Page 23 - ZF-Catalog-ZSi-Foster-08-19-R3

Column - Design Fundamentals
Columns are structural members that are loaded parallel to their length. Most columns are vertical and are used to carry loads from a higher level to a lower level. However, any member subjected to compression loads, such as a diagonal or prop brace, is a column.
A column fails by “buckling”, which is a sudden loss of straightness and subsequent collapse. Allowable column load is dependent on:
(a) The length of column
(b) The type of loading
(c) The support conditions
(d) The column’s cross-sectional shape and material.
Column Length
The column length is measured from braced point to braced point. A braced point is where the column is restrained from lateral movement (translation) in all directions.
Column Loading – Concentric Loading
Loads applied to the center of gravity of the column cross-section are considered concentric. A beam that passes over and rests on the top of a column is an example of concentric loading.
Column Loading – Eccentric Loading
Any load which is not concentric is eccentric. The amount of eccentricity (in inches) has a major effect on the load-carrying capacity of any particular column. A load that is transmitted to a column using a standard fitting bolted to the slot face, is considered eccentric.
The load tables give allowable loads for both concentric (loaded at C G ) and certain eccentric (loaded at slot face) loading. Allowable loads for other eccentric loading must be determined by a qualified design professional.
Support Conditions
Based on the support conditions, an appropriate "K" value is selected. This “K” value, which mathematically describes the column end conditions, is used in the column design equations. The most common support condition combinations are as follows:
Support Conditions - Pinned Top –
Fixed Bottom
The top is restrained against lateral movement (translation) but is allowed to rotate. The bottom is restrained against rotation and lateral movement.
This is a common support condition and is used to construct the allowable column load applied at the Slot Face tables.
Support Conditions - Pinned Top –
Pinned Bottom
Both ends are restrained against lateral movement (translation) but, are allowed to rotate.
Support Conditions - Fixed / Free Top –
Fixed Bottom
The top is restrained against rotation, but is allowed to move laterally. The bottom is restrained against rotation and lateral movement (translation).
Cross-Sectional Shape
K=0.80
Support Conditions - Fixed Top –
Fixed Bottom
Both ends are restrained against rotation and lateral movement (translation).
The cross-sectional shape of a column member determines the value of its “Radius of Gyration” or "r". In general, a member with a large "r" makes a better column than a member with a small "r". Each axis of a column has a different "r". Typically the axis with the smallest "r" determines the final design.
K=1.00
K=1.2
K=0.65
ZSi-Foster Engineering Catalog
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