Steel Connection Design Guide -- Bolted and Welded Connections per AISC 360-22

Steel connections transfer forces between members. Inadequate connection design is a leading cause of structural failures -- connections must be designed for every applicable limit state. This guide covers bolted and welded connection design per AISC 360-22 Chapter J, including the governing equations, limit states, and practical detailing requirements.

This is a limit-states design reference. For catalog descriptions of when to use each connection type, see Steel Connection Types Explained. For worked numerical examples, see Connection Design Examples. For the end-plate moment connection specifically, see End Plate Connection.

Bolt Limit States per AISC 360 Section J3

Bolts in structural steel connections must be checked for the following limit states. Every limit state must be verified -- satisfying shear alone is not sufficient.

Bolt Shear (J3.6)

The nominal shear strength of a single bolt:

Rn = Fnv * Ab
phi = 0.75

where Fnv = nominal shear stress from AISC 360 Table J3.2. For Group A bolts (A325, F1852): Fnv = 54 ksi with threads included in the shear plane (N), 68 ksi with threads excluded (X). For Group B bolts (A490, F2280): Fnv = 68 ksi (N), 84 ksi (X).

For a 3/4 in. diameter A325-N bolt: Ab = 0.442 in^2, phi Rn = 0.75 x 54 x 0.442 = 17.9 kip per shear plane.

Thread condition matters. A bolt with threads in the shear plane (the standard condition unless the bolt length is carefully specified) has only 79% of the strength of the same bolt with threads excluded. This is a common and costly detailing error -- specifying "A325 bolts" without the thread condition defaults to threads included (N), and the fabricator may supply bolts long enough that threads cross the shear plane.

Bolt Bearing and Tearout (J3.10)

Bearing at bolt holes, with deformation permitted:

Rn = 2.4 * d * t * Fu    (bearing)
Rn = 1.2 * Lc * t * Fu   (tearout, per bolt edge)
phi = 0.75

where d = bolt diameter, t = connected material thickness, Fu = tensile strength of the connected material, Lc = clear distance from the hole edge to the adjacent hole edge or material edge.

For standard holes, the hole diameter dh = d + 1/16 in. The clear distance Lc = s - dh (for interior bolts) or Le - dh/2 (for edge bolts), where s is bolt spacing and Le is edge distance.

When deformation at the bolt hole at service load is a design consideration (standard case), the bearing limit is governed by 2.4 d t Fu. When deformation is not a concern (oversize holes or slotted holes parallel to the load), the limit increases to 3.0 d t Fu.

Example: 3/4 in. A325 bolt bearing on 3/8 in. A36 plate (Fu = 58 ksi). dh = 13/16 in. Edge distance Le = 1.5 in. Lc = 1.5 - 13/32 = 1.094 in. Tearout: 1.2 x 1.094 x 0.375 x 58 = 28.6 kip. Bearing: 2.4 x 0.75 x 0.375 x 58 = 39.2 kip. Tearout controls. phi Rn = 0.75 x 28.6 = 21.4 kip per bolt.

Bolt Tension (J3.6)

Rn = Fnt * Ab
phi = 0.75

where Fnt = 90 ksi for Group A (A325) and 113 ksi for Group B (A490) from Table J3.2.

For combined tension and shear, the available tensile strength is reduced per J3.7:

F'nt = 1.3 Fnt - (Fnt / (phi Fnv)) * frv <= Fnt

where frv = required shear stress = Vu / (Ab x number of bolts). This interaction is important for moment connections where bolts carry both shear from gravity load and tension from the moment couple.

Block Shear Rupture (J4.3)

Block shear is a combined tension-and-shear failure where a block of material tears out of a connection element. It governs for coped beams, gusset plates, and shear tabs.

Rn = 0.60 Fu Anv + Ubs Fu Ant <= 0.60 Fy Agv + Ubs Fu Ant
phi = 0.75

where Anv = net area in shear, Ant = net area in tension, Agv = gross area in shear, Ubs = 1.0 for uniform tension stress (most cases).

The two terms represent shear rupture + tension rupture, capped by shear yielding + tension rupture. Block shear often controls the design of beam copes and thin shear tabs where the bolt group is close to the beam end.

Example -- Beam cope block shear: W18x55 beam coped at top flange only, shear tab 3/8 in. thick x 12 in. deep, three 3/4 in. bolts at 3 in. spacing. Ant = (1.5 edge + 3x2 bolt) x 0.375 = 2.81 in^2 net. Anv = (12 - 2.5 x 0.8125) x 0.375 = 3.74 in^2 net. Agv = 12 x 0.375 = 4.50 in^2 gross.

Tension rupture + shear rupture: 0.60 x 58 x 3.74 + 1.0 x 58 x 2.81 = 130 + 163 = 293 kip. Tension rupture + shear yield: 0.60 x 36 x 4.50 + 1.0 x 58 x 2.81 = 97 + 163 = 260 kip (controls). phi Rn = 0.75 x 260 = 195 kip.

Weld Limit States per AISC 360 Section J2

Fillet Weld Strength (J2.4)

The design strength of a fillet weld per unit length:

phi Rn = phi * 0.60 * FEXX * (leg * 0.707)
phi = 0.75

where FEXX = electrode classification strength (70 ksi for E70XX), leg = fillet weld leg size. The 0.707 factor converts leg size to effective throat dimension (leg x sin 45 degrees).

For E70XX electrodes: phi Rn per 1/16 in. of leg size = 0.75 x 0.60 x 70 x (0.0625 x 0.707) = 0.75 x 42 x 0.0442 = 1.39 kip/in per 1/16 in. leg.

Fillet weld capacity table (E70XX):

Minimum fillet weld size per AISC 360 Table J2.4 is governed by the thicker connected part. For 3/4 in. thick material, minimum weld = 1/4 in. For 1/4 in. thick material, minimum = 1/8 in.

Maximum fillet weld size along edges: For material less than 1/4 in. thick, weld leg <= material thickness. For material 1/4 in. or thicker, weld leg <= material thickness - 1/16 in.

CJP Groove Weld Strength (J2.4)

Complete-joint-penetration groove welds develop the full strength of the weaker connected base metal. For tension normal to the effective area:

phi Rn = phi * Fy * Ag    (base metal yielding)
phi Rn = phi * Fu * Ae    (base metal rupture)

where the phi factors are 0.90 and 0.75 respectively. With matching filler metal (E70XX on A36/A992 base metal, E80XX on A572 Gr 50), the weld metal strength does not control -- the base metal controls.

PJP Groove Weld Strength (J2.4)

Partial-joint-penetration groove welds have their effective throat specified on the design drawings. The strength is computed the same as fillet welds: phi Rn = phi x 0.60 x FEXX x effective throat. PJP welds are common for column splices where full penetration is unnecessary and the reduced weld volume saves cost.

Bolt Group Analysis -- Eccentric Shear

When a bolt group resists shear applied eccentrically (the standard case for shear tabs and clip angles), the bolts carry both direct shear (divided equally among bolts) and a couple from the eccentric moment.

Elastic Method (Conservative)

The moment M = V x e is resisted by bolt forces proportional to their distance from the bolt group centroid. The force in bolt i is:

ri = M * di / (sum di^2)

where di is the distance from bolt i to the bolt group centroid. The direct shear is V / n per bolt. The combined force is the vector sum, and the most heavily loaded bolt is checked against the single-bolt shear capacity.

Instantaneous Center Method (AISC 360 J3.9)

The more accurate IC method accounts for bolt ductility and load redistribution beyond the elastic range. The bolt group capacity is:

phi Rn = phi * C * rn

where rn is the single-bolt shear strength and C is a coefficient from AISC Manual Tables 7-6 through 7-13, depending on bolt spacing, number of bolts, and eccentricity. The C coefficient is always larger than the elastic method prediction (typically 20-40% higher for practical geometries), reflecting ductile redistribution.

Practical Detailing Requirements

Minimum Edge Distance (AISC 360 Table J3.4)

These are minimum fabrication limits. The bearing/tearout check often requires larger edge distances.

Minimum Bolt Spacing (J3.3)

Minimum center-to-center spacing = 2-2/3 x d (bolt diameter). Preferred spacing = 3d. Maximum spacing for seal-tight connections = 12 x t (thinner plate thickness) but not exceeding 6 in. For painted or unpainted weathering steel: max = 14 x t, max 7 in.

Hole Types (Table J3.3)

Oversize and slotted holes reduce bearing capacity and are only permitted in slip-critical (SC) connections where the faying surfaces are prepared to a specified slip coefficient.

Connection Types -- Key Design Parameters

Worked Example -- Single-Plate Shear Tab Design

Given: W18x55 beam (tw = 0.390 in., d = 18.1 in.), Vu = 65 kip, supported by W14x90 column flange. A36 plate. 3/4 in. A325-N bolts. E70XX electrodes.

1. Determine number of bolts: Try 4 bolts. Single shear per bolt: phi Rn = 0.75 x 54 x 0.442 = 17.9 kip. Four bolts: 4 x 17.9 = 71.6 kip > 65 kip. OK for shear. But eccentricity must be considered.

2. Plate thickness (shear yielding): Try PL 3/8 in. Ag = 0.375 x 12 in. depth = 4.50 in^2. phi Rn = 0.90 x 0.60 x 36 x 4.50 = 87.5 kip > 65 kip. OK.

3. Eccentric shear -- IC method: e = a/2 = distance from bolt line to weld line. Standard shear tab: a = 3 in. For 4 bolts at 3 in. spacing, e = 3 in., from AISC Manual Table 7-6: C = 4.37 (approximately). phi Rn = 0.75 x 4.37 x 17.9 = 58.7 kip < 65 kip. Increase to 5 bolts.

4. Five-bolt check: C for 5 bolts at 3 in. spacing, e = 3 in.: C = 5.60. phi Rn = 0.75 x 5.60 x 17.9 = 75.2 kip > 65 kip. OK.

5. Plate thickness (bearing): 5 bolts bearing on 3/8 in. plate. Lc per bolt (3 in. spacing): Lc = 3 - 13/16 = 2.1875 in. Tearout: 1.2 x 2.1875 x 0.375 x 58 = 57.1 kip per bolt. With 5 bolts: phi Rn = 0.75 x 5 x 57.1 = 214 kip >> 65 kip. OK.

6. Block shear: Ant = (1.5 + 4x3 - 4.5x13/16) x 0.375 = (13.5 - 3.66) x 0.375 = 3.69 in^2. Anv = (12 - 4.5x13/16) x 0.375 = (12 - 3.66) x 0.375 = 3.13 in^2. Agv = 12 x 0.375 = 4.50 in^2.

Rn = 0.60 Fu Anv + Ubs Fu Ant = 0.60 x 58 x 3.13 + 1.0 x 58 x 3.69 = 109 + 214 = 323 kip. Cap: 0.60 Fy Agv + Ubs Fu Ant = 0.60 x 36 x 4.50 + 1.0 x 58 x 3.69 = 97 + 214 = 311 kip. phi Rn = 0.75 x 311 = 233 kip > 65 kip. OK.

7. Weld (plate to support): Use 1/4 in. fillet weld E70XX. Weld length = 12 in. Capacity: 2 x (12 x 5.57) = 133.7 kip (double-sided). Required demand including eccentricity: weld must resist shear + moment. For the AISC Manual Table 10-2 method with C = 0.539 (a=3, l=12): phi Rn = 0.75 x 0.539 x 12 x (1.0 per 1/16) x 1.39 = not straightforward. Alternative: size weld directly.

Required kip/in = Vu / (2L) + Mu / (2 x L^2/6) = 65/(2x12) + 65x3/(2x144/6) = 2.71 + 4.06 = 6.77 kip/in. 1/4 in. fillet capacity = 5.57 kip/in per side. Two sides = 11.14 kip/in > 6.77 kip/in. OK.

Final shear tab: PL 3/8 x 4-1/2 x 1'-0 with five 3/4 in. A325-N bolts at 3 in. spacing. 1/4 in. fillet weld both sides of plate to support. 3 in. setback.

Common Connection Errors

1. Designing for shear only and ignoring eccentricity -- AISC 360 requires that the bolt group be designed for the eccentric moment produced by the shear acting at the bolt group centroid. Forgetting this can underestimate bolt forces by 30-50%.

2. Mixing bolt grades on the same project -- A325 and A490 bolts have different head markings, different strengths, and different pretension requirements. Use one grade per connection. Mixed grades on-site produce unverifiable assemblies.

3. Specifying slip-critical where bearing is sufficient -- Slip-critical connections require faying surface preparation (Class A, B, or C slip coefficient per RCSC) and pretensioned bolts. They cost roughly 30% more to fabricate and inspect. Only specify SC where slip at service load must be prevented: column splices in braced frames, connections subject to fatigue, and connections with oversize/slotted holes.

4. Ignoring block shear in beam copes -- A cope reduces the beam web depth at the connection and concentrates the reaction into a smaller shear area. Block shear often controls the beam cope design. Check both the supported beam cope and the supporting member (e.g., shear tab or clip angle).

5. Weld all-around on shear tabs -- Welding across the top edge of a shear tab (in addition to the vertical edges) restrains the simple shear connection and introduces unintended moment into the column. Standard practice is to weld the vertical edges only, leaving the top and bottom free.

Related Tools and References

• Steel Connection Design Examples
• Steel Connection Types Explained
• End Plate Connection Design
• Bolt Hole Reference
• Weld Electrodes Reference
• Gusset Plate Connection
• How to Verify Calculations

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All connection designs must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) for the specific loads, materials, and building code applicable to the project. The site operator disclaims liability for any loss arising from the use of this information. Results are PRELIMINARY -- NOT FOR CONSTRUCTION.