Transcript Connectors

Connection Design
Types of Connections
• Three forces: Axial, shear and moment
• Many connections have 2 or more
simultaneously.
• Connections are usually classified according to
the major load type carried.
– Shear
– Moment
– Axial: splices, bracing, truss connectors, hangers…
Economic Considerations
• Shear Connections:
– Design for specified factored loads
– Allow use of single-plate and single-angle shear
connections
– Do NOT specify full-depth connections or rely on
AISC uniform load tables
Economic Considerations
• Moment connections:
– Design for specified factored moments and shears.
– Provide a breakdown of the total moment
• Gravity, seismic, wind are treated separately
• This is needed for column web doubler plate calcs
– If stiffeners are required, allow use of fillet welds
instead of complete joint penetration welds
– To avoid use of stiffeners, consider redesign with a
heavier column to avoid them.
Economic Considerations
• Bracing Connections
– In addition to providing brace force, also provide
beam shear and axial transfer force.
– The transfer force is not necessarily the beam axial
force obtained from FEA
– Misunderstanding of the transfer force can lead ot
uneconomic or unsafe connections
Strength Limit States: Tension
• Either tension yielding or fracture govern.
Design strength for yielding in the gross
section is
– F R n = f s y Ag
• Design strength for fracture in net section is
– f R n = f s u An
• f = 0.9 for yield, 0.75 for fracture
• sy = yield strength; su = tensile strength; Ag = gross
area; An = net area.
Tension
• Sometimes entire gross area or net area
cannot be considered effective.
– For example, brace attaching to a large gusset:
Gross area is based on the Whitmore section
– Or, connecting elements, such as angles, where
only one leg of the angle is connected, a shear lag
factor must be included in the calculation of net
area.
Shear
• Either shear yielding or fracture govern. Design
strength for yielding in the gross section is
– F Rn = f 0.6 sy Ag
• Design strength for fracture in net section is
– f Rn = f 0.6 su An
• Due to resistance provided by the flange, net
shear fracture will govern capacity of flanged
members only when BOTH flanges are coped.
Bending
• Either tension yielding or fracture govern.
Design strength for yielding in the gross
section is
– F R n = f s y Zg
• Design strength for fracture in net section is
– f R n = f s u Zn
Bending: Plastic section
• Zn = Zg (1 - dh/b)
– Where dh = hole diameter and b = bolt spacing
• This is exact for even number of rows, and
slightly conservative for odd number.
Localized Limit States
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•
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Bearing at bolt holes
Bolt tear-out
Block shear
Local web yielding
Local web crippling
• Local web
compression
buckling
• Local flange bending
• Axial yield line
• Plate Plastification
Bearing at Bolt holes
• Large compressive stresses can occur where the
shank of the bolt bears on the connected
material.
– f Rn = f 2.4 db t su
• Where f = 0.75, db = bolt diameter, t = thickness of material.
• If deformation at the bolt hole under service
loads is not a design consideration, the bearing
strength can be determined as
– f Rn = f 3.0 db t su
Bolt Tear-out
• Shear fracture where bolt tears out through
the material.
• If deformation at bolt hole is a concern, use previous
equation or this (whichever is smaller)
• f Rn = f 1.2 Lc t su < f 2.4 db t su
if we don’t care about hole def,
f Rn = f 1.5 Lc t su< f 3.0 db t su
where Lc = length of connector tearing out
Possible failures
• For each bolt, have to check
– Bolt shear
– Bearing on main material at bolt
– Bearing on connection material at bolt
– Bolt tear out through main material
– Bolt tear out through connection material
7/8” A490X bolts
1.5”
3”
3
1.5”
1
3/8 PL (A36)
W8x15
4
2