Karel:
I use 8.5%.
John:
I have spent several hours pursuing the question of the tensile strength of a multi-strand thread/rope being the sum of the individual parts over the past two weeks, and have "Googled" it to death! I still have not come up with a definitive answer; so I posed it to a PhD physicist friend of mine last evening; and he did not have an immediate answer.
The problem goes all the way back to the mid-1700's when a French man of science first published a paper in which he claimed the the tensile strength of multiple strands was greater than the sum of the total. He subsequently published a paper in which he retracted his earlier claim, and stated that the the tensile strength of the whole was less than the sum of the individual strands. This is what was published in the 1911 version of Encyclopedia Britannica, and seemingly has been taken as 'gospel' since that date.
It only stands to reason that work has been done in this realm since the 1700's, but I have found Google to be a waste of time in trying to find references to more recent studies; even in the area of "the physics of rope". I did find one site where they answer questions from school teachers, and where three different individuals took a stab at an answer. All there were highly qualified; with nothing definitive, other than to lead one to believe that the sum is equal to or greater than the total of the individual components.
I personally, and intuitively, agree with one who said something to the effect that: "... intuitively, the end product is equal to or greater than the sum...". otherwise, why do they make ropes of varying diameters, and numbers of strands, to meet the needs for dealing with heavier and heavier 'loads' (increasing tensile stress)?
Almost everything I found had to do with WHY ropes break!
Of little value to us, at least as I see it.
Thus, at the moment, I would have to say that the jury is still out on the subject, in terms of something definitive on the subject that is more recent than the mid-1700's!
aged sage