Structural Bolts: A325 vs. A490, Grades and How to Tension Them Right
A structural bolt is not a hardware-store bolt with a better pedigree. It is a designed component of the steel frame, specified by grade, installed to a documented pretension, and inspected before the connection is signed off. Get the grade wrong, or call a pretensioned joint snug-tight, and you have a connection that looks finished and behaves nothing like the drawing. This guide covers what the ASTM F3125 grades actually mean, what pretension values apply to common diameters, the four installation methods recognized in the field, and how to choose the tooling that gets you there repeatably.
What Is a Structural Bolt?
A structural bolt is a high-strength, heavy-hex fastener used to connect steel members in buildings and bridges. Unlike a common bolt, it is installed to a specified minimum pretension so the joint clamps tight, and it is covered by the ASTM F3125 specification.
F3125 consolidated the older standalone standards into one document with four grades. Grade A325 is a medium-carbon or alloy steel bolt with 120 ksi minimum tensile strength. Grade A490 is a quenched and tempered alloy steel bolt at 150 ksi. Grades F1852 and F2280 are the twist-off tension-control versions of A325 and A490, with a splined end that shears off when the installation torque is reached.
Three details separate structural bolts from everything else in the fastener bin. First, the head is heavy hex, which puts more bearing area under the head than a standard hex bolt. Second, the assembly is a system: the bolt, the nut, and the hardened washers are matched, and mixing lots or substituting a hardware-store washer invalidates the installation. Third, the threads are typically excluded from or included in the shear plane by design, which is the difference between an X-type and an N-type connection on the drawing.
Joints themselves come in three types. Snug-tightened joints only need the plies brought into firm contact. Pretensioned joints require the bolt to be tensioned to a specified minimum. Slip-critical joints require that pretension plus a prepared faying surface, because the connection resists load through friction rather than by bearing on the bolt shank.
What Is a Structural Bolt Used For?
Structural bolting shows up wherever steel meets steel and the connection has to be accounted for:
- Beam-to-column connections — shear tabs, clip angles, and end plates in commercial and industrial framing.
- Moment connections — flange plates and stiffeners where the joint has to transfer rotation, not just shear.
- Bracing and truss work — gusset plates carrying axial load into the frame.
- Bridge and highway steel — almost always slip-critical, almost always inspected.
- Crane rails, monorails, and equipment bases — joints that see cyclic and reversing loads where slip is unacceptable.
- Pipe rack and process structures — refinery and plant steel where field bolting beats field welding on schedule.
How to Tension Structural Bolts
Pretension is the whole point. Four installation methods are recognized, and every one of them starts from the same place.
- Bring the joint to snug-tight first. Snug-tight means the plies are in firm contact, typically the full effort of an ironworker on a spud wrench or a few impacts from an impact wrench. Work from the most rigid part of the joint outward.
- Turn-of-nut. After snug, rotate the nut a defined additional amount: one-third turn for bolt lengths up to four diameters, one-half turn for four to eight diameters, and two-thirds turn for eight to twelve diameters. Match-mark the nut and the protruding thread so the rotation is verifiable.
- Calibrated wrench. Set torque against a tension calibrator each shift, for each lot, diameter, and length. Torque is a proxy for tension, and lubricant condition changes the relationship, so recalibration is not optional.
- Direct tension indicators. DTI washers have raised protrusions that flatten as the bolt stretches. When the gap closes to the specified feeler-gauge dimension, the bolt is at tension. Fast to install and fast to inspect.
- Twist-off tension-control bolts. A splined end shears at a preset torque using a shear wrench. Verification is visual: the spline is gone or it is not.
Minimum pretension is a real number, not a feel. For Grade A325, a 3/4-inch bolt requires 28,000 lbs, a 7/8-inch bolt requires 39,000 lbs, and a 1-inch bolt requires 51,000 lbs. Grade A490 runs higher: 35,000 lbs at 3/4 inch and 49,000 lbs at 7/8 inch. Those values, along with the field inspection requirements, come out of the RCSC specification published through AISC's current standards library. For the erection-sequence rules that govern when bolts must be in place, see OSHA's steel erection standard for beams and columns.
A325 vs. A490 Structural Bolts
Both are F3125 grades and both look similar in the box. The differences are in strength, coating rules, and how forgiving they are.
| Grade A325 | Grade A490 | |
|---|---|---|
| Minimum tensile strength | 120 ksi | 150 ksi |
| Material | Medium carbon or alloy steel | Quenched and tempered alloy steel |
| Pretension, 7/8 in. | 39,000 lbs | 49,000 lbs |
| Hot-dip galvanizing | Permitted | Not permitted |
| Head marking | A325 or F3125 Grade A325 | A490 or F3125 Grade A490 |
| Reuse | Limited reuse with engineer approval | Never reuse |
The galvanizing rule is the one that bites people. A490 bolts are not hot-dip galvanized because the process introduces hydrogen embrittlement risk at that strength level, so corrosion protection has to come from a different system. If your torque strategy is the question rather than the grade, our guide to hydraulic torque wrenches covers the tooling side, and our breakdown of spud wrenches and how to use one covers the fit-up work that has to happen first.
Structural Bolt FAQs
A490 bolts are stronger than A325 bolts, with 150 ksi minimum tensile strength versus 120 ksi. A490 is quenched and tempered alloy steel and requires higher pretension at every diameter. A490 also cannot be hot-dip galvanized because of hydrogen embrittlement risk, and it cannot be reused, while A325 may be reused once with engineer approval.
Snug-tight means the connected plies are brought into firm contact, not that the bolt is at a specified tension. In practice it is the full effort of an ironworker on a spud wrench or a few impacts from an impact wrench. Snug-tight is the finished condition for some joints and only the starting point for pretensioned and slip-critical joints.
A 7/8-inch Grade A325 bolt requires 39,000 lbs of minimum pretension. The comparable values are 28,000 lbs at 3/4 inch and 51,000 lbs at 1 inch. Grade A490 bolts at the same diameters require more: 35,000 lbs at 3/4 inch and 49,000 lbs at 7/8 inch. These minimums are set by the RCSC specification.
A490 bolts must never be reused, and galvanized A325 bolts must not be reused either. Plain-finish A325 bolts may be reused once if the engineer of record approves, because tensioning permanently stretches the bolt into its plastic range. Touching up or retightening a previously pretensioned bolt does not count as reuse under the specification.
Crews use spud wrenches for fit-up, impact wrenches with impact sockets for snugging, and either a shear wrench, a calibrated torque wrench, or a hydraulic torque wrench for final tensioning. Direct tension indicator washers and a feeler gauge are common on inspected work. A tension calibrator on site is required whenever the calibrated wrench method is used.
A slip-critical connection resists load through friction between the connected plies rather than by bearing against the bolt shank. It requires full pretension plus a specified faying surface condition, which means no paint, no mill scale contamination, and no loose debris. Bridges, crane rails, and joints with load reversal are commonly designed as slip-critical.
Ready to Get Started with Structural Bolts?
Midland Tool has supplied Michigan's steel trades since 1962, and structural bolting is one of the places where having the right tooling on the truck decides the day. Browse our bolts and fasteners selection, spec the tensioning side from our torque wrenches category, and outfit the crew from sockets, ratchets and attachments. Ask us about the StockUp program if you want bolt inventory managed on site, and about our bolting services and 24/7 emergency support when a connection cannot wait until Monday. If you need the governing document, the full text lives in ASTM F3125/F3125M, the high-strength structural bolt specification.
