How to Choose Structural Bolts: ASTM A325, A490, EN 14399 and IS 3757

ASTM A325 and A490 are the two high-strength structural bolt standards in the American system. A325 bolts have a minimum tensile strength of 120 ksi (about 830 MPa) for sizes up to 1 inch and are the standard choice for steel frames. A490 bolts reach 150 to 170 ksi (about 1040 to 1170 MPa) and are used where a higher strength is needed. Since 2015 both are also published within ASTM F3125, which groups the structural bolt grades in one specification, so you may see the grade written as F3125 Grade A325 or A490.

In Europe, EN 14399 covers high-strength structural bolting assemblies for preloading, in system HR (bolts of class 8.8 and 10.9 with hexagon nuts) and system HV (class 10.9, with thicker nuts, for hot-dip galvanized joints). In India, IS 3757 covers high-strength friction grip (HSFG) bolts. EN 15048 is for non-preloaded structural bolting. Always state which standard the project drawings call out. For ordinary bolts see our hex bolts guide.

A325 and A490 bolts are made as Type 1 (medium-carbon or alloy steel) and Type 3 (weathering steel, which forms a protective rust layer like Cor-Ten steel and is used with weathering steel frames). Type 2 has been withdrawn. If the frame is made of weathering steel, use Type 3 bolts so that the colour and corrosion behaviour match.

A490 bolts must not be hot-dip galvanized. At that strength the steel is sensitive to hydrogen embrittlement, and a failure can be sudden. A325 bolts can be galvanized, and then the nut must be oversize-tapped, lubricated and tested with the bolt as a lot. If you need a galvanized joint of higher strength, speak to the designer before you order.

Structural bolts rely on the bolt, the nut and the washer working as one assembly, with lubrication and coating checked together. A nut from another lot, or one with a dry coating, may need much more torque to achieve the same pretension, and that changes the result of every tightening method. This is why the standards require lot identification and rotational capacity testing of the assembly, and why we supply the heavy hex bolt, the heavy hex nut (ASTM A563 DH or equivalent) and the hardened washer (ASTM F436, see our washers guide) as a set.

Keep the sets together in sealed cartons, labelled with the lot number, until they are installed. Do not mix lots on a joint, and do not reuse an installed A490 or galvanized A325 assembly.

Joints are either snug-tight (bolts tightened with a few impacts or the full effort of an ironworker), pretensioned, or slip-critical, where friction between plates carries the load. Pretensioned and slip-critical joints need a method that proves the bolt has reached the minimum tension. The accepted methods are turn-of-nut (tighten snug, then turn by a set angle), a calibrated wrench, twist-off tension control bolts (ASTM F1852 or F2280), and direct tension indicator washers (ASTM F959).

The method is chosen by the contractor and the engineer, and it affects what you order. Twist-off bolts have a splined end that shears at the correct torque and are popular for site speed and easy inspection. Tell your supplier which method will be used so that the lubrication and the washers are right.

• Snug-tight: no preload required; plain heavy hex assembly.
• Turn-of-nut or calibrated wrench: heavy hex bolt with lubricated nut, tested in a bolt-tension device on site.
• Tension control (twist-off): bolts with a splined end, nut and washer supplied and lot-tested as one assembly.

For structural work do not accept a plain commercial invoice. Request a mill test certificate for the bolt, nut and washer showing the heat number and mechanical results, a lot test report for rotational capacity, the coating thickness report for galvanized parts, and the manufacturer's mark and grade on each bolt head. Third-party inspection before dispatch is available for projects that require it.

Check that the markings on the product match the certificate (see how to read a mill test certificate): the A325 or A490 mark, or the HR or HV marks and the manufacturer's code for EN 14399. A bolt without markings is not a structural bolt.

Global Fasteners Exports is a trading and export company: our structural bolts are made by partner factories and we handle inspection, packing, documents and shipping. Because we buy in volume from those factories, prices are typically up to 30% lower than the market price.

For structural bolts, send the project standard (ASTM, EN 14399 or IS 3757), the grade and type, and the tightening method, so the assembly is supplied and lot-tested as one set.

Structural bolt sets are heavy, so containers are filled by weight. Send the standard, type, sizes and lengths, finish and the installation method, and we confirm the lot arrangement and the container quantity.

• Samples or detailed photos can be arranged before a bulk order; sample and courier costs are normally adjusted against the first order.
• Standard payment terms are 50% advance to confirm the order and the balance before dispatch.
• Documents: commercial invoice, packing list, certificate of origin and bill of lading, plus mill test certificates and inspection reports on request.
• Different products can be combined in one consolidated FCL or LCL shipment.