Structural Engineering & Statics
where the load goes
Concept

Flotation — where it appears

The failure of a substructure below the water table by being lifted by the water's uplift. It is resisted by weight rather than strength, so the check is a ratio of the structure's weight to the water's push, and it changes with every storey added and every rise of the water.

Named by 3 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 Under a light building the first thing the pump was holding up is the building. The hours after the pump stops at which each check on the basement is used up — the slab's 0.2 mm cracks and the building's flotation — against the number of storeys over it, with the water table risen 3.0 m and ground recovering in 12 hours; drawn at 48 hours where a check is never used up. The cracks' deadline is 9.5 h whatever the building. Flotation's is at once under 2 storeys, 1.0 h under 4 storeys, 4.7 h under 5 storeys, 10.1 h under 6 storeys, never under 8 storeys; with 5 storeys or fewer it comes first.

The building held down by its pump

A basement whose water table has risen above its design level is often given back its design head by a pump under the slab, and from then on it is safe while the pump runs. When the pump stops the water comes back on the ground's own clock, and the checks the pump was protecting are lost in an order the building's weight decides. Under eight storeys only the slab's cracks were ever at risk; under five, the building's flotation check goes first, four and three-quarter hours after the pump stops, and the slab's strength is never in question at all. A structure that was safe by its weight has become one that is safe by a machine, and the machine's reliability is now one of its loads.

equilibrium · Uplift The silt hands the head back over decades, a season at a time. The basement is under five storeys, its water table 2.0 m above the design head and swinging ±0.5 m over the year, highest in April; pumped drains under its slab start 30 times as conductive as the ground. The head under the slab above the design head, against the years since the drains were new, for drains silting with time constants of 4, 8 and 16 years; dots, a standpipe read every October on the 8-year drains. With those drains the head reaches flotation in the wet season of year 24.1; the reading first exceeds 0.25 m in year 14.8, 9.4 years before.

The drain that fails a season at a time

A basement held at its design head by drains and a pump can fail suddenly, when the pump stops, or slowly, when fines settle in the drains and the head under the slab creeps up while the pump keeps running. The slow failure gives a standpipe read once a year a decade of warning, in its own time constants — if it is read in the wet season. And long before the silt floats the building on its own, it has made every stop of the pump more likely to.

equilibrium · Uplift

Named alongside it

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

UpliftDrainageHydrostatic pressureWater tableBallastBuoyancyControl perimeterCrack widthEffective depthEquilibriumFactor of safetyFree body

All concepts