Designers now using the Eurocodes will be familiar with Category B, slip-resistant at serviceability and Category C, slip-resistant at ultimate, as set out in Table 3.2 of BS EN 1993-1-8. In the Eurocodes, it is also clear that for Category B connections, the design slip resistance is compared to the serviceability loads.
Turning back to BS 5950 may led to some uncertainty about which loads to use when calculating slip resistance, particularly for connections designed to be non-slip in service. Clause 6.4.2 specifies the slip resistance PsL as:
• For connections designed to be non-slip in service:
PsL = 1.1Ks μP0
• For connections designed to be non-slip under factored loads:
PsL = 0.9Ks μP0
In both cases, the resistance should be compared to the ultimate loads. This is made clear by the note at the end of clause 6.4.1: NOTE The resistance of a friction grip connection to slip in service is a serviceability criterion, but for ease of use is presented in a modified form, suitable for checking under factored loads.
For connections which are designed to be non-slip in service, BS 5950 does not reduce the loads, but rather increases the calculated resistance of the bolts (compare the 1.1 factor with 0.9 in the above expressions) to give an equivalent result.
AD 274 gives advice on the capacity after slipping, covered in clause 6.4.4 of BS 5950. This is an important check for connections designed to be non-slip in service and is designed to ensure that if it slips, the connection does not fail at ultimate loads.
Contact: Richard Henderson
Tel: 01344 636555