Each pier would rest on cement and stone laid obliquely so that the curving columns that bore the weight of the tower would exert their thrust at right angles to the mast. In each excavator twenty foot deep quick setting cement was poured. Over the cement, blocks of limestone were laid and in turn, these were capped with two layers of ultra hard cut stone (as used for the Arc de Triomphe). Embedded in the centre of each mass were two great anchor bolts twenty six feet long and four inches in diameter, to which a cylindrical flanged iron shoe was attached. The column would be bolted to the shoe and locked into the stone mass, pressure from the columns was to be 10.2 pounds per square inch, whereas the stone could actually support 421 pounds per square inch.
These foundations were in place by June 30, five months after starting. These practices at this stage involved no new innovations, except that Eiffel placed a piston in the hollow of each shoe, which could be moved by water under pressure. These hydraulic jacks permitted him the ability to raise or lower each of the sixteen columns thus, ensuring the perfect horizontal place for the piers when the time came.
Each jack was capable of lifting 900 tonnes, Eiffel calculations indicated that pressure on the earth beneath the foundations would range from 58 – 64 pounds per square inch depending on wind force and which pier was being considered. Well within the limits of the substrata.
The Structural Frame
Built to celebrate the science and engineering achievements of the age, soaring 320.7m (including a mast), weighing 7000 tonnes, the structure consists of two visibly distinct parts.
A base composed of a platform resting on four separate supports (pylons or bents) joined into a rigid whole by horizontal bands, above, a slender tower, created as the bents taper upwards, rising above a second platform to merge in unified columns. (Appendix 2)
In the construction of the tower, the curve of the base pylons was precisely calculated so that the bending and shearing forces of the wind were progressively transformed into forces of compression, which the bents could withstand more effectively. (4)
Eiffels use of wrought iron in an open-lattice design produced such a light structure that the tower has approximately the same weight as the air that surrounds it. (The mass of air in a box just large enough to enclose the tower [125 m x 312m] is 6.28 million kilograms which is 86% of the towers 7.3 kilograms. (5)
As aforementioned, the tower consists of four arched legs that taper inwards to form a single column to the top level at a height of 275 metres (level 1 being 57 metres, level 2 115 metres). The top level contained a suite of rooms, a spiral staircase and a flagpole, which brought the height to 312 metres.
As Eiffel himself said, “my primary concern in designing the tower was wind resistance”. It was the resultant mathematical conclusion that determined the shape of the tower. (6)
In modern terms, Eiffel’s calculations stated that ‘the torque from the wind on any part of the tower from a given height to the top is equal to the torque of the weight of the same parts’.
The greatest difficulty in erecting the tower was the bonding of the four main pillars at the first floor. (Appendix 3) The structure was made of prefabricated puddle iron. Several other cast iron parts were used including the 16 trusses connecting the masonry and the structure.
On July 1 the tower began to rise, the four colossal box piers inclined towards the centre of the base area at an angle of 54 degrees. The pieces were hoisted into position by tall steam powered cranes. Girders used weighed no more than 3 tonnes.
Temporary pins were used initially then replaced with rivets as related pieces were moved into position. As the columns began to approach the tops of the cranes, Eiffel used creeper cranes that mounted in the soon to be elevator tracks, (Appendix 4) these thirteen ton cranes could pivot 360 and be moved as required. Owing to centre of gravity calculations, it was estimated that the columns would collapse at a height of 92 feet, while the columns would not be joined until the height of 180 feet the solution was temporary bracing. (3)
These 12 wooden derricklike pylons 90 feet in height were built and placed beneath the 3 columns of each pier. Once in place, four platforms each 150 feet high and 82 feet long were placed in the centre of the piers.
The first platform was completed in March 1888, the second level in early July 1888.
Envelope
To all intents and purposes the tower was not designed for any sort of building envelope, to that end the author will briefly discuss the functioning envelopes put in place to provide protection to visitors and staff of the tower.
On the first level was a rectangular promenade 930 feet long and nine feet wide, nine decorative arcades on each side protected this platform. Four restaurants were also built on this level, here the construction of these features being very limited involved iron frameworks and wood providing the main materials used.
On the second floor radiating from the elevator in the centre were a print and pressroom, a small bar and a pastry shop, again, iron framework and wood were used.
The third platform was completely enclosed behind glass windows.
Above this was a small apartment with rooms for scientific experiments and for use by Eiffel, again wood was the main construction material. (Appendix 5)
Summary
On completion in March 1889, the tower was the worlds tallest structure until surpassed by New York’s Chrysler building decades later. The safety record for the construction was phenomenal with only one death (compared to other such projects).
The overall cost was 6% less than the budget of $16 million.
The main parts for highlighting are the facts that mathematical calculations proved so successful that only minor changes were made to the original work plan during construction. Secondly, this was the first time prefabricated materials were used on such a scale, lastly, despite massive professional opposition to the task, Eiffel, an engineer rather than an architect, had achieved what many had doubted possible.
Statistics
Number of Iron structural components 15000
Number of Rivets 2.5 million
Weight of Foundations 277.602 Kilograms (306 tons)
Weight of Iron 8092.2 tons
Total Weight including elevator system 9441 tons
Height of first platform 189 feet
Height of second platform 379 feet 8 inches
Height of third platform 905 feet 11 inches
Maximum sway at top caused by wind 4 ¾ inches
Maximum sway at top caused by metal dilation
Size of base area 412 feet square (2.5 acres)
References
- A bit of history “The Official Eiffel Tower Site”www.tour-eiffel.fr
- Eiffel Tower facts “The Official Eiffel Tower Site”www.tour-eiffel.fr
- Joseph Harris “The Tallest Tower”(Regnery Gateway, Washington 1975)
- Joseph Gallant “The Shape Of The Eiffel Tower”
- Joseph Gallant “Physics Of The Tower”
- The Eiffel Tower Equation “wwwusers.imaginet.fr/~chourd/
All pictures kindly reproduced with the permission of the official Eiffel Tower Site
