I. Introduction
According to ACI Code 2.2, a structural element with a ratio of height-to-least lateral dimension exceeding three used primarily to support compressive loads is defined as column. Columns support vertical loads from the floor and roof slabs and transfer these loads to the footings. Columns usually support compressive loads with or without bending. Depending on the magnitude of the bending moment and the axial force, column behavior will vary from pure beamaction to pure column action. Columns are classified as short or long depending on their slenderness ratios. Short columns usually fail when their materials are overstressed and long columns usually fail due to buckling which produces secondary moments resulting from the P - D effect. Columns are classified according to the way they are reinforced into tied and spirally reinforced columns. Columns are usually reinforced with longitudinal and transverse reinforcement. When this transverse reinforcement is in the form of ties, the column is called “tied”. If the transverse reinforcement is in the form of helical hoops, the column is called “spirally reinforced”. Since failure of columns often cause extensive damage, they are designed with a higher factor of safety than beams. II. Types of Columns 1. Tied Column A tied column, shown in Figure 1, is a column in which the longitudinal reinforcement bars are tied together with separate smaller diameter transverse bars (ties) spaced at some interval along the column height. These ties help to hold the longitudinal reinforcement bars in place during construction and ensure stability of these bars against local buckling. The cross sections of such columns are usually square, rectangular, or circular in shape. A minimum of four bars is used in rectangular and circular cross sections.  2. Spirally-Reinforced Column They are columns in which the longitudinal bars are arranged in a circle surrounded by a closely spaced continuous spiral, shown in Figure 2. These columns are usually circular or square in shape. A minimum of six bars is used for longitudinal reinforcement.  3. Composite Column A composite column is a column made of structural steel shapes or pipes surrounded by or filled by concrete with or without longitudinal reinforcement, shown in Figure 3.  Behavior of Tied and Spirally-Reinforced Columns Axial loading tests have proven that tied and spirally reinforced columns having the same crosssectional areas of concrete and steel reinforcement behave in the same manner up to the ultimate load, as shown in Figure 4.a. At that load tied columns fail suddenly due to excessive cracking in the concrete section followed by buckling of the longitudinal reinforcement between ties within the failure region, as shown in Figure 4.b.   Reference: Click here |
