Modular Support Systems Technical Guide, Edition 2
Allowable loads corresponding to deflection limits in this technical guide are based on“ simply-supported” and“ uniformly loaded” conditions. See page 36 for modification factors to be applied to allowable load values for other conditions.
Column Design Fundamentals
Columns are structural elements that resist applied loads primarily in axial compression along their length.
A column’ s capacity is based on its unbraced length( noted as“ height” in load tables), support conditions, cross-sectional properties, material properties, and load location( i. e. whether applied at the column’ s center of gravity or at the face of the profile).
Column Unbraced Length
Such a load induces a bending moment in the column at the point of load application and, consequently, results in a lower allowable column load compared to a concentrically loaded condition. Columns with higher slenderness ratios are particularly sensitive to eccentrically applied axial loads.
Column Support Conditions
A column’ s allowable load capacity depends considerably on its support( end) conditions. Variations in support conditions for a column are addressed via an Effective Length Factor, K, which modifies a non-pinned-end column’ s unbraced height to represent that of an equivalent pinned-end column. Accordingly, the K value for a pinned-end column is 1.0. See the figure below for applicable K values to be used for certain column support conditions.
The unbraced length of a column represents the distance between braced locations. Braced locations along a column are restrained against lateral movement( perpendicular to the length of the column). The larger the unbraced height of a column, its ability to resist applied loads is decreased. The tendency for a column to buckle about an axis under consideration is highly dependent on its slenderness ratio, KL / r, where K is the effective length factor( explained to the right), L is the unbraced length of the column, and r is the radius of gyration for the cross section. The AISI S100-16 / CSA S136-16 Specification recommends that such ratios not exceed 200 for compression members. Buckling must be considered in both principal directions for a column, with the smaller buckling load controlling the design. Remember that for members that are not doubly symmetric( e. g channels), the radius of gyration will be different for each principal direction.
Column Loading- Concentric
A load applied at the center of gravity, C. G., of a column cross section is considered concentric. Such a load does not induce a bending moment in the column at the point of load application.
Column Loading- Eccentric
A load applied away from the center of gravity, C. G., of a column cross section is considered eccentric( e. g. a load at the slotted face of an MT channel).
Fixed-end condition. Both ends of column are restrained against translation( lateral movement) and rotation.
Top end of column is restrained against translation but permitted to rotate. Bottom of column is restrained against translation and rotation.
Pinned-end condition. Both ends of column are restrained against translation but are allowed to rotate.
Top end of column is restrained against rotation but allowed to translate( move laterally). Bottom end of column is restrained against translation and rotation.
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