-- | ----------- | --------------- | | 3/16" | 0.133 | 4.18 | | 1/4" | 0.177 | 5.57 | | 5/16" | 0.221 | 6.96 | | 3/8" | 0.265 | 8.35 | | 7/16" | 0.309 | 9.74 | | 1/2" | 0.354 | 11.14 | | 5/8" | 0.442 | 13.92 |
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)
| Bolt Diameter | Minimum Edge (Sheared) | Minimum Edge (Rolled/Gas Cut) |
|---|---|---|
| 1/2" | 7/8 | 3/4 |
| 5/8" | 1-1/8 | 7/8 |
| 3/4" | 1-1/4 | 1 |
| 7/8" | 1-1/2 | 1-1/8 |
| 1" | 1-3/4 | 1-1/4 |
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)
| Hole Type | Diameter | Use Case |
|---|---|---|
| Standard | d + 1/16 | Default for all bolted connections |
| Oversize | d + 3/16 (d <= 7/8) or d + 1/4 (d >= 1) | Slip-critical only, field adjustment |
| Short slot | (d + 1/16) x (d + 1/4) | Slip-critical, one-direction adjustment |
| Long slot | (d + 1/16) x (2.5 d) | Slip-critical, major adjustment |
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
| Connection Type | Primary Forces | Key Limit States | Typical Bolts | Typical Weld |
|---|---|---|---|---|
| Shear tab (single plate) | Shear only | Bolt shear, bolt bearing, block shear, plate shear yielding | 3-6 A325-N | Fillet (plate to support) |
| Double angle | Shear only | Bolt shear, angle shear rupture, block shear | 3-6 A325-N per leg | Optional shop weld |
| End plate (shear) | Shear only | Bolt shear, plate bearing, end plate shear | 4-8 A325-N | Fillet (plate to beam) |
| End plate (moment) | Tension + shear | Bolt tension, prying, plate flexure, column flange bending | 4-8 A325 per flange | CJP or fillet |
| Column splice | Axial + moment | Bolt shear, plate tension, bearing | 4-8 A325 per flange | PJP or CJP |
| Base plate | Axial + shear | Concrete bearing, plate bending, anchor tension | 4 anchor rods | Fillet (plate to column) |
| Gusset plate (brace) | Axial | Block shear, bolt bearing, plate buckling, weld to gusset | 4-8 A325-N | Fillet |
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
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%.
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.
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.
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).
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.
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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.