| Racking: | (english) The distortion of a rectangular shape to a skewed parallelogram. |
| Reaction: | (english) A reaction is a force exerted by a support on an object: sometimes called support reaction. Using this definition, a reaction is an external force. |
| Resultant: | (english) The resultant of a system of forces is a single force or moment whose magnitude, direction, and location make it statically equivalent to the system of forces. |
| Rigid: | (english) An idealized concept meaning something which does not deform under loading. In fact, all objects deform under loading, but in modelling it can be useful to idealize very stiff objects as rigid. |
| Roller support: | (english) In two dimensions, a roller support restrains one translation degree of freedom. |
| Rotation: | (english) Motion of an object where the path of every point is a circle or circular arc. A rotation is defined by a point and vector which determine the axis of rotation. The direction of the vector is the direction of the axis and the magnitude of the vector is the angle of rotation. |
| Section Modulus: | (english) A property of a cross sectional shape, which depends on shape, and orientation. Section modulus is usually denoted S, and S = I/c, where I = moment of inertia about an axis through the centroid, and c is the distance from the centroid to the extreme edge of the section. |
| Shear: | (english) An system of internal forces whose resultant is a force acting perpendicular to the longitudinal axis of a structural member or assembly: sometimes called shear force. |
| Shear strain: | (english) Strain measuring the intensity of racking in the material. Shear strain is measured as the change in angle of the corners of a small square of material. |
| Shear modulus: | (english) The ratio of shear stress divided by the corresponding shear strain in a linear elastic material. |