How do structures carry loads?
Consider again the example of the load path when you sit on a chair.
- The load (your weight) acts on the seat of the chair the seat carries the load by bending.
- The seat carries the load to the chair legs the legs carry the load by axially compressing.
- The legs push down on the floor which transfers the load to the walls the floor carries the load by bending.
- The walls carry the load to the footings the walls carry the load by axially compressing.
- The footings carry the load to the earth the footings and earth carry the load by compressing.
There are four load paths – two which involve bending and two which involve axial compression. Bending and axial compression are called the structural actions. All load paths can be described as one of four types:
All structures on earth can be considered to carry their loads in a combination of these four ways.
1. When the load acts directly along the axis of the members of a structure, a structure carries its load by shortening or lengthening – either in tension (pulling) or in compression (pushing). Tensile axial loads cause the member to lengthen, compressive axial loads cause the member to shorten.
2. When the load causes an applied moment about an axis perpendicular to the length of the member, a structure carries its load by curving. Moments can be applied directly to a member, or more commonly they occur when loads act transversely to the member. They cause the members to curve, and they set up internal bending moments in the member.
3. When the load acts transversely to the members of a structure, a structure carries its load by distorting. This sets up internal shear forces in the member. As we shall see, shear forces and bending moments are very closely related.
4. When the load causes an applied moment about the axis of the members of a structure, a structure carries its load by twisting. This type of applied moment will be called a torque. This structural action will not be dealt with in this unit.