Framing Systems

• Force transfer between elements: The reactions of one structural system correspond to equal and opposite loads on the systems that supports it.

• Load path: Load path is the sequence of transfers from one element to the next, from the point where a load is applied to the earth.

• Free body diagrams to isloate elements: Free body diagrams can be used to isolate a structural element in a larger system (such as a girder of a bridge) and examine the forces acting on it.

Supports and Equilibrium

• Common support types: There are three common types of supports defined in two dimensions: roller, pin, and fixed. They are categorized according the degrees of freedom that they restrain.

  • A roller restrains one axis of translation, allowing translation along the perpendicular axis, and allowing rotation.

  • A pin support restrains translation completely, allowing rotation.

  • A fixed support restrains translation and rotation.

Supports and Stability

• 3 DOF for rigid body in 2D. A rigid body in two dimensions has three degrees of freedom (DOF): two translations and one rotation.

• Stability for rigid body in 2D: If the supports for a rigid structure restrain less than 3 DOF, the structure is unstable.

• Parallel lines of restraint: If the lines of action of the support restraints are all parallel, the structure will be unstable: unrestrainted in translation (it can roll away).

• Concurrent lines of restraint: If the lines of action of the support restraints converge at a point, the structure will be unstable: unrestrained in rotation about that point.