Why structural washers matter more than people think
On many PEB sites, slotted holes are used to allow alignment and tolerance.
But when a standard small washer is placed over a slot, it doesn’t cover the opening properly.
The washer bends.
The nut sinks into the slot.
The joint loses preload.
This is where a structural washer becomes critical.
What goes wrong with normal washers
Standard flat washers are designed for round holes.
Slotted holes are larger and elongated.
If washer OD is too small:
- Load is not distributed
- Washer can deform
- Nut can pull into slot
- Preload drops
- Joint becomes unstable
Why OD24–OD25 works for M12 slotted joints
For M12 connections in steel structures:
- Bolt diameter = 12 mm
- Slot width often = 18–20 mm
- Washer OD must be larger than slot
OD24–OD25 washers:
- Fully cover the slot
- Distribute load evenly
- Prevent deformation
- Maintain joint preload
How wrong washers end up on site
Standardization mindset
Procurement orders “M12
washers” without specifying OD.
Suppliers ship standard OD (~22 mm).
Cost pressure
Larger washers cost slightly more.
So smaller ones are substituted.
Drawing ambiguity
Drawings show washers but not OD.
This leads to assumptions.
Where structural washers are required
- PEB primary frames
- Bracing connections
- Moment connections
- Seismic or wind-load joints
Anywhere slots are used, structural washers are needed.
What happens if this is ignored
- Joint slippage
- Loss of preload
- Re-torquing cycles
- Audit remarks
- Long-term fatigue damage
How to prevent washer OD issues
Specify washer OD in drawings and BOQ
Use structural washers for slotted holes
Inspect incoming washers
Reject undersized washers
Train site staff to identify washer sizes
Final thought
A washer is not just a washer.
In slotted joints, washer OD decides whether the joint works as designed.
That’s why a structural washer is not an upgrade — it’s a requirement.