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. You set the footing length L and thickness H. The two columns are placed by the left column position and their centerline spacing, both measured along the footing width. The footing width and the right cantilever are calculated. Columns can be concrete columns or pedestals, steel or generic columns on steel base plates, or wood columns.

Footing width and bearing check

The footing width B is set to twice the distance from the left edge to the service axial load resultant. That distance is taken as the largest across the service load combinations, pairing the left and right column loads from each combination. The resultant eccentricity from the footing centroid is checked against B/12. If the right column or its base plate would extend past the right edge, the calculator flags it and reports a recommended minimum left column position.

Bearing uses the largest service axial load plus the footing and soil weight, spread uniformly over the footing plan. The gross bearing pressure q_gross is compared to the allowable bearing capacity q_a:

utilization = q_gross / q_a ≤ 1.0

Load combinations

Service (ASD) combinations per ASCE 7-22, Chapter 2 are used for soil bearing, sliding, and uplift. LRFD combinations per ASCE 7-22 Ch. 2 and ACI 318-19 Ch. 13 are used for concrete strength checks. Strength checks use the factored combination with the largest total axial load. You can graph any combination in the diagrams.

Stability checks

Sliding factors of safety are computed in each direction a lateral load acts, for every service combination. Sliding resistance is the friction coefficient times the service axial load plus the foundation weight:

FS_sliding = F_resist / H_total ≥ FS_min

For unrestrained footings, an uplift factor of safety is reported for service combinations with net uplift. Minimum factors of safety for sliding and uplift are user-defined.

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

Along the footing width, the footing is analyzed as a beam carrying the factored column loads and a uniform upward soil pressure. Positive moment demand is taken at the column faces and at the maximum moment. The governing negative moment between the columns is checked against the top bars. Across the footing length, each column produces a cantilever moment checked against the bottom bars in that direction. Factored moment demand M_u is compared to the factored flexural capacity:

utilization = M_u / (phi × M_n) ≤ 1.0

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, in both directions. 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, and is cut to three sides where it meets a footing edge:

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 and top reinforcement in both directions, for straight bars or standard 90° hooks. For straight bars it is reduced when excess reinforcement area is provided. Available development distance is measured from the critical section to the 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 at each column-footing interface is checked against the concrete bearing capacity at the top of the footing, with dowels contributing for concrete columns. 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 its column centerline
• 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