Beam Splice Function and Load Transfer

A beam splice is a field connection that joins two beam segments at a point of reduced moment, typically located near the inflection point (zero moment) under gravity loading. The splice transfers three internal forces from one beam segment to the other: bending moment (via flange plates), vertical shear (via a single web plate or double web plates), and any coincident axial force.

Design philosophy (AISC Manual Part 14):

  1. Flange plates are designed for the full flange force at the splice location: P_f = M_u / (d - t_f), where d is the beam depth and t_f is the flange thickness. This conservatively assumes the moment couple acts at the flange centroids.
  2. The web plate is designed for the total vertical shear V_u at the splice location. Moment on the web bolt group due to load eccentricity must be accounted for.
  3. If the splice is at the point of inflection (zero moment under gravity load), a minimum moment capacity per the project specification (often 25% of the beam's flexural capacity) should be provided for robustness, erection stability, and progressive collapse resistance.

Flange Plate Design — AISC Manual Section 14-2

The flange splice plate(s) transfer the flange force from one beam segment to the other. Because flange splices are loaded predominantly in tension or compression parallel to the beam axis, the bolt group is concentrically loaded (no eccentricity in the bolted connection), simplifying the bolt shear check.

Flange Plate Sizing:

Gross area yielding: phi Rn = 0.90 x Fy x Ag >= P_f Net section fracture: phi Rn = 0.75 x Fu x Ae >= P_f

Where Ae is the effective net area accounting for bolt hole deductions. For a single line of bolts in the direction of force, the net width is b_net = b_plate - n_holes x (d_h + 1/16 in.), where d_h is the nominal bolt hole diameter.

Flange Plate Bolts:

The bolts in the flange splice are in direct shear (no eccentricity). For n bolts in the flange plate connection on each side of the splice:

Flange plate yield and fracture on the gross/net section through the bolt holes must also be checked. The Whitmore section (projecting 30 degrees from the first bolt row to the last, bounded by the plate width) is used for the effective tension width in flange plates per AISC Manual Part 9.

Flange Plate Worked Example — W24x76 Splice

Given: W24x76 beam (d = 23.9 in., b_f = 8.99 in., t_f = 0.680 in., t_w = 0.440 in.), splice location M_u = 240 kip-ft, V_u = 45 kips. A992 steel (Fy = 50 ksi, Fu = 65 ksi). Use A572 Gr. 50 splice plates.

Step 1 — Flange force: P_f = M_u / (d - t_f) = 240 x 12 / (23.9 - 0.680) = 2,880 / 23.22 = 124.0 kips.

Step 2 — Flange plate size: Required gross area: Ag >= 124.0 / (0.90 x 50) = 2.76 in^2. Use a 9 in. x 3/8 in. plate: Ag = 9 x 0.375 = 3.38 in^2 > 2.76 in^2. OK. phi Pn_yield = 0.90 x 50 x 3.38 = 152 kips > 124 kips. OK.

Step 3 — Net section fracture (4 bolts per side, 13/16 in. holes for 3/4 in. bolts): Net width = 9.0 - 2 x (13/16 + 1/16) = 9.0 - 2 x 0.875 = 7.25 in. (two bolts in the critical row across the plate width). Ae = 7.25 x 0.375 = 2.72 in^2 (assume shear lag factor U = 1.0 for plates loaded in direct tension). phi Pn_fracture = 0.75 x 65 x 2.72 = 132.6 kips > 124 kips. OK.

Step 4 — Flange bolts: Use 8 bolts (4 per side). Required shear per bolt = 124.0 / 4 = 31.0 kips. 3/4 in. A325-N provides 17.9 kips. NOT OK. Try 7/8 in. A325-X (threads excluded): phi Rn = 0.75 x 68 x 0.601 = 30.7 kips. Still marginal. Use 7/8 in. A490-X: phi Rn = 0.75 x 84 x 0.601 = 37.9 kips > 31.0 kips. OK with 4 bolts per side. Alternatively, increase to 6 bolts per side with 3/4 in. A325-N: 124.0 / 6 = 20.7 kips > 17.9 kips. NOT OK. Use 7/8 in. A325-N with 6 bolts: 124.0 / 6 = 20.7 kips, phi Rn = 24.4 kips. OK.

Final flange plate design: 9 in. x 3/8 in. A572 Gr. 50 plate, 12 total 7/8 in. A325-N bolts (6 per side, 3 rows x 2 columns).

Web Plate Design — AISC Manual Section 14-3

The web splice plate transfers vertical shear across the splice. Because the web bolts are eccentric to the shear plane (the beam web centreline), the bolt group is subjected to both direct shear and an eccentric moment. This requires bolt group analysis per AISC Manual Part 7 (Instantaneous Centre of Rotation method per Table 7-6 through 7-13).

Web Plate Sizing:

Gross shear yielding (AISC Equation J4-3): phi Vn = 0.60 x Fy x Agv >= Vu, where Agv is the gross shear area of the web plate.

For a single web splice plate on each side of the beam web:

Web Bolt Group — Eccentric Shear:

Per AISC Manual Table 7-7 (eccentrically loaded bolt groups, instant centre method): For a single-column bolt group with eccentricity e to the shear plane: C = coefficient from Table 7-7 (depends on number of bolts n, spacing s, and eccentricity e). Required capacity: Vu <= C x phi x rn

For a double-column bolt group (2 columns x n rows), Table 7-8 provides C coefficients for vertical load eccentric to the bolt group centroid.

Web Plate Worked Example (continued):

Given: V_u = 45 kips, W24x76 web t_w = 0.440 in.

Step 1 — Web plate: Use a single 3/8 in. x 16 in. web plate (slightly thinner than beam web for economy). Shear area on each side = 16 x 0.375 = 6.0 in^2. phi Vn_shear = 1.00 x 0.60 x 50 x 6.0 = 180 kips > 45 kips. OK.

Step 2 — Web bolts (single column of 5 bolts, 3/4 in. A325-N, 3 in. spacing, e = 3 in.): From AISC Manual Table 7-7 for n = 5, e = 3 in., s = 3 in.: C = 2.52 (interpolated). phi Rn_per_bolt = 17.9 kips. Design shear capacity: C x phi Rn = 2.52 x 17.9 = 45.1 kips. 45.1 kips > 45 kips. OK.

Final web plate design: 3/8 in. x 16 in. A572 Gr. 50 plate, 5 total 3/4 in. A325-N bolts per side (single column).

Bearing and Tearout Checks — J3.10

Per AISC 360-22 Section J3.10, the design bearing strength at bolt holes must be checked for both the splice plates and the beam material:

phi Rn = 0.75 x (2.4 x d x t x Fu) for standard holes with adequate edge distance and spacing. The edge distance must satisfy Lc >= 1.5 d for full bearing strength; otherwise the tearout check governs: phi Rn = 0.75 x (1.2 x Lc x t x Fu), where Lc is the clear distance from the hole to the edge or adjacent hole in the direction of force.

For the flange plate (3/8 in., Fu = 65 ksi, 7/8 in. bolts), edge distance e = 1.5 in.: phi Rn_bearing = 0.75 x 2.4 x 0.875 x 0.375 x 65 = 38.5 kips per bolt. 38.5 kips > 20.7 kips (demand per flange bolt for the 6-bolt alternative). OK.

For the web plate (3/8 in., Fu = 65 ksi, 3/4 in. bolts), edge distance e = 1.5 in.: phi Rn_bearing = 0.75 x 2.4 x 0.75 x 0.375 x 65 = 33.0 kips per bolt. Bearing on the beam web (t_w = 0.440 in., Fu = 65 ksi): phi Rn_bearing = 0.75 x 2.4 x 0.75 x 0.440 x 65 = 38.6 kips per bolt. Bolt shear at 17.9 kips governs in all cases for the web. This is the intended design hierarchy.

Slip-Critical Requirements

For splice connections subject to fatigue (crane runway girders, bridge girders) or where slip at the splice would impair serviceability, the bolts must be designed as slip-critical per AISC 360-22 Section J3.8. The slip resistance per bolt is:

phi Rn = mu x Du x h_f x T_b x N_s

Where mu is the mean slip coefficient (0.30 for Class A, 0.50 for Class B surfaces), Du = 1.13 for standard holes, h_f = 1.0 for no filler or standard filler, T_b is the minimum bolt pre-tension from AISC Table J3.1 (28 kips for 3/4 in. A325, 39 kips for 7/8 in. A325), and N_s = 1 for single-shear connections.

For 3/4 in. A325 bolts with Class A faying surface (unpainted clean mill scale): phi Rn_slip = 1.00 x 0.30 x 1.13 x 1.0 x 28 x 1 = 9.5 kips per bolt.

This is substantially lower than the bearing shear strength of 17.9 kips. Slip-critical design typically governs when required, necessitating more bolts or larger diameters than the equivalent bearing-type connection.

Frequently Asked Questions

Where should beam splices be located for optimal design?

Beam splices should be located near points of contraflexure (zero moment) under governing load combinations. For simply supported beams under uniform gravity load, there is no point of zero moment — the splice must be designed for the moment at its location. For continuous beams, the splice is placed at approximately the quarter-span point (0.2L to 0.25L from the support) where the moment is approximately 25% of the maximum negative moment at the support. The exact location is coordinated with the steel fabricator to balance splice forces against shipping length constraints (typically 40-65 ft for unrestricted highway transport).

How do I design the flange splice plate width relative to the beam flange?

The flange splice plate width should match or slightly exceed the beam flange width to provide adequate edge distance for the splice bolts and to avoid a net section that is narrower than the beam flange. A common rule is to use the beam flange width minus 1/4 in. (to avoid interference with the flange-to-web fillet on the beam) and then verify that the required bolt layout fits within the available width with minimum edge distances satisfied.

Should web splice plates be single or double shear?

Double shear (web plates on both sides of the beam web) is preferred for larger beams (W24 and up) because it eliminates eccentricity on the web bolts and doubles the bolt shear resistance (N_s = 2 for slip-critical). However, double web plates require access from both sides of the beam, which may be impractical in congested areas. Single shear web plates are common for W21 and smaller beams and are always eccentric to the shear plane, requiring the ICC method for bolt group analysis.

Educational reference only. All design values are per AISC 360-22 and the AISC Steel Construction Manual 16th Edition. Verifiy bolt group C-coefficients against the current AISC Manual Tables 7-6 through 7-13. Designs must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE). Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent professional verification.