# COLUMN BUCKLING CALCULATOR

Column buckling calculator for buckling analysis of compression members (columns).

When a structural member is subjected to a compressive axial force, it's referred as a compression member or a column. Compression members are found as columns in buildings, piers in bridges, top chords of trusses. They transmit weight of an object above it to a lower one. During this transmission, they are compressed.

If a long slender bar is loaded, it will bend and buckle before it yields. Due to the sudden nature of the buckling, special attention is needed during design. This failure mode is different  from the yield or fatigue and named as instability.

Compression members are classified according to slenderness ratio and loading type (central or eccentric loading) and analyze methods for each category is different. Columns categorization, formulas for the determination of critical loads for different categories and loading types can be summarized as follows;

According to design requirements, different end conditions such as fixed, guided, pinned and free end can be used in compression members. Effect of end conditions shall be taken into account for the determination of critical loads for compression member.

 END CONDITIONS

### Column Buckling Calculator:

 INPUT PARAMETERS Parameter Value Column length [L] mm m inch ft Cross-sectional area [A] mm^2 cm^2 inch^2 ft^2 Second moment of area [I] mm^4 cm^4 inch^4 ft^4 Distance to the neutral axis [c] mm m inch ft Eccentricity [e] mm m inch ft Design factor [nd]** Modulus of Elasticity [E] GPa ksi Yield strength [Sy] MPa psi Compressive yield strength [Syc]* Applied Force [P] N kN lbf End conditions Free-Free Pinned-Free Pinned-Pinned Guided-Free Guided-Pinned Guided-Guided Fixed-Free Fixed-Pinned Fixed-Guided Fixed-Fixed

Note 1: Use dot "." as decimal separator.

Note 2: * If not known, use same value with yield strength

Note 3: ** Appropriate design factor shall be selected to cover material nonuniformity, uncertainty of service conditions, calculation and analysis inaccuracy etc.

 RESULTS Parameter Value Effective length constant [C] * 1.2 Radius of gyration of column [r] --- mm m inch ft Slenderness ratio of column [S] --- Effective slenderness ratio of column [Seff] --- Critical load for failure [Fc] --- N kN lbf Allowable load(includes nd) [Fa] --- Factor of safety [fos]** --- Column Category

Note 1: * Suggested values per manual of steel construction.

Note 2: ** Shall be larger than design factor. Green color means safe, red color means not safe according to input parameters.

### Definitions:

Allowable stress: If a calculated maximum stress of a member for the expected service conditions is less than a certain value and if this certain value has a proper margin against failure stress, then this value is called allowable stress. The allowable stress shall be less than failure stress with some margin because of uncertainty of the conditions of service, nonuniformity of material, and inaccuracy of stress analysis. In other words, allowable stress is a maximum load which can be safely placed on a structure.

Buckling: Sudden failure of a structural member subjected to compressive stress, where the compressive stress at the point of failure is less than the ultimate compressive strength of the material. The failure is due to the instability of the structure.

Failure stress: Loss of function stress for the design. Yield strength, proof strength, ultimate tensile strength, critical load for instability (buckling) can be selected for failure stress but this selection depends design, material in use and type of analysis that will performed.

Design factor (nd):The ratio of failure stress to allowable stress. The design factor is what the item is required to withstand .The design factor is defined for an application (generally provided in advance and often set by regulatory code or policy) and is not an actual calculation.

Eccentricity: If a load act on a member with some offset to the centroid of a member, it’s called eccentric loading. The perpendicular distance from the line of action of the load to either principal central axis is the eccentricity with respect to that axis.

Effective length : The effective length of a column is the distance between successive points which has zero moment.

Effective slenderness ratio: The ratio of effective length of column to the radius of gyration both with respect to the same axis of bending. The ratio is used as a means of assessing the stability of the element. Higher slenderness ratio results lower failure loads.

Factor of Safety (Safety Factor): The ratio of failure stress to actual/expected stress. The difference between the factor of safety (safety factor) and design factor is: The factor of safety gives the safety margin of designed part against failure. The design factor gives the requirement value for the design. Safety factor shall be greater than or equal to design factor.

Modulus of elasticity (Young’s modulus): The rate of change of unit tensile or compressive stress with respect to unit tensile or compressive strain for the condition of uniaxial stress within the proportional limit. Typical values: Aluminum: 69 GPa, Steel: 200GPa.

Neutral axis: The line of zero fiber stress in any given section of a member subject to bending; it is the line formed by the intersection of the neutral surface and the section.

Neutral surface: The longitudinal surface of zero fiber stress in a member subject to bending, it contains the neutral axis of every section.

Proportional Limit: The largest value of stress up to which a linear relation still exist between stress and strain (Hooke’s Law).

Radius of Gyration (Area): The distance from an axis at which the area of a body may be assumed to be concentrated and the second moment area of this configuration equal to the second moment area of the actual body about the same axis.

Stability: The structure's ability to support a given load without experiencing a sudden change in its configuration.

Yield strength: The stress at which a material exhibits a specified permanent deformation or set. Example: Al6061-T6: 145 MPa

### Supplements:

 Link Usage Sectional Properties Calculator of Profiles Sectional properties needed for the compression/buckling analysis can be calculated with sectional properties calculator.

### List of Equations:

Column Buckling Formulas and Calculation Steps

### Examples:

 Link Usage Compression of Round Column An example about the calculation of allowable load on a compression member. Sectional properties of the member are also calculated. Eccentric Compression Load On I-Beam An example about the calculation of factor of safety of a compression member which is loaded eccentrically. Sectional properties of the member are also calculated.