How to Choose the Right Bolt Grade for Steel Structures
What the Grade Number Means
A bolt grade such as 8.8 is two numbers. The first is roughly one hundredth of the tensile strength in MPa, and the second is the ratio of yield strength to tensile strength.
Grade 8.8 therefore gives about 800 MPa tensile strength and 640 MPa yield strength. Class 10.9 gives 1000 MPa and 900 MPa, and class 12.9 gives 1200 MPa and 1080 MPa.
Matching the Grade to the Load
Light structures such as handrails, cladding supports and signage normally use class 4.8 or 8.8 bolts, where the joint carries modest loads.
Steel frames, mezzanine floors and crane supports need class 8.8 or 10.9, often as a slip-critical connection with controlled tightening.
Bridges, towers and heavy machinery usually specify 10.9 or ASTM A325 / A490, because the design relies on high preload and resistance to fatigue.
Corrosion and the Environment
The finish matters as much as the grade. Hot-dip galvanizing gives the longest life outdoors and is standard for structural bolting.
Zinc plating suits indoor assemblies and lighter exposure. For coastal or chemical environments, stainless 304 or 316 removes the coating question entirely.
What to Put on the Enquiry
To get an accurate quotation, send the standard, the size and length, the grade, the thread length and the finish. If the joint has a torque specification, include it.
Daosan produces class 4.8 to 12.9 bolts to DIN, ISO and ANSI standards and can supply test reports with each lot.
Need Help Choosing the Right Fastener?
Tell us the standard, size, grade and finish you need and our team will come back with a price and lead time, usually within 24 hours.
