Heavy Hex Bolts
Manufacturer
India · Mumbai
Anatomy of a Heavy Hex Bolt — Built for Structural Steel
A heavy hex bolt is a conventional headed hex bolt, but with a significantly larger head and across-flats dimension than a standard hex bolt at the same thread size — the identical dimensional upsizing principle used on our heavy hex nuts, applied here to the bolt head. The larger head accepts higher installation torque without rounding and provides greater bearing area against the steel members being joined, matched to a heavy hex nut on the opposite side.
This is the headed alternative to our stud bolts page — where a stud bolt has no head at all and is torqued entirely through two nuts for pressure-retaining flange service under ASTM A193, a heavy hex bolt has a conventional head and single nut, and is manufactured to an entirely different standard family — ASTM A325 and A490 — because it is intended for structural steel building and bridge connections, not pressure vessel or piping flanges. This is a genuinely different service category with its own governing codes, inspection requirements, and installation verification methods.
The single most important engineering decision on a structural bolted connection is whether it is designed as slip-critical or bearing-type — a distinction with no equivalent anywhere else in our bolt range, and covered in full detail below.
Why Heavy Hex Bolts — The Standard Structural Steel Fastener
Larger Bearing Area at the Same Thread Size
The wider head and matched nut provide meaningfully greater contact area against the connected steel members than a standard hex bolt, reducing local bearing stress on high-load structural connections.
Withstands Higher Installation Torque
The larger across-flats dimension resists rounding under the higher torque or turn-of-nut rotation structural preload verification methods require, compared with a standard hex bolt head at the same thread size.
Governed by Codes Engineers Already Expect
ASTM A325 and A490 are the default specification every structural engineer, fabricator, and inspector expects for bolted steel connections — specifying a heavy hex bolt under these standards avoids any ambiguity in a structural drawing review.
Compatible With Standard Tension Verification Hardware
The larger head geometry accommodates direct tension indicator (DTI) washers and calibrated wrench tensioning equipment designed around the heavy hex head profile, covered in detail below.
Slip-Critical vs Bearing-Type Connections
A structural bolted connection can be designed to carry its load in one of two fundamentally different ways, and this choice has no equivalent anywhere else in our bolt range. In a bearing-type connection, the joint is permitted to slip slightly under load until the bolt shank itself comes into direct bearing contact with the sides of the bolt holes — load is then carried by the bolt shank resisting shear, similar in principle to how a standard through-bolt carries load in most everyday connections.
In a slip-critical connection, the joint is designed so that it must never slip at all under service load — instead, the connected steel plates are clamped together so tightly by the bolt’s own tension that load is transferred entirely through friction between the clamped surfaces, with the bolt shank never actually bearing against the hole at all under normal conditions. Slip-critical connections require significantly higher, precisely verified bolt preload than bearing-type connections, since the entire load path depends on that clamping friction rather than the bolt shank’s shear strength.
Which type a given connection requires is an engineering design decision made on the structural drawing, not a choice made at the point of bolt selection — but it directly determines both the bolt tensioning method required and the surface preparation needed on the connected steel (slip-critical connections typically require a specified faying surface condition to achieve a reliable friction coefficient). Getting this distinction right, and installing to the correct tension for the connection type specified, is the central quality concern on any structural bolting project.
ASTM A325 vs ASTM A490
Both are structural bolting grades — the difference is strength class and the resulting design load capacity.
ASTM A325
The standard structural bolt grade for the majority of building and bridge connections, available in Type 1 (medium carbon steel) and Type 3 (weathering steel, matching unpainted weathering steel structures).
ASTM A490
A higher-strength alloy steel structural bolt grade, used where connection design loads exceed what A325 bolts can economically carry, allowing fewer or smaller-diameter bolts to carry an equivalent structural load.
Bolt Tension Verification Methods
A structural bolt’s design strength depends on it actually reaching its specified tension — verification is not optional on slip-critical connections.
Turn-of-Nut Method
The nut is first brought to a “snug-tight” condition, then rotated a further specified fraction of a turn (commonly 1/3 to 1 full turn depending on bolt length and grade) — the geometric rotation itself, not a torque reading, provides the tension verification.
Direct Tension Indicator (DTI)
A washer with small raised protrusions is installed under the bolt head or nut; as the bolt is tensioned, the protrusions compress to a specified gap, measured with a feeler gauge — a physical, visually verifiable confirmation of achieved tension.
Calibrated Wrench Method
A torque wrench calibrated against a representative sample of the actual bolt lot being installed applies a torque value correlated to the required tension for that specific lot, grade, and lubrication condition.
Heavy Hex Bolt Material & Strength Options
Grade selection follows the connection’s structural design requirement and environmental exposure.
Heavy Hex Bolt Configurations & Related Hardware
A325 and A490 grades across the full size range, matched with heavy hex nuts and DTI washers. M12–M64.
ASTM A325 Heavy Hex Bolt
Standard structural grade per ASTM A325 / ASME B18.2.1. M12–M64. The default specification for general structural steel building and bridge connections.
ASTM A490 Heavy Hex Bolt
High-strength structural grade per ASTM A490. M12–M64. For high-load and seismic-resistant structural connections.
Direct Tension Indicator Washers
DTI washers matched to bolt grade and size for verified tension installation, especially critical on slip-critical connections requiring documented preload confirmation.
Custom Heavy Hex Bolts
Non-standard lengths, weathering steel compositions, and drawing-specific configurations. Contact sales@akbarfasteners.in with grade, size, and connection type requirement.
Heavy Hex Bolt Size Chart — M12 to M64
Across-flats dimension and head height compared with a standard hex bolt at the same thread size.
| Thread Size | Across Flats (mm) | Head Height (mm) | Standard Hex Equivalent (mm) |
|---|---|---|---|
| M12 | 21.0 | 7.9 | 19.0 |
| M16 | 27.0 | 10.2 | 24.0 |
| M20 | 34.0 | 12.7 | 30.0 |
| M24 | 41.0 | 15.1 | 36.0 |
| M30 | 50.0 | 19.1 | 46.0 |
| M36 | 60.0 | 22.6 | 55.0 |
Heavy Hex Bolts Application Areas
Structural steel connections wherever verified, code-compliant preload is a design requirement.
Structural Steel Buildings
moment and shear connections
Bridge Construction
slip-critical bridge joints
Industrial Plant Structures
structural platform connections
Tower & Transmission Structures
high-wind and seismic bolting
Rail Infrastructure
platform and gantry structures
Power Generation Structures
structural support connections
Offshore & Marine Structures
corrosion-resistant platform steel
Solar & Wind Structural Mounting
foundation-to-frame connections
Heavy Hex Bolt Standards Coverage
Same ISO 9001:2015 quality system as our complete fastener range. Grade and dimensional compliance verified against ASTM A325/A490 on every production lot.
Complete Documentation Package
Grade / Heat Number
Tensile · Hardness
Head · Across Flats
Conformance
On Request
Related Bolt Types & Materials
Heavy hex bolts for structural steel connections. Heavy hex nuts as the standard matched pairing. Stud bolts for the headless flange-joint alternative. SS 316L for corrosive-environment structural steel.
Heavy Hex Nuts
The standard matched nut for every heavy hex bolt connection — the identical dimensional upsizing principle applied to the nut side of the joint.
View Heavy Hex Nuts →Stud Bolts
The headless alternative for pressure-retaining flange service, torqued through two nuts rather than a single head-and-nut structural connection.
View Stud Bolts →SS 316L Fasteners
For heavy hex bolt applications in marine, coastal, or corrosive-environment structural steel where carbon and alloy steel are insufficient.
View SS 316L →Full Bolt Range
Complete bolt manufacturing capability at Akbar Fasteners — heavy hex, stud, socket head, and all specialty forms across every material grade.
View Bolt Range →Need Heavy Hex
Bolts?
Grade (A325/A490) · thread size · connection type · quantity. Akbar Fasteners Pvt. Ltd. responds within 24 hours.
Export: +91 8696 32 4646 · Support: +91 800 9315 333 · export@akbarfasteners.in