The AISC allows short slotted holes

[SWNORRIS]

The AISC allows short slotted holes in any or all plies of slip critical connections, without regard to direction of loading, which is not something that's addressed in Section J.3 2(e). We have multiple situations where a diagonal brace connects to a plate at a column flange near the base, and a plate at the bottom flange of a beam. I asked to use short slotted holes, in the direction of the slope, but I was wondering if this would actually reduce the bolt value. These are not erection bolts... so the diagonal brace will not be field welded after erection. If that were the case, I don't think the hole size would be an issue. I know that the bolt value can be re-calculated using the short slots, if their use actually decreases the bolt value. There are two 3/4 A325 bolts at each connection. My question is.... does the use of short slots in this condition reduce the bolt value? I've been told that the use of short slotted holes will turn this connection into a slip critical connection, which in turn, reduces bolt value.

 

[BridgeSmith]

The way Ive understood it, the use of slots decreases the bearing capacity for the hole (technically, the steel around the hole), but does not affect the shear capacity of the the bolt, or the allowable slip force for a slip critical connection.

Using a slotted hole doesn't automatically make the connection slip critical, either as far as I know. If movement of the connection would be detrimental to performance, then the connection should be designed as slip critical, whether it has slots or not.

 

[TLHS]

Using a slot doesn't turn it into a slip critical connection. Making it a slip critical connection (and following all of the design requirements) is what allows you to use a slot. Those values are smaller than bearing capacities, but not as some sort of penalty for using a slot. They're simply the capacities available when using a friction style connection. They're not really bolt shears at that point, they're connection shears due to friction caused by bolt pretension.

If you start putting slots in the direction of loading and using slip critical connections, I'd make sure to put significant quality control requirements on it. Normally you have the bearing connection to save you, but the amount of slip in a slot is starting to get a bit more dramatic.

 

[sbisteel]

There is a significant difference in the capacity of a SC bolt in a short slot if the slot is parallel with the load, compared to the slot oriented transverse to the load. Reference J3.8 for more information.

 

[SWNORRIS]

I understand that, but these are parallel to the load. So, in my mind, regardless of whether the hole is a standard 13/16 round hole, or a 13/16 short, horizontal slot; the bolt will still end up bearing on the bottom edge of the hole. So how does slip critical even come into play? Even if the 3/4 bolts are set dead center of the holes, they could only drop 1/32, and I simply can't understand how that could be a concern.

 

[sbisteel]

Is the load subject to reversal of direction? Will the brace see both compression and tension?

If your slot is parallel to the load, there is too much room for the bolt to move in the slot to consider a bearing type of connection. You must use SC bolts

If the slot is transverse to the load, the RCSC code (4.1,4.2 & 4.3) allows you to use pre-tensioned joints in which the bolts are designed with bearing values but installed with full pretension. However the RCSC code also states that "joints in which slip is detrimental to the performance of the structure" require SC connections, many engineers will run with that and require SC bolts even though you are only looking at ~1/16" of possible motion.

 

[SWNORRIS]

We're using TC bolts.

 

[BridgeSmith]

The slip force is the force that can be applied to a connection with fully tensioned bolts before it the plates slide against each other. The capacity of the connection in bearing is the lower of the shear capacity of the bolt or the bearing capacity of the side of the bolt hole that the bolt bears against. The slip force, I think, is always smaller than the bearing capacity, but the slip force capacity is checked against the Service loading, while the bearing capacity is checked against the Strength (or factored design) loading.

Is the long dimension of the slots parallel to the loading, or perpendicular? If the slotted part moves to the end of the hole, is that amount of movement unacceptable? If so, you must design it as a slip critical connection - the applied service load must be less than the slip force. If movement to the limits of the slot is acceptable, it's designed as a bearing type connection. In either type, the capacity of the bolts in shear and the capacity of the plates between the hole and the edge of the plate in the direction of the applied force must both be more than the design load, with all applicable load factors applied.