Combined footing

A rectangular footing under two columns, designed to ACI 318-19 (13.3.4 and Chapter 8) with ASCE 7-22 loads. The footing is analysed as a beam along its length under the factored net soil pressure and the column loads; the columns' transverse beams, punching and bearing are checked at each column.

Inputs of the combined footing #

• Footing: length L, width B, thickness h, soil cover and unit weights. For a uniform soil pressure under D + L the length should be twice the distance from the left end to the resultant of the column loads; the calculation suggests that length when yours differs.
• Columns: the centre of each column from the left end (x1 < x2) and its size (c1 along L, c2 across), and the columns' concrete f'c. The columns are on the footing's centreline.
• Column loads per column and load type, unfactored: P and the moment M in the long direction (positive moves the resultant toward the right end).
• Soil: allowable pressure and its basis (gross or net).
• Reinforcement: top and bottom longitudinal bars (size and number across B), transverse bottom bars (size and spacing), which bottom layer is outermost, covers; λs option (13.2.6.2 lets you neglect it).

Auto-design (editor toolbar) varies the width (1/2-ft steps), thickness (2-in. steps), longitudinal bottom and top bars and transverse bars; the length when unlocked (column positions kept). Objectives: lowest cost index (concrete, formwork, reinforcement); least concrete, then cost. Kept unless you select them: length L. Every other input is kept; see Auto-design.

Checks performed #

The drawings (screen, report, DXF and PDF sheet) show the footing plan with the two columns (marks C1, C2) and the bottom bars both ways, section A along L with the top and bottom longitudinal bars, the transverse bars at their spacing and the soil cover, the factored shear and moment diagrams along L under it, and section B across the footing at column 2. Bars are drawn over the full length as designed; cut-offs, hooks, splices and column dowels are not part of the calculation and the sheet notes say so.

Limitations #

• Rectangular footings only (no trapezoidal or strap footings); columns on the centreline; moments across the width are not entered.
• The transverse-beam width (c1 + d/2 each side) is common textbook practice, not a code rule; confirm it against your practice.
• Verified by hand calculation only; no published combined-footing example is on file.

Verification examples #

Published worked example (backend/tests/test_verify_combined_footing.py): the free StructurePoint example that reproduces Wight, Reinforced Concrete Mechanics and Design, 7th Ed., Example 15-5 (ACI 318-14): 25 ft 4 in × 8 ft, an exterior 16 × 24 in column at the property line and an interior 24 in column 20 ft away.

This example found an error, now fixed: the moment transferred by eccentric shear at an edge column was taken as Vu times the eccentricity of the column; it is now the moment of the column load and of the soil reaction inside the section about the section's centroid (ACI 318-19 R8.4.4.2.3), which raised vu at the exterior column from 184 to 192 psi.

Hand calculation of the default design: 21.5 ft × 7 ft × 36 in, columns at 1 ft and 17 ft.