A three-hinged arch is a statically determinate structure consisting of two beams or arch halves connected by a pin (hinge) at the crown and at each of the two supports. Because the three hinges eliminate any bending moment at those points, the arch can span long distances without being sensitive to settlement or temperature differences.
Working principle
The three hinges make the structure statically determinate: there are exactly as many unknown reactions as equilibrium equations. This brings important advantages:
- No locked-in stresses — temperature, shrinkage or settlement cause no extra stresses
- Easy to analyse — solvable using only the three equilibrium equations
- Independent of foundation — uneven settlement is accommodated by the hinges
- Prestressing possible — tie rods or cables keep the structure slim
Components
- Two arch (or beam) halves — glued laminated timber, steel truss or concrete
- Crown hinge — a joint at the crown, often a steel pin connection
- Base hinges — two foundation joints
- Tie rod or tie plate — resists the horizontal thrust
Applications
Three-hinged arches are used for:
- Halls and warehouses — spans up to 50 m
- Sports halls — clear open space without columns
- Swimming pools — aggressive environments need simple structures
- Riding halls and stables — laminated timber arches over the arena
- Bridges — historic cast-iron and steel three-hinged bridges
Pros and cons
Advantages:
- Robust against settlement and temperature differences
- Easy to transport in two parts
- Quick to erect on site
Disadvantages:
- Requires more material than a built-in frame
- Hinges need careful detailing
- Not always the most economic for short spans
Comparison with other frame types
| Type | Statics | Advantage |
|---|---|---|
| Three-hinged arch | Determinate | Unaffected by settlement |
| Two-hinged arch | Indeterminate | Material saving |
| Fixed-end frame | Indeterminate | Stiffest |
| Trussed arch | Variable | Aesthetic |
Related terms
- Portal frame
- Arch
- Glued laminated timber
- Statically determinate
- Tie rod
- Pin joint
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