Combined Footing (IBC 2024)

How it calculates

The Combined Footing (IBC 2024) calculator analyzes and sizes a rectangular footing supporting two columns to IBC 2024 with ACI 318-19 concrete design and ASCE 7-22 load combinations. Geometry is set by footing length, width, and thickness, with two columns positioned independently along the length axis. Each column can be a concrete pedestal or a steel base plate.

Load combinations and bearing check

The calculator generates ASD load combinations per ASCE 7-22, Chapter 2 to evaluate service-level soil bearing, and LRFD combinations per ASCE 7-22 Ch. 2 and ACI 318-19 Ch. 13 for concrete strength checks. The governing combination is identified as the one producing the maximum effect for each check. Gross bearing pressure q_gross is compared to the allowable bearing capacity q_a:

utilization = q_gross / q_a ≤ 1.0

Eccentricity is tracked in both X and Y axes. For large eccentricities where the resultant falls outside the kern (Zone 2), an iterative procedure solves for the bearing pressure profile with partial lift-off of the footing.

Stability checks

Overturning and sliding factors of safety are computed for each axis using ASD service-level loads:

FS_overturn = M_resisting / M_overturning ≥ FS_min

FS_sliding = F_resist / H_total ≥ FS_min

An uplift safety factor is also reported. Minimum factors of safety for overturning, sliding, and uplift are user-defined.

Flexural design (ACI 318-19, Cl. 22.2)

The footing is treated as an inverted beam loaded by the upward net soil pressure and downward column loads. Critical sections for positive moment (bottom reinforcement, between and outside the columns) and negative moment (top reinforcement between columns) are identified from the bending moment diagram. Factored moment demand M_u is compared to the nominal flexural capacity:

utilization = M_u / (phi × M_n) ≤ 1.0

Separate checks run for X-axis (longitudinal bars spanning between columns) and Y-axis (transverse bars) reinforcement in both positive and negative bending. Compression reinforcement is not considered in the bending strength.

One-way shear (ACI 318-19, Cl. 22.5)

One-way shear demand V_u is taken at the critical section, a distance d from each column face. No shear reinforcement is assumed; capacity is provided by concrete alone:

utilization = V_u / (phi × V_c) ≤ 1.0

Two-way (punching) shear (ACI 318-19, Cl. 22.6)

Punching shear is checked independently at each of the two columns. The critical perimeter b_o is located at d/2 from the column face:

utilization = v_u / (phi × v_c) ≤ 1.0

v_u is the factored shear stress on the critical perimeter and v_c is the punching shear strength per ACI 318-19, Cl. 22.6.

Development of top and bottom reinforcement (ACI 318-19, Cl. 25.4)

Required development length l_d is calculated for bottom reinforcement (positive bending) and top reinforcement (negative bending) in both axes, and reduced when excess reinforcement area is provided. Available development distance is measured from the critical section to the nearest bar end. If insufficient, the calculator automatically evaluates whether a plain concrete design for that direction passes and updates the sheet results accordingly.

Column-footing interface bearing (ACI 318-19, Cl. 22.8)

Bearing stress at each column-footing interface is checked against concrete bearing capacity at the top of the footing. For steel base plates, only concentric axial bearing is checked at the interface; base-plate design itself is run in a separate calculator.

Assumptions

• Each column assembly (pedestal, base plate, steel section) is concentric about the column centerline at its X position
• Column design itself is run separately; this sheet checks the footing only
• Excess reinforcement area reduces required development length
• Footing is treated as rectangular only when B and L differ enough that inner bar spacing is less than outer bar spacing
• Column self-weight is not included in uplift resistance