Thrust

Rules that use the word

The flatter the arch, the harder it pushes. An arch pushes outward on its supports as well as down. For the same load and span the horizontal push grows as the rise falls: halve the rise and you roughly double the thrust.

An arch stands if its thrust line fits inside it. Trace the path of the resultant force through the stones from crown to foundations. If a line can be found that stays everywhere within the masonry, the arch will stand; where it touches an edge, a hinge will open, and four hinges make it collapse.

Make the end piers of an arcade the strongest. In a row of arches the thrusts of neighbouring arches cancel over each intermediate pier, which may therefore be slender; at the ends of the row nothing balances them, so the outermost piers must be made wider or buttressed.

Span the nave in timber. Cover a wide nave with triangulated timber trusses rather than a vault: their tie-beams take the outward thrust, so the walls need only carry weight and may stand on slender columns.

Buttress the dome where the semidomes cannot. Where no semidome takes the thrust of the dome, on the north and south, carry it across the aisles to massive buttress piers that rise above the roofs, and fill the great arches with thin walls pierced by windows.

Make a shallow dome steeper. A shallow dome pushes harder outward than a steep one of the same span: when the first dome of Hagia Sophia fell, it was rebuilt with a higher rise so that its thrust would be more nearly vertical.

Where it appears in the book

In the chapters The Arch, Vaults, Domes, The Roof, Frame and Skin, Byzantium and the Early Church, Romanesque and Gothic, The Islamic World.

Nearby in the glossary

Thermal bridge · Thermal mass · Tholos · Tholos tomb · Thrust line · Tierceron · Tile-hanging · Tokonoma