Bolts Loads Types in a Bolted Joint 1

[struclearner]

Dears,
I need your help regarding clearing my concepts about bolts loading and the scenarios, when one or other type or combined loading occurs simultaneously.
Following are the types of loads on a Bolt in a Bolted Joint,
- Tensile/Axial Load; This load comes from Pre-Load and from external Tensile or Bending Loads. Tensile loads from bending comes when the location of the clamped members is pulled aprt and bolts in the area also see the portion of that load.
How the tensile load due to bending is calculated, is the Bending Stress Formula of MC/I is used to calculate the tensile stresses due to bending and combined with direct tensile stresses coming from F/A.

- Bending Load: When the bending load is seen by a Bolt, or when a Bolt undergoes bending deformation?
The bolt sees bending loads, when the joint slips due to external load (acting perpendicular to Bolt axis) by overcoming the friction present at the Bolt head/member and clamped members interface, when one of the clamped member moves relative to other and puts a perpendicular force on the Bolt, causing the bending of the bolt?

- Shear Load: When a Shear load is seen by a Bolt?
Shear load will act only, when the slipping of the joint occurs, bringing bolt into contact with member's hole and the shear loads acts at the Bolt cross section. When there is no slippage of the joint, Bolt will not see this shear load?

I came across about the Bolted Joint evaluation by calculating the External and Internal Threads Shear stresses of a tapped joint under the Bolt Tensile load (combined pre-load and external tensile load). How the Bolt Tensile Load (parallel to Bolt axis, which cause Tensile Stresses perpendicular of Bolt Cross Section) can cause Shear Stresses (parallel to Bolt Cross Section)?
Thanks a lot for your valuable comments helping to understand the Bolt loading and loading origin.

Reference for External & Internal Shear Stress:

https://mechanicalc.com/reference/bolted-joint-analysis#external-thread-shear

 

[r13]

Not the best sketch, but should answer your questions 1 and 2. For the third, the bolt will not see the shear until the shear friction due to bolt torque is exceeded, and the bolt is forced into bearing.

Your last question seems related to the inclination of the thread. Is it?

 

[struclearner]

The tensile force due to pure tensile or/and bending load is pretty much easy to understand.
The evaluation of the Bolted Joint by checking the Bolts threads shear stresses from tensile force in the Bolt is a little hard to understand, as the tensile force is acting along the Bolt axis and shear stresses acts parallel to the cross-section plane of the Bolt.

 

[strucbells]

In structural applications usually the shear stresses in the bolt threads are taken care of by a minimum thread engagement length so that the bolt cross-sectional area in tension controls (ie will fail before the threads). We don't typically check those.

 

[r13]

This may help.

 

[struclearner]

Hello r13,
Thanks for the clarification.
1- To calculate the Thread Shear Strength, following formula is mentioned,
Shear Stress = Bolt Tensile Force/Thread Shear Area
The bolt tensile/axial force is taken, the projection of this force parallel to thread flank surface responsible for shear stresses might be taken care of by the shear area.
Reference:

https://mechanicalc.com/reference/bolted-joint-analysis#external-thread-shear

2- In the Book, AN INTRODUCTION TO THE DESIGN AND BEHAVIOR OF BOLTED JOINT, Third Edition, JOHN H. BICKFORD,
While calculating the force required to strip (shear) the threads, shear area of the threads and material shear strength is taken.
The force obtained from this, must be force acting parallel to thread flank surface and from this the tensile force can be calculated using the thread angle.
Thanks for the sharing. The things are now better understandable to me, regarding the shear stresses on the threads caused by tensile force on the bolt.

 

[r13]

Glad to be helpful.