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Factors for selection; strap and combined footings

Short answer

Factors: soil bearing, load, settlement, groundwater, seismic, cost. Combined footings support two or more columns when they are close or an exterior column is at the property line. Strap footings join an exterior footing at the property line to an interior footing with a rigid beam, used when column spacing is large.

What are the factors that influence the selection of a foundation? Under what circumstances are strap footings and combined footings adopted?

Factors influencing selection of foundation

  • Bearing capacity and compressibility of soil, and depth of the firm layer.
  • Magnitude and type of loads, and the allowable settlement of the structure.
  • Groundwater level, frost, scour and seasonal moisture change.
  • Seismic condition, adjacent structures and site restrictions.
  • Availability of materials, equipment and skilled labour, and total cost.

Combined footing

A single footing supporting two or more columns. It is used when:

  • Columns are so close that their separate footings would overlap or touch.
  • An exterior column is at the property line, so an isolated footing cannot be centred under it.
  • The soil is weak and a larger area is needed.
  • Differential settlement between neighbouring columns must be reduced.

Rectangular combined footing is used when the loads are similar; trapezoidal when the loads differ much or the space is limited at one end.

Strap (cantilever) footing

Two isolated footings joined by a rigid beam (strap). It is used when:

  • The exterior column is on the property line and its footing cannot extend outside.
  • The distance between columns is large, so a combined footing would be uneconomical.
  • The strap takes the eccentric moment of the edge footing and transfers it to the inner footing, so both footings get uniform pressure.

✓ Answer checked by Claude Sonnet 5.5

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