Can a nut be used as a washer for oversized holes in steel connections?

No. Per AISC 360-22 Section J3.2, using a nut as a washer is NOT permitted for oversized or slotted holes. The reasons: (1) Bearing area — an F436 hardened washer distributes clamp-up force over a much larger area than a nut's chamfered bearing face. A 3/4" F436 washer has approximately 1.47 in² bearing area vs roughly 0.66 in² for the nut face. (2) Hardness — F436 washers are through-hardened to Rockwell C 38-45, while A563 nuts are not designed to serve as bearing washers and may deform under pretension loads. (3) Thickness — F436 washers have specified minimum thickness (5/32" for 3/4" bolts); nuts have variable thickness with a chamfered surface that creates uneven bearing. (4) Flatness — F436 washers are manufactured flat within 0.010" to provide uniform bearing; the chamfered nut bearing surface cannot achieve this. AISC requires hardened washers under the element being turned during installation for standard holes, and F436 washers over oversized holes in outer plies. AS 4100 Clause 9.3 similarly requires washers for oversized hole connections. If a non-standard configuration is proposed, the engineer of record must provide written justification, verify bearing area adequacy, and specify additional inspection. The simplest fix: install an F436 washer — cost is negligible (~$0.10-0.50 per washer) and it eliminates a compliance issue.

When are oversized holes permitted in steel connections?

Per AISC 360-22 Table J3.1 and Section J3.2, oversized holes are permitted ONLY in slip-critical (SC) connections, NOT in bearing-type connections. In bearing-type connections (snug-tight or pretensioned), only standard holes are allowed because load transfers through bolt bearing against the hole edge — oversized holes would permit slip before the bolt engages. In slip-critical connections, oversized holes are permitted because load transfers through friction, so hole clearance does not affect strength. However, F436 hardened washers must be installed over oversized holes in outer plies. For short-slotted holes: permitted in slip-critical connections only. For long-slotted holes (transverse to load): permitted in slip-critical connections but limited to one ply; a plate washer or continuous bar with standard holes must cover the slot. Standard hole dimensions per AISC Table J3.3: for 3/4" bolt, standard hole = 13/16", oversized = 15/16" (16% larger area). For 1" bolt, standard = 1-1/16", oversized = 1-1/4" (38% larger area). The reason bearing-type connections prohibit oversized holes: if the bolt can move 1/16-1/8" before bearing, the connection displaces under load, which could cause serviceability problems, progressive connection failure, or redistribution of forces not accounted for in design.

What are the bolt bearing and tearout limit states per AISC 360?

Per AISC 360-22 Section J3.10, bolt bearing strength at bolt holes is: Rn = 1.2 × Lc × t × Fu ≤ 2.4 × d × t × Fu for standard, oversized, and short-slotted holes loaded parallel to the edge, or Rn = 1.5 × Lc × t × Fu ≤ 3.0 × d × t × Fu for long-slotted holes with slot perpendicular to load. Where: Lc = clear distance from hole edge to material edge or adjacent hole edge in the load direction, d = bolt diameter, t = material thickness, Fu = tensile strength of connected material. The 1.2Lc term represents tearout (shear rupture along two planes from hole to edge), and the 2.4d cap represents pure bearing (ovalization of hole under bolt). When Lc ≥ 2d, bearing controls at 2.4d × t × Fu; when Lc < 2d, tearout controls at 1.2Lc × t × Fu. For LRFD, φ = 0.75. Example: 3/4" A325 bolt in 3/8" A36 plate (Fu = 58 ksi) with edge distance Le = 1.5": Lc = 1.5 − 0.5 × (13/16 + 1/16) = 1.5 − 0.44 = 1.06". Tearout: φRn = 0.75 × 1.2 × 1.06 × 0.375 × 58 = 20.7 kips. Bearing cap: φRn = 0.75 × 2.4 × 0.75 × 0.375 × 58 = 29.4 kips. Tearout controls at 20.7 kips. Minimum edge distance per AISC Table J3.4 for 3/4" bolt is 1-1/8" (sheared edge) or 1" (rolled/thermal cut), so 1.5" is adequate.

What are the minimum weld sizes per AISC 360?

AISC 360-22 Section J2.2b specifies minimum fillet weld sizes based on the thinner part joined to ensure adequate heat input and fusion: for material thickness ≤ 1/4", minimum weld = 1/8"; for t > 1/4" to 1/2", minimum = 3/16"; for t > 1/2" to 3/4", minimum = 1/4"; for t > 3/4", minimum = 5/16". Maximum fillet weld size at edges: for material thickness < 1/4", maximum = material thickness; for t ≥ 1/4", maximum = t − 1/16". The maximum prevents melting away the edge. Minimum weld length: 4 × weld size or 1-1/2", whichever is larger (J2.2b). Intermittent fillet welds: minimum length per segment = 1-1/2" or 4 × weld size. Fillet weld effective throat = 0.707 × leg size. Design strength per unit length: φRn = φ × 0.60 × FEXX × 0.707 × w where FEXX = electrode classification strength (70 ksi for E70XX), φ = 0.75. For E70XX electrodes: 1/4" fillet provides φRn = 0.75 × 0.60 × 70 × 0.707 × 0.25 = 5.57 kips per inch of weld length. A 3/16" fillet provides 4.18 kip/in. Weld metal volume: 1/4" fillet ≈ 0.031 lb/ft of electrode per inch of weld. For a 4" long 1/4" fillet both sides of a clip angle, capacity = 4 × 2 × 5.57 = 44.6 kips.

What are the F436 hardened washer requirements for structural bolts?

ASTM F436 hardened washers are required per AISC 360-22 Section J3.2 under the following conditions: (1) Under the element (nut or bolt head) that is turned during installation — always required for pretensioned bolts; (2) Over oversized holes in outer plies — required regardless of connection type; (3) Over short-slotted holes in outer plies when the slot axis is perpendicular to the load; (4) For A490 bolts over 1" diameter — required even for standard holes; (5) When the outer ply material has specified minimum yield strength less than 40 ksi. F436 washers are NOT required for snug-tight connections with standard holes in typical structural steel (Fy ≥ 36 ksi) when the turned element bears directly on steel. F436 washers are through-hardened to HRC 38-45 with minimum thickness: 1/2" bolt → 0.097", 5/8" → 0.122", 3/4" → 0.122", 7/8" → 0.136", 1" → 0.136", 1-1/8" → 0.136", 1-1/4" → 0.136", 1-3/8" → 0.163", 1-1/2" → 0.163". For slotted holes, plate washers (not standard F436) are required: minimum thickness = 5/16" for bolts up to 1", 3/8" for bolts > 1" per AISC J3.2. The washer prevents the nut or bolt head from embedding into the connected material as pretension is applied, ensuring the full clamping force develops in the bolt shank rather than being lost to bearing deformation.

Steel Connection Code Compliance — Bolt, Weld & Bearing Checks

Q: What are the minimum weld sizes per AISC 360?

AISC 360-22 Section J2.2b specifies minimum fillet weld sizes based on the thinner part joined to ensure adequate heat input and fusion: for material thickness ≤ 1/4", minimum weld = 1/8"; for t > 1/4" to 1/2", minimum = 3/16"; for t > 1/2" to 3/4", minimum = 1/4"; for t > 3/4", minimum = 5/16". Maximum fillet weld size at edges: for material thickness < 1/4", maximum = material thickness; for t ≥ 1/4", maximum = t − 1/16". The maximum prevents melting away the edge. Minimum weld length: 4 × weld size or 1-1/2", whichever is larger (J2.2b). Intermittent fillet welds: minimum length per segment = 1-1/2" or 4 × weld size. Fillet weld effective throat = 0.707 × leg size. Design strength per unit length: φRn = φ × 0.60 × FEXX × 0.707 × w where FEXX = electrode classification strength (70 ksi for E70XX), φ = 0.75. For E70XX electrodes: 1/4" fillet provides φRn = 0.75 × 0.60 × 70 × 0.707 × 0.25 = 5.57 kips per inch of weld length. A 3/16" fillet provides 4.18 kip/in. Weld metal volume: 1/4" fillet ≈ 0.031 lb/ft of electrode per inch of weld. For a 4" long 1/4" fillet both sides of a clip angle, capacity = 4 × 2 × 5.57 = 44.6 kips.

Q: What are the F436 hardened washer requirements for structural bolts?

ASTM F436 hardened washers are required per AISC 360-22 Section J3.2 under the following conditions: (1) Under the element (nut or bolt head) that is turned during installation — always required for pretensioned bolts; (2) Over oversized holes in outer plies — required regardless of connection type; (3) Over short-slotted holes in outer plies when the slot axis is perpendicular to the load; (4) For A490 bolts over 1" diameter — required even for standard holes; (5) When the outer ply material has specified minimum yield strength less than 40 ksi. F436 washers are NOT required for snug-tight connections with standard holes in typical structural steel (Fy ≥ 36 ksi) when the turned element bears directly on steel. F436 washers are through-hardened to HRC 38-45 with minimum thickness: 1/2" bolt → 0.097", 5/8" → 0.122", 3/4" → 0.122", 7/8" → 0.136", 1" → 0.136", 1-1/8" → 0.136", 1-1/4" → 0.136", 1-3/8" → 0.163", 1-1/2" → 0.163". For slotted holes, plate washers (not standard F436) are required: minimum thickness = 5/16" for bolts up to 1", 3/8" for bolts > 1" per AISC J3.2. The washer prevents the nut or bolt head from embedding into the connected material as pretension is applied, ensuring the full clamping force develops in the bolt shank rather than being lost to bearing deformation.

Steel connections must comply with specific code provisions for bolts, welds, bearing, and hole types. This page addresses common code compliance questions that arise during connection design, including the frequently asked question about using a nut as a washer in oversized holes.

Common Code Compliance Questions

Can a Nut Be Used as a Washer for Oversized Holes?

This is a frequent question in structural steel connection design. The short answer: it depends on the code and the connection type.

AISC 360-22 Position

Per AISC Specification Section J3.2:

Standard holes: Hardened washers are required under the element (nut or bolt head) that is turned during installation
Oversized holes: F436 hardened washers are required over oversized holes in outer plies
Slotted holes: F436 washers are required, and for long slotted holes, a plate washer or continuous bar with standard holes must be used to cover the slot

Using a nut as a washer is NOT permitted by AISC 360 for the following reasons:

Bearing area: A nut does not provide the same bearing area as a proper F436 washer. The washer is designed to distribute clamp-up force over a larger area.
Hardness: F436 washers are through-hardened to a specific hardness range. Standard nuts (A563) are not designed to serve as washers.
Thickness: F436 washers have specific thickness requirements (approximately 5/32 inch for 3/4 inch bolts) that are not met by a nut.
Flatness: Washers are manufactured flat to provide uniform bearing. Nuts have a chamfered bearing surface.

Exceptions and Workarounds

If a nut must be used in a non-standard configuration:

Structural rationale: The engineer of record must provide a written justification
Bearing check: Verify the nut provides adequate bearing area on the connected material
Clamp-up verification: Ensure the assembly can develop the required pretension (for slip-critical connections)
Special inspection: Additional inspection requirements may apply

AS 4100 Position

AS 4100 Clause 9.3 requires washers under nuts and bolt heads for:

Connections with oversized holes
Connections where the bolt hole diameter exceeds 1.3 × bolt diameter
Turn-of-nut pretensioning method

AS 4100 does not explicitly address using a nut as a washer, but the general requirement for proper bearing distribution implies that standard washers should be used.

Oversized Holes — When Are They Permitted?

AISC 360-22 Rules

Connection Type	Standard Holes	Oversized Holes	Short Slots	Long Slots
Bearing-type	Permitted	NOT permitted	NOT permitted	NOT permitted
Slip-critical	Permitted	Permitted	Permitted	Permitted (one ply only)
Tension-only	Permitted	Permitted	Permitted	Case-by-case

Key points:

Bearing-type connections (most common): ONLY standard holes are permitted. No oversized holes.
Slip-critical connections: Oversized holes are permitted, but require hardened washers (F436) and proper pretensioning
The reason is simple: oversized holes allow bolt movement, which is unacceptable when the bolt bears against the hole edge to transfer shear

Oversized Hole Dimensions (AISC Table J3.3)

Bolt Diameter (in)	Standard Hole (in)	Oversized Hole (in)
1/2	9/16	5/8
5/8	11/16	13/16
3/4	13/16	15/16
7/8	15/16	1-1/16
1	1-1/16	1-1/4

Bolt Bearing and Tearout Compliance

Bearing Strength at Bolt Holes

Per AISC 360-22 Section J3.10:

For standard holes (deformation at service load is a concern):

φRn = φ × 1.2 × Lc × t × Fu (φ = 0.75)

For standard holes (deformation is NOT a concern):

φRn = φ × 1.5 × Lc × t × Fu (φ = 0.75)

For oversized or slotted holes:

Use a reduced coefficient (see AISC Table J3.10).

Lc = clear distance between the edge of the hole and the edge of the adjacent hole or edge of material.

Tearout Check

Tearout occurs when the material behind the bolt shears out along two parallel planes:

φRn = φ × 1.2 × Lc × t × Fu (same as bearing, but Lc is the edge distance)

If tearout governs, increase edge distance or add more bolts.

Minimum Edge Distance Compliance

Per AISC Table J3.4M:

Bolt Diameter (in)	Min Edge Distance — Cut Edge (in)
5/8	7/8
3/4	1
7/8	1-1/8
1	1-1/4

These are minimums. Bearing capacity may require larger edge distances.

Weld Compliance Checks

Minimum Fillet Weld Size (AISC Table J2.4)

Thinner Part (in)	Min Weld Size (in)
≤ 1/4	1/8
> 1/4 to 1/2	3/16
> 1/2 to 3/4	1/4
> 3/4	5/16

This is a prescriptive minimum for heat input and fusion, not a strength requirement.

Fillet Weld Strength

φRn = φ × 0.60 × FEXX × (0.707 × w) × L (φ = 0.75)

where:

FEXX = electrode classification strength (e.g., 70 ksi for E70XX)
w = fillet weld leg size
L = effective weld length (actual length minus 2 × w for start/stop craters)
0.707 = converts leg size to throat size for equal-leg fillet

E70XX Weld Strength

For E7018 (most common structural electrode):

FEXX = 70 ksi (483 MPa)
Fillet weld shear strength = 0.60 × 70 = 42 ksi
Design shear strength per 1/16 inch of leg per inch of length = 0.928 kips/in

Maximum Weld Size

Along edges of material:

Material < 1/4 inch thick: max weld size = material thickness
Material ≥ 1/4 inch thick: max weld size = material thickness - 1/16 inch

Slip-Critical Connection Compliance

When Slip-Critical is Required

Per AISC Section J3.1, slip-critical connections are required for:

Connections subject to fatigue loading with reversal of load direction
Joints where bolts connect to oversized holes or slotted holes with loads parallel to the slot
Joints where slip would compromise the structural integrity

Pretensioning Requirements

Bolt Diameter	A325 Min Pretension (kips)	A490 Min Pretension (kips)
5/8	19	24
3/4	28	35
7/8	39	49
1	51	64

Pretension = 0.70 × Fnt × Ab

Installation Methods

Turn-of-nut: Rotate the nut a specified number of turns from snug-tight (typically 1/3 to 1 full turn depending on bolt length and geometry)
Calibrated wrench: Torque to achieve required pretension (requires daily calibration)
Direct tension indicators (DTI): Compressible washers that indicate proper tension
Twist-off bolts: Bolt has a spline that shears off at the correct tension

Faying Surface Requirements

For slip-critical connections, the faying surfaces must meet specific requirements:

Surface Class	Condition	Slip Coefficient μ
A	Unpainted clean mill scale	0.33
B	Unpainted blast-cleaned	0.50
B	Hot-dip galvanized and roughened	0.50
C	Unpainted roughened (4 mil amplitude)	0.40

Painted surfaces: verify slip coefficient with manufacturer data. Most standard paints reduce μ to 0.05-0.15, effectively preventing slip-critical behavior.

Connection Design Checklist — Code Compliance

Bolt type specified (A325, A490, Grade 8.8) and matched to application
Hole type matches connection type (standard for bearing, oversized only for slip-critical)
Proper washers specified (F436 for oversized holes, hardened for pretensioning)
Edge distances ≥ minimum per AISC Table J3.4
Bolt spacing ≥ 2.67d center-to-center
Bearing capacity checked per AISC J3.10
Tearout capacity checked for all bolts near edges
Fillet weld size ≥ minimum per Table J2.4
Fillet weld throat provides adequate strength
Weld length ≥ 4w (minimum effective length)
Slip-critical designation applied where required
Faying surface class specified (A, B, or C)
Pretensioning method specified for slip-critical connections
Shop drawing notes include all connection requirements

Frequently Asked Questions

Can I use a nut as a washer? No. Per AISC 360-22, standard F436 hardened washers must be used under nuts and bolt heads, especially for oversized holes. Nuts do not provide the correct bearing area, hardness, or flatness.

When are oversized holes permitted? Only in slip-critical connections. Bearing-type connections must use standard holes. Oversized holes are also permitted in tension-only connections at the discretion of the engineer.

What is the minimum fillet weld size for 1/2 inch plate? 3/16 inch per AISC Table J2.4. This ensures adequate heat input and fusion.

How do I check bolt bearing? Calculate φRn = 0.75 × 1.2 × Lc × t × Fu where Lc is the clear distance from hole edge to adjacent hole or material edge. Verify φRn ≥ required shear per bolt.

What is the slip coefficient for galvanized connections? Class B: μ = 0.50 for hot-dip galvanized and roughened surfaces. Untreated galvanized surfaces may have lower slip coefficients.

What weld electrode should I use for A992 steel? E7018 (SMAW) or E70T-1 (FCAW) with FEXX = 70 ksi. The electrode strength should match or exceed the base metal strength requirements.

Bolted Connections Calculator — Automated AISC bolt checks
Welded Connections Calculator — Fillet and groove weld capacity
Bolt Hole Sizes — Standard hole dimension tables
Bolt Bearing & Tearout — Bearing and tearout calculations
Min Weld Size — Minimum weld size by material thickness
Bolt Grades — A325, A490, Grade 8.8 properties
Connection Types — Shear, moment, and specialty connections

Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.