This article is about bearing connection vs slip critical connection in steel structure. Know basic difference between bearing and slip critical

Bearing vs slip critical connections is little bit confusing question ❓ In this article you will have clear idea about bearing and slip critical connections. Bolted structural steel connections are one of the most common connection types in structures due to their low cost and efficient installation. 

Bolted connections transfer force between two connected members through either the dowel action of a bolt or through friction in the connection. The American Institute of Steel Construction (AISC) specifies two performance categories for high-strength bolted connections: 

Fig 2: N-type X-type bolts

1. Bearing-type connections
2. Slip-critical connections

Recommended for you: Types of bolts in steel structure 

Bearing-type connection

Bearing-type connections are among the simplest means of transferring external force from one joint member to another. Bearing-type connections rely only on the bolt shank to transfer an applied force from one joint member to the other. Bolts in a bearing-type connection are typically tightened only enough to keep the joint members in full contact. Therefore, in a bearing-type connection, the effects of friction between the plates are negligible.Fig.1: Bearing-type connectionFig. 1 shows a bearing-type connection of a lap joint and the associated free body diagrams of each component. The free body diagram of the bolt shank demonstrates that the applied load, P, is transferred from plate to plate through shear in the bolt. Because applied load, P, creates an unbalanced moment and the bolt holes are slightly larger than the nominal diameter of the bolt, some minor rotation of the bolt will occur, creating both tension and shear in the bolt shank. However, the induced tension in the bolt is generally negligible compared to the shear.Bearing-type connection is further classified into two types. The threads of a bolt may either be included (N bolts) in the shear plane or excluded (X bolts) from the shear plane.

• N type: Bearing type connection where the threads of the bolt are included in the shear plane.
• X type: Bearing type connection where the threads of the bolts are excluded from shear plane.

The shear plane is the plane between two or more pieces under load where the pieces tend to move parallel from each other, but in opposite directions.

The most commonly used bolt is an ASTM A325N 3/4"bolt with the threads included in the shear plane.

Slip critical connection 

Slip-critical connections do not rely on bearing between the plate and the bolt to transfer the load between joint members. Instead, pre-tensioning of the bolt creates a clamping force between the connected members and the joint relies on friction to transfer the load between joint members. Fig. 3: Slip critical connection 
Slip-critical connections are often used to prevent fatigue failure of the bolt in cases where the direction of the load frequently changes. Fig. 3 shows a slip-critical connection and the associated free body diagrams of the components. The pretension in the bolt compresses the joint members together. The resulting frictional forces provide the resistance necessary to prevent plates from sliding past one another and the plates from bearing on the bolt.The "turn of the nut" method is also widely used to achieve that state of friction. The faying surfaces of slip-critical connections must be properly prepared in order to maximize friction forces between the surfaces joined. Usually, this requires cleaning, descaling, roughening, and/or blasting of the faying surfaces. Painting the faying surfaces with a class B primer also allows being in accordance with most of the design that asks for Slip-critical joint.

When to use Slip critical joints

Per the RCSC Specification (RCSC ), Slip-critical joints are only required in the following applications involving shear or combined shear and tension:

• Joints that are subject to fatigue load with reversal of the loading direction.
• Joints that utilize oversized holes.
• Joints that utilize slotted holes, except those with applied load approximately normal (within 80 to 100 degrees) to the direction of the long dimension of the slot.
• Joints in which slip at the faying surfaces would be detrimental to the performance of the structure.

Slip Critical Connections are expensive because of faying surface preparation, tightening and inspection requirements.SC-Connections are not needed for typical framing connections and most moment connections and bracing connections.SC-Connections may be needed when dynamic or vibration loads are present or may be used to control drift in frames and are required in some moment connections.

Hi,

Scenario 1:

I\'ve got a beam supported by a hanger column, for which the connection is a column base plate and then 4 Bolts through the top flange of the beam below.

My question is related to whether it is best to use snug tight (S) or tension Bearing (TB) bolts for this connection.

My understanding is that the TB bolts will put a small amount of pre-load into the bolt, which is then relieved by the tension force applied under loading, until a point where the load applied exceeds the pre-load, when the load applied will be eating into the residual bolt tension capacity.

I\'m wanting to use snug tight bolts, though I\'m a little nervous that the bolts might loosen over time due to minor vibrations and movements. I\'ve not got a situation where cyclic or vibration is really any issue, though I\'m just worried I might be missing something.

Is this a scenario where TB bolts would be more appropriate? Or snug is OK?

Scenario 2:

If I have some packing plate (say up to 25mm) between the hanging column and the beam top flange (to allow for tolerance/deflections during construction) and I have a combined shear and tension load to resolve at the connection, would I need TB bolts to allow enough clamping force to transfer the shear and avoid any bolt bending? If so, then once the preload is fully relieved, will I have any issues?

Thanks in advance!

Ben.

Scenario 1:

1. If a "pin" [i.e. shear] is otherwise acceptable, then S [as S provides limited preload - hence not incurred value effective, and will loosen], or
2. For any other application [i.e. "significant" capacity required, or loosening must be prevented], then use: TB, or TF [provides preload].

If the joint preload is exceeded, the connection has failed.
Refer following as a start:



Scenario 2:

Refer scenario 1, and please review AS Cl 9.3.2.5.


For your safety, and the safety of others, please consider consulting a: competent, and qualified; engineer.