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There is no universal torque setting for a structural bolt. The connection drawings and project specifications determine whether the joint is snug-tight, pretensioned or slip-critical. Snug-tight bolts are tightened until the connected plies are in firm contact. Pretensioned and slip-critical joints require a specified minimum bolt pretension installed with an accepted, verified method. A generic torque chart cannot safely replace that requirement.
For a quick reference, the 2018 edition of ANSI/AISC 360-16 lists a required pretension of 28 kips for a 3/4-inch Group A (A325-type) bolt and 35 kips for a 3/4-inch Group B (A490-type) bolt. Those are force targets, not torque settings. Torque can be used only when the calibrated-wrench method is specified and the wrench has been calibrated for the actual bolt lot, diameter and condition.
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Why one torque number cannot answer the question
Torque is the twisting moment applied to the nut or bolt head. Bolt pretension is the tensile force clamping the connected steel together. The relationship between them changes with thread friction, nut friction, lubrication, galvanizing, coatings, dirt, temperature and the condition of the threads. Two bolts receiving the same torque can therefore develop very different pretension.
That is why the American Institute of Steel Construction (AISC) states: “Torque is an invalid measure for a pre-tensioned installation unless it is calibrated.” A torque value taken from a general chart may be suitable for neither the bolt assembly nor the joint in front of you.
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Start with the drawings, erection specifications and the engineer of record’s requirements. They should identify the required joint condition and, for pretensioned work, the permitted installation method. U.S. structural-steel work is governed by the adopted edition of AISC 360 and the RCSC Specification referenced by the project; the contract documents control if they are more specific.
Identify the required joint condition
Snug-tight joints
A snug-tight joint has no prescribed minimum bolt tension. Tighten the assembly until the connected plies are in firm contact. RCSC practice described by AISC permits a few impacts of an impact wrench, resistance to a suitable non-impacting wrench, or the full effort of an ironworker using an ordinary spud wrench.
“Firm contact” does not mean applying a particular number of foot-pounds. It means the faying surfaces and plies are pulled together so that gaps caused by ordinary erection tolerances are removed. Several passes may be needed because tightening one bolt can loosen another in the same connection.
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Pretensioned joints
A pretensioned joint must be tightened to the minimum bolt pretension specified by the applicable standard. The required force is developed after the joint has first reached snug-tight condition. The installation method named in the contract documents determines how that force is achieved and verified.
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Slip-critical joints
A slip-critical joint is a pretensioned connection in which resistance to slip at the faying surfaces is part of the design. It still requires the specified minimum pretension, but the condition of the contact surfaces, coatings and holes can also be structurally significant. Do not treat “slip-critical” as a different torque chart; it is a design and surface-preparation requirement implemented with a verified pretensioning method.
What the AISC pretension table actually tells you
AISC Table J3.1 defines minimum pretension as 0.70 times the minimum tensile strength of the bolt, rounded to the nearest kip. The values below are from the 2018 edition of ANSI/AISC 360-16 and apply to the listed bolt groups and diameter.
| Bolt description | Diameter | Minimum pretension target | What it is not |
|---|---|---|---|
| Group A, A325-type | 3/4 inch | 28 kips | Not a universal foot-pound torque value |
| Group B, A490-type | 3/4 inch | 35 kips | Not a universal foot-pound torque value |
A kip is 1,000 pounds-force. These figures describe bolt tension, not the setting on a torque wrench. Diameter, bolt group, assembly condition and the specified installation procedure must all match before a table value can be applied.
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Accepted ways to install pretensioned bolts
The RCSC/AISC practice recognizes several methods. Each has a different field check; none should be selected merely because a particular tool is available.
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| Method | How the target is reached | Required control | Torque sensitivity |
|---|---|---|---|
| Turn-of-nut | After snug-tightening, rotate the nut by the specified angle for the bolt length and joint geometry. | Verify snug-tight condition, required rotation, bolt length and washer arrangement during pre-installation testing. | Not based on a torque number; friction still affects the snug-tight step. |
| Calibrated wrench | Use a tension-controlled torque setting established for the actual assembly. | Pre-installation verification and calibration for the actual lot, diameter and condition; AISC states calibration is daily. | High. Lubrication, coating and thread condition can materially change tension. |
| Twist-off tension-control bolt | A splined end breaks off when the assembly reaches the manufacturer’s specified tension range. | Use the correct assembly and perform the required pre-installation verification. | Not set by a field torque value, but assembly condition and tool operation still matter. |
| Direct-tension indicator | A washer or indicator device changes shape as the specified tension is developed. | Check indicator gap or protrusion with the approved feeler gauge and follow the product procedure. | Not based on a generic torque number. |
| Combined method | Use a specified initial tightening followed by a prescribed rotation or other final operation. | Verify every stage and the exact combined-method sequence. | Depends on the individual stages; never infer a torque from one stage alone. |
Pre-installation verification
Before production tightening, assemble representative fasteners in the same configuration that will be used in the work: the same bolt lot, diameter, grade, nut, washers, coatings, lubrication and installation equipment. Demonstrate that the selected procedure achieves the required pretension and record the result as required by the project specification.
When a torque wrench is acceptable
You can use a torque wrench on high-strength structural bolts when the contract documents specify the calibrated-wrench method and the wrench is calibrated for the actual conditions. Do not choose a value from a hardware-store chart or a generic “A325 torque” table.
What must be calibrated
- The wrench or installation tool must be checked against a suitable calibration device.
- The calibration must represent the actual bolt diameter, grade or group, lot, nut and washer arrangement.
- Thread lubricant, galvanizing, paint, dirt and other coatings must match the production assembly.
- Calibration must be performed at the frequency required by the governing specification; AISC guidance states daily calibration for the calibrated-wrench method.
If any of those conditions changes, the established torque may no longer produce the required tension. Recalibrate or repeat the verification rather than assuming the old setting remains valid.
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A correct sequence reduces the chance that early bolts lose tension as later bolts pull the joint together. AISC’s guidance is that “installation should commence at the tightest part of the joint and progress toward the free edges.”
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- Confirm the joint designation. Read the connection detail and specification to determine snug-tight, pretensioned or slip-critical requirements.
- Check the assembly. Verify bolt length, bolt group or grade marking, matching nut, required washers and the orientation of any direct-tension indicator. Replace damaged or contaminated components.
- Bring the plies together. Install enough bolts to align the connection without forcing holes or distorting the steel. Tighten from the most constrained or tightest area outward until all plies are in firm contact.
- Install washers as specified. Washer location and quantity can differ by assembly and bolt orientation. Do not omit or substitute them without approval.
- Apply the specified pretension method. Use the verified turn-of-nut, calibrated-wrench, twist-off, direct-tension-indicator or combined procedure named in the documents.
- Make additional passes. Large or flexible connections may require more than one cycle because tension redistributes as neighboring bolts are tightened.
- Inspect and document. Record the method, verification results, tool or lot information and any rejected or replaced fasteners required by the inspection plan.
Common mistakes and how to correct them
Using a generic torque chart
Problem: A chart gives a foot-pound number but does not account for the actual friction conditions. Correction: Follow the specified method and perform pre-installation verification; use torque only after calibrated-wrench testing establishes the setting.
Treating A325 and A490 values as interchangeable
Problem: A 3/4-inch Group A/A325-type bolt and a 3/4-inch Group B/A490-type bolt have different AISC pretension targets: 28 and 35 kips respectively. Correction: Identify the bolt group from the markings and documents before selecting the procedure.
Skipping snug-tightening
Problem: Pretensioning before the plies are in firm contact can leave gaps and produce an unreliable connection. Correction: Achieve snug-tight condition first; AISC states, “The snug-tight condition must be achieved prior to pre-tensioning.”
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Problem: Tool wear, a new bolt lot, different lubricant or a changed coating can alter the torque-tension relationship. Correction: Follow the daily calibration requirement and repeat verification whenever the assembly condition changes.
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Tightening from the free edge inward
Problem: This can trap gaps near the connection’s tightest region. Correction: Start at the tightest part and work toward the free edges, repeating passes as needed.
Substituting fastener components
Problem: A different nut, washer, coating or lubricant changes the assembly behavior and may invalidate the verification. Correction: Use the specified compatible assembly or obtain written direction from the engineer of record.
Field inspection checklist
- Connection drawings identify the joint as snug-tight, pretensioned or slip-critical.
- Bolt diameter, group or grade, length, nut and washer configuration match the approved documents.
- Faying surfaces and coatings meet the design requirements, especially for slip-critical joints.
- Snug-tight condition was achieved before pretensioning.
- The chosen installation method was verified before production work.
- Torque tools, if used, have current calibration records tied to the actual lot and assembly condition.
- Turn-of-nut rotations, direct-tension-indicator readings or twist-off results are recorded as required.
- Final inspection confirms the tightening sequence progressed from the tightest area toward free edges.
What to do when the drawings do not state a torque
Do not fill the gap with a torque-table guess. Ask the general contractor, steel erector or engineer of record to clarify the joint designation and required installation method. If the documents call for pretension, request the applicable minimum force, accepted method and verification procedure. If they call for snug-tight, confirm that no pretensioned or slip-critical requirement applies.
For an existing connection with an unknown history, do not retighten every bolt to an arbitrary value. Document the bolt markings, connection detail and observed condition, then obtain an engineering assessment. Retrofitting a connection can require checking bolt strength, hole condition, plate bearing, slip resistance and the effect of loosening or replacing fasteners.
How to specify a replacement structural bolt assembly
“3/4-inch structural bolt” is incomplete. A replacement request should identify the applicable standard or specification, bolt group or grade, diameter and length, matching nut, required washers, coating or finish, and whether the joint is snug-tight, pretensioned or slip-critical. ASTM F3125 assemblies may be designated by grade or group, but the project documents and markings govern the acceptable replacement. A replacement that fits the hole can still be wrong if its strength, coating or nut-and-washer configuration differs.
Quick Recap
Bottom line for the most common questions
- How tight should structural bolts be? As tight as the joint designation requires: firm contact for snug-tight joints or the specified minimum pretension for pretensioned and slip-critical joints.
- What torque should I use for an A325 bolt? No universal value. Use the specified installation method; a torque value is valid only after calibration for the actual assembly.
- Do structural bolts need to be torqued? Not necessarily. Turn-of-nut, twist-off, direct-tension-indicator and combined methods are also accepted when specified and verified.
- What is the torque for a 3/4-inch structural bolt? The standards provide pretension force, not one universal torque. A 3/4-inch Group A/A325-type target is 28 kips; Group B/A490-type is 35 kips under the cited AISC table.
- Can I use a torque wrench on high-strength bolts? Yes, but only under a specified calibrated-wrench procedure with the required pre-installation and daily calibration controls.
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