Why "Prying Force" is important?

This article is all about 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"?

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.

To read the extended version, which states about the different modes of prying action, kindly go through the link in the first comment.