----------------------- | ---------------- | -------------------------------- | ------------------------ | ------------ | | Flange plate yielding | J4.1 (phi=0.90) | Cl 7.2 (phi=0.9) | Cl 6.2.3 | Cl 13.2 | | Flange plate rupture | J4.2 (phi=0.75) | Cl 7.2 (phi=0.9) | Cl 6.2.3, gamma_M2=1.25 | Cl 13.2 | | Block shear | J4.3 (phi=0.75) | Cl 9.1.6 | Cl 3.10.2 | Cl 13.11 | | Bolt shear | J3.6 (phi=0.75) | Cl 9.2.2.1 (phi=0.8) | Table 3.4, gamma_M2=1.25 | Cl 13.12.1.2 | | Bolt bearing/tearout | J3.10 (phi=0.75) | Cl 9.2.2.4 | Table 3.4 | Cl 13.12.1.2 | | Compression plate buckling | E3 (phi=0.90) | Cl 6.3 | Cl 6.3.1 | Cl 13.3 | | Splice location | No specific rule | Cl 9.1.4 (minimum design action) | Cl 6.2.7.1 | Cl 10.4 |

Key difference: AISC does not mandate a minimum design force at a splice (beyond the calculated demand). AS 4100 Clause 9.1.4 requires that splices be designed for the greater of the calculated demand or a proportion of the member capacity. EN 1993-1-8 and CSA S16 similarly require minimum splice strength proportional to the member capacity, typically 50-75% of the section capacity.

Step-by-Step Example

Problem: Design a bolted flange-plate moment splice for a W18x50 beam at a location with Mu = 150 kip-ft and Vu = 40 kips. A325-N bolts, 7/8-inch diameter. A36 splice plates.

Step 1 -- Flange force: W18x50: d = 18.0 in, tf = 0.570 in, bf = 7.50 in. Tf = 150 × 12 / (18.0 - 0.570) = 1800 / 17.43 = 103.3 kips.

Step 2 -- Number of flange bolts: phi × rn per bolt (single shear, A325-N 7/8"): 0.75 × 54 × 0.6013 = 24.4 kips. n_required = 103.3 / 24.4 = 4.23 bolts. Use 6 bolts (2 rows of 3) for symmetry. Total flange bolt capacity = 6 × 24.4 = 146.4 kips > 103.3 kips. OK.

Step 3 -- Flange splice plate size: Try plate: bp = 7 in, tp = 1/2 in. A36 (Fy = 36 ksi, Fu = 58 ksi). Gross yielding: phi × Rn = 0.90 × 36 × 7.0 × 0.50 = 113.4 kips > 103.3. OK. Net section (3 holes across): An = (7.0 - 3 × 1.0) × 0.50 = 2.0 in^2. dh = 7/8 + 1/8 = 1.0 in. phi × Rn = 0.75 × 58 × 2.0 = 87.0 kips < 103.3 kips. **FAILS.** Increase plate width to 9 in. Revised An = (9.0 - 3 × 1.0) × 0.50 = 3.0 in^2. phi × Rn = 0.75 × 58 × 3.0 = 130.5 kips > 103.3. OK.

Step 4 -- Web splice (shear only): Vu = 40 kips. Try 4 bolts (single column, double shear through web). phi × rn per bolt (double shear): 2 × 24.4 = 48.8 kips per bolt. n = 40 / 48.8 = 0.82. Use 4 bolts minimum for stability. Web plate: 3/8" x 10" A36. phi × Vn = 0.60 × 36 × 10.0 × 0.375 = 81.0 kips > 40 kips. OK.

Result: Flange plates = 9" x 1/2" A36 with 6 each 7/8" A325-N bolts per flange. Web plate = 10" x 3/8" A36 with 4 each 7/8" bolts in double shear. Controlling limit state: flange plate net section rupture at 0.79 utilization.

Common Design Mistakes

• Locating the splice at the point of maximum moment: Splices should be placed at or near inflection points (points of contraflexure) where moment is minimal. Placing a splice at midspan of a simply supported beam forces the splice to carry the full beam capacity, requiring heavy plates and many bolts.
• Using the same number of bolts on flange and web: The flange bolts carry moment (high force), the web bolts carry shear (typically much lower force). Sizing both groups identically wastes material on the web side.
• Forgetting to check the compression flange plate for buckling: The gap between beam ends (typically 1/2" to 1") plus clearance to the first bolt row creates an unsupported length. Thin, wide compression plates can buckle locally in this region.
• Not considering erection clearance: Field splices need enough room for a bolt wrench between the flanges. Standard practice requires a minimum gage of 5-1/2" and clearance of 1-1/2" from bolt center to the k-region of the beam.
• Ignoring the shear lag factor U for narrow splice plates: When the splice plate width is significantly less than the beam flange width, not all elements of the flange are directly engaged. The shear lag factor U may be less than 1.0, reducing the effective net area.
• Not designing for minimum splice strength in seismic zones: For special moment frames (AISC 341), beam splices must be designed for the probable maximum moment at the splice location, which can exceed the design-level moment by a factor of 1.1RyMp.

Frequently Asked Questions

What is the difference between a moment splice and a shear splice? A shear splice (also called a simple splice) is designed only to transfer vertical shear across the splice point and is placed where the beam moment is zero or nearly zero — typically at points of contraflexure in continuous beams. A moment splice must transfer both the full design moment and the shear, requiring flange plates to carry the flange couple (tension and compression forces) plus a web plate to carry shear and sometimes a portion of the moment. Moment splices are significantly more material- and bolt-intensive than shear splices, which is why locating them at or near contraflexure points is preferred whenever framing layout permits.

How is the moment demand distributed between flange plates and the web plate in a moment splice? The simplest and most common approach assigns the entire moment to the flanges as a force couple: each flange plate carries a force equal to the design moment divided by the beam depth (approximately M/d), with one flange in tension and the other in compression. The web plate then carries only the design shear. A more rigorous approach distributes moment proportionally to the relative stiffnesses of the flange and web cross-sections: the flanges carry the moment in proportion to their area times distance from the neutral axis, while the web carries the remainder in addition to the shear. The flange-couple simplification is conservative for the flanges and is standard practice for most building splice designs.

Why are bolted beam splices preferred over welded splices at points of contraflexure? Points of contraflexure are often located near the middle of a span, away from column faces, making field welding difficult and quality control expensive compared to bolting. Bolted splices are erected by positioning the splice plates and installing fasteners — a straightforward field operation with well-established inspection procedures. Field welding at height requires qualified welders, controlled preheat, weather protection, and ultrasonic or radiographic inspection of full-penetration welds. For routine building frames, the cost and quality-control advantages of bolted splices at low-moment locations make them the standard industry choice.

What is the difference between net section and gross section for a tension splice plate? Gross section is the full cross-sectional area of the plate without deductions. Net section is the gross area minus the area of bolt holes (using the hole diameter plus 1/16 inch for punching damage per AISC 360). Tension capacity is checked on both sections: gross section controls yielding under the yield strength Fy, while net section controls fracture under the ultimate strength Fu, reduced by the shear lag factor U when the tension force is not uniformly distributed. Net section fracture often governs for bolted splice plates with wide bolt patterns, because Fu × Ae can be less than Fy × Ag even though Fu > Fy.

How many 7/8-inch A325-N bolts are needed for a 100-kip shear splice? For a 7/8-inch A325 bolt in a bearing-type connection with threads in the shear plane (N condition), the nominal shear strength is Fnv = 54 ksi per AISC 360 Table J3.2. Bolt shear area = π/4 × (0.875)² = 0.601 in². Design shear strength per bolt in single shear: φrn = 0.75 × 54 × 0.601 = 24.3 kips. For 100 kips: bolts required = 100 / 24.3 = 4.1 → 5 bolts minimum. In double shear, φrn doubles to 48.6 kips per bolt, giving 100 / 48.6 = 2.1 → 3 bolts minimum. Bearing and block shear must also be checked to confirm the governing limit state.

What governs flange plate thickness in a moment splice — tension or compression? On the tension flange, the critical checks are gross yielding (Fy × Ag ≥ Tflange) and net section fracture (Fu × Ae × U ≥ Tflange), with the net section check often governing when multiple bolt holes reduce the net area. On the compression flange, local buckling of the plate can govern if the unsupported length between the bolt group and the column face is long relative to the plate width; the plate is checked as a short column under the flange compression force. In many practical splices the tension-side net section check sizes the plate, and the same plate thickness is used on both flanges for symmetry and fabrication simplicity.

Related pages

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• Disclaimer (educational use only)
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• Bolted Connection Checklist

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