ASTM F3125 covers inch and SI (metric) products as separate standards, and they should not be mixed as exact equivalents. A325 and A490 are furnished in inch sizes from 1/2 to 1-1/2 in., while A325M and A490M are furnished from M12 to M36. Inch conversions below are for orientation only.

AISC states that 3/4-, 7/8- and 1-in. diameters are the industry-standard sizes and provide adequate design strength for the vast majority of connections in steel structures. Larger diameters are permitted, but they can need larger clearances and edge distances, and they can be harder to tension.

Standardizing on a few diameters helps in several ways:

• Fabrication: fewer punches, drills and templates.
• Erection: fewer tools and sockets on site. See the Socket Size Chart for wrench sizes.
• Inspection: a limited set of sizes is easier to check.
• Inventory and cost: fewer SKUs and less risk of mix-ups.

Gross shank area is the circular area of the nominal diameter. It is used in several structural calculations, but it is not the same as thread tensile stress area, and it is not an allowable bolt strength.

Thread area is smaller than gross area. AISC 360-22 uses a tensile-stress-area approach in its updated bolt provisions, and AISC noted that the older approximate thread-area assumption becomes less accurate for larger bolts and threaded rods. The thread tensile stress area follows the ASME B1.1 formula below, using the standard coarse thread series (UNC) pitches for structural bolts.

AISC Table J3.3 lists nominal hole dimensions for four hole types. For bolts 1-1/8 in. and larger, the table gives formulas, so those rows below are calculated from the formulas. The same dimensions are used for fabrication and for engineering calculations, and they affect bearing, tearout, net section, block shear, slip resistance and installation.

The default condition. A standard hole is 1/16 in. larger than the bolt in the sizes in this chart.

Larger round holes used only where the connection type permits. They typically need hardened washers and have restricted use in bearing and slip-critical joints.

Allow limited adjustment in one direction, with slot direction mattering to the force path.

Allow greater movement and adjustment, and are more restricted in use. Installers may not enlarge holes freely in the field. Hole use depends on the connection type and the AISC and RCSC requirements. For general-purpose hole sizes, see the Bolt Hole Size Chart and the Clearance Hole Size Chart.

AISC requires the center-to-center spacing of standard, oversize or slotted holes to be generally not less than 2⅔ times the nominal bolt diameter, and prefers 3 times the diameter where practical. AISC also requires adequate clear distance between holes.

AISC also limits how far apart bolts can be, so the plates stay together and do not buckle or let moisture in between them. For painted members, or unpainted members not subject to corrosion, the maximum spacing is generally 24 times the thickness of the thinner connected part or 12 in. Closer limits apply for unpainted weathering steel. Compression members and staggered patterns have their own conditions.

Edge distance is measured from the center of a bolt hole to the nearest edge of the connected part. End distance is the same measurement in the direction of force. The required value depends on bolt diameter, hole type, edge condition (sheared versus rolled or thermally cut), force direction, and bearing and tearout checks. There is no universal “1.5d” rule.

Lesser edge distances are permitted only where the applicable provisions of AISC Chapter J are satisfied, and AISC allows a small reduction at holes where the required strength is 25 percent or less of the maximum. Edge distances less than one bolt diameter are not permitted without approval from the engineer of record. For plate dimensions, see the Steel Plate Thickness Chart.

Fabrication drawings use a small set of layout terms. Knowing them makes bolt patterns easy to read.

Structural bolt length comes from the grip of the joint plus the nut, washers and required thread projection, not from the diameter. It also depends on bolt style, thread length and the project specification. ASME B18.2.6 gives the dimensional requirements for heavy-hex structural bolts, including length and thread length.

Grip is the combined thickness of the connected plies the bolt passes through. For washers, add each washer thickness in the length calculation. Adding more plies, a beveled washer or a DTI washer changes the length needed.

Standard heavy-hex structural bolts are partly threaded, so the shank and thread lengths vary with diameter and length. The thread must extend enough for the nut to seat with the required projection, and whether threads fall in the shear plane affects the available shear strength.

F3125 Supplementary Requirement S1 allows A325 bolts with nominal lengths of four bolt diameters or less to be threaded full length, and they are marked A325T. For a 3/4 in. bolt that means up to 3 in. long, and for a 1 in. bolt up to 4 in. long. A490 has no equivalent A490T provision. Because the threads run through the whole bolt, they cannot then be excluded from the shear plane.

Heavy-hex structural bolts have a wider head than ordinary hex bolts. ASME B18.2.6 sets the dimensions. Use it, not ordinary SAE hex-bolt tables, for structural bolts and for wrench and clearance checks.

The head height, bearing face and clearance for sockets and tension-control tools also come from ASME B18.2.6 and the tool maker. For tool sizes, see the Socket Size Chart.

High-strength bolting uses nuts made to ASTM A563/A563M or ASTM A194/A194M. For F3125 Grade A325 and A490, ASTM A563 Grade DH (or Grade DH3 for weathering-steel Type 3 assemblies) is the standard heavy-hex nut. Other A563 grades may be permitted for some plain A325 assemblies under RCSC, and galvanized assemblies need the correct nut and lubricant combination. Check the RCSC nut table for each bolt grade.

Heavy-hex nuts share the head width across flats shown above. For nut thickness, ASME B18.2.6 lists 39/64 in. for 5/8 in., 47/64 in. for 3/4 in. and 55/64 in. for 7/8 in. nuts. For general nut dimensions, see the Nut Size Chart.

ASTM F436/F436M governs hardened steel washers used with structural bolts. Washers are required at specific positions depending on the installation method and hole type. Oversize and slotted holes need hardened washers, and beveled or clipped washers and DTI washers have their own rules in RCSC.

For SAE, USS and other washer types, see the Washer Size Chart.

AISC 360-22 moved away from the older Group A, B and C labels to strength-based group names: Group 120, Group 144 and Group 150. The names reflect minimum tensile strength classes in ksi.

The A325 and F1852 class, with a 120 ksi minimum tensile strength.

The newer Grade 144 class, 144 ksi, available as heavy hex and twist-off up to 1-1/4 in.

The A490 and F2280 class, in the 150 ksi range. See the Bolt Grade Chart for a broader comparison with general-purpose bolt grades.

AISC defines the nominal shear strength of a bolt as the nominal shear stress times the gross bolt area. The stress depends on the bolt group and on whether threads are included in or excluded from the shear plane. Single shear counts one plane, and double shear counts two.

Bolt tension resistance depends on the group, the tensile area, the applied axial force, installation condition, combined shear and tension, and prying effects. The material tensile strength of the bolt is not the same as the available strength of the connection, because the connected parts, bolt layout and prying all reduce what the joint can carry.

Many structural bolts see shear and tension at the same time, as in moment connections, hangers and bracing. AISC Chapter J requires an interaction check for combined loading. Adding a shear capacity and a tension capacity together is not a valid calculation.

Structural joints work in one of two ways, and the choice is part of the design, not just the tightening method.

Use the 2025 RCSC Specification as the principal high-strength bolting reference for both. Do not imply that either joint type is universally stronger.