Parts of an Arch- Part 4 | Building Construction |PART 4

The intrados is the inner curve – that’s the part you see when you look up at the arch from underneath. The extrados is the outer curve on top – it’s where the pressure and load travel before moving to the supports.
In short:
👉 Intrados = inside curve
👉 Extrados = outer curve
These curves aren't just shapes – they control how the arch holds weight and stays strong.

🧱 1. Load Path Understanding
The extrados is where external loads like weight or pressure act first. It helps engineers visualize how forces move through the arch. The intrados shows how those forces are transferred underneath — often where tension or stress might occur.

📐 2. Design and Detailing
In design drawings, these terms define exact curves and allow accurate calculation of thrust lines, centers of curvature, and radius. For structural analysis, identifying these curves helps determine where reinforcements or extra support might be needed.

🏗️ 3. Construction Execution
Builders use intrados/extrados to set formwork and centering correctly. Especially important in arches with variable thickness or decorative features, so they know what face to align to.

🎨 4. Aesthetic and Architectural Clarity
Architects refer to the intrados for visible interior finishes (like carvings or tiles). Extrados is used for external weathering protection and surface treatments.

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