Encased composite column
A W-shape encased in a rectangular reinforced concrete section, to AISC 360-22 Chapter I: the detailing limits, axial compression and tension, the axial force and strong-axis moment interaction by the plastic stress distribution (Commentary Method 2 Simplified with AISC Manual Table 6-2a) or Chapter H, shear, and the load transfer between the steel and the concrete. Concrete-filled HSS and box columns are the filled HSS column type.
Inputs of the encased column #
- Section: the encased W or HP shape and its steel; the encasement h1 (along the flanges) by h2 (along the web); f'c and the concrete unit weight (Ec = wc^1.5 √f'c); four corner bars or eight (three per face), their size, the face-to-center distance and Fysr; the ties and their spacing.
- Length and loads: K and L about each axis; the axial force, the strong-axis moment and the shear by load type (ASCE 7-22 combinations), the moments from a second-order analysis (your statement); the interaction method.
- Load transfer: where the external force enters (the steel section or the encasement), and steel headed stud anchors (number within the load introduction length, diameter, Fu) or a transfer strength you enter (direct bearing, your design).
Auto-design varies the encasement size (its proportions kept) and the bar size; the steel shape too when you unlock it.
Checks performed #
| Check | Basis |
|---|---|
| f'c 3 to 10 ksi, Fy ≤ 75 ksi, Fysr ≤ 80 ksi | 360-22 I1.3 |
| Steel core ≥ 1 % of Ag; bars ≥ 0.4 % (≤ 8 %, ACI 318-19 10.6.1.1); ties No. 3 at 12 in. or No. 4 at 16 in., ≤ half the least dimension; bars to the core | I2.1a |
| Compression Pno, EIeff with C1, Pe, Pn; not less than the bare steel member | I2.1b, Eqs. I2-2 to I2-7; Chapter E |
| Tension Fy As + Fysr Asr | I2.1c, Eq. I2-8 |
| Axial force and strong-axis flexure, points A, B, C reduced by λ = Pn/Pno | I5; Commentary Eqs. C-I5-1a, b; Manual Table 6-2a |
| Or Chapter H interaction (Method 1) | H1.1 |
| Shear on the steel section | I4.1(a), G2.1 |
| Load transfer V'r by studs (Qnv = Fu Asa) or entered | I6, Eqs. I6-1, I6-2a; I8.3a |
Limitations #
- Strong-axis bending only; weak-axis and biaxial bending, and Method 2 with point D, are not built.
- Four or eight bars only: Table 6-2a assumes bars at the faces and at the centerline.
- The neutral axis of point B in the web or below the steel uses the Table 6-2a equations as cited (to confirm against the Manual); Example I.11 exercises the flange case.
- The cage's other ACI 318 details (crossties, splices), AISC 341 D1.4b seismic detailing and the second-order analysis are yours.
Verification examples #
Verification status: 28 values compared in 7 examples (7 published, 0 hand calculations from the cited equations): 28 within tolerance, 0 recorded deviations. Code basis: partial. Only some provisions were read in the target edition; the rest rests on older editions, examples or summaries.
AISC Design Examples v16.0 I.8 (load transfer), I.9 (axial compression), I.10 (axial tension) and I.11 (combined force, Method 1 and Method 2 Simplified).