The internal forces of each member of a structural system are distributed in a particular way across that member's cross-sectional area. Stress is a measure of the intensity of this force on a single unit of area. Common units of measure of stress are KN/m2, N/mm2, #/in2, #/ft2, or kips/in2. The distribution of the stress may be constant across a cross-sectional area, or it may be variable. It can vary due to the loading conditions, the material or geometry of the structural member.
Structural members are subjected to a variety of types of stress. They can be divided into primary and secondary stresses. Primary stresses are Compression, Tension, Torsion and Shear. Secondary stresses are the result of deformations of the structural system, or member, which cause additional loads within each member to occur (For example, a beam expanding in the summer sun pushing on a fixed column). These stresses will be discussed in more detail in a later lecture. Initially, the focus will be on simple tension and compression stresses for members loaded symmetrically along an axis, which causes uniformly distributed internal stress.
The Pirelli Office Block, designed by Ponti and engineered by Nervi, is a wonderful example of a structure in which the columns express not only the relationship between the moment and the building, but of the relationship between direct stress and size as well. The stress in the column was intended to remain as constant as possible.
Stress/Area Relationship
Compressive Stress
A stress prism is a diagram which represents the stress distribution across the section of a loaded member. The volume of the stress prism is equal to the total force acting on the cross-section.
Drawing a Stress Prism
More Stress Prisms