Why fully threaded bolts cause problems in shear connections
On many steel sites, installers use whatever bolts are available.
Often that means fully threaded bolts are used everywhere — including in shear and bearing-type connections.
At first, everything seems fine.
Bolts tighten.
Torque is applied.
Inspection passes.
But weeks later:
- Nuts loosen
- Torque drops
- Joints slip
- Retightening becomes routine
This happens because fully threaded bolts behave differently than partial-thread bolts in structural joints.
What goes wrong in a shear connection
In a shear or bearing-type joint:
- The bolt shank should carry shear
- The threads should stay outside the shear plane
- When threads sit inside the joint:
- Contact area is reduced
- Bearing capacity drops
- Stress concentrates at thread roots
- Micro-slippage begins
This leads to preload loss and joint movement.
Why partial thread bolts perform better
Partial-thread bolts have:
- Smooth shank through the joint
- Threads only above the nut
- Better shear capacity
- Lower stress concentration
This makes them ideal for:
- Bearing-type joints
- Structural connections
- High-load steel frames
How fully threaded bolts end up in structural joints
1. Availability
Fully threaded bolts are easier to stock and source.
2. Cost pressure
They are sometimes cheaper.
3. Lack of awareness
Installers don’t always understand thread location importance.
4. Drawing ambiguity
Drawings often specify bolt size and grade, not thread type.
Typical structural bolt thread practice
For M16, M20, M24 structural joints:
- Use partial thread bolts
- Ensure unthreaded shank spans the joint thickness
- Keep threads out of shear plane
What happens if this is ignored
- Nut loosening
- Bolt slippage
- Joint deformation
- Fatigue damage
- Re-torquing cycles
- Audit remarks
How to prevent thread-related joint failures
Specify partial thread bolts in BOQ
Indicate grip length in drawings
Inspect bolt thread length before installation
Train site staff on thread placement
Reject fully threaded bolts for bearing joints
Final thought
A bolt is not just its diameter and grade.
Thread placement matters.
Using fully threaded bolts in shear connections is like using the wrong tool quietly.
It works at first — and then it fails.