Compression Members

Compression Members: Structural elements subjected only to axial compressive forces Stress:Uniform over entire cross section Loads are applied along a longitudinal axis through the centroid In reality: (a) Secondary Bending due to Load Eccentricities - Ignore (b) Computed Bending Moment - Treat as Beam Column

Types of Compression Members Columns Vertical Members that support vertical loads Trusses and Bracing Systems Compressive Forces Struts Smaller compression members not classified as columns

Compression Members Compression members are susceptible to BUCKLING BUCKLING – Loss of stability –Axial loads cause lateral deformations (bending-like deformations) P is applied slowly P increases Member becomes unstable - buckles

Column Theory Axial force that causes Buckling is called Critical Load and is associated to the column strength P cr depends on Length of member Material Properties Section Properties

Let’s Have Some Fun

Column Theory - Euler Buckling

Elastic Buckling

Compression Members – Stress-Strain Behavior Tension Test Data

Compression Members – Stress-Strain Behavior Coupon tests different than short column tests

Compression Members – Stress-Strain Behavior Rolled Shapes

Compression Members – Stress-Strain Behavior Rolled Shapes Tips of flanges and web cool down first and shorten Hot liquid areas follow

Compression Members – Stress-Strain Behavior Rolled Shapes Tips of flanges and web have solidified Hot liquid areas cool down and shorten further Solidified flanges resist shortening Residual Compressive stresses in flanges Residual tensile stresses in remaining Residual stresses ksi

Compression Members – Stress-Strain Behavior

Column Strength Consider a Long Slender Column Material Behavior ELASTIC BUCKLING

Column Strength Consider a Shorter Column Material Behavior INELASTIC BUCKLING

Column Theory - Very Short Columns Consider a Very Short Column PLASTIC BEHAVIOR Yielding takes place before any instabilities

Column Theory - Column Strength Curve