Design of Concentrically Braced Frames

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Constance Lindsey
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1 Design of Concentrically Braced Frames Anindya Dutta, Ph.D., S.E. Example Configurations X-Braced Inverted V (Chevron) 2 Story X-Braced 1

2 Example Configurations V (Inverted Chevron) Zipper Special Concentrically Braced Frames Primary location of energy dissipation are the braces Braces dissipate energy by tension yielding and compression buckling 2

location of energy dissipation are the braces Braces

3 Special Concentrically Braced Frames Special Concentrically Braced Frames 3

4 Special Concentrically Braced Frames Column Axial Load Distribution Special Concentrically Braced Frames Column Axial Load Distribution 4

5 Special Concentrically Braced Frames Beam Design Axial Load Special Concentrically Braced Frames Beam Design: Flexure 5

6 Special Concentrically Braced Frames Basic Design Procedure Calculate the demand based on ASCE 7 Analyze the structure; find brace forces Size the fuses i.e. braces Capacity design other non yielding members Special Concentrically Braced Frames Basic Design Procedure 4. Capacity design other members Use expected brace capacity Eliminate conservative design assumptions Do not use φ factors for expected strength 6

the structure; find brace forces Size the fuses i.e. braces Capacity design other non yielding members

7 Requirements for Member Design Slenderness Bracing member slenderness KL / r 4 E / Braces with 4 E / F is permitted in y KL / r 200 frames where columns are designed for Ry times nominal strength of the brace elements This load need not exceed the axial loads from inelastic analysis or the max load that can be developed by the system F y Special Concentrically Braced Frames Slenderness 7

strength of the brace elements This load need not exceed the axial loads from inelastic analysis

8 Requirements for Member Design Brace Effective Length Requirements for Member Design Brace Effective Length 8

9 Requirements for Member Design Brace Effective Length: End Fixity Requirements for Member Design Brace Effective Length 9

10 Requirements for Member Design Required Strength If UAnt<Agross then Fu(UAnt)>RyFyAg Max load indicated by analysis that can be transferred to the brace by the system Requirements for Member Design Lateral Force Distribution All compression or tension system (generally not allowed) Sum of horz. Comp. on either compression or tension 0.7V NG Along any line of bracing at least 30% but not more than 70% of the force is to be resisted by brace in tension Exception allowed when compression braces are designed for Amplified (Ω) load combinations of ASCE 7 10

allowed) Sum of horz. Comp. on either compression or tension 0.

11 Requirements for Member Design Lateral Force Distribution 0.3V Tension 0.7V 0.3V Tension 0.7V 0.3V Compression 0.7V 0.3V Compression 0.7V OK OK Requirements for Member Design Width-Thickness Limitations Members to be seismically compact. Follow requirements of Table I

12 Requirements for Member Design Local Buckling Connections to be designed for expected yield strength of member in tension RyFyAg This force need not exceed the max load indicated by analysis that can be transferred to the brace by the system 12

RyFyAg This force need not exceed the max load indicated by

13 13

14 Pin ended Fixed ended 14

15 Design flexural strength of the connection (if fixed) φr 1. 1R n Design compressive strength of the connection if pinned along with proper detailing y φr 1. 1R n M y P p n 2t Offset 15

16 2t Concrete Filled Deck Tearing of Gusset: No Hinge Zone 16

17 Folding of Gusset: Hinge Zone Gusset Compression Estimate the max. compression force from the brace: Consider true brace length Consider connection fixity Consider material overstrength Shortcut: t Tension strength is always a greater than compression strength 17

Consider connection fixity Consider material overstrength

18 Gusset Compression Gusset Compression 18

19 Gusset Compression Gusset Compression 19

20 Design of gussets Design of gussets: Uniform Force Method R uc θ P u β A ub wp P u sinθ-a ub Rub β e b R uc +R ub -P u cosθ ec α α 20

21 Design of gussets: Uniform Force Method P u ec V uc V = β uc P u H uc = P u r r H uc eb θ H ub = α Pu Vub = Pu r r V ub V ub H ub where R ub -V ub P u sinθ-a ub r = ( α + e ) 2 ( ) 2 c + β + e b P u sinθ-a ub -H ub R ub Design of SCBF: Specials for Chevron 21

22 Design of SCBF: Specials for Chevron Design of SCBF: Specials for Chevron 22

Design of reinforced concrete columns. Type of columns. Failure of reinforced concrete columns. Short column. Long column

Design of reinforced concrete columns Type of columns Failure of reinforced concrete columns Short column Column fails in concrete crushed and bursting. Outward pressure break horizontal ties and bend