Column Structural Members

• Columns are structural members that support axial compressive loads, with cross-sectional dimensions less than their height.
• Columns support vertical loads from floors, roofs, and transmit them to the foundations.
• Columns are a special case of a compression member that is vertical.
• A slender or long column is weakened by moments induced by slenderness.
• Short columns are stocky enough that slenderness can be ignored.

Types of Columns

• Columns can be classified based on slenderness ratio, position of loads, bracing, cross-section shape, and type of reinforcement including being short or long.
• Slenderness Ratio: Defines the column as short, subject to crushing failure or long, subject to buckling failure
• Position of Loads: Defines the column as axially loaded (concentric), uniaxially, or biaxially loaded
• Bracing: Defines the column as being unbraced/sway, or braced/non-sway
• Cross Section/Shape: Defines the column as rectangular, square, circular, L shaped etc.
• Type of Reinforcement: Defines the column as being tied, spiral or composite

Position of Loads

• All columns are subjected to a bending moment (BM) on one or both axes, which may be due to end restraint from monolithic placement of floor beams and columns.
• Accidental eccentricity from imperfect alignment or floor load imbalances/lateral loads will also cause bending moment
• Axially loaded columns have load (BM) on the axis.
• Uniaxially loaded columns have load (BM) on one axis.
• Biaxially loaded columns have load (BM) on both axes.

Bracing

• Braced columns receive lateral stability from walls/bracing, whereas unbraced ones rely solely on columns.
• Braced columns aren't designed to withstand lateral loads; braced frames resist lateral loads (wind, earthquake) through shear walls/bracing.
• Sidesway/joint translation is prevented in braced columns, but happens in unbraced ones, where steel structures are often designed using the bracing method.
• Most reinforced concrete (RCC) structures are designed using the unbraced method.
• Braced columns better resist earthquakes.
• It is hard to provide an opening for braced columns but easy for unbraced.

Reinforcement in Columns

• Lateral reinforcement in columns, like ties or spirals, maintains the longitudinal bars' position while concrete is placed.
• It prevents longitudinal reinforcement buckling by containing the concrete cover.

Spiral vs. Tied Columns

• In a spiral column, when the shell spalls off the core has strength stemming from triaxial stress.
• As a result, a spiral column could undergo large deformation, and reach a second maximum load.
• In tied columns When the concrete core crushes, reinforcement buckles outward between ties, and this typically occurs suddenly, without warning.
• Spiral columns are more ductile, give warning of impending failure, and allow load redistribution but occurs at very high strains and corresponds to a shortening of about 1 inch in an 8 foot high column.
• When spiral columns are eccentrically loaded, the second maximum load might fall below the initial maximum.
• Spiral columns have a strength-reduction factor of 0.75.
• Tied columns have a strength-reduction factor of 0.65.

Longitudinal Reinforcement

• ACI Code Section 10.6.1.1 limits the area, Ast, of longitudinal reinforcement in tied and spiral columns to not less than 0.01 times the gross area, Ag.
• The pg = Ast/Agmust be not less than 0.01 besides as allowed in ACI Code Section 10.3.1.2.
• It must not be more than 0.08Ag (0.06Ag in columns of special moment frames designed to resist earthquake forces).
• Creep transfers load from the concrete to the reinforcement under sustained loads.
• Creep and the shrinkage of modern concretes on the transfer of vertical compression stresses from the concrete to the longitudinal (vertical) bars uphold the lower limit of 0.01.
• Although the code allows a maximum steel ratio of 0.08, it is generally very difficult to place this amount of steel in a column when particularly lapped splices are used.
• The minimum number of bars in a rectangular column is four, while in a circular/spiral column, it is six (ACI Code Section 10.7.3.1).
• Concrete cover for reinforcement shall follow section 20.6.1

Ties and Spirals

• Ties shall consist of a closed loop of deformed bar with spacing.
• Clear spacing must be at least (4/3) * dagg, where dagg is the maximum size of the coarse aggregate
• Center-to-center spacing shouldn't exceed 16db of longitudinal bar, 48db of tie bar, or the member's smallest dimension.
• Minimum diameter of the tie bar shall be No. 3 enclosing No. 10 or smaller longitudinal bars and No. 4 enclosing No. 11 or larger longitudinal bars or bundled longitudinal bars
• Spirals consist of continuous bars/wire with even spacing, the clear spacing conforming to these requirements :
  • At least the greater of 1 in. and (4/3)dagg
  • Not greater than 3 in
• Spiral bar/wire diameter for cast-in-place construction must be 3/8 in.
• The amount of spiral reinforcement is defined by using a spiral reinforcement ratio, ps, equal to the ratio of the volume of the which may be expressed as or
• Rectilinear reinforcement must satisfy these arrangements:
  • Every corner and alternate longitudinal bar shall have lateral support provided by the corner of a tie with an included angle of not more than 135 degrees
  • No unsupported bar shall be farther than 6 in. clear on each side along the tie from a laterally supported bar

Axially Loaded Columns

• The steel drum filled with sand analogy, demonstrates effect of lateral pressure is exerted by the sand, this causes hoop tension.
• If the pile is not confined, like in the drum, it is not able to support any load.
• Strength effect of a spiral can be visualized by considering that confinement increases the load carrying capacity of column

Account for the Effect of Accidental Moments

• ACI Code Sections 10.3.6.1 and 10.3.6.2 specify that the maximum load on a column must not exceed 0.85 times the load from the ACI formula for spiral columns.
• Load on a column must not Exceed 0.8 times the load from the ACI formula for tied columns

Eccentrically Loaded Columns

• Columns can be classified in terms of equivalent eccentricity (if bi-axially loaded).
• Large eccentricity columns are subject to tension, which causes tensile yielding of steel which requires strain.
• Small eccentricities will lead to failures stemming from crushing of concrete accompanied by yielding of steel on the more heavily loaded side.
• Balanced eccentricity refers to one specific length that leads to failures by simultaneous yielding of concrete or tensile steel which leads to crushing.

Interaction Diagram

• Different combinations of axial load and moment create failures.

Column Interaction Diagram Points

• Point A represents pure axial load, referring to uniform axial compression without moment. It is the column's largest axial load.
• Point B represents the zero tension, the onset of cracking and tensile stresses in concrete are ignored.
• Region A-C represents compression-controlled failures stemming from crushing before extreme tensile layer reinforcement yields. They are called compression-controlled columns
• Point C represents balanced failure from the maximum compressive strain corresponding to the tensile strain in the reinforcement.
• Point D, the tension-controlled limit corresponds to the strain distribution with .003 compressive strain on the top and .005 tensile strain on the tensile layer of steel.
• Region C-D is the transition region in moments and loads.
• Region D-E is tension-controlled which experiences tensile yielding.

Choice of Column

• Use spiral columns for eccentricity ratios less than 0.1 because spiral columns have better load capacity.
• A column using ties with bars farthest from the axis of bending is most efficient with greater eccentricity ratios.
• Columns using ties with bars in faces are appropriate when both moments exist on both axes, but spiral columns are infrequently used in non seismic locations.

Column Size Estimate

• There are formulas (Equations 11-19a, 11-19b) for estimating the required size of a column which accounts for spiral and tied columns
• These may underestimate if there are moments present