Design of Compression Members

Vocabulary practice flashcards covering terminology, failure modes, formulas, and structural detailing requirements for steel compression members in limit state design.

Compression Member

A straight structural member subjected to two equal and opposite compressive forces applied at its ends.

Ideal Compression Member

A theoretical compression member that is perfectly straight, free of geometric imperfections or initial crookedness, and loaded concentrically such that the center of gravity of the load coincides with the center of gravity of the member cross section.

Column (Stanchion or Post)

A vertical compression member used in buildings to support floors or girders, typically designed to resist heavy compressive loads.

Strut

A relatively small-span and lightly loaded compression member commonly used in roof trusses and bracing systems.

Continuous Strut

A compression member that extends continuously over multiple joints, such as a top chord member of a truss bridge girder or the principal rafter of a roof truss.

Discontinuous Strut

A compression member that extends between two adjacent joints only, such as the vertical or inclined web members in a roof truss.

Principal Rafter

The top chord compression member in a roof truss.

Boom

The principal compression member in a crane structure.

Effective Length ($KL$)

The distance between the points of contraflexure in a buckled compression member, obtained by multiplying the actual unsupported length ($L$) by the effective length factor ($K$).

Stability Bracing

Bracing provided in a structural frame to stabilize the structure against buckling or instability under the action of vertical gravity loads.

Non-Sway Frame

A framed structure in which lateral translation is prevented, or one where lateral sway is sufficiently small such that second-order $P\text{--}\triangle$ effects are negligible.

Inter-Storey Drift

The relative difference in lateral deflection between the top and bottom ends of a column within a specific storey, used to evaluate lateral frame stiffness.

Slenderness Ratio ($\lambda$)

The ratio of the effective length of a compression member to the appropriate radius of gyration of its cross section ($\lambda = \frac{KL}{r}$).

Unstiffened Element

A constituent cross-sectional plate element that is supported along only one edge parallel to the direction of compressive stress.

Stiffened Element

A constituent cross-sectional plate element that is supported along both parallel edges in the direction of the compressive stress.

Local Buckling

A failure mode characterized by isolated wrinkling or buckling of thin constituent plate elements (flange or web) before overall column buckling occurs.

Flexural Buckling

An overall column buckling mode where the member fails by lateral deflection and bending about the axis having the largest slenderness ratio (least radius of gyration).

Torsional Buckling

A buckling mode in which a compression member fails by twisting about its longitudinal axis, occurring when the torsional rigidity is substantially less than its flexural rigidity.

Flexural-Torsional Buckling

A coupled failure mode in which an axially loaded compression member bends and twists simultaneously, characteristic of unsymmetrical or singly symmetrical cross sections.

Euler Critical Load ($P_{cr}$)

The theoretical elastic buckling load for an ideal, pin-ended column, given by $P_{cr} = \frac{\pi^2 EI}{L^2}$, which depends solely on bending stiffness and length rather than steel yield strength.

Squash Load ($P_y$)

The ultimate plastic compressive capacity of a short, stocky column given by $P_y = f_y A$, where the entire cross section yields without prior buckling.

Tangent Modulus ($E_t$)

The instantaneous slope of the material stress-strain curve at a critical stress level exceeding the proportional limit, utilized in calculating inelastic buckling strength via $P_{cr} = \frac{\pi^2 E_t I}{L^2}$.

Residual Stresses ($f_r$)

Locked-in self-equilibrating internal stresses formed in structural steel members primarily due to non-uniform cooling during rolling or welding operations.

Imperfection Factor ($\alpha$)

A parameter in IS: 800–2007 (values: $0.21$, $0.34$, $0.49$, and $0.76$ for curves $a$, $b$, $c$, and $d$) that accounts for cross-sectional geometry, fabrication method, and initial out-of-straightness.

Stress Reduction Factor ($\chi$)

A non-dimensional factor applied to yield strength to determine design compressive stress, defined as $\chi = \frac{1}{\phi + \sqrt{\phi^2 - \bar{\lambda}^2}}$.

Latticed Column

A built-up compression member composed of two or more distinct structural shapes held at a specified spacing by lacing bars or batten plates to act as a unified section.

Component Column

An individual longitudinal main segment of length $a_1$ between two consecutive lattice or batten connections in a built-up column.

Lacing System

A system of inclined flat or angle bars oriented at $40^\circ$ to $70^\circ$ to the column axis, designed to transfer transverse shear and hold component shapes parallel and equidistant.

Batten Plate

A transverse plate or rolled shape connected across the open faces of a built-up column perpendicular to the ribs, enabling the member to resist transverse shear as a Vierendeel frame.

Transverse Shear ($V_t$)

The design shear force perpendicular to the longitudinal column axis, taken as $2.5\%$ of the factored axial load, which must be resisted by the lacing or battening system.

Tie Plate

A plate provided at the extreme ends of laced or battened columns to prevent distortion of the open cross section caused by unbalanced lateral forces.

Column Splice

A structural connection provided along the height of a column to join two sections together due to length limitations, erection requirements, or transitions in section sizes.

Encased Column

A structural steel compression member surrounded by dense concrete of at least M-15 grade with nominal stirrup reinforcement to enhance fireproofing, prevent corrosion, and increase load capacity.

Non-Dimensional Effective Slenderness Ratio ($\bar{\lambda}$)

The normalized slenderness parameter defined as $\bar{\lambda} = \sqrt{\frac{f_y}{f_{cc}}} = \sqrt{\frac{f_y (KL/r)^2}{\pi^2 E}}$, representing the transition between plastic yielding and elastic buckling.