Column base plate
An exposed column base: a W-shape, rectangular HSS, round HSS or pipe column on a steel base plate, bearing on a concrete pier or footing through a grout pad, with anchor rods. Concrete bearing to AISC 360-22 J8, plate bending by the AISC Manual Part 14 and AISC Design Guide 1, 3rd Ed. (2024) Chapter 4, anchor rods to AISC 360-22 J3 and the anchorage in the concrete to ACI 318-19 Chapter 17, with ASCE 7-22 load combinations.
Inputs of the base plate #
- Column and plate: the column (AISC Shapes Database v16.0), the plate length N (along the column depth, the direction of the moment and the shear), width B, thickness tp and steel.
- Concrete support: the plan dimensions and depth of the pier or footing, concentric with the plate. They give A2 (the largest area geometrically similar to the plate, within a 1:2 frustum in the depth, J8) and the edge distances of the rods. f'c is the concrete strength (2,500 to 10,000 psi); the grout thickness enters the shear lug eccentricity and the ACI 0.80 grout-pad factor on the rod shear (17.7.1.2.1).
- Loads: service loads per load type (D, L, Lr, S, R, W, E: axial P with compression positive, moment M about the column major axis, shear V along N), combined by ASCE 7-22 2.3.1 or 2.4.1, or the required strengths entered directly for the chosen method. See Loads and combinations.
- Anchor rods: diameter, ASTM grade (F1554 Gr. 36, 55, 105, A449, A36), spacing between the two rod lines (along N) and of the outer rods along B, rods per line (1 to 4), headed (heavy hex nut, or a plate washer area Abrg) or hooked (hook length eh), and the embedment hef.
- Shear transfer (the engineer's choice): friction (μ, 0.4 prefilled per DG1 4.3.5), the anchor rods (the number of rods that take shear: all of them with welded plate washers or a setting plate; rod bending over half the washer-to-grout distance, DG1 4.3.4) or a shear lug (width, height below the plate, thickness, steel, fillet weld).
- Code options: whether the ACI 318 anchorage is checked, cracked concrete, supplementary reinforcement for tension and for shear (φ per Table 17.5.3(b)) and the edge reinforcement factor ψc,V.
Auto-design (editor toolbar) varies the plate length N and width B (1-in. steps), thickness tp (1/8-in. steps) and rod diameter; the rod spacing follows the plate size; plate grade when unlocked. Objectives: lightest (plate + rods steel weight); lowest cost index (steel). Kept unless you select them: plate steel. Every other input is kept; see Auto-design.
Checks performed #
| Check | Basis |
|---|---|
| Concrete bearing | J8: Pp = 0.85 f'c A1 √(A2/A1) ≤ 1.7 f'c A1, φc = 0.65, Ωc = 2.31 |
| Bearing under axial load and moment | DG1 uniform bearing stress: e ≤ ecrit (Y = N − 2e, Eqs. 4-39 to 4-44) or e > ecrit with rod tension (Eqs. 4-55 to 4-59; no real solution = plate too small) |
| Plate bending, bearing side | Axial: l = max(m, n, λn'), Manual Eqs. 14-2 to 14-6; moment: m with Y (Eqs. 4-51, 4-52) and n; Mn = Fy tp²/4 (F11) and tp,req |
| Plate bending, tension side | Rods beyond the flanges: x = f − d/2 + tf/2, M = T x / B (Eqs. 4-61, 4-62); rods between W flanges: the web cantilever a over beff = min(2a, d − 2tf) (Example 4.7-3) |
| Rod tension (AISC) | DG1 Eq. 4-22: Rn = Fu Ase, φ = 0.75, Ω = 2.00 |
| Rod shear and combined stresses (AISC) | J3.7: Fnv = 0.450 Fu; F'nt by Eqs. J3-3a/b with the rod bending stress (DG1 4.3.4) |
| Friction | DG1 Eq. 4-30: min(0.65 μ Pu, 0.75 × 0.2 f'c Ac, 0.75 × 800 psi Ac), LRFD combinations |
| Shear lug | ACI 318-19 17.11.2 bearing and 17.11.3 breakout (φ = 0.65); lug flexure (F11) and shear yielding (J4-3); fillet welds (J2.4, kds = 1.5); the lug moment V (G + hef,sl/2) enters the bearing analysis (17.11.1.1.2) |
| Anchorage in the concrete | ACI 318-19 Chapter 17 for the rod group (see Anchors in concrete): steel, breakout, pullout, side-face blowout, shear breakout, pryout, interaction |
ACI 318 is strength design: the anchorage, friction and lug-concrete checks always use the LRFD combinations. With ASD and required strengths entered directly, those checks cannot be made and an advisory says so.
Drawings #
The drawing sheet (screen, PDF and DXF) has two views: the plan (concrete support, base plate, column section, anchor rods, the shear lug hidden below the plate, the rod spacing and edge distances, and the plate and support sizes) and the section along N (column stub, plate, grout pad, the support hatched, the rods with their headed or hooked end at hef, the shear lug with its designed fillet welds, rod projection, embedment and thickness). Only what the calculation designs is drawn; the column-to-plate welds, hole and washer sizes, rod length, leveling hardware and the support reinforcement are not part of it, and the sheet notes say so. The nuts and plate washer are the standard hardware of the rod diameter. On 11 x 17 both views share one sheet; on letter sheets a larger plate takes a continuation sheet.
Limitations #
- Moment about the column major axis only; biaxial moments and a minor-axis moment are not covered. The bearing is the DG1 uniform (rectangular) stress block; the triangular (elastic) distribution of DG1 Appendix B is not offered.
- Rods in two lines across N, symmetric about the plate centre. Rods inside the walls of an HSS or pipe are reported as not feasible. For HSS and pipe columns the tension-side lever uses the wall centre line (DG1 states Eq. 4-61 for W columns).
- Not checked: column-to-plate welds, plate washers and holes (Manual Table 14-1), the base plate local bending at the lug, anchor reinforcement (ACI 17.5.2.1), seismic anchorage (17.10; use the Anchors in concrete calculation for it).
- Combined perpendicular and parallel shear on one edge is not a code rule: shear acts along N only here.
Verification examples #
AISC Design Examples v16.0 (J.6, K.9) and AISC Design Guide 1, 3rd Ed. (2024):
| Example | What it checks | Published | ILX Calcs |
|---|---|---|---|
| J.6 | W12×96, 22 × 22 plate on a 24 × 24 pedestal | φcPp = 875, Pp/Ωc = 583 kips, tmin = 1.54 in. | 875.2, 582.9, 1.541 |
| K.9 | HSS6×6×½ on a footing | Pp = 862, φcPp = 560, Pp/Ωc = 373 kips, tmin = 0.917 / 0.918 in. | 861.9, 560.2, 373.1, 0.917 / 0.918 |
| DG1 4.7-1 | Axial, A2 = A1 | φcPp = 729, X = 0.960, λn' = 3.11, tmin = 1.36 in. | 729.3, 0.959, 3.112, 1.361 |
| DG1 4.7-2 | Axial with confinement | φcPp = 723, tmin = 1.21 in. | 716.0, 1.211 |
| DG1 4.7-3 | Uplift, rods between the flanges | ru = 17.5, φRn = 20.1, tp,req = 0.882 in., φNcbg = 76.3 kips | 17.5, 20.1, 0.882, 76.6 |
| DG1 4.7-5 | Shear lug | Y = 0.0967 in., T = 6.41, φVbrg = 70.9, φVcb = 26.1, ru = 7.91 kip/in. | 0.097, 6.41, 70.7, 26.1, 7.91 |
| DG1 4.7-6 | Rods in tension, shear and bending | ft = 42.2, φF'nt = 26.5 ksi (1⅛ in.); 20.3, 32.6 ksi (1½ in.) | 42.2, 26.5; 20.4, 32.6 |
| DG1 4.7-8 | Friction; A2 ≥ 4A1 | φVn = 129, φcPp = 1,440 kips, tmin = 0.896 in. | 129.0, 1,436.5, 0.897 |
| DG1 4.7-9 | Pure moment | Y = 0.886 in., T = 47.0 kips, tp,req = 1.06 / 1.31 in. | 0.885, 47.0, 1.062 / 1.307 |
| DG1 4.7-10 | Small moment | ecrit = 5.02 in., Y = 14.0 in., tp,req = 1.16 in. | 5.023, 14.0, 1.158 |
| DG1 4.7-11 | Large moment | Y = 8.60 in., tp,req = 1.92 / 0.755 in. | 8.585, 1.918 / 0.749 |
| DG1 4.7-12 | Uplift with a large moment | qmax = 42.0 / 27.9 kips/in.; Y = 0.300 in., T = 113 / 68.4 kips | 41.99 / 27.97; 0.274 / 0.247 in., 111.5 / 66.9 kips (deviation) |
| DG1 4.7-13 | Uplift with a low moment, all rods in tension | largest rod tension 28.1 / 16.9 kips | 28.12 / 16.88 |
Every value is within 1 % (LRFD and ASD, in tests/test_verify_steel_base_plate.py) except the DG1 4.7-12 bearing length and rod tension: the Example writes Y = 17.5 in. − 17.2 in. = 0.300 in., the difference of two rounded numbers; unrounded, Eq. 4-65 gives 0.274 in. (LRFD) and 0.247 in. (ASD), and the rod tension is 1.3 % and 2.2 % lower (documented deviation DG1-4712-ROUND). The 4.7-2 bearing strength is 1.0 % lower because ILX takes A2 geometrically similar to the plate as J8 defines it (518 in.²; the example uses 528 in.²). Where an example leaves out the plate size or the rod pattern, the value used is stated with the example in the test file.