mammuth wrote:Except press fitting bolts normal bolts are not designed for shear!!! For shear the bolts should clamp the two surfaces well (they should also be not painted and not too smoth) so the surfaces create the friction against shear. For this you need 10.9 or better....
Not quite true, a bolted connection in shear has the following failure modes, (probably a couple more as well)
1) direct shear of the face of the bolt
2) necking of the bolt
3) bearing failure of the plate
4) bearing failure of the bolt
5) Block shear of the plate/beam flange/web
6) local buckling failure of the plate
7) prying action from plate deflection
None rely directly on the "tightness" of the bolt, in fact as the clamping force increases the other strength properties decrease. Grade 8.8 bolts are perfectly suitable for 99% of application, 10.9 and 12.9 are not typically used in structural work. You would typically use a 2mm oversize clearance hole i.e. if you use a M16 bolt, drill an 18mm hole, M20 = 22mm hole etc
There are however connections which utilise plate friction which use special Gr10.9 (and higher) bolts called High Strength Friction Grip (HSFG) bolts and also Tension Control Bolts (TCB's) these are designed to utilise the clamping force and offset the loss of bolt capacity against friction between the plates to increase the shear capacity of the connection.
The friction connection relies on correct preparation of the mating surfaces and the correct torque applied to the bolt. There is a British Standard and a Euronorm for the correct surface preparation
TCB's also do not induce any torque or twist into the bolt stem or plate as the tool holds the nut end of the bolt and snaps off when the correct nut torque has been achieved. TCB's are great but you need a special tool to install them (you can hire them in the UK by the day)
HSFG bolts typically have a special load indicating washer which have nodules on the washer that compress when you tighten the nut, but you need to use a feeler guage to check the gap is correct
Hope that helps