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Design of Bracing Connections in Concentrically Braced Frames
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Reference Materials
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D i f S i iReference Materials
Design of Seismic‐Resistant Steel Building
Structures
3. Concentrically Braced Frames
Prepared by:Michael D. EngelhardtUniversity of Texas at Austinwith the support of theAmerican Institute of Steel Construction.
Version 1 ‐March 2007
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Brace End
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Brace Net Section Fracture
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Gusset Plate
U d i d b h i G t Pl t B kliUndesired behaviour – Gusset Plate Buckling
Desired behaviour – Gusset Plate Bending and Brace Buckling
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Bad Designs of Bracing Connections
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HSS Bracing Connections
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gusset plate
rectangular HSS bracing member
Check HSS bracing member for limit state of net i fsection fracture
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AISC:Ru= Ry Fy AgGB50011‐2010: 1.2Abrfy
Required axial tension strength of brace for limit state of fracture of the netstate of fracture of the net section
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Ru= Ry Fy AgRu = 1.2Abrafy
Critical Net SectionAe = U AnAe < Ag due to:e g
slot (An < Ag ), andshear lag (U < 1)
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Ru= Ry Fy Ag
Limit state: fracture of net section (0.75) Ae (Rt Fu) ≥ Ry Fy Ag
OR:For A500 Gr B rectangular HSS:
ksi464.1A yye
FRFR
AA
750
14.1ksi583.175.0
ksi464.1AA
g
e
utg FRA 75.0
Need to Reinforce Net Section (Ae need not exceed Ag )
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Reinforcing net section of bracing member....
I d th t t ti t I recommend that net section to be reinforced such that yielding occur in the brace member, not at the connection
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Gusset Plate in Tension 1.2Abrfy/betg <= f
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Also check block shear rupture of bracing member....
Lt = design wall thickness of HSS
Pu= Ry Fy Ag1.2Abrfy
Pn = (0.75) ( 4 L t x 0.6 x 64.2 ksi) ≥ 1.4 x 46 ksi x Ag
A557.0L g = minimum length of welded overlap
tL g p
needed based on block shear rupture in HSS bracing member
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PP PP
Plastic Hinge
PP
For "pinned" end braces: flexural plastic hinge will form at mid‐length only. Brace will impose no bending moment on connections and adjoining members.j gMust design brace connection to behave like a "pin"
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2t
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2t
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2t
Concrete floor slab
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2t
Concrete floor slab
Styrofoam
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> 2t
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>2t>2t
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> 2t2t
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Gusset Plate in Compression
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Concentric Work Point Location and h U if F M h dthe Uniform Force Method
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The Uniform Force Method
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Gusset Plate Design
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Beam and Column Web Yielding and Stability Checkingy g
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Beam Midspan
Check gusset buckling
Check gusset yield, gusset net section fracture, gusset block shear fracture local Check gusset buckling,
beam web crippling, etc.block shear fracture, local beam web yielding, etc.
1.1 Ry PnRy Fy Ag
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Connection at Beam Midspan
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Connection at Beam Midspan
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Gusset Plate
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Ordinary Concentrically Braced Frames
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Ordinary Concentrically Braced Frames
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Ordinary Concentrically Braced Frames