Structural Engineering & Statics
where the load goes
Concept

Buoyancy — where it appears

The upward push of water on anything below the water table, equal to the weight of the water it displaces. On a basement it is the water pressure at the slab times the slab's area, and it acts whether or not the structure is watertight, so weight, anchors or drainage must answer it.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

A basement is a boat. A 20 by 30 m substructure dug 6 m into ground whose water table stands 2 m down. The head on the underside of the base slab is 4.0 m, so the pressure there is 39.2 kN/m² over the whole plan — 23.5 MN of it, pushing upward. Nothing about the structure changes that number. What resists it is weight: 18.7 MN of concrete and whatever is built above, giving a factor of 0.80. The structure floats if the water reaches 2.82 m below the ground, and a base slab alone would have to be 1.64 m thick to hold it down.

A basement is a boat

Every load in this collection presses down and is resisted by strength. Hydrostatic uplift presses up, is resisted by weight, and does not care what is built on it — so the check contains no material property at all. It is a ratio of two weights, and one of them is water.

equilibrium · Uplift Closer piles add pull-out the block cannot pay for. For 600 mm piles 15 m long under a 20 × 30 m raft, in ground whose shaft friction is 0.30 of the effective vertical stress and whose submerged unit weight is 10 kN/m³: the summed pull-out of every pile on a square grid (thin, rising steeply as the spacing closes), the submerged weight of the block of ground the group would lift (dashed, 90 MN whatever the spacing), and the group's capacity, the lesser (solid). The two meet at 2.06 m — 3.4 pile diameters — where s² = π·d·β·L/2. Closer than that the group lifts the block and the extra piles add nothing; the most piles worth having is about 142.

The piles that would lift the ground

A basement too light to stay down can be pinned down with tension piles, each holding by the friction on its shaft. Add enough of them, close enough together, and the ground between them stops being something they grip and becomes something they carry: the group lifts out as a block, and the block's weight is all it can hold. For 600 mm piles 15 m long that happens closer than 2.06 m apart, a spacing with no raft, no water and no load in it — and every pile past that point is paid for twice.

equilibrium · Uplift

Named alongside it

The objects these essays reach for when they reach for this one.

Free bodyUpliftBallastControl perimeterDrainageEffective depthEffective stressEquilibriumFactor of safetyFlotationFrictionHydrostatic pressure

All concepts