Posted by Alex J [216.54.94.2] on Monday, January 20, 2014 at 07:47:38 :
In Reply to: Re: Bolts are SOP, drilled for bigger size. posted by Sherman in Idaho [72.47.153.112] on Sunday, January 19, 2014 at 22:47:11 :
Most all connections are now designed as slip critical. The holes are overized on purpose and the bolts just act as a clamping mechanisim and the internal shear of the hardware is not accounted for. The reason being is if you start to have a hardware shear failure there was no real "warning". Now, if there is tolerance in the holes the plates are painted after assembly so if there is a slip critical failure the indication is visible by unmatching paint lines. This is in major structural connections such as a fabricated girder splice and not in minor connections such as diaphragms or lateral bracing.
The comments about grade 5 vs grade 8...
Grade 8 is good for getting more tension from the bolt as its stress range is higher so you can really torque them down before they start to elongate. If you can clamp your connected plates together then this is great. However, if your connection can slip and you are in fact relying on the bolt shear for capacity, then you need to know whether or not that bolt is going to shear or the plates fastened are going to tear. In most cases, the plates fastened are going to tear in a situation with automotive connections. Grade 5 is more than sufficient usually. If people want to know how you would "calculate" that I can share a few pages from my AISC Steel Construction Manual, and even a few pages from my Machinery's Handbook that both describe "block shear" vs "bolt shear" in structural connections that disregard "ply friction"