“In-plane” describes forces, loads or stiffness acting within the flat plane of a wall, floor or panel — as opposed to “out-of-plane”, where the load acts perpendicular to that plane. It is an important concept when assessing the strength and stability of a building.
In-plane versus out-of-plane
A wall, floor or panel behaves very differently depending on the direction of the load:
- In-plane: the element is usually very strong and stiff. A flat plate can carry large forces within its own plane without deforming significantly. This is known as diaphragm action.
- Out-of-plane: perpendicular to the plane, the same element is weak. A thin wall bends much more easily under a perpendicular load.
Diaphragm action in the structure
The high in-plane stiffness is used deliberately in structures to absorb horizontal forces and channel them down to the foundation. Think of loads from:
- wind
- earthquakes
Floors and shear walls then act as stiff diaphragms. The floors collect the horizontal forces and pass them to the shear walls, which in turn carry them down to the foundation. This distinction between in-plane and out-of-plane strength is essential when designing the stability of a building.
Related terms
- Structure
- Load
- Load-bearing wall
- Load-bearing floor
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