Connections

General

• The terms connection and support are closely related, and are often used interchangeably in informal discussion.

• In formal modelling, connection refers to force-displacement constraints among members in a model, while support refers to force-displacement constraints between the model and objects outside the model.

• When a connection allows a particular type of motion, it does not exert the corresponding internal force.

  • A connection that allows relative rotation between two members exerts no internal moment. When a free body diagram is cut through that point, the internal moment is known to be zero.

  • A connection that allows axial movement (e.g. an expansion joint in a bridge) exerts no axial force.

Articulation

• Connections can be used to manipulate the distribution of internal forces in a structure.

• By releasing internal force quantities (as a hinge or pin connection releases moment) connections can make an indeterminate structure determinate (as with the continuous beam example).

• As with supports, many connections are not close to extreme ideals such as pinned and moment-resisting, but lie in-between.

Eccentricity

• One of the key concepts in designing structural connections is avoiding accidental eccentricity.

  • Eccentricity refers to a distance offset between balancing forces, resulting a moment equal to the force magnitude times the distance.

• In structural frameworks there are two basic strategies for avoiding eccentricity:

  • Organize about a plane of symmetry: Use the "sandwich principle" combining single and double members and connectors so that all centerlines lie within a plane of symmetry.

    

  • Converge member centerlines: When several members join at a connection, arrange them so that their centerlines converge at a common point.