A plate lug is checked at the pin hole, above the hole, in bearing, at its base where it meets the shell or pad, and at the base welds. The tool runs two complete sets of checks on the same geometry. They use different loads and different allowables, and the report lists them separately. Passing one is not credit toward the other.
The load on the plate
The lug load is resolved into three components in the plate's axes: along its base (the vessel axis), normal to the base (the pull), and across the plate (the side load). For a lug pointing up or down the load across the vessel axis is the pull and the lateral part is across the plate; for a lug to the side the two swap. The hole and edge checks take the full resultant P = √(along² + normal² + across²), which is conservative for the hole region. The base checks use the components separately.
Route 1: Moss / AISC allowable stress, on W_L
- Net section at the pin hole: P / ((A − D1)·t) ≤ 0.45·F_y, with A = 2·(side width) + D1 and t including cheek plates, t + 2·t_p.
- Shear above the hole: P / ((R − D1/2)·t) ≤ 0.4·F_y.
- Bearing: P / (D2·t) ≤ 0.9·F_y, D2 the pin diameter.
- Base: σ_a/(0.6F_y) + σ_bx/(0.6F_y) + σ_by/(0.75F_y) ≤ 1, in-plane bending 0.6F_y, across-plate bending 0.75F_y; base shear ≤ 0.4·F_y.
- Base fillet welds: required leg = line force / (0.707·F_s), F_s 108.9 MPa for E70XX and 93.8 MPa for E60XX (15.8 and 13.6 ksi, Moss 10-3), line force from the pull and the two moments normal to the weld lines and the shears along them.
Route 2: ASME BTH-1-2023, on W_E
No impact factor is applied to the load (BTH-1 3-5.1: the design factor already covers it). The strength is divided by N_d, set by the Design Category: A is N_d = 2.00 (peak impact up to 50 %), B is 3.00 (up to 100 %, the tool's default), C is 6.00. The hole region uses the pinhole expressions:
Bearing is P/(D_p·t) ≤ 1.25·F_y/N_d (3-55). The base is a solid rectangular bar: tension with bending about both axes (3-35) against F_t = F_y/N_d, F_by = 1.25·F_y/N_d (3-25) and F_bx from (3-20), (3-22) or (3-24) by the slenderness L_bd = h·B/t², with C_LTB = C_b = 1; shear ≤ F_y/(N_d·√3) (3-28); combined √(f² + 3·f_v²) ≤ F_y/N_d (3-37); weld shear stress 0.6·E_xx/(1.2·N_d) (3-57).
Tail lug at 30°, 40 mm plate, hole 76, pin 70, edge R = 150
| Inputs | F_y 260, F_u 485 MPa (example values); side width 150 − 38 = 112 mm; t = 40 mm; N_d = 3 |
| Load, Moss | W_L resultant 449.0 kN |
| Net section | 449000/((300 − 76)·40) = 50.1 MPa against 0.45·260 = 117.0: ratio 0.43 |
| Shear above hole | 449000/(112·40) = 100.2 MPa against 0.4·260 = 104.0: ratio 0.96 |
| Bearing | 449000/(70·40) = 160.4 MPa against 0.9·260 = 234.0: ratio 0.69 |
| Load, BTH-1 | W_E resultant 249.5 kN; D_p/D_h = 0.921 so C_r = 1; b_eff = min(160, 103.3, 112) = 103.3 mm |
| P_t, P_b, P_v | 1112.9, 906.5, 941.6 kN: ratios 0.22, 0.28, 0.26 |
| Bearing | 249460/(70·40) = 89.1 MPa against 1.25·260/3 = 108.3: ratio 0.82 |
The two routes govern at different places (shear above the hole for Moss, bearing for BTH-1). Neither is a margin on the other.
Limits
- The checks are hand formulas on the plate and its base. They know nothing of the shell's local stress at the lug; that is the job of the FE model (see the article on the FE model).
- Cheek plates enter only as extra thickness at the pin. The weld check treats the base as lines of weld on the plate and any end gussets.
- With K_L below 1.8 the Moss route alone does not reach F_y/3 of ASME B30.20; the tool says so.