Shear Plane of Bolts 1

[Tim76]

Can anyone explain to me what is meant in terms of connection types, when the threads are included or exluded in the shear plane? In practice when would use use one or the other? Thanks.

 

[chichuck]

Tim76:

I presume you are talking about structural steel connections using high-strength bolts. These are dealt with in the AISC Manual of Steel Construction 9th Edition as well as in most structural steel textbooks.

Briefly, there are two types of connections using high strength bolts (A325 or A490).

1) slip critical connectins, these are designed assuming that the connection has failed when slip occurs between the connected parts. This slip occurs when the friction between the connected parts is exceeded by the loads on the connection. This friction arises from the pretension on the bolts. The slip is quite small, and has nothing at all to do with the fact that the bolts are smaller than the holes they are placed in. (Standard holes for a 3/4" dia bolt is 13/16" dia.) At the connectin failure, the bolt has not failed (ie has not broken) Some of the bolts may be bearing on the sides of the bolt holes, others may not.

2) bearing connections, these are designed assuming that the connected parts already have slipped, (this is a real condition), that all the bolts are in bearing (this is also real, after slip, and after increase in applied loads, some deformatinos occur in the bolts and the steel parts to reach that condition), and as more load is applied, finally the connectin fails, either by breaking bolts or by yield or fracture of the connected parts. The if the connection fails in the bolts, it is important to know if the threads are included in the shear plane or not. The shear plane is the common surface between connected parts. If the threads are included, the bolt will fail, through an area equal to the root area lf the bolt threads. If the threads are excluded, then the bolt fails through an area euqal to the gross area of the bolt. See the AISC manual for values of gross area and root thread area for different bolt diameters. This is why you see larger values for capacity of bolts with threads excluded than you see for bolts with threads included. This is also a detailing issue, and when you design connections, you probably cannot determine which condition you have, it depends on thickness of parts connected, bolt thread length. I cannot think of a situatino whaere you'd know with enough certainty that you can get threads excluded, so I always design for the threads included case, that results in: slightly lower bolt capacity, generally a few more bolts in a connection than I'd have with threads excluded. It also results in more restful sleep at night. If you do run into a situatino where you can be sure the threads are excluded, then feel free to design the connections that way.

I hope this is somewhat clear.
'

Regards,

 

[JAE]

There is actually a THIRD type of connection in AISC -

The Pre-Tensioned bolted connection. This is exactly like the slip critical connection except that inspection of the torquing methods is not required.

See Section 4 of The AISC "Specification for Structural Joints Using ASTM A325 or A490 Bolts" where all three types are described.

 

[dbuzz]

I tend to be conservative and adopt the lower value for shear stength, i.e. assume that threads are included in the shear plane.