Threaded hole

Tapped hole or threaded hole refers to a standard nut threads provided on a steel plate to act as a standard nut to receive bolts in a connection.

One of the major challenges in designing a threaded hole for connection is the lack of proper design guidelines in any of the international design codes. Other challenge is the difference in standard nut material properties from the plate material properties.

On Westgate Tunnel Project, I was required to design tapped holes or threaded holes for few structural connections on steel box girders bridge and steel portals. The connections were to be designed as bearing connections for snug-tight condition. That’s when I had to research a bit more into this topic.

Modes of failure

For a tapped bolted connection subject to static tensile forces, there are three possible modes of failure:

• Bolt breaking – when the capacity of the threads are higher than the bolt capacity with sufficient thread engagement length.
• Bolt thread stripping – for shorter thread engagement with plate material stronger than bolt itself.
• Internal stripping of threaded hole – when the plate material is relatively weak compared to the bolt and the length of engagement is insufficient, the threads on the plates can strip off.

Factors affecting the strength of threaded hole connections

1. Dimensional or geometric factors and material factors.
   1. Tensile Stress Area of Bolt
   2. Shear Area of External Threads
   3. Shear Area of Internal Threads
   4. Length of Thread Engagement
2. Ultimate strength of External Thread Material
3. Ultimate strength of Internal Thread Material
4. Ratio of Shear Strength to Tensile Strength
5. Nut Dilation
6. Relative strength of nut to bolt threads
7. Coefficient of Friction
8. Applied Torque
9. Number of threads in grip

When the material strength and hardness properties of bolts are significantly higher than plate material, the plate material is carefully chosen to avoid impractically long threads. This would usually be the case for Gr. 350 steel plates when used with bolt grades 8.8 and above. Longer threaded hole typically more than 1.5 bolt dia leads to galling of threads, therefore, Specialised plates such as Bisplates are used to minimize excessive tapped lengths.

Design Reference for design of threaded holes

The purpose of this article was only to direct the readers to a reliable design document which I came across during my design task. “Analysis and Design of Threaded Assemblies” by E. M. Alexander is an excellent guide for steel designers.