Seismic Performance of New and Older CBFs Dawn Lehman and Charles Roeder (PIs) Po-Chien Hsiao (GSRs) University of Washington

Prior Earthquake Damage Column Fracture Beam Damage Connection Damage Incipient Collapse

Investigating the Seismic Performance of CBFs Qualifying Seismic Performance – Experimental observations of damage – Collapse? Few, if any, tests Quantifying Seismic Performance – Response History Analyses – Fragility Functions – System impact: type, height, model Equalizing Seismic Performance – R factor – FEMA P695?

Investigation of 3 Types of CBFs NCBF – Designed Prior to 1988 on West Coast (still designed today on East Coast) – No consideration of ductile detailing or limits SCBF – Current AISC design – Borne out of understanding that prior (pre-1988) does not result in satisfactory performance – Restrictions on brace geometry and recommendations for gusset plates SCBF-BDP – Adopts current brace design limits – More rational, balanced design of gusset plate and welded connections

Summary of CBF Design Provisions Current SCBF Designed for brace material overstrength Accommodate out-of- plane rotation of brace. Recommendations: use 2t p provisions Pre-1988 (NCBF) No limit on KL/r No limit on b/t Nominal tension capacity of the brace No provisions accommodating out-of-plane rotation of the brace BRACE GUSSET PLATE CONN. Proposed BDP Use  factors. Design connecting weld for gusset plate strength. Accommodate out-of- plane rotation of brace with elliptical clearance (corner) or 6t p (midspan) Kl/r <~ 100 b/t – seismically compact (1997)

Laboratory Observations

Comparison: Single-Story CBF Tests W12x72 Columns W16X45 Beams HSS 5x5x3/8 Brace Actuator Strong Wall Strong Floor Load Beam

SCBF: Clearance types

NCBF: Connection Variations Extensive! Some Examples…

Tested Pre-1988 Connection Bolted end-plate connection Relative to SCBF: – Shorter brace-to- gusset length – Gusset and associated connections are typically weaker than brace

Comparison of Three Tests Current Design (Post 1997) Improved (Balanced) Design Older (Pre-1988) Design

Balance Design Response: Brace 1. Hinging at Center 2. Cupping 3. Tearing 4. Fracture

Improved SCBF: Extensive Yielding in Gusset Brace buckling and yielding Extensive yielding and OOP rotation of gusset plate Yielding of beams and columns

Current Design using 2t Linear Clearance Inelastic action included – Brace yielding and buckling Overall failure mode – Fracture of the gusset plate-to-frame welds Drift Range: -1.3% to 1.6% (2.9%) Weld Fracture

Comparison of L-2t p and E-8t p CURRENT DESIGN BALANCED DESIGN

Response of pre-1988 CBF

Variation in CBF Performance Brace fracture Weld fracture Connection fracture Pre-1988 NCBF Current SCBFBDP SCBF

Analytical Modeling of CBFs Composite fiber sections 10 beam-column elements with initial imperfection through entire length Spring-type model of gussets Simple connection Rigid elements Increased strength element

Review of SCBF Model 1.Buckling behavior of the brace is a key elements in the SCBF seismic response. 2.Significant deformation of the gusset plate connections and included in model. Stiffness is important (can not model as pin or fully restrained). Variations in the design are important. 3.Local yielding of the beams and columns must be simulated.

Model Implementation: SCBF Model Brace Fracture Connection Model Spring-type of Shear Tab Proposed model of gusset plate connections Rigid Links

Adaption of SCBF Model to NCBF Model 1.Buckling behavior of the brace is still key elements in the SCBF seismic response. 2.Deformation of the gusset plate connections is not; stiffness is. 3.Connection fracture must be simulated 4.Post-fracture “moment frame” response is important.

Fracture triggered (K e and D limit were calibrated to NCBF32.) Disp. Load KeKe D limit Model Implementation: NCBF Proposed spring-type model of gusset plate connections combined with axial fracture model Axial Fracture Model of Connection Calibrated by NCBF32 Model Connection Model Connection Fracture

Analytical Simulation of Tests ImprovedCurrent Pre-1988 (NCBF)

Predicting Performance of CBFs

Ideal Progression of Response – BDP follows this progression – Current overlooks gusset yielding. Alternative failure mode. – Neither accurately accounts for or estimates system collapse Brace BucklingBrace Yielding Gusset Plate Yielding Column and Beam Yielding Brace Fracture

Performance States (ATC)

Dynamic Response Analysis Idealized 3, 9 and 20 story buildings (SAC SMRF) buildings 40 Seattle ground motions. 2% and 10% in 50 yr. events Scaling procedure depended on: – Hazard Level – Building Height – Type of CBF (NCBF vs. SCBF) Gravity Frame to capture second-order effects; degradation not modeled.

4 6.1m 3.96m 18’ Braced Frames 4 9.1m 5 9.1m 6 9.1m 5 9.1m 3.96m 3.96m 5.5m Idealized Buildings

Rigid member Shear connection Model (x12) Leaning Col. Half of gravity frames (11.5 times of gravity col., A, I, Mp) Gravity loads of the floor Slave nodesMaster nodes Pin Center Line of the braced frame Gravity Frame Model 2/50 3-story 9-story

T1T1 T2T2 T1T1 T2T2 BDP-SCBF Pre-1988 NCBF AISC-SCBF Ground Motion Scaling: 3-Story Frame

9-story SCBF 20-story SCBF Ground Motion Scaling: Taller Buildings Scaling based on first and second modes Scaling based on first and second modes

Response

Impact of Connection Model: SCBF

Impact of R Factor on Performance: 3-Story Frame 3.96m

Impact of R-Factor on Performance: 9-Story Frame

Impact of R Factor on Performance: 20-Story Frame 3.96m 18’

Comparison: Building Height Impact of building height as significant as R

SCBF vs. NCBF VS.

NCBF vs. SCBF

FEMA P695: Overview To quantify R, C d,  Incremental dynamic analysis – Analogous to a “virtual” shake table test. Increase earthquake intensity until “collapse” Use IDA results to compute median spectral acceleration values at MCE and “collapse” Compare resulting ratios with limits Pass or Fail evaluation.

CsCs S MT Ŝ CT SD MT SD CT SD MT /1.5R CMR 1.5C d 1.5R MCE Ground Motions Collapse Level Ground Motions Spectral Displacement Spectral Acceleration (g) Collapse Assessment: FEMA P-695 Analysis

S MT Ŝ CT S MT Ŝ CT IDA: 3-story vs. 20-story 3-story 20-story

Scaling Method M1 [S T (T o )]M2 [Sa(T o )] Building3-Story20-Story3-Story20-Story R Factor S MT (g) Ŝ CT (g) CMR SSF ACMR Accep. ACMR20% 1.73 Pass/FailFailPassFail Pass FailPass Collapse Evaluation SCBF FEMA 695

Pre-1988 NCBF Incremental Dynamic Analysis SCBF NCBF S MT Ŝ CT S MT Ŝ CT

Collapse Evaluation NCBF vs. SCBF

 Maximum Shear To ELF Story Shear

CdCd

Conclusions Pre-1988 CBF vulnerable to “premature” connection failure. Retrofit methods untested; largely absent in ASCE-31. Represent a serious, largely unstudied hazard Seismic evaluation based on performance results in reliable and important results including: – Performance evaluation depends on modeling assumptions and ground-motion scaling method – Performance of low-rise structure depends on R. – Performance of SCBF depends on R and building height. Recommendation: R factor of 6 for mid-rise SCBFs; R factor of 3 for low-rise SCBFs.

Conclusions Collapse evaluation is difficult and misleading Results are contrary to earthquake observations and experimental findings. For example : – 20-story SCBF computed to have increased seismic vulnerability relative to 3-story SCBF – Pre-1988 CBF sustains significant damage at lower levels of seismic excitation, yet exceeds performance of SCBF from FEMA 695 evaluation. Recommend careful(re-)consideration of this approach as a design basis Current C d is low for low rise buildings;  value is appropriate.