Design of Bracing Connections in Concentrically Braced Frames

Reference Materials

Design of Seismic-Resistant Steel Building Structures

Prepared by:Michael D. EngelhardtUniversity of Texas at Austinwith the support of theAmerican Institute of Steel Construction.

Version 1 - March 2007

3. Concentrically Braced Frames

Reference Materials

Brace End

Brace Net Section Fracture

Gusset PlateUndesired behaviour – Gusset Plate Buckling

Desired behaviour – Gusset Plate Bending and Brace Buckling

Bad Designs of Bracing Connections

HSS Bracing Connections

gusset plate

rectangular HSS bracing member

Check HSS bracing member for limit state of net section fracture

AISC:Ru= Ry Fy Ag

GB50011-2010: 1.2Abrfy

Required axial tension strength of brace for limit state of fracture of the net section

Ru= Ry Fy Ag

Ru = 1.2Abrafy

Critical Net SectionAe = U An

Ae < Ag due to:slot (An < Ag ), andshear lag (U < 1)

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:

14.1ksi583.175.0

ksi464.1AAg

e

Need to Reinforce Net Section (Ae need not exceed Ag )

ut

yy

g

e

FRFR

AA

75.0

Reinforcing net section of bracing member....

I recommend that net section to be reinforced such that yielding occur in the brace member, not at the connection

Gusset Plate in Tension 1.2Abrfy/betg <= f

Pu= Ry Fy Ag

1.2Abrfy

Also check block shear rupture of bracing member....

Lt = design wall thickness of HSS

Pn = (0.75) ( 4 L t x 0.6 x 64.2 ksi) ≥ 1.4 x 46 ksi x Ag

tA557.0

L g = minimum length of welded overlap

needed based on block shear rupture in HSS bracing member

For "pinned" end braces: flexural plastic hinge will form at mid-length only. Brace will impose no bending moment on connections and adjoining members.Must design brace connection to behave like a "pin"

PP

PP

Plastic Hinge

2t

2t

Concrete floor slab

2t

Concrete floor slab

Styrofoam

2t

>2t

> 2t

>2t

> 2t

Gusset Plate in Compression

Concentric Work Point Location and the Uniform Force Method

The Uniform Force Method

Gusset Plate Design

Beam and Column Web Yielding and Stability Checking

1.1 Ry Pn

Ry Fy Ag

Check gusset buckling, beam web crippling, etc.

Check gusset yield, gusset net section fracture, gusset block shear fracture, local beam web yielding, etc.

Beam Midspan

Connection at Beam Midspan

Connection at Beam Midspan

Gusset Plate

Ordinary Concentrically Braced Frames

Ordinary Concentrically Braced Frames

Ordinary Concentrically Braced Frames