Concrete-filled HSS column

A rectangular or round HSS, a pipe or a built-up box of four plates, filled with concrete, designed as a composite member to AISC 360-22 Chapter I (LRFD or ASD), with ASCE 7-22 load combinations when you enter loads by type. It classifies the section for local buckling and checks compression, tension, flexure about both axes, shear and the interaction of axial force and flexure. It can also check the load transfer between the steel and the concrete at a connection.

Inputs of the filled column #

• Section: an HSS or pipe from the AISC Shapes Database v16.0, or a built-up box of width B, depth H and plate thickness t. HSS use the design wall thickness and the corner radii of Manual Part 1 (outside 2t, inside t), so the concrete area and inertia are compatible with the database steel properties. A box has sharp corners and four equal plates; plate extensions past the corners are ignored, as Design Example I.7 does.
• Steel: A500 Gr. B or C, A1085 and A53 Gr. B for HSS and pipe; A572 Gr. 50 and A36 for box plates. Section I1.3 limits Fy to 75 ksi.
• Concrete fill: f′c from 3 to 10 ksi for normal weight concrete, 3 to 6 ksi for lightweight (I1.3), and the unit weight wc from 90 to 155 pcf, which sets Ec = wc^1.5 √f′c. Stronger materials belong to Appendix 2, which this type does not cover.
• Lengths: the member length L (used for B1), and Kx Lx and Ky Ly for the column strength. K is your decision: prefilled 1.0, the value of the direct analysis method (C3).
• Loads: service loads by type, with ASCE 7-22 combinations, or required strengths entered directly. Axial force is positive in compression. Moments at the top, bottom and midheight about each axis; Vx is the shear that goes with Mx (in the plane of bending about x) and Vy the one with My. A shear left blank is computed from the moments.
• Second-order effects: moments from your second-order analysis, or Appendix 8 amplification. For B1, I1.5 gives EI* = 0.64 EIeff with the direct analysis method and EIeff with the effective length method. B2 is your story value.
• Interaction: the H1.1 equations by default (Method 1 of Commentary I5). The other option is the plastic stress distribution for compact sections (Method 2 Simplified, Commentary Eqs. C-I5-1a and b) and Eqs. I5-1a and b for noncompact and slender sections.
• Load transfer (I6): whether the external force enters through the steel or through the concrete, and the transfer mechanism, direct bond or direct bearing. For direct bond you can enter the load introduction length (0 takes two times the least transverse dimension, I6.4b). For direct bearing enter the concrete area A1 under the internal plates.

Auto-design varies the shape within the given family (rectangular HSS, round HSS or pipe), lightest first, or the plate thickness of a box, and the steel grade when you unlock it. Concrete, loads and the box's outside dimensions stay as entered. An optional limit caps the depth.

Checks performed #

The shear strength follows the 2022 Specification, which adds the concrete to the steel. Kc is 1 unless the section is compact with a short shear span (10 for rectangular and 9 for round below Mu/(Vu d) = 0.5, linear up to 0.7). A section counts as compact here only when it is compact for compression and for flexure about both axes.

Limitations #

• No internal bars. The strengths ignore longitudinal reinforcement (Asr = 0), including its contribution to fire resistance. Fire design is not covered.
• Interaction methods. The strain compatibility and effective stress-strain methods (I1.2b, I1.2d) are not provided. Eqs. I5-1 are written for one bending axis: with biaxial bending a noncompact or slender section falls back to H1.1 and the steps say so. The full five-point curve of Method 2 is not offered, only Method 2 Simplified, which avoids the unsafe region near point D that Commentary I5 warns about.
• Noncompact and slender rectangular HSS in flexure. My and Mcr use the stress blocks of Design Example I.7 with sharp corners. All but a few HSS are compact, so this rarely applies to a tube.
• Noncompact round sections in flexure. Neither the Specification nor the Manual gives equations for My of a round filled section. The type applies the I3.4b definition directly (steel linear to Fy at the extreme fibre, concrete linear to at most 0.7 f′c) over the circle. No published example confirms this: review it for such a section.
• Load transfer. Concurrent loading of the steel and concrete (I6.2c) and shear connectors (I6.3b) are not calculated, and direct bond is taken over the whole perimeter (the end condition of Design Example I.3). For a force entering on fewer faces, use the perimeter and force of each face yourself. The connection itself is not designed.
• Construction stage. The bare tube under loads placed before the concrete cures, and the fluid pressure of the concrete, are noted but not calculated (I1).
• Not covered: W-shapes and other encased members, Appendix 2 high-strength materials, seismic provisions (AISC 341), and the net section of a tension connection.

Verification examples #

Verification status: 151 values compared in 36 examples (36 published, 0 hand calculations from the cited equations): 148 within tolerance, 3 recorded deviations. Code basis: the provisions it uses were read in the target edition.

The tests compare the type with these published values, each read from the source at the page cited.

Two documented differences, both on bare steel strengths and neither affecting these composite results: I.7 states 1,980 and 541 kips for the bare box, without showing the calculation, and the type gets 1,882 and 509 kips. Those figures reproduce with the HSS wall factors of Table E7.1 and a flat width of H − 3t, while the type reads the Specification for a plate box. The Steel Tube Institute table lists 821 kips for the bare HSS20×20×3/8, which fits a 0.375 in. wall, while the article's 0.349 in. wall gives 730 kips by hand and 729 kips here.

What is checked only by hand calculation. No free published worked example was found for a round filled section, for Eqs. I5-1 or for Kc above 1. The tests check these against hand calculations from the cited equations: round Pno, EIeff and Pn; the Table 6-4 plastic moment against an independent strip integration (within 1.5 %, Table 6-4 assumes a thin wall); Eqs. I5-1 and Table I5.1 as printed; and Kc. The Companion to the Manual, Volume 2, Tables 4-A to 4-F (the filled HSS strength tables) was not available, so only the Table 4-B cells that Examples I.4 and I.6 print are compared.