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Steel Tension Member 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 Tension Member Design Using The 14th Edition AISC Steel Construction Manual:
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Design of Steel Tension Members:
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Trial-and-error tension member selection is a difficult process. But, Part
5 of the AISC Steel Construction Manual contains tension member selection tables
that make it a much simpler process, by enabling the designer to select a member
based on the required axial tension capacity. However, the tables give the
capacity of the member and the capacity of the member if it is connected using a
fully welded section. If any other design is used for the connection the capacity
of the member must still be calculated based on the connection used on the member.
This process is described below.
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1. From the tables choose a tension member that has a tensile yielding capacity
slightly larger than the required axial tensile strength.
Alternatively calculate the
required gross area for tensile yielding and choose a member whose section
area is equal to or larger than the area required.
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2. Decide on the type of connection that will be used, if a fully welded end section
will be used, the selection table has the rupture capacity for the tension member
if each of its elements were weld connected to carry the tension load. In
this case select the member which has both a tensile yield strength and a tensile
rupture strength larger than the required tensile strength. At this
point the chosen member will be adequate if using a fully welded section connection.
If not go to step 3.
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3. Calculate the net area, Ant, of the cross section of the member
at the connection. See AISC D3 for how to determine the net area. The
net area, An, of a member is the sum of the products of the thickness and the net
width of each element.
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4. Calculate the shear lag factor, U = 1 - x/L. Where x is the distance from the plane of the
connection to the centroid of the section and L is the length of the connection (distance between outermost bolts).
Or alternatively enter table D3.1 and determine the shear lag factor, U, for the connection to
be used. Some of the common shear lag factors are given in the table below:
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Shear Lag Factors, U, For Common Connections
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Description
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Shear Lag Factor,
U
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1
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All tension members where the tension
load is transmitted directly to each of
cross-sectional elements by fasteners or
welds. (except as in Cases 3, 4, 5 and 6)
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U
= 1.0
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2
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Plates where the tension load is transmitted
by longitudinal welds only.
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I
≥ 2w . . .U
= 1.0
2w >I
≥1.5w . . .U
=0.87
1.5w >
I ≥w
. . .U
=0.75
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3
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All tension members where the tension
load is transmitted by transverse welds
to some but not all of the cross-sectional
elements.
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U
= 1.0
and
An
= area of the directly
connected elements
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4
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W, M, S or HP Shapes or Tees cut from
these shapes: with flange connected
with 3 or
more fasteners per line in direction of
loading
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bf
≥ 2/3d
. . .U
= 0.90
bf
< 2/3d
. . .U
= 0.85
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5
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W, M, S or HP Shapes or Tees cut from
these shapes: with web connected with
4 or
more fasteners in the direction of loading
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U
= 0.70
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6
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Single angles with 4 or more fasteners
per
line in direction of loading
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U
= 0.80
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7
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Single angles with 2 or 3 fasteners
per
line in the direction of loading
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U
= 0.60
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l
= length of connection, in. (mm);
w = plate width, in. (mm)
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5. Calculate the tensile
rupture strength using AISC D2 (b) of the net section. Yield strengths
and tensile strengths for common shapes can be found in the table below.
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Common Shapes - Yield Strengths and Tensile Strengths:
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Shape
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Fy (Ksi)
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Fu (Ksi)
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W-shape
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50
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65
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Angle
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36
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58
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WT
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50
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65
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Rect. HSS
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46
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58
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Round HSS
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42
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58
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Double Angle
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36
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58
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Steel Pipe
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35
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60
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6. Compare the tensile rupture strength calculated in step 4 to the tensile yield
strength of the member from the table. Is the member subject to a perpendicular
shear force? If so go to step 7, if not then the tensile strength of the member
will be the lesser of tensile yielding and tensile rupture strength.
If the calculated tension strength is lower than the required tension strength,
select a member with a higher tension yielding capacity/larger area and start over.
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7. If the tension member is subject to both tension and perpendicular shear at the
connection, use AISC J4.3 to
check the block shear of the member. The tension capacity of the member
will then be the lower of tension yielding, tension rupture and block shear.
If the calculated tension strength is lower than the required tension strength,
select a member with a higher tension yielding capacity and start over.
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Analysis of an Existing Member:
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The steps below are for the analysis of an existing tension member. An existing
tension member can be analyzed quite easily by calculation.
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1. Determine the gross area, Ag of the member and calculate the tensile yield strength, AISC D2 (a).
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2. Calculate the net area of tension, Ant, of the member connections and determine
the shear lag factor, U of the connection.
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3. Calculate the
tensile rupture strength, AISC D2(b).
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4. If a perpendicular shear force exists on the member, check block shear using AISC J4.3.
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5. Member tension capacity is the lower of tensile yielding, tensile rupture and
block shear.
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