Thesis Defense Presentation

Enhanced Seismic Performance of Multi-Story Special Concentrically Braced Frames

Using a Balanced Design Procedure

Masters Thesis DefenseDecember 8, 2009

Eric J. LumpkinMasters Candidate

Charles W. Roeder Professor – Principal Investigator (PI)

University of Washington

Dawn E. Lehman Associate Professor – Co-PI


Po-Chien Hsiao PhD Candidate


K.C. Tsai International Collaborator

NCREE

Laura N. LowesAssociate Professor – Committee


• SCBFs – Special Concentrically Braced Frame– High seismic regions

– Economic (less steel and fabrication)

• Traditional design philosophy– Inherently stiff and inverted

truss systems

– Transfer forces through brace axial loads

• Design requirements in AISC seismic manual and specification

Overview of SCBFs

• Majority of seismic response dominated by brace buckling and tensile yielding

• Brace to framing element connections– Gusset plate – pinned ends

– Moment connection – fixed ends

Overview of SCBFs

S N

h1

h2

h3

S N

h1

h2

h3

Current Design May Lead to…

Overly large gusset plates

Undesirable failuremechanisms

Excessive beam/column damage

Soft Stories

Summary of Project

University of California - Berkeley

NCREE

University of Minnesota - MAST


Objective

To improve the seismic design of

SCBF systems and connections through

advanced models and full scale

system testing

Ntp

Ntp

Elliptical Clearance ModeBalanced Design Procedure

Investigation of Midspan Gussets

• Investigation of unique gusset plate configurations– Chevron configuration

– Gussets on either side of a beam

• Modification of middle gusset plate clearances• Eliminate middle gusset edge stiffeners • Lead to Tomorrows Concentrically Brace 2 (TCBF2)

Extension to Three Story Testing

NS

Ntp Ntp

TCBF 2 Overview

TCBF 2-HSS

TCBF 2-WF

TCBF 2-IP

Construction Drawings

Moment Connection

Shear Connection

Reusable Test Frame

Experimental Setup

Test Specimen

OOP Frame

OOP Kicker

Actuators

Strong Wall

Strong Floor

7.8” Composite Slabs

Most Accurate SCBF Specimens Tested

• TCBF2-HSS– OOP buckling HSS5x5x3/8 brace

shape (U.S.) on each story

– 3/8” middle and corner gusset plates

• TCBF2-WF– OOP buckling H175x175x7.5x11

brace shape (Japan) on each story

– 10-mm middle and corner gusset plates

• Similar braces sizes to UW specimens

TCBF2 Specimens

• TCBF2-IP– HSS125x125x9 In-plane buckling

braces on each story• In-plane buckling achieved with a

20-mm knife plate• 18-mm gusset plate to remain

elastic• Majority of inelastic action in knife

plate hinge region

TCBF2 Specimens

• Loading Protocol– Pseudo-static

– Cyclic

– Increasing amplitude based on yield displacement (Δy)

– Adopted from SAC recommended protocol

Experimental Setup

Δy

• Designed using a combination of two methods– Balanced design procedure and limit states

• Used to ensure strength and ductility

– Non-linear, finite element modeling (Hsaio, 2009)• Used to establish connection clearances

TCBF2 Design

• AISC Design– Ensures strength with

resistance factors (φ) for varying limit states (φRn)

– φ factors based strength, safety and statistically extreme considerations

– Higher φ factors for limit states that are easier to predict or outcome is less severe

– Connection is stronger than brace

Balanced Design Procedure• Balanced Design Procedure

– Ensures strength AND ductility with balance factor (β) for varying limit states (βRn)

– β factors based on achieving ductility while preventing undesirable failure mechanisms

– Higher β factors for limit states that increase connection ductility (gusset plate yielding)

– Connection stronger than brace

• Vertical offset (Ntp) from the beam clearance to allow for gusset plate ductility and rotation (TCBF2-HSS, WF)

TCBF2 Design - Middle Gusset Plates

2tp 4tp 6tp

Ntp Ntp

• FEM used to establish hinge region dimensions (Hsiao, 2009)– Parametric study

TCBF2 Design – Knife Plates

Ntkp

260-mm x 20-mm (1tkp)

210-mm x 25-mm (2tkp)

260-mm x 20-mm (2tkp)

TCBF 2 Connection Summary

8t

6t

Corner Gusset Plates Middle Gusset Plates

• 8t elliptical clearance

• Based on single story, single bay UW testing

• Balanced design procedure

• 6t linear offset from beam

• Close proximity to beam eliminates need for edge stiffeners to control buckling


Knife Plates

• 2t linear clearance from gusset plate


2t

• Test results broken into four parts

Test Results

HSS OOP vs. WF OOP TCBF2-HSS vs. TCBF2-WF

HSS OOP vs. HSS In-PlaneTCBF2-HSS vs. TCBF2-IP

Overall Test Comparison

Beam/Column Demands

Test Results – Overall

Backbone Frame Response

1st Story 2nd Story 3rd Story Frame

TCBF2-HSS 4.38% 4.32% 2.13% 3.82%

TCBF 2-WF 5.21% 5.56% 2.99% 4.86%

TCBF2-IP 3.99% 4.56% 1.99% 3.48%

Sustained Drifts

kip

TCBF2-HSS 516.4

TCBF 2-WF 490

TCBF2-IP 449

Maximum Resistance


Test Results




Beam/Column Demands

• OOP buckling specimens hysteretic response• WF brace achieve higher story drifts, however prone to

loss of resistance

• WF braces have a more “pinched” hysteretic behavior

Test Results-HSS vs. WF

TCBF2-HSS TCBF2-WF

Frame Drift :-2.43% to 2.43%Frame Drift :-1.74% to 2.08%

• Energy dissipation– On an equal drift basis, HSS braces dissipate more energy

than WF braces

– WF braces dissipate more energy overall

Test Results-HSS vs. WF

TCBF2-HSS TCBF2-WF

• Brace fracture achieved in both specimens

Test Results – HSS vs. WF

TCBF2-HSS TCBF2-WFNS NS

• Comparison of first story brace buckling

Test Results - HSS vs. WF

TCBF2-HSS TCBF2-WF

Test Results – HSS vs. WFNS

TCBF2-HSS TCBF2-WF

• Larger initial spike in WF brace displacement •HSS buckling more gradual•WF brace achieve higher OOP displacements

1.2% SD

2.1% SD

Wide Flange

HSS

Test Results – HSS vs. WFTCBF2-HSS TCBF2-WF

Corner Gussets Corner Gussets

Middle Gussets Middle Gussets

• 6tp vertical clearance permits substantial OOP gusset rotation

• So does 8tp elliptical clearance


NS

NS

TCBF2-HSS TCBF2-WF

• Interface weld tearing


TCBF2-HSS TCBF2-WF

2.15% SD 3.05% SD


Test Results




Beam/Column Demands

• OOP buckling specimens• Similar hysteretic behavior

• Identical energy dissipation capabilities

Test Results-In Plane Buckling

TCBF2-HSS TCBF2-IP

Frame Drift :-1.74% to 1.74%Frame Drift :-1.74% to 2.08%

Test Results-In Plane Buckling

Test Results – In Plane BucklingNS

• In-plane buckling connections are long – resulting in shorter braces•In-plane braces achieve slightly higher normalized buckled displacements than OOP buckling HSS braces

• Knife plate connections are fundamentally different than gusset plate connections– Inelastic rotation and yielding is isolated to a confined

region as opposed to over an area.

Test Results – In Plane Buckling

• In-plane rotating knife plates achieve substantial inelastic rotations

Test Results - In Plane Buckling

Knife Plate

Gusset Plate

• 3NBr buckled OOP instead of the intended In-plane direction– Construction error resulted in a hinge

region that was 1.55tkp (31-mm) instead of 2tkp (40-mm)

– Indicates 2tkp is on the threshold of what is permissible to initiate in-plane buckling

– Inelastic tearing in hinge region– Large rotational demands in confined

region

Test Results – In Plane Buckling


Test Results




Beam/Column Demands

• Two predominant forces – axial and moment

• Axial due to dead weight of structure and vertical component of brace forces (Pc, Pt)

• Inelastic moments arise when column bending increases as braces buckle and fracture

Test Results – Column Demands

NS

Pt

Pt

Pt

Pc

Pc

Pc

• Results in column axial forces that are 50% nominal tensile yield capacity

• Inelastic bending demands are very high, moments slightly exceed Mp (1.1Mp)

Test Results – Column Demands

NS

Brace Buckling

Moment Axial Load

• Predominant force in SCBF beams are moments, due to:– Unbalanced brace vertical

resultants as buckling and fracture occurs – especially at third story beam

– Frame action

Test Results – Beam Demands

NS

Pt

Pt

Pt

Pc

Pc

Pc

Test Results – Beam Demands

NS

First Story BeamMoments due to uneven brace buckling and brace fracture

Third Story BeamMoments due to unbalanced vertical brace forces

Second Story BeamMoments due to frame action

• AISC recommends designing chevron beams for full unbalanced load (non-composite capacity)

• However, no significant damage or deflections were noted during testing

• Deflections did not exceed 4-mm

Test Results – Beam DemandsNS

Pt 0.3Pn

• General– All specimens achieved increased drift ranges when

compared to current AISC design SCBFs

– Balanced design procedure maximized system ductility by extending inelastic behavior beyond brace

– Behavior of single story, single bay corner gusset is also applicable to multi-story SCBFs

– Specimens exhibited good distribution of inelastic behavior over first two stories

– Decreased contribution from third story

Conclusions

• Connection clearances

– 6tp vertical clearance on middle gusset plates

• Provides for substantial ductility and OOP rotational ability

• Controls gusset plate buckling and eliminates need for edge stiffeners

– 8tp elliptical clearance

• Allows for substantial OOP rotation on corner gusset plates

• Valid only for corner gusset plates because stiffening is required on midspan gussets

– 2tkp clearance performed well however may require modification

Conclusions

Ntp Ntp

Ntp

Ntp

Ntkp

• Brace type (HSS vs. Wide-flange)– Type of brace has a large effect on SCBF performance

– Wide flange braces more ductile (can achieve approximately 25% higher story drifts than HSS braces)

– However, wide flange braces:• Increase connection demand and damage• More rapid loss of compressive load after buckling than

HSS braces• May increase risk for a soft story• Have a more pinched hysteretic behavior than HSS

braces and thus dissipate less energy on an equal drift basis

Conclusions

• In-plane buckling HSS braces– Viable option for SCBFs

– Achieve comparable drifts to OOP buckling HSS braces

– Energy dissipation equal for OOP and in-plane buckling braces

– Increased post-earthquake serviceability

– Buckling not impeded by walls, exteriors or studs

Conclusions

• Investigation of larger knife plate clearances (2.5tkp – 3tkp)

• Test SCBF specimen with a pseudo-dynamic loading protocol with varying brace sizes at each level– More accurate representation of seismic loading

• Further evaluate chevron brace configuration to assess demands on beam

Future Work

Acknowledgements

• K.C. Tsai and the NCREE staff• My advisors – Charles Roeder and Dawn Lehman• Po-Chien Hsiao, Kelly Clark, Jake Powell • NEES and NSF• Our advisory committee

Questions/Comments?

Thesis Defense Presentation

Documents

Transcript of Thesis Defense Presentation