Concrete-Filled Column

How it calculates

This calculator designs and analyses concrete-filled composite columns to AISC 360-22 Chapter I, covering axial compression, tension, flexure, combined interaction, shear and serviceability under uniaxial bending. The workflow classifies the section, builds the composite section properties, then evaluates each limit state under either LRFD or ASD.

Section input and geometry limits

The column can be a round HSS, a square or rectangular HSS from the section database, or a built-up box defined by its outside width, outside depth and plate thickness. For a built-up box the outside dimensions must exceed twice the plate thickness, so the wall geometry remains physically valid.

Material inputs are the steel grade, the concrete compressive strength f'c and the concrete unit weight. Chapter I bounds both: f'c must be at least 3 ksi with an upper limit for normalweight concrete under Cl. I1.3, and the unit weight must lie between 90 and 155 lb/ft³ per Cl. I1.5. The structural steel area must also comprise at least 1 percent of the gross composite area for the member to qualify as composite.

Section classification

Section classification follows AISC 360-22 Table I1.1a for axial compression and Table I1.1b for flexure. For round HSS the slenderness parameter is the diameter-to-thickness ratio D/t, compared against the compact limit 0.15·E/Fy, the noncompact limit 0.19·E/Fy for compression, and a maximum permitted value of 0.31·E/Fy. For rectangular sections the flange b/t and web h/t ratios are checked separately against limits expressed as multiples of the square root of E/Fy.

Where the slenderness exceeds the maximum permitted value, the composite provisions of Chapter I do not apply and the calculator says so rather than extrapolating beyond the code.

Composite section properties

The plastic axial compressive strength of the composite section is

Pp = Fy × As + C2 × f'c × Ac

where As and Ac are the steel and concrete areas and C2 is the code coefficient that differs between round and rectangular fill. Plastic section moduli are computed for the steel and concrete components separately, so the plastic neutral axis and the flexural capacity follow from the filled geometry rather than from the bare steel section.

Axial compressive and tensile strength

Compressive strength is evaluated per Cl. I2.2b, taking the composite plastic strength together with the elastic critical buckling load derived from the effective stiffness of the filled section and the effective length K·L. The effective length factor is entered directly, with a warning where K falls outside the usual 0.5 to 2.0 range so that assumed end fixity gets a second look.

Tensile strength per Cl. I2.2c is taken on the steel section alone, since the concrete fill is not relied on in tension.

Flexural strength

Flexural strength follows Cl. I3.4b and depends on the section classification. Compact sections develop the full plastic moment of the composite section, while noncompact and slender sections are limited by first yield or local buckling as appropriate to the classification band.

Combined axial and flexure

The interaction method is selected from the classification results. Where the section is compact for both axial and flexural actions, the plastic distribution method of Cl. I1.2 is used to build the interaction curve. Otherwise, and for members in net tension, the Cl. I1.5(b) method applies. In both cases the result is reported as a single demand-to-capacity ratio:

utilization = DCR ≤ 1.0

Shear strength

Shear capacity is taken per Cl. I4.2 on the steel section, with the appropriate resistance factor under LRFD or safety factor under ASD.

Serviceability

Short-term deflection is computed for the applied lateral and eccentric load cases and compared against the governing limit, which can be set as a span ratio, an absolute value, or both. The calculator reports the governing deflection, the governing limit and the critical span ratio, so the controlling serviceability case is visible alongside the strength checks.

Design method and load combinations

Selecting LRFD reports design strengths φPn, φMn, φTn and φVn against factored demands. Selecting ASD reports allowable strengths Pn/Ω, Mn/Ω, Tn/Ω and Vn/Ω against service demands. Load combinations are generated to IBC 2024 from the individual load inputs, and the governing combination for each limit state is identified in the summary.