Structural Engineering & Statics
where the load goes
Concept

Tension pile — where it appears

A pile designed to hold a structure down rather than up, resisting uplift from buoyancy or overturning by shaft friction in the ground around it. Its capacity is the friction along its length, and the ground it pulls on must itself be heavy enough not to lift with 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 How long the pumps must run is set by how fast the water comes back. The floors that must be cast before the pumps stop under a 20 × 30 m basement dug 6 m below a water table at the surface, its 0.9 m slab and walls weighing 18.7 MN against 35.3 MN of uplift when the water is back, so that the weight stays ahead of the water at every instant, against the water's recovery time on a logarithmic scale, for a floor every 5, 7 and 10 days. If the water returns at once, five floors — the building has to be nearly heavy enough on its own. With a floor every 5 days: 3 at a 3-day recovery, 2 at 7, 0 at 14; a floor every 7 days: 4 at a 3-day recovery, 3 at 7, 1 at 14; a floor every 10 days: 4 at a 3-day recovery, 3 at 7, 2 at 14. Dashed, the closed form for weight added continuously at a floor a week, which runs about one floor below the steps: a floor arrives at the end of its cycle, not through it.

The water that comes back while the floors go up

A basement below the water table is built dry, inside an excavation kept pumped, and the building that will one day hold it down is not there yet. When the pumps stop, the water comes back over days and the frame goes up a floor a week, and the basement is safe only if the weight stays ahead of the water at every instant. How long the pumps must run is not set by how heavy the building is. It is set by one ratio — how much weight the frame adds while the ground refills — and for a basement that needs five floors on it to hold itself down, a seven-day recovery asks for three and a fourteen-day one for one.

equilibrium · Uplift

Named alongside it

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

Hydrostatic pressureUpliftBallastBuoyancyConstruction sequenceControl perimeterDrainageEffective depthEquilibriumFactor of safetyFlotationFree body

All concepts