A haunched portal frame — Dutch kniespant — is a frame in which the vertical column and the sloping rafter form one continuous member, joined at an angled, rigid connection known as the knee or haunch. Because that connection carries moment, the frame is stable in its own plane without extra bracing in the roof slope.
The bases can be pinned — combined with a pin at the apex this gives a three-pinned frame — or fixed into the foundation. That choice decides the force path: fixed bases pass a moment into the foundation, pinned bases do not.
Materials
- Steel — rolled sections with a stiffened haunch, by far the most common form
- Reinforced concrete — precast or cast in situ, with reinforcement running continuously through the knee
- Timber and glulam — the knee is made as a glued finger-jointed or laminated connection, or with steel gusset plates and dowels
Where it is used
- Sheds, warehouses and livestock buildings with a clear span and a pitched roof
- Industrial buildings where internal columns are unwanted
- Canopies and carports at smaller scale, often in timber
Practical tips
- The knee governs the design — bending moment peaks there; the haunch, weld or glue line at that point is structurally essential, not decoration
- Watch the horizontal thrust at the base — a portal pushes its feet apart; the foundation or a tie must take that thrust
- Always have the frame designed by an engineer to the Eurocodes; for light structures the snow-plus-wind combination is usually critical
- Stability across the frames does not come free — that needs wind bracing or stiff wall and roof diaphragms
- Detail corrosion protection at the knee of a steel frame carefully: it collects moisture and dirt in its many angles
Related terms
- Portal frame
- Truss
- Three-pinned frame
- Roof truss
- Glulam
- Eurocode
- Gable roof
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