Seismic Assessment of Existing Chevron Braced Frames Designed in accordance with the 1980 Canadian

Code Requirements

Y. Balazadeh Minouei, S. Koboevic & R. Tremblay

CEISCE Annual Seminar Quebec City May 2, 2014

Context

• Before 1989, no ductile seismic detailing requirements in CSA S16 standards

• Possible seismic deficiencies for steel structures built before the 1990’s

Context (cont’d) Previous study (Jiang et al. 2012) • Assessment of the building using NBCC 2010 and CSA 2009 • Tension-compression chevron braced frame:

- Braces, columns, beams and brace connections had insufficient strength;

- Governing failure mode for all brace connections: Block shear failure

CSS: Cross Sectional Strength OMS: Overall Member Strength AT&B: Axial Tension and Bending

Objectives

Study the seismic response of a tension-compression chevron-braced frame located in Vancouver, BC, designed in accordance with the requirements of 1980’s Canadian seismic norms by using nonlinear time history analysis

Apply different numerical modelling techniques to examine the effect of nonlinear behaviour of different structural members on the global seismic response of the building

Investigate in detail the impact of modelling approaches with varying complexities on the assessment of beam seismic response

Design of the building studied • Structure design based on NBCC 1980 and CSA-S16.1-M78 in Vancouver • W-shape sections for beams and columns • Back-to-back double angle sections for bracing members • V=ASKIFW • T Rayleigh method • Structural members CSA-G40.21-300W steel

5 @ 9144 = 45 720

X-Bracing

b)

X-BracingNote: All dimensions in mm

Chevron Bracing

Chevron Bracing

4572

5 @

914

4 =

45 7

20

9 @

396

2 =

35 6

58

N

Numerical modelling

Model B Elastic beam-column

elements for beams and columns

Nonlinear beam-column elements for braces

Model C

Nonlinear beam-column elements for braces and

beams

Elastic beam-column elements for columns

Assessment procedure

• Assessment: ASCE 41-13 • Analysis: Nonlinear time history

• Seismic loads: NBCC 2010 spectrum • Member capacity: CSA S16-09

Ground motions

Ground motions(cont’d)

Earthquake name Year Recording station Scale factor

Northridge 1994 Castaic Old Ridge Route 1.485

Northridge 1994 Beverly Hills - 14145 Mulhol 1.620

Loma Prieta 1989 Hollister - South & Pine 1.485

Loma Prieta 1989 Palo Alto - SLAC Lab 1.620

Landers 1992 Barstow 1.755

San Fernando 1971 Hollywood 1.755

Hector Mine 1999 Joshua Tree 1.890

Assessment procedure

Slender: 8Δc Compression: Stocky: 7Δc

Tension : 9ΔT Brace evaluation

(collapse prevention performance level)

Use a two-dimensional model

No accidental torsional effects

ASCE 41 standard

Assessment procedure (cont’d)

Brace action Axial deformation (mm) Acceptance criteria Analysis results

Tension 96.01 (9ΔT) 617.31 Compression 49.70 (8Δc) 932.40

Assessment results (Model B)

Assessment results (Model B) (cont’d)

t = 3.8 s

t = 3.8 s

Assessment results (Model C)

Assessment results (Model C) (cont’d)

t = 3.8 s

Beam buckling response

Beam buckling response

Beam buckling response

A B

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

16 nonlinear beam-column elements

A A A A A A BB B A A A A A A A

Gusset plate

Beam buckling response (cont’d)

FModel 1

FModel 2

F

Beam buckling response (cont’d) FModel 3

FModel 4

Beam buckling response (cont’d)

FModel 5

Conclusion

• Model B - Elastic Beams:

- Buckling of the braces at the first level; - Large flexural demands on the beam at that level; - The resulting large storey drift imposed bending

moments on the columns and caused soft-storey response;

- Brace connections were found to have insufficient capacity.

Conclusion (cont’d)

• Model C - Inelastic Beams:

- Buckling of the beam at the first level within its half-length;

- Large storey drifts and, thereby, high flexural demand on the columns

- Braces did not buckle at the first floor and the brace connection capacities were not reached.

Conclusion (cont’d)

• Further assessment of the beam buckling response in chevron braced frames was performed using five different beam models;

- Beam in-plane buckling response and strength are influenced by the presence of the braces and the gusset plate connections;

- The distribution of stresses and strains were sensitive to the modelling employed for braces and the gusset plates;

- Beam compressive strength can be predicted by current code equations assuming an effective length equal to the beam half-length.

Ongoing/Upcoming work

• Use nonlinear beam-column elements for the columns

• Evaluate the seismic response of the structure for different probabilities of exceedance (5%/50 year, 10%/50 year)

Acknowledgments

Funding from the Natural Sciences and Engineering Research Council of Canada (NSERC) for the Canadian Seismic Research Network (CSRN)

Thank you