel to guarantee consistent mechanical properties across the millions of tonnes required. Puddled iron had well-understood properties, a long fabrication history, and excellent resistance to the fatigue loading that would result from continuous wind-induced oscillation over a long service life.

Puddled iron's tensile strength is approximately 300 to 370 MPa, compared to modern structural steel at 250 to 345 MPa depending on grade. Its resistance to fatigue crack propagation, while lower than the best modern steels, was sufficient for a structure expected to stand for 20 years (its original intended lifespan). The slag inclusions characteristic of wrought iron actually impede crack propagation, providing fracture toughness that early Bessemer steel lacked.

The Curved Leg Profile and the Wind Load Rationale

The most distinctive visual feature of the Eiffel Tower is the curved profile of its four legs, which sweep outward dramatically at the base and converge to a relatively slender shaft above the second level. Eiffel did not adopt this form for aesthetic reasons. In contemporary writings and presentations, he explicitly described the curve as the geometric solution to the wind pressure problem.

The reasoning is rooted in the concept of the resultant pressure diagram. At each horizontal cross-section through a leg, the wind force applied to the structure above that section creates a bending moment at that cross-section. The structure must resist this moment. For a tall tower, the moment is largest at the base and decreases toward the top. If the four legs are spread outward, the couple arm between windward and leeward legs increases, allowing the tower to resist the bending moment as an axial force couple (compression in the leeward legs, tension in the windward legs) rather than as internal bending in individual members. The wider the spread at the base, the more efficiently the tower converts wind-induced overturning moment into axial forces in the legs.

Koechlin calculated the wind pressure distribution on the tower and determined the shape of the leg such that at every elevation, the tangent to the curved leg profile exactly aligned with the direction of the resultant of the wind pressure above that elevation. This means the leg is always loaded as a compression strut without bending, an elegant structural optimisation that anticipates the mathematics used in modern form-finding methods. The practical result is that the tower's form is, in a rigorous sense, the three-dimensional analogue of a variable-depth shear wall whose depth is tailored to match the applied moment diagram.

Lattice Truss Construction and Wind Permeability

Each leg and each horizontal platform of the Eiffel Tower is a lattice truss rather than a solid section. The lattice consists of diagonal and horizontal iron members riveted together to form an open grid. This construction serves two critical structural functions simultaneously. First, the lattice provides very high stiffness-to-weight ratio: the open geometry creates a large second moment of area for a given amount of material, resisting bending efficiently. A solid section of the same weight would be far less stiff.

Second, the open lattice allows wind to pass through rather than being blocked by a solid surface. The drag force on a lattice structure is a fraction of the drag that would act on a solid surface of the same overall dimensions. Eiffel was acutely aware of this aerodynamic advantage. The solidity ratio, the fraction of the tower's silhouette that is solid iron rather than open air, is approximately 0.3 over most of the tower height. This dramatically reduces the wind force compared to a solid tower of the same height and width. In modern terms, the drag coefficient for such a lattice is roughly 1.3 to 1.5 compared to approximately 2.0 for a solid rectangular section.

The first platform sits at 57.6 metres, the second at 115.7 metres, and the third at 276.1 metres. The horizontal platforms, in addition to serving as viewing areas, act as stiff ring frames that tie the four legs together and prevent differential deflection between them. They function similarly to the floors of a building frame, distributing lateral loads between the legs.

Foundation Design and the Role of Hydraulic Jacks

The four legs spread to a 125-metre square footprint at grade. Each leg bears on a separate massive concrete foundation block. The Champ de Mars site includes sandy alluvial soil near the Seine, with groundwater close to the surface on the river side. The two foundations adjacent to the Seine were constructed using compressed-air caissons to excavate below the water table to firm bearing material at approximately 5 metres depth. The two inland foundations are conventional mass concrete on dense gravel.

The foundations are oriented at 45 degrees to the building's face, so each leg bears on its own pad and the legs do not share footings. Each foundation receives the load from a single leg as a large inclined compressive force. The inclination of the leg at the base is approximately 54 degrees from horizontal, so the foundation experiences both a vertical force and a significant horizontal thrust component that must be resisted by friction and passive earth pressure against the foundation sides.

During construction, temporary hydraulic jacks were installed under each leg shoe to allow precise three-dimensional adjustment as the legs were brought to their final position. This was essential because the fabrication tolerances of the lattice sections, while excellent for the era, inevitably accumulated over 57 metres of structure. The jacks allowed corrections of a few millimetres to bring the first platform's anchor points into exact position for connection. This was the first large-scale use of hydraulic jacks for structural alignment and represented a significant construction innovation.

Thermal Expansion, Deflection, and Long-Term Behaviour

The Eiffel Tower expands and contracts significantly with temperature. The coefficient of thermal expansion of iron is approximately 12 x 10^-6 per degree Celsius. Over the 300-metre height and a temperature range of roughly 40 degrees Celsius between winter and summer in Paris, the tower can grow or shrink by up to 15 centimetres vertically. More significantly for its silhouette, the sun heats the south-facing ironwork far more than the shaded north-facing side, causing the tower top to lean away from the sun by up to 18 centimetres on hot days as the sun-exposed leg expands more than the shaded legs. This thermal behaviour was anticipated in Eiffel's calculations and is entirely within the elastic range of the structure.

Wind causes the tower to oscillate. Eiffel designed the structure to limit horizontal deflection at the summit to 70 mm under the design wind load, an extremely tight limit for a 300-metre structure. In practice, measured deflections under storm conditions have been consistent with this estimate. The natural period of the tower is approximately 10 to 12 seconds for the fundamental sway mode, placing it in a frequency range well below the peak energy content of typical wind gusts, which helps prevent resonant wind-induced oscillation.

Maintenance is continuous. The tower is repainted every seven years using a brownish-grey paint applied in three graduated shades, darker at the base and lighter at the top, to give a visually uniform appearance against the sky. Each repainting consumes approximately 60 tonnes of paint. The structure has been modified over its life to add broadcast antenna masts, elevating the total height to approximately 330 metres, and to install meteorological and telecommunications equipment. The ironwork itself has required no significant structural repairs in over 135 years of service, a tribute to the quality of the original design and the durability of low-carbon wrought iron in the Paris climate.