SRC Column Design

Steel-Reinforced Concrete (Encased Composite) — AISC 360-22 Chapter I §I2.1. W section inside rectangular concrete column with longitudinal rebar. Squash load Pno, EIeff, axial & flexural strength, P-M interaction diagram.

Input Parameters

kip · in
Concrete Section
in
in
ksi
Steel Section
Longitudinal Rebar
in
Cover & Column Height
in
ft
Both ends fixed — K = 0.65
Applied Loads (LRFD)
kips
kip-ft
kip-ft
Mu must include second-order (P-δ) amplification per AISC Ch. C before entry.
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Enter section geometry and loads — results appear here.

📚 SRC Column Design — Theory & Code Background

What Is an SRC Column?

A Steel-Reinforced Concrete (SRC) column — also called an encased composite column — consists of a structural steel shape (typically a W section) fully encased in reinforced concrete. The concrete provides lateral bracing to the steel, increases axial and flexural capacity, and improves fire resistance. The steel section provides significant ductility and erection strength before concrete placement.

SRC columns combine high axial and flexural strength with excellent ductility, making them common in high-rise building cores, transfer levels, and seismic regions.

AISC 360-22 §I2.1 — Encased Composite Columns

Section I2.1 governs the design of encased composite columns. The method:

  • Computes the squash load Pno = FyAs + 0.85f'cAc + FyrAr
  • Computes the effective stiffness EIeff using coefficient C1
  • Applies Euler buckling (same §E3 formulas as steel columns)
  • Checks P-M interaction via the Plastic Stress Distribution Method (PSDM)
AsCross-sectional area of W steel section (in² / mm²)
ArTotal area of longitudinal reinforcement (in² / mm²)
AcNet concrete area = Ag − As − Ar (in² / mm²)
AgGross concrete section area = B × H (in² / mm²)
C1Concrete stiffness modifier = min(0.25 + 3(As+Ar)/Ag, 0.7)
IsMoment of inertia of steel section about bending axis (in⁴ / mm⁴)
IrMoment of inertia of rebar about bending axis (in⁴ / mm⁴)
IcMoment of inertia of concrete about bending axis (in⁴ / mm⁴)

Minimum Requirements — §I2.1a

  • Steel section ratio: ρs = As/Ag ≥ 1% and ≤ 8% (AISC §I1.3)
  • Steel yield: Fy ≤ 75 ksi (525 MPa)
  • Concrete strength: 3 ksi ≤ f'c ≤ 10 ksi (21–69 MPa, normal-weight)
  • Rebar yield: Fyr ≤ 80 ksi (550 MPa)
  • Minimum concrete cover: 1.5 in (38 mm) clear to stirrups per ACI 318 §20.6.1.3
  • Clear gap between W section and rebar: ≥ 1.0 in (25 mm)
  • Clear gap between W section and concrete face: ≥ 1.5 in (38 mm)

Note: No compactness classification (b/t check) applies to encased members — the concrete fully restrains local buckling of the steel section.

Squash Load Pno — §I2.1b

Pno = FyAs + 0.85f'cAc + FyrAr

This is the nominal compressive strength without length effects (pure squash). The 0.85 factor reflects that concrete in columns is less fully stressed than in pure compression tests (sustained load, imperfections).

Effective Stiffness EIeff — §I2.1b

EIeff = EsIs + EsIr + C1EcIc
C1 = min(0.25 + 3(As+Ar)/Ag,  0.7)

C1 accounts for creep and sustained loading effects on concrete stiffness. A higher steel ratio increases C1, raising the effective stiffness. The cap of 0.70 limits unconservative overestimation for very steel-heavy sections.

Column Strength Pn — §I2.1 / §E3

Pe = π²EIeff / (KL)²
If Pno/Pe ≤ 2.25:  Pn = Pno × 0.658Pno/Pe
If Pno/Pe > 2.25:  Pn = 0.877Pe

Design strength: φcPn with φc = 0.75 (LRFD). The governing axis is the one with the smaller φcPn.

Concrete modulus: Ec = 33wc1.5√f'c (US, psi)  /  4700√f'c (SI, MPa). Fixed unit weight: wc = 145 pcf (23.6 kN/m³).

PSDM Interaction Envelope — §I3.4

For compact composite members (encased sections are always compact — no b/t limit applies), the P-M interaction is checked via the Plastic Stress Distribution Method (PSDM). The full cross-section capacity at each axial level is found by sweeping the Plastic Neutral Axis (PNA) from the top to the bottom of the section.

At each PNA position y:

  • W section: compression (±Fy) above and below PNA
  • Rebar: each bar is at ±Fyr depending on position
  • Concrete: 0.85f'c in compression only (above PNA)

φ-Scaled Design Envelope

  • P-axis: φc = 0.75, capped at φcPn (column buckling limit)
  • M-axis: φb = 0.90

Radial D/C Check

A ray is cast from the origin through the demand point (Mu, Pu). The DCR is the ratio of demand distance to envelope intersection distance. DCR ≤ 1.0 → adequate.

For biaxial bending: each axis is checked independently against its own PSDM envelope (the x-axis envelope differs from y-axis for non-square sections). Governing DCR = max(DCRx, DCRy).

Worked Example — W10×49 in 24×24 in Concrete, f'c = 5 ksi, L = 14 ft

Given: B = H = 24 in, f'c = 5.0 ksi, W10×49 (A992, Fy = 50 ksi), 8-#8 bars (Fyr = 60 ksi), cover = 1.5 in, dst = 0.394 in, K = 0.65, L = 14 ft.

Step 1 — Section Properties

Ag = 24 × 24= 576 in²
As (W10×49)= 14.4 in²
Ar = 8 × 0.79 (#8)= 6.32 in²
Ac = 576 − 14.4 − 6.32= 555.3 in²
ρs = 14.4/576= 2.5% ✓ (1–8%)

Step 2 — Material Properties

wc = 145 pcf, f'c = 5.0 ksi
Ec = 33×1451.5×√(5000)/1000≈ 4074 ksi
C1 = min(0.25 + 3×(14.4+6.32)/576, 0.7)= min(0.358, 0.7) = 0.358

Step 3 — Squash Load Pno

FyAs = 50 × 14.4= 720 kips
0.85f'cAc = 0.85 × 5 × 555.3= 2360 kips
FyrAr = 60 × 6.32= 379 kips
Pno= 3459 kips

Step 4 — EIeff and Column Strength

Is,x (W10×49)= 272 in⁴
Ir,x = Σ Abar×yi²≈ 620 in⁴
Ic,x = 24⁴/12 − Is,x − Ir,x≈ 26,700 in⁴
EIeff,x = 29000×272 + 29000×620 + 0.358×4074×26700≈ 64,900,000 kip·in²
KL = 0.65 × 14 × 12= 109.2 in
Pe,x = π²×64,900,000/109.2²≈ 53,600 kips
Pno/Pe = 3459/53600= 0.065 < 2.25
Pn = 3459 × 0.6580.065≈ 3341 kips
φcPn = 0.75 × 3341= 2506 kips

P-M interaction via PSDM: demand Pu = 500 kips, Mux = 150 kip-ft → DCR ≈ 0.36 (OK).

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