An option.

IMHO, cela me paraît plus précis que "to bear".

That's what is means, now native English-speaking people can find a better phrasing.

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Note added at 11 mins (2011-03-25 10:01:40 GMT)
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That is, for example, if they break when you apply a traction of 100 kilos, then you must not apply more than 55 kilos.

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Note added at 13 mins (2011-03-25 10:03:49 GMT)
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To explain the word, it means that when using the cables, you don't "ask" them to perform over 55%, literally speaking.

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Note added at 31 mins (2011-03-25 10:22:18 GMT)
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"Traction exceeding 55%" is better English.

the cables must be sufficiently strong (strong enough) so that their normal working load is not greater than 55 % of their breaking strain

the French sentence is badly phrased, probably because of the maths involved... if the limit was 50 % we could put : the cables must have a breaking strain of double that of the normal working load ;-)

Seasteel offers up to 40% higher breaking strain than standard polypropylene ropes and up to 70% higher breaking strain than ...
www.gaelforcemarine.co.uk/Ropes-and.../Commercial-Ropes/

BTW, for lifts, I believe the breaking strain/working load ratio is 8 to 1

The French really is atrociously written! There might be a reason for that, however, something to with the noyer le poisson, in which case you would presumably have to come up with something equally as obtuse, but before you can make that decision and phrase your translation accordingly you have first to understand what they want (or don't want) to say.

As it is written you get the impression there must be something inherent to the cables that will prevent them being overloaded. I know: a strain gauge connected to a transmitter that reverses the ships engines or turns them on to counter adverse windage or tidal effect putting too much strain in the hawsers .... Yes, I'm being daft!

What it has to mean is that, in light of the forces expected to be called into play, the cables must be of sufficient strength that those forces will not be greater than 55% of the cable strength. IOW, if you have an expected maximal force of 55 units, the nominal breaking strength of your cable must be at least 100 units.

IOW, the French is approaching the matter arse-about-tilt: you don't want to decide your cable strength and then see how much force will be exerted by the ships mooring them, but rather determine the mooring forces (tide, windage, tonnage, etc.) and from there determine the required strength of the cables.

"Steel mooring lines shall be such that the forces exerted upon them are no greater than 55% of their breaking strength under normal service conditions. "

I'd have to have a couple of stiff whiskeys to clear my brain, to be sure that there is actually a difference in meaning, but I'd actually be happier with :

"Steel mooring lines shall be such that the forces exerted upon them under normal service conditions are no greater than 55% of their breaking strength "

even if that is not - assuming we are not missing appropriate punctuation - what the French appears to say.

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Note added at 6 hrs (2011-03-25 16:30:22 GMT)
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Apologies to those concerned: I read only the "title words" of previous answers before answering myself, considering that the problem was not the asked word but the entire phrase. I now see others have addressed this problem before me.