aplombs des génératrices

07:51 Jul 17, 2018 

The discussion is sort of missing the point :-) What Tony is saying correctly describes the situation, but not in the same terms as the ST. He's working with the circumferences which are duals of the generators. Think of a generator as straight line in 3D, move it parallel to itself along a closed curve C and it sweeps out a "cylinder". If C is a circle, then this is what we commonly think of as a cylinder. But it can be a more complex closed curve and there are standards for non-cylindrical pipes. Consider joints, grouts, fittings ...

A generator doesn't have a base and one shouldn't try to interpret what that term might possibly mean.

07:09 Jul 17, 2018 

I don't quite see why you are acting so suprised at my 2 comment: "really rather obvious" refers to the fact that the logical way of expressing the distance between 2 cylindrical objects would be to measure from the closest points on their circumference — more practical, for example, than measuring between thier (abstract) centre-lines. With the further refinment as kindly pointed out by Robin that they might not lie in the same horizontal plane, so measuring between the vertical tangents produces a different result from measuring on the diagonal between their nearest points.
Your own suggestion, however, 'at the base of', is indeed a "highly unwelcome ambiguity" — in normal EN, that would be interpreted as 'at the lowest point of, which in the case of 2 cylindrical objects would be at the lowest point on their circumference, vertically benath the centre-line; this would of course yield a quite different, erroneous measurement!
Whether or not you happen to have come across the expression I suggest, the fact is that is is a typical standard expression used in lots of fields, and correctly mirrors the exact technical meaning of the phrase used in FR — though no doubt equally rare!

06:12 Jul 17, 2018 

A "highly unwelcome ambiguity" ??? For something that you qualify as "really rather obvious"??? (!)
Il y a peut-être mieux, je n'en doute pas! Chose certaine, "l'aplomb" (= perpendicularity) et "à l'aplomb de" sont usuels en construction. En 14 ans de génie-conseil, j'ai dû taper ça des centaines de fois. Et crois-moi, pour avoir eu le grand déplaisir de faire un an de trigo, s'il avait fallu que je tape un "vertical tangent(s) to the circumference(s)" dans un devis technique ne serait-ce qu'une fois, je m'en souviendrais!

18:40 Jul 16, 2018 

That's exactly what I have expressed in my suggestion — but sadly, the mere dictionary suggestion of 'at the base of' would NOT work here — indeed, that would introduce a highly unwelcome ambiguity. It's important to get inside the head of an engineer in order to undesrtand the way it is being expressed in FR, and the best way to express that in EN of a comparable register.

17:40 Jul 16, 2018 

"the bases of the nearest generators" doesn't mean anything.

17:28 Jul 16, 2018 

En fait, ça dit que si la ligne pilote est posée dans la même tranchée que la canalisation d'eau, la distance horizontale entre les (verticales des) génératrices les plus proches de cette tranchée (=canalisation+ligne pilote) sera d'au moins 40 cm.

"À l'aplomb de" est une autre façon de dire "à la verticale de": on trace une ligne verticale imaginaire sur le côté de chacune des génératrices qui sont côte à côte; la distance entre ces deux lignes doit être d'au moins 40 cm. En anglais, on dit tout simplement "at the base of". Ex.: à l'aplomb du mur" = at the base of the wall (R&C). Dans votre cas, ça donnerait quelque chose comme "the minimum horizontal distance between the bases of the nearest generators shall be 40 cm".

10:59 Jul 16, 2018 

It's the "minimum horizontal distance" between the water pipe and the pilot line. The horizontal distance between them must be at least 40 cm.