The design of the Eiffel Tower is the result of detailed analyzes carried out by 50 engineers and industrial designers, who produced 700 general plans and 3,600 shop drawings. The first concern of the engineers is to prevent the tower from overturning, a difficulty overcome thanks to the bell-shaped layout of its four pillars, which provide it with sufficient stability. The 7,341 tons that the tower weighs are thus firmly seated.
The second concern is to prevent the tower from deforming or swinging too much under the action of the wind, which is why the structure must be very rigid. This is obtained thanks to the junction of the four pillars of the tower by means of a large lattice beam, at the height of the first floor, and the triangulation system. The basic structural unit of the Eiffel Tower is the triangulated quadrilateral:each of its four pillars is formed by 28 of these quadrilaterals, or panels, which measure between 6 and 11 m on each side; there are four on the section that goes to the first floor, 57.63 m high. This system allows the almost total stiffening of the tower facing the wind. The maximum horizontal swing at the top is 7 cm, which assumes a height ratio of 1/4,285, much less than that usually found in tall buildings, which is typically more than 1/1000.
The stages of an elevation to heaven
During the first five months of work, the foundations are put in place. They consist of a compact gravel bed several meters deep, on which are placed heavy concrete blocks. On these blocks are built large stone supports, which anchor the four pillars of the tower. The construction of the north and west foundations, the closest to the Seine, is particularly complex, because the area is marshy and unstable:it is necessary to dig 5 m below the water table to reach firm ground. To carry out the excavation, Eiffel uses a system of pneumatic caissons, which were introduced in England in 1830, but have never before been applied to a work having the dimensions of the Eiffel Tower.
For the assembly of the first floor, pyramid-shaped wooden scaffolding is used to support the pillars. Four load towers are then built, on which are mounted the four large beams of the first floor. By connecting these four beams to the inclined pillars, these are stabilized. Starting from the first floor, on each of the four pillars are mounted steam-powered climbing cranes, which glide along the pillars and hoist the tower sections.
The progress of the work, regular, is about ten meters per month. In September 1888, the second floor was reached, 115 m high. From there, the tower takes the form of a proper pylon, and the building process becomes easier. The last phase, the installation of the lifts, is another unprecedented technical success:the companies Édoux, Otis and Combaluzier are installing three types of lifts, meeting the challenge of climbing to 276 meters.
The most delicate moment of construction is the joining of the four pillars to form the first floor of the tower, since the structures must fit together with millimeter precision. Eiffel knew that there would inevitably be a misalignment. This is why, in each of the pillars, a cavity has been created that serves as a piston, into which pressurized water is injected until the desired leveling is obtained. After this stage, the construction becomes simpler.
Workers with iron discipline
On the tower, 150 to 250 workers are busy at the same time. Their job is to assemble the parts that another hundred workers make in the Eiffel workshops in Levallois-Perret, near Paris, from where they arrive by rail. Their main task is to unite the different parts by means of rivets, the ancestors of screws. To set the rivets, teams of four men are formed:the apprentice operates the forge and heats the rivet red hot; the pimp introduces it into the hole already made in the workshop, and secures it by the head; the riveter strikes the rod to form the opposite head; finally, the batter completes the work at the mace. During the first phase, 40 teams set some 4,200 rivets per day. In total, the Eiffel Tower has 2.5 million rivets. The workers were hired among the carpenters of Paris, accustomed to working at a certain height and therefore not very sensitive to vertigo. In fact, there was only one fatal accident, which moreover occurred outside working hours. More than the height, the big problem for the workers was the cold, especially during the freezing winter of 1888-1889. The working day is 9 hours (12 hours in summer). Responding to the demands of the workers, who went on strike twice at the end of 1888, because they considered that they were insufficiently paid, Eiffel offered them performance bonuses and improvements in their working conditions, such as a canteen at the first floor, where they can warm up the food they bring from home.