Grade 8.8 bolt ordered but Grade 5.6 delivered is more common than people admit
On paper, everything looks right.
The BOQ says Grade 8.8 bolt.
The purchase order says Grade 8.8.
The invoice says Grade 8.8.
But on site, something feels off.
The bolt feels lighter.
The head marking looks different.
Torque values feel inconsistent.
Washers deform too easily.
Later during inspection, someone checks the marking.
It’s not 8.8.
It’s 5.6.
This situation — Grade 8.8 ordered, Grade 5.6 delivered — is one of the most dangerous silent failures in PEB and steel structure projects.
Because nothing looks wrong… until something fails.
Why the difference matters
A Grade 8.8 bolt has:
- Higher tensile strength
- Higher proof load
- Higher fatigue resistance
- Better performance under dynamic load
A Grade 5.6 bolt is meant for:
- Light-duty joints
- Non-structural connections
- Low-load applications
Using 5.6 where 8.8 is required reduces joint capacity significantly.
In wind zones, seismic zones, or large-span PEB frames, this becomes a structural risk.
How this mismatch actually happens
1. Visual similarity
To the untrained eye, Grade 8.8 and 5.6 bolts look identical.
Same size.
Same finish.
Same hex shape.
Only the head marking tells the truth.
2. Supplier substitution
When Grade 8.8 stock is unavailable:
- Some suppliers ship 5.6 instead
- Expecting no one will notice
- Especially under urgent timelines
- 3. Mixed inventory
In warehouses, 5.6 and 8.8 bolts often sit near each other.
During picking:
- Wrong cartons are selected
- Wrong pallets are loaded
- Wrong grade travels to site
- 4. Lack of incoming inspection
Many sites do not:
- Check head markings
- Verify mill test certificates
- Cross-check batch numbers
So wrong grades pass through silently.
How to catch wrong bolt grades before installation
1. Check head markings
Grade markings are stamped on the bolt head:
- 8.8 will be marked “8.8”
- 5.6 will be marked “5.6”
Always visually verify before use. - 2. Demand MTCs
Ask for Material Test Certificates with:
- Heat number
- Mechanical properties
- Batch traceability
- 3. Separate storage
Store 8.8 and 5.6 bolts separately and label clearly.
Never mix. - 4. Train site staff
Even basic training on grade identification prevents major risks.
Typical Grade 8.8 bolt usage in PEB
Common applications:
- Primary frame connections
- Bracing connections
- Portal frame joints
- Heavy secondary members
Typical sizes:
- M12 × 30 / 35 mm
- M16 × 40 / 45 / 50 mm
- M20 × 50 / 60 mm
Coatings:
- HDG for outdoor
- Zinc plated for indoor
What happens if this is ignored
- Reduced load capacity
- Increased fatigue risk
- Joint loosening
- Structural deflection
- Audit failures
- Warranty and liability exposure
And most dangerously — a false sense of safety.
Final thought
A bolt is a small part.
But the grade defines the strength.
Grade 8.8 ordered, Grade 5.6 delivered is not a paperwork error — it’s a structural hazard.
Catching it early saves far more than fixing it later.