Strut Services Group, Inc.

Unistrut Alabama

Column Loading Information

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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:

  • the length of column,
  • the type of loading,
  • the support conditions, and
  • 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 Unistrut Metal Framing 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 – FIXED TOP – FIXED BOTTOM

Both ends are restrained against rotation and lateral movement (translation).

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

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.


Installation Torque

Bolt torque values are given to ensure the proper connection between Unistrut Metal Framing components.  It is important to understand that there is a direct, but not necessarily consistent, relationship between bolt torque and tension in the bolt. Too much tension in the bolt can cause it to break or crush the component parts.  Too little tension in the bolt can prevent the connection from developing its full load capacity.  The torque values given have been developed over many years of experience and testing.

 

Installation Torque
BOLT SIZE

1/4″-20

3/16″-18

3/8″-16

1/2″-13

3/8″-11

3/4″-10

Recommended Torque Ft-Lbs (N-m)

6  (8)

11  (15)

19  (26)

50  (68)

100  (136)

125  (170)

Max Torque Ft-Lbs (N-m)

7  (9)

15  (20)

25  (34)

70  (95)

125  (170)

135  (183)

These are based on using a properly calibrated torque wrench with a clean dry (non-lubricated) Unistrut fitting, bolt and nut. A lubricated bolt or nut can cause extremely high tension in the connection and may lead to bolt failure. It must be noted that the accuracy of commercial torque wrenches varies widely and it is the responsibility of the installer to ensure that proper bolt torque has been achieved.

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