Eccentrically Braced Frames

Eccentrically Braced Frames

Presented by: Gustavo Cortes and Jose Monarrez

Outline 

Design • Philosophy • Preliminary design (ASCE -7) • Force distribution • Design procedure (AISC specifications and AISC Seismic provisions) • Design summary



Performance Evaluation • Evaluation requirements (FEMA 356) • Findings and conclusions

Outline 

Design • Philosophy • Preliminary design (ASCE -7) • Force distribution • Design procedure (AISC specifications and AISC Seismic provisions) • Design summary



Performance Evaluation • Evaluation requirements (FEMA 356) • Findings and conclusions

Philosophy 



To restrict the inelastic action to the links, and to design the framing around the links to sustain the maximum forces that can be delivered by the links.

EBF Advantages 

If designed properly will achieve: • High elastic stiffness, • Stable inelastic response under cyclic lateral loading, • Excellent ductility, and • Energy dissipation

Preliminary design 





TEN-STORY OFFICE BUILDING DESIGN

Building project is located in Los Angeles, California, USA (2/3) Maximum Credible Earthquake (MCE)

MCE with Design Sa Soil factors

MCE Period (sec) T=0.2 T=1.0

SM (g) 2 0.75

2 1.1

SD (g) 1.33 0.733

Design Data         

Occupancy Category: I Seismic Design Category: E Importance Factor: 1 Seismic Weight W: 21,037 kips Response Modification Coefficient: R=7 System Overstrength Factor: Ωo=2 Deflection Amplification Factor: Cd=4 Fundamental Period: Ta=1.1sec Load combinations per ASCE 7-05: • (1.2 + 0.2SDS)D + ΩoQE + L + 0.2S • (0.9 – 0.2SDS)D + ΩoQE + 1.6H

Base Shear and Force distribution Floor Level

Fx (kip)

10

193

9

200

8

173

7

146

6

120

5

96

4

73

3

51

2

32

1

15 ∑ Base Shear = 1099 kip

Design Procedure The Eccentrically Braced Frame Scope: To have significant inelastic deformation in the links “fuses”  

Link (yielding and strain hardening) 



Beam outside of the Link (Elastic) 



Beam-column action will control size of frame members

Brace (Elastic) 



Link length will control the stiffness and ductility of the syst em

Must be stronger than demands generated by the link

Column (Elastic) 

Forces from braces and beams.

Link Design •

Factored Loads applied to the link 

From Analysis: • • •

Ultimate Axial Force Ultimate Shear Force Ultimate Moment Force

•

Post-yielding behavior of link was controlled by shear yielding.

•

Short links (4ft) are expected to yield in shear. (related to the shear span).

•

Link rotation angle is the inelastic angle between the link and the BOL when the total story drift = design story drift.

•

Links are protected zones

1) 2) 3) 4) 5) 6)

Check Local buckling Determine the shear strength of the link Check link rotation angle Check lateral bracing requirements Check stiffener requirements Design of the welds connecting the stiffeners to the beam

Beam outside-of -the Link (BOL) Design 

Determine the amplified Loads • The strength in the beam outside -of -the link is based on the expected shear of the link • The resulting link end moment is based on the expected shear of the link. • The over-strength factor is to be used in the proportioning of BOL elements that should remain in the linear range of response beca use non-linear response is not acceptable.

1) Check Local Buckling 2) Determine unbraced length 3) Consider Second order Effects

Brace Design 

Determine the amplified loads Note: No part of brace connection shall extend over the link length

1) Check Local Buckling 2) Determine the unbraced length 3) Consider second order effects 4) Check combined loading 5) Check shear strength

Column Design 



Strength was determined based on the load combinations of ASCE 7 -05 The columns met the seismically compact requirements

1) Calculate the effective length and slenderness ratio of the column 2) Check slenderness limits

Design Summary

Plans…

Performance Evaluation for the EBF Building

Modeling Parameters 

As per FEMA 356 for an EBF: • The rotation at the link: QCE  θ  y = K e e



The Moment at yield depends on the length of the link: e≤

1.6 M CE  V CE 

QCE  = V CE  = 0.6 F  ye Aw

Mathematical Representation 

Moment rotation relation-link B

IO

LS

CP C D

A

E

Modeling Parameters and  Acceptance Criteria 

FEMA 356, Table 5-6 a

b

C

IO

LS

CP

Mathematical Representation 

Force-deformation relationship-brace B

IO

LS

CP

C D

A

E

BSE – 1 (Proportional to Cvx) Failure of 2nd Floor Link

19 in

Earthquake Hazard Level 



Basic Safety Earthquake – 1 (BSE – 1): 10%/50 yrs. Basic Safety Earthquake – 2 (BSE – 2) : 2%/50yrs. (MCE) 2

δ  t  = C 0C 1C 2C 3 S a 

T e

4π 

2

g

For a very rigid building, the Te is small (less than 1.0) – results in a small target displacement

BSE – 1 (cont.)

9.5 in

BSE – 2 (Proportional to Cvx)

14.4 in

BSE – 1 (Uniform Load)

9.5 in

BSE – 2 (Uniform Load)

14.4 in

 Acceptance Criteria NOTES: Hazard Level

Load Pattern

IO

LS

CP

BSE – 1

Cvx

2

0

0

Two links at the first floor.

Uniform

6

0

0

The two links at floors 1 to 3.

Cvx

4

0

0

The two links at floors 1 and 2.

Uniform

2

2

2

First floor – 2 CP

BSE – 2

Second floor – 2 LS Third floor – 2 IO

Performance Evaluation

Concluding Remarks 



The building is very rigid – our δt requirements are small Even though our building satisfies the Basic Safety Objectives we do not feel very comfortable with its behavior

Recommendations 



It is highly recommended that a performance evaluation be conducted as part of the design Modify the beam sizes to increase ductility – hinges formed only at floors 1 to 4