slip critical connections vs bearing type

huangyhg

slip critical connections vs bearing type
consider a standard hole 3/4 inch diameter a325 bolt in shear.  
allowable stress for bearing type (n) connection is 9.3 kips, and for slip critical (f) connection is 7.7 kips     
slip critical connection are required to be inspected in the field for correct tension with torque testing devicess, or with direct tension indicators, etc., while the bearing type is not required to be inspected.
so how come the n connection which doesnt have to be checked gets a larger allowable stress then the slip critical connection which is inspected.
i would think that something which does not require inspection would get a lower allowable stress.
it is because of the intent. in a bearing connection you can use the plastic shear strenght to the factored level, and the equivalent thing is portraited in getting your allowable shear strength. however, the main requirement of a slip critical connection is to avoid the connection slippage at service level, that for most of the cases should cover a very big percentage (say over 99.5%)of the actual peak situations experienced in service, and typically not less than 95% of the expected peaks of solicitation that corresponding to the code-specified loads. to get such reliability, experience proves that lesser allowable strengths must be permissible. of course you will have for the same geometry an equal or bigger limit load for the slip critical connection (with proper element), but this becomes less paramount once we consider that the true goal of the slip critical connection would have failed.
bearing value seems awfully low compared to slip critical...
ishvaaag is right - the two have different limit states.  the bearing capacity is based on bolt shear - a limit state where the bolt literally fails.
the slip critical connection limit state is slippage, not bolt failure.  so the load is lower because after torqing, the load that causes slip is lower than the load that causes bolt failure.
the difference between the n bolt and the slip critical is not that great as the n bolt has a further penalty in that threads are not excluded from the shear plane as is the case with the x bolts.
you still have to check the connection for bearing if originally designed as slip critical in case slippage occurs.
additional comments on slip connections can be found here
if you are interested