English

It is one thing to design a bridge.  It is another thing to build it.  Planning and executing the construction of a bridge is often very complicated, and in fact may be the most ingenious (ingénieur- engineer) parts of the entire enterprise.  An incomplete structure is often subjected to stresses and oscillations that would not arise after completion. 
Even before any actual construction isdone, substantial work may be needed in the form of tests.  Boreholes will be made to check the condition of the ground, in conjunction with any available geological maps.  Records of wind speed anddirection will be consulted, and new measurements made if necessary.  In the case of a river or sea crossing, records of water levels and velocities will be needed.  Models of the bridge or of parts maybe tested aerodynamically and hydrodynamically, and of course mechanical tests will be made.  Computer simulations will supplement these tests, enabling a great variety of applied forces to beinvestigated.  There may also be investigations into the effects on people and on the natural environment.  It may even be necessary to overcome opposition to the construction, from a variety of objectors. There was much opposition, for example, to the construction of the Skye bridge.  In older times, ferry operators could be extremely vociferous about a bridge proposal.
Let´s look at the living world,to see how things are done there.  Starting at the beginning, an immature form is likely to be vulnerable.  A half built medieval castle must have been hard to defend.  In modern times, a half builttank or aircraft is vulnerable if bombs can be dropped on the factory. 
The bridge builder, like the mammals, has to nurse the embryo structure through difficult stages.  Very often, the stressesdiffer considerably from those of the complete structure, and can be more concentrated.  The collapse of several box girder bridges in the 1960s damaged the reputation of this type for a period, until...