What is horizontal thrust?
Horizontal thrust is the sideways force that develops in structures with sloping or curved members, such as rafters, arches and vaults. In Dutch it is called spatkrachten. The force such a member exerts on its support splits into a vertical component (the weight going down) and a horizontal component: the thrust. This tends to push the supports apart. Walls or piers that cannot resist it start to lean outwards or crack.
Where it occurs
- Couple roof. Two rafters leaning against each other push outwards on the walls at their feet. A tie beam, loft floor or anchorage at the foot must take this force.
- Arches and vaults. Every arch pushes sideways against its supports. The flatter the arch, the greater the thrust for the same load.
- Bridges. In arch bridges the thrust is resisted by heavy abutments or by tie rods under the deck.
Solutions
- Tie member. A beam or rod connecting the feet of the rafters or the arch takes the thrust as a tensile force.
- Buttress or flying buttress. In Gothic churches, buttresses and flying buttresses resist the thrust of the vaults.
- Mass. Thick walls and heavy piers can absorb the horizontal force through their own weight.
- Wall anchors. Anchors tie walls to floors and roof, so that the whole works together.
Application in alterations
- Do not remove beams without advice. When converting a loft, collar beams, tie beams or loft floor joists are sometimes removed. If such a beam acts as a tie, the walls can be pushed outwards.
- Watch for signs. Façades leaning outwards, horizontal cracks below the wall plate or loose anchors point to horizontal thrust.
- Have a structural engineer do the calculations if you alter a roof or arch, or modify a load-bearing wall under a roof.
Signs of horizontal thrust to watch for
- Façades leaning outwards: Points to horizontal thrust
- Horizontal cracks below the wall plate: Points to horizontal thrust
- Loose anchors: Points to horizontal thrust
Common mistake: thinking that a roof only bears down vertically on the walls. Without a tie connection, every pitched roof also pushes sideways.
Related terms
- Buttress
- Flying buttress
- Collar beam
- Wall plate
- Roof structure
- Pointed arch
- Mechanical stress
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