Hello P.J.
i don't have that BS under my hand, nevertheless the effect is known from similar standards.
640 Mpa is the materials nominal yield strength (8x8* 10).
560 MPa is the permitted stress of that material under tension load.
The bolt shall not be tensioned to full yield strength, but instead to a lower value which then still allows for operational loads.
Pls. refer explanatory notes, for
- pretensioning
- allowed loading when thread is in shear plane / not in s. p.
- combined loading (tension and shear)
There's some threads on EngTips on this one, too.
Regards
Roland Heilmann