Tension Members

1
Tension Members

• Structural members that are subjected to axial
  tensile force (truss members, cables in
  suspension bridges, bracing for buildings,).
• Any cross-sectional configuration may be used,
  since the only determinant of strength is the
  cross-sectional area.
• Circular rods and rolled angle shapes are
  commonly used.

2
Gross and Net Areas

• The usual practice is to drill or punch standard
  holes with a diameter 1/16 in. larger than the
  fastener diameter.
• To account for possible roughness around the
  edges of the hole, AISC requires the addition of
  1/16 in. to the actual hole diameter.
• Thus, the effective hole diameter is 1/8 in.
  larger than the fastener diameter.

3
Example

• Ag 5 x ½ 2.5 in2
• An (5-2 x ¾) x ½ 1.75 in2

4
Design Strength

• A tension member can fail by reaching one of the
  two limiting states yielding or fracture.
• To prevent yielding and accompanying excessive
  deformation, the stress on the gross section(Ag )
  should be less than Fy .
• To prevent fracture, the stress on the net
  section(An) must be less than Fu.

5
Nominal Strength

• Nominal strength in yielding, Pn
  Fy Ag
• Nominal strength in fracture, Pn
  Fu Ae
• Ae is the effective net area. The smaller of
  these is the design strength of the member.

6
Strength Reduction Factor

• Strength reduction factor for yielding,
  ?t 0.90
• Strength reduction factor for failure,
  ?t 0.75 
• The resistance factor is smaller for fracture
  than yielding, reflecting the more serious nature
  of reaching the limit state of fracture.

7
Effective Net Area

• When all elements of the cross section are not
  connected ( Ex only one leg of an angle is
  bolted to a gusset plate), shear lag occurs.
• The connected element becomes overloaded and the
  unconnected part is not fully stressed.
• This can be accounted for by using a reduced, or
  effective, net area. 

8
Effective Net Area

• For Bolted Connections Ae
  U An
• If all elements of the cross section are
  connected, then, U 1
• If not, use the recommended values of the
  reduction factor, U (see next slide)

9
Recommended Values for U
10
Effective Net Area

• For welded connections
  Ae U Ag
• For any W-, M-, S-, or tee shape connected by
  transverse weld alone
  Ae area of connected element

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Ae For Welded Connections

• For plates or bars connected by longitudinal
  welds at their ends
• U1 l ? 2w
• U0.87 2w ? l ? 1.5w
• U0.75 1.5w ? l ? w
• l length of the pair of welds ? w
• w distance between the welds

13
Staggered Fasteners

• The net area will be maximized if the fasteners
  are placed in a single line.
• If more than one line is needed (limited
  connection length), staggering the fasteners
  minimizes the reduction in cross-sectional area.

14
Staggered Fasteners

• wn wg ? d ? s2 / 4g
• wn net width
• wg gross width
• ? d sum of the hole diameters
• s (pitch) spacing of two adjacent holes (
  parallel to direction of the load)
• g (gage) transverse spacing of lines of bolts

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Staggered Fasteners

• If more than one failure pattern is conceivable,
  all possibilities should be investigated, and the
  one corresponding to the smallest load capacity
  should be used.
•