An incline which starts from the circular alley of the lighthouse, and reaches the bank of the Seine, connects the Park of the Universal Exhibition with the quays of arrival of the steamships.
As nothing is useless at an Exhibition, care was taken to use the bridge, which does not interrupt traffic on the Quai d'Orsay, for the special exhibition of a new system.
We are a long way from the primitive footbridges made of a tree trunk thrown over a river; and if the construction of bridges has made immense progress in recent times, either from the mechanical point of view or from the point of view of economy, it is to the multiplication of railways that we owe the achievement.
The heaviest loads, the most daring arches, the greatest distances, are supported, thrown, crossed, with an ease which astonishes our mind if only observation and calculation make us meditate for a moment on the difficulties to be overcome. The gigantic strides of these metallic arches thrown over space, with a boldness that must not exclude solidity, form a whole scientific poem whose solution belongs to engineers and builders.
It is impossible to imagine the precautions and prudence that govern the experiments to which a newly built bridge is subjected.
When the Quai d'Orsay bridge was completed, the engineers of the Ponts et Chaussées, Messrs Buffet and Foulard, with the assistance of Mr Cheysson, themselves presided over the following experiments: A first load of 500 kilograms per square metre of the deck forming the surface of the bridge was imposed; this constituted a total weight of 200,000 kilograms.
The calculations predicted a regular and predictable deflection, and the experiment justified the predictions.
After this first test, two single-axle cars with a load of 12,000 kilograms and five horses were driven over the bridge.
No deformation occurred. The bridge was crossed abreast, in the opposite direction, crosswise and in a straight line, by this same team, and the thirty devices placed to measure the lowering and raising of the bridge did not indicate a variation of more than seven millimetres at each of the points observed.
Everything was favourable to solidity, and from that moment on the bridge was given over to the circulation of cars of all loads and all types of carriages.
These kinds of experiments have something solemn about them, when one thinks that on their deductions depends the life of a certain number of men who will cross this daily crowded passage, and one cannot help admiring without reserve, the intelligent solicitude which watches over the safety of public roads in the name of science.
It is not enough to build a bridge in the desirable conditions of permanence and solidity; the multiplication of transport routes, and the enormous expenses with which they burden the budgets of companies, cities, and States, demanded an economic solution of indisputable importance.
Here we enter into the details of the special merits of the exposed bridge.
Cast iron and iron have hitherto been used almost alone, either in competition with stone or in competition with wood, to build bridges for different purposes.
A new metal is being used here for the first time: Bessmer steel.
The use of this metal has a higher safety factor than iron and cast iron. The ratio between the breaking loads of these two metals is as high as six to ten; long-span structures are therefore possible, and the reduction or elimination of support points in the middle of the obstacles to be crossed is already a significant saving.
The span of the exposed bridge is 25 metres over a width of 21 metres between railings.
All the truss arches are made of Bessmer metal; the bridge itself is made of the arch type with lattice spandrels, which have considerable advantages from a construction point of view.
They provide the possibility of distributing the load due to the deck on all points of the span arches with proportional and mathematical equality. One does not need to be an engineer to understand that the weight of a load, concentrated by a constructional defect on any one point, presents a serious danger, whereas a distribution on all the supporting points reduces the general weight and divides the effects of deflection.
In mechanics as in politics, the motto adopted by Catherine of Russia is true: Divide ut impera: Divide if you want to win.
Simplifying the construction elements, and reducing the loads to a span of proven solidity guaranteed by the nature of the metal itself, in addition to obtaining a notable economy in the execution, such is the intrinsic merit of the Quai d'Orsay bridge.
Science in positive calculations always aims more at the useful than the pleasant. For us, who would like art to be fraternally and in all circumstances allied to science, we have examined this bridge, whose mechanical qualities are now known to us, from the point of view of form.
We would perhaps have wished that the monotony of the straight line which exclusively composes the top of the triangles with curvilinear bases situated on the vertical facades of the right and left of the bridge, had been broken up a little by some ornaments, intended to render its general aspect less severe and less acute to the eye.
However, the latticed spandrels are less harsh in design than some of the fittings used, for example the T-irons, and in the end give an almost original character to the whole construction itself. Finally, if the artistic charm is not the dominant point of this remarkable bridge, we hasten to add that its serious and practical qualities have earned it our full attention and that we make it our duty to acknowledge them by pointing them out.
©L'Exposition Universelle de 1867 Illustrée