Shallow foundation types: strip, isolated, combined, strap, mat

One-line orientation

The shallow family is one idea — spread the load onto near-surface soil — expressed at different scales: a line (strip), a point (isolated), two points (combined/strap), or the whole footprint (mat). The exam tests which one fits the load geometry.

Key points

• Strip / continuous (wall) footing: runs along the length of a wall to distribute a linear load evenly.
• Isolated / spread (column) footing: supports a single point load (one column). Isolated pads may be connected with grade beams for lateral stability.
• Combined footing: supports two or more columns that are too close to distribute their loads individually — a single pad serves them together.
• Strap footing: separate exterior and interior column footings are linked by a strap beam that does not bear on the soil. The beam transfers the exterior column’s eccentric effect to the interior footing so the soil pressures can balance.
• Mat / raft foundation: a very thick, heavily reinforced concrete slab under the entire footprint. Spreads load over a large area; suited to heavy loads, non-homogeneous or low-bearing soil, and tight structural spacing.

Confusions / comparison

Type	Supports	Load geometry	Note
Strip / wall	A wall	Linear (line)	Continuous along the wall
Isolated / spread	One column	Point	May be tied with grade beams
Combined	Two+ close columns	Two points, one pad	Columns too close to separate
Strap	Eccentric exterior column + interior column	Two pads joined by a non-soil-bearing strap beam	Transfers the eccentric effect to balance soil pressure
Mat / raft	Whole structure	Entire footprint	Thick, heavily reinforced; weak soil/heavy load

Related

→ Shallow vs deep foundations (this topic): why shallow at all · Mat vs floating (this topic): the two large-area options compared · Grade beam vs tie beam (this topic): the beams that tie isolated footings · Slab-on-grade & FPSF (this topic): the at-grade slab case.