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Steel Column Design: |
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This design guide is intended to provide guidance for the safe and economical design
of steel columns. This design guide and the corresponding calculations are based
on the 14th edition of the AISC Steel Construction Manual. All calculations
can be performed for Load Resistance Factor Design, LRFD or Allowable Strength Design,
ASD.
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Steel Column Design Using The 14th Edition AISC Steel Construction Manual:
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Column Selection By Column Tables:
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Trial-and-error column selection is a difficult process. But, Part 4 of the
AISC Steel Construction Manual contains column selection tables that make it a much
simpler process, by enabling the designer to select a column based on the required
column capacity. The tables assume that buckling will occur first about the
minor axis. However, the process below includes a step for checking to insure
buckling does not occur about the major axis.
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1. From the required combination load equations for LRFD or ASD, determine the required
load on the column. The loading should be either factored loading for LRFD
or the unfactored loading for ASD. From the selected combination load calculate
the required design axial load on the column, Pu. Note:
the design loading on the column should also include the column's own weight
as part of the dead load.
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2. Determine the effective length factor for the major axis or x-axis, Kx and for
the minor axis or y-axis, Ky. To aid in selection of the
effective length factor, use the table below.
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End no. 1
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End no. 2
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Design K
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Built-in: rotation fixed,
translation fixed
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Built-in: rotation fixed,
translation fixed
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0.65
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Built-in: rotation fixed,
translation fixed
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Pinned: rotation free,
translation fixed
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0.80
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Built-in: rotation fixed,
translation fixed
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Rotation fixed,
translation free
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1.20
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Built-in: rotation fixed,
translation fixed
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Free: rotation free,
translation free
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2.10
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Pinned: rotation free,
translation fixed
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Pinned: rotation free,
translation fixed
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1.0
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Pinned: rotation free,
translation fixed
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Rotation fixed,
translation free
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2.0
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3. Calculate the effective length assuming that buckling will
occur about the minor axis: effective length = KyLy.
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4. Enter the table and select a column that will support the required load with
an effective length KL = KyLy.
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5. Calculate the effective length for buckling around the major axis, KxLx.
Now calculate the equivalent ratio effective length to be used in the table to check
major axis buckling capacity of the same beam. KxLx' = KxLx/(rx/ry)
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6. Enter the table and check the column capacity using effective length KL = KxLx'.
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7. If the column can support the load with KL = KxLx' then stop the column is
adequate. If the column cannot support the load then choose a larger column
that will support the load at KL = KxLx'.
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Analysis of an Existing Column:
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The steps below are for the analysis of an existing beam. An existing beam
can be analyzed quite easily with the selection tables in the AISC Steel Construction
Manual. Also the properties of the beam can be analyzed by calculation using
the "Specifications For Structural Steel Buildings" which is part of the
AISC Steel Construction Manual, and which contains the calculations upon which all
of the various column tables are based and the properties and strengths were calculated.
Or, a combination of calculation as well as reference to the tables in the manual
can be used.
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1. Determine the properties of the column that is being analyzed. Determine
the loading on the column. Through measurement etc., identify the column and
determine the column shape
properties, area (A), radius of gyration about the x-axis (rx) and the radius
of gyration about the y-axis (ry). Determine the unbraced lengths of lateral
bracing, of the x-axis (Lx) and of the y-axis (Ly).
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2. Determine the values of the effective length factors Kx and Ky. Use
the table below as required.
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End no. 1
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End no. 2
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Design K
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Built-in: rotation fixed,
translation fixed
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Built-in: rotation fixed,
translation fixed
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0.65
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Built-in: rotation fixed,
translation fixed
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Pinned: rotation free,
translation fixed
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0.80
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Built-in: rotation fixed,
translation fixed
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Rotation fixed,
translation free
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1.20
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Built-in: rotation fixed,
translation fixed
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Free: rotation free,
translation free
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2.10
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Pinned: rotation free,
translation fixed
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Pinned: rotation free,
translation fixed
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1.0
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Pinned: rotation free,
translation fixed
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Rotation fixed,
translation free
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2.0
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3. Using the effective length, KL = KyLy, enter the column load tables and
find the allowable load on the column (If analyzing by calculation go to step 5).
Then calculate the equivalent effective length for major axis buckling, KxLx'
= KxLx/(rx/ry). Enter the tables using the effective length, KL = KxLx'
and check the capacity of the column. The capacity of the column will be the
lower of the two values obtained.
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4. Compare the lower of the two values obtained in step 3 above to the loading
on the column. If the capacity of the column is greater than than the load
on the column, the column is adequate.
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5. If analyzing the column by calculation using the calculations found in
"Specifications For Structural Steel Buildings" of
the AISC Steel Construction Manual, first determine the required section properties
of the column, A, rx and ry respectively and then check the compactness of the column
using table B4.1. Calculations for the compression compactness of common shapes
are given below.
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6. Using the compactness of the beam choose the proper calculation and calculate
the strength of the column.
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7. Compare the calculated strength of the column with the loading on the column.
If the calculated capacity is greater than the actual column loading the column
is adequate.
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