Combined Foundations

What Are Combined Foundations?

Combined foundations (also called combined footings) are single footings that support two or more columns. They are used when isolated pads would overlap, columns are closely spaced, or significant eccentricity makes a single pad inefficient. A combined foundation shares and redistributes column reactions, helping control contact pressures and differential settlements while simplifying reinforcement and formwork where space is tight.

This guide explains when combined footings out-perform isolated pads, common geometries and detailing, load-path logic, geotechnical checks (bearing, sliding, overturning), settlement prediction, groundwater and frost considerations, seismic/lateral behavior, and construction QA/QC. It links to core topics including Site Characterization, Geotechnical Soil Testing, Bearing Capacity, and alternatives such as Shallow Foundations, Mat Foundations, and Pile Foundations.

Combined foundations are about balance—sharing column reactions, keeping the resultant within the kern, and limiting differential settlement without jumping to a mat.

When Are Combined Footings the Right Choice?

Use a combined footing when an isolated pad becomes impractical, yet a mat would be excessive. Typical triggers:

• Close Column Spacing: Adjacent isolated footings would overlap, or the clear distance is too small for constructability.
• Property Line/Eccentric Load: An exterior column near a boundary causes high eccentricity; pairing it with an interior column balances the resultant.
• Unequal Column Loads: One column is much heavier; a combined footing redistributes contact pressures to keep soil stresses within limits.
• Utility or Architectural Constraints: Obstructions make symmetric pads infeasible; a single combined footing can thread between constraints.
• Serviceability Controls: Differential settlement and rotation limits govern (see Settlement Analysis).

Types & Geometry

Geometry follows load magnitudes, column spacing, and boundary constraints. Common types include:

• Rectangular Combined Footing: Two columns supported on a single rectangle. Efficient when loads are similar and columns are aligned.
• Trapezoidal Combined Footing: Plan width varies so the centroid of footing area aligns with the resultant of column loads—ideal for unequal loads or edge conditions.
• Strap (Cantilever) Footing: Two isolated pads connected by a stiff beam (the “strap”). The strap transfers moment so the edge footing avoids excessive eccentricity.
• Combined Wall-Column Footing: Column and short wall segment share a footing; useful near shear walls and cores.
• Beam-and-Slab Combined Footing: Deeper ribs under column lines with thinner slab between to control punching and flexure without excessive thickness.

Load Paths, Straps & Soil–Structure Interaction

In a combined footing, column reactions flow into a shared slab (and strap beam where used), then into the soil. The design objective is to shape the plan area and stiffness so that the resultant passes near the footing centroid and the soil contact pressure remains compressive and well distributed.

• Strap Action: A stiff strap converts an eccentric edge column into a statically favorable system by coupling it to an interior footing. The strap ideally spans over soil (no bearing) so it carries moment, not soil reaction.
• Stiffness Tuning: Depth and reinforcement of the strap/footing control rotation compatibility; overly flexible straps won’t balance the edge column effectively.
• SSI Modeling: Include realistic subgrade modulus and coupling between columns in Geotechnical Modeling and structural analysis; iterate reinforcement and geometry.

Bearing Capacity, Eccentricity & Settlement Checks

Proportion the footing so service/factored contact pressures remain below allowable/limit states and the base stays fully compressive. Then check settlements—often the governing serviceability criterion.

Use parameters from a robust program of Geotechnical Soil Testing—including Triaxial, Atterberg Limits, Permeability, and compaction tests such as the Standard Proctor Test. Predict immediate and consolidation settlements (see Soil Consolidation) and check differential rotation along the footing length.

Groundwater, Frost & Durability

Environmental ground conditions directly affect constructability and long-term performance. Characterize seasonal groundwater and freezing depth; design drainage and durability details accordingly (see Groundwater in Geotechnical Engineering).

• Drainage: Provide free-draining subbase and subdrains to limit pore pressure and capillary rise.
• Frost: Place the base below frost line and specify non-frost-susceptible materials beneath footings.
• Aggressive Exposure: Use low-permeability concrete and appropriate cover; seal joints and penetrations.
• Authoritative References: For durable national guidance, consult FHWA and USACE.

Seismic & Lateral Load Considerations

Combined foundations must resist sliding and overturning under seismic and wind loads. Evaluate site class, potential liquefaction, and lateral spreading using authoritative hazard data (see USGS).

• Sliding: Check base friction and available passive resistance; keys may be used where code-permitted.
• Overturning: Ensure resultant stays within kern under lateral combinations; deepen/thicken or add strap ribs as needed.
• Liquefaction: If susceptible, densify, drain, or shift to a mat/piled solution; see Liquefaction.
• Adjacent Excavations: Coordinate with Retaining Wall Design and Earth Retaining Walls to limit ground movement impacts.

Design Workflow: From Ground Model to Details

A transparent, iterative workflow ensures the combined footing meets both geotechnical and structural performance targets.

• 1) Investigate: Borings, CPT/SCPT, lab tests; map variability and hazards—start with Site Characterization.
• 2) Parameterize: Derive stiffness, strength, compressibility; validate via Geotechnical Data Analysis.
• 3) Proportion Footing: Select rectangular or trapezoidal plan; align centroid with resultant; consider strap for boundary conditions.
• 4) Serviceability: Model SSI and settlements; check rotations and differential movement (see Settlement Analysis).
• 5) Strength: Check bearing, punching/two-way shear, one-way shear, flexure, sliding, and overturning; refine thickness and reinforcement.
• 6) Detailing: Rebar development, column pedestals, sleeves, anchor coordination, joints, drainage.
• 7) Tools: Document assumptions and sensitivity in Geotechnical Design Software.

Construction Methods, Monitoring & QA/QC

Field execution should match design assumptions. Specify acceptance criteria and documentation so performance is verifiable.

• Subgrade Prep: Proof-roll; remove soft pockets; place and compact a leveling pad (see Compaction Test).
• Formwork & Reinforcement: Maintain cover; resolve congestion at column pedestals and strap intersections; provide clear bar schedules.
• Concrete: Low-permeability mix for wet exposure; continuous pours to minimize cold joints; curing per spec.
• Instrumentation: Where risk warrants, set settlement pins and heave markers; monitor groundwater during dewatering.
• Reporting: Compile tests, photos, and as-builts into final Geotechnical Reporting.

FAQs: Combined Foundations

How do I choose between rectangular and trapezoidal plans?

If column loads are similar and spacing is regular, a rectangular plan is efficient. If loads differ or one column sits near a boundary, a trapezoid helps align the centroid with the resultant to control eccentricity.

When should I switch to a strap footing?

Use a strap when an edge footing would be highly eccentric; the strap couples to an interior footing and carries moment so both pads maintain uniform compression.

When is a mat foundation better?

If many combined footings start to merge, or if differential settlements are tight across a large grid, a Mat Foundation can simplify construction and improve performance.

What references won’t change URL often?

National repositories such as FHWA, USACE, and USGS provide stable, authoritative guidance.

Conclusion

Combined foundations bridge the gap between isolated pads and mats. By smartly sharing column loads, aligning the resultant with the plan centroid, and managing settlement and lateral demands, they deliver reliable performance and construction economy. Start with a high-quality ground model and testing (Soil Testing), confirm bearing and settlements (Bearing Capacity, Settlement Analysis), and iterate geometry with Geotechnical Modeling. For boundary-driven designs, consider strap action; for larger grids or tighter settlement criteria, step up to Mat Foundations or Deep Foundations. Continue exploring adjacent topics across our hub—Retaining Wall Design, Ground Improvement Techniques, and Soil-Structure Interaction—to round out your foundation strategy.