Structural Engineering & Statics
where the load goes
Concept

Arch — where it appears

A curved member that carries a transverse load principally as compression along its own line, at the price of a horizontal thrust at each springing. Its efficiency depends on how close its shape is to the funicular of the load it carries, and its design is usually decided by whatever receives the thrust.

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

Also named here as thrust — the same set of essays touches all of them, so they are one junction rather than several.

first · antisymmetric · C 49.7second · symmetric · C 117.1dashed is the rib before it moved · drawn hugely exaggerated

The arch that leans instead of squashing

A masonry arch is asked whether a line of thrust fits inside it. A steel rib is asked a different question entirely: it is a column carrying an axial force along its whole length, and the mode it buckles in puts one half up and the other half down while the crown moves sideways.

stability · arch buckling 0.10×0.32×3×10×00.20.40.60.81tie stiffness, relative to the one drawnthrust ÷ its rigid-tie value, and bending ÷ its worstthrustbending, to 7422 kNmas built: 2168 kN, tie stretches 68 mmfunicular wL²/8f = 2250 kN · rigid tie 2229 · the difference is the rib's own shortening

The thrust that never reaches the ground

Every arch on this site has ended at the same sentence — the foundation is where an arch is really decided. A tie changes the sentence without changing the arithmetic: the horizontal force is still there, still the same size, and it now closes on itself through a bar at deck level.

structures · tied arch

Named alongside it

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

ThrustAntisymmetric modeAxial shorteningBending momentBracingBucklingCompatibilityCritical loadDeckEffective lengthEigenvalueFlexibility

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