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Why "Prying Force" is important?

August 21, 2020 Posted by Parishith Jayan , , No comments
Our previous post is about structural steel connections, emphasizing its importance and complexity. This post is sort of continuation of the previous post, which is going to emphasize on the additional tension force that is developed in the bolted connections or to make it simple, it is all about "Prying Force".

Before moving into the force aspect, we should know the behavior of prying.

So, what is prying action? 

Let's consider a steel plate that is bolted to the table (similar to the following image). Now we are about to apply the force, F on one end A of the steel plate. What would be the reactive force that is offered by the bolt to hold the steel plate in its position? Is it "F"?

structures-simplified, prying action, prying force
No, the reactive force required is greater than F, since the other end B of the steel plate is pushing against the table. It will develop force Q at the end B.

So, the bolt requires a reactive force, which is equal to F+Q to hold the steel plate to the table, this particular behavior is called "Prying Action" and the additional tensile force, Q is termed as "Prying Force". 

Why it is important?

While determining the tension force of a bolted connection, it is hard to separate the discussion of bolt tension from the connecting element. So, the behavior of connecting elements needs to be incorporated in determining the bolt forces. Since they can incorporate additional forces due to the prying action.

As discussed in the above example, every bolted connection, when subjected to tensile load eccentrically, will try to separate the connecting part from the connected surface, due to the eccentricity in the applied tensile load and bolt placement, addition prying forces will be created which has to be resisted by the bolts. 

If the bolts are not checked for this additional force, eventually it may lead to the failure of the connection bolt.

Different Modes of Prying Action

Normally, the concept of prying action will be presented in terms of T-stub in textbooks and codal provisions.
structures-simplified, prying action, prying force, prying

As we see in the figure, the tensile load is not concentric and acting at some eccentricity to the center of the bolt which may induce prying force on the bolt. The behavior of the T-stub can be analyzed in three different modes.

Mode 1: Complete Flange Yielding (Thin Plate & Strong Bolt)

This mode occurs with a flexible connection flange (thin plate) and a strong bolt. What will happen in this situation, on the application of tensile load, the flexible flange tries to elongate. Whereas the bolt is rigid and won't undergo much of an elongation, holding the connecting flange tight to the surface (resulting in the double curvature). This makes the counterpart of the connecting plate to induce a very strong prying force (refer the attached image below).

Mode 2: Bolt Failure with Flange Yielding

In this mode, the flange will be relatively rigid compared to the bolt. In this case, when a tensile load is applied, the plate elongates. Since the bolts are flexible, they indeed elongate with the flange and allows the flange to form a single curvature (refer the attached image below). Here, the bolt is not resisting much, so the prying force created is little compared to the previous mode.

Mode 3: Bolt Failure (Thick Plate & Weak Bolt)

This occurs with thick plates and weak bolts. As the tensile load is applied, the plates which are thick enough won't elongate that easily. Meanwhile, since the bolts are weak, they fail even before the plate elongates resulting in no prying force.

structures-simplified, prying action, prying force, thin plate, thick plate

Conclusion

  • The key take away from the post is that "the prying force if neglected could result in failure of the connection". 
  • Based on the connection geometry, the prying force can result in 40% of the tension in the bolt. 
  • The calculation of prying force is not as simple as it seems, it is complex and involves a lot of variables.
  • The prying force will be maximum with thin plates and strong bolts.
  • Thick plates and weak bolts won't form a prying action.

References

  1. SCI-P207 - Joints in Steel Construction - Moment Connections
  2. IS 800-2007 - General Construction in Steel - Code of Practice

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