Design of Bracing Connections in Concentrically Braced Frames

Reference Materials

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

Brace End

Brace Net Section Fracture

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

Bad Designs of Bracing Connections

HSS Bracing Connections

gusset plate

rectangular HSS bracing member

Check HSS bracing member for limit state of net i fsection fracture

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

Ru= Ry Fy AgRu = 1.2Abrafy

Critical Net SectionAe = U AnAe < Ag due to:e  g

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:

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 )

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

Gusset Plate in Tension 1.2Abrfy/betg <= f

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

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"

2t

2t

2t

Concrete floor slab

2t

Concrete floor slab

Styrofoam

> 2t

>2t>2t

> 2t2t

Gusset Plate in Compression

Concentric Work Point Location and h U if F M h dthe Uniform Force Method

The Uniform Force Method

Gusset Plate Design

Beam and Column Web Yielding and Stability Checkingy g

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

Connection at Beam Midspan

Connection at Beam Midspan

Gusset Plate

Ordinary Concentrically Braced Frames

Ordinary Concentrically Braced Frames

Ordinary Concentrically Braced Frames