Braced Frame Gusset Plate Connections for Seismic Design Charles W. Roeder and Dawn E. Lehman University of Washington, Seattle WA Research Project Funded by NSF Grant CMS

Content of Presentation Why are we doing this? What are our global goals? How do we hope to accomplish goals? Research efforts to achieve goals? –Evaluation of existing design methods –Proposed modifications –Experimental program –Analytical Investigation

Braced Frame Gusset Plate Connections

Why are we doing this? Concentrically braced frames (CBFs) readily meet multiple performance objective for seismic design Stiff and strong to meet functional performance objectives (IO and O) If detailed and designed properly braced frames can develop ductility needed to meet LS and CP performance limit states

Why are we doing this? Primary source of ductility in braced frames is inelastic action in brace  Inelastic brace buckling and tensile yield with Special CBFs (SCBFs)  Axial yield in tension and compression with buckling restrained CBFs (BRCBFs)

Why are we doing this? Seismic Response of Isolated Braces Traditional Braces Buckling Restrained Braces Experimental Results Show Isolated Braces Perform Well

How do we achieve the goals - Gusset Plate Connections Influence Seismic Performance of Braced Frame Systems oBrace performance depends on connection stiffness and strength oDesign rules (e.g. AISC Uniform Force Method) are approximate oNot directed toward ductility requirements oUneconomical connections may result if not designed for actual SCBF or BRCBF demands oOther connections buckled, fractured or otherwise failed prematurely

Global Objectives of Current Research Project Develop a performance-based design strategy for both SCBF and BRCBF gusset plate connections that provides economical design and improved seismic performance.

Research Objectives Meet Multiple Performance Objectives Limit Yielding to Brace for Seldom Events Permit Yielding in Connection for Rare Events Controlling Failure Mode Permitted in MCE Prevent Undesirable Failure Modes for All Events

How Do We Achieve These Goals? Determine Primary Yield Mechanism –Brace Yielding/Buckling for SCBF systems –Brace Yielding for BRCBF systems Balance other yield mechanisms and failure modes (rare events only) R yield <  F yield Balance undesirable failure modes R yield <  F failure Develop IMPROVED Seismic Design Procedure for Braced Frame Systems: Balance Yield Mechanisms and Failure Modes

How Do We Achieve These Goals? Evaluate yield mechanisms and failure modes in SCBFs, BRCBFs and their gusset plate connections Develop Balance Factors to assure good system performance –Balance factor, , similar to  factor but assures ductility not resistance  always less than or equal to 1.0 Smaller when consequences of specific failure mode are severe or unpredictable Larger when consequences of specific failure mode are mild Provide prescriptive details if required to prevent some failure modes Develop IMPROVED Seismic Design Procedure for Braced Frame Systems

Research efforts to achieve goals? Review seismic performance of BRCBF and SCBF systems Examine existing design procedures and research results Evaluate past research results to determine best available models and initial balance conditions Linear and nonlinear FEM analysis to better understand frame and gusset plate performance Experiments -- Approx. 25 large scale tests of brace, surrounding frame (beam and column) and gusset plate connection -- to evaluate performance and refine methods Combine these observations to prepare design recommendations

Summary: Current Research Status and Results

Current Research Status and Results Identified Yield Mechanisms and Failure Modes Gathered Previous Experimental Data Evaluated Current Design Models for: Buckling of Gusset Plate Connection Failure Modes Elastic FE Analysis of Connection Behavior Initial Development of Experimental Setup Initial Design of Reference Specimens

Review of Past Research and Performance Identified appropriate yield mechanisms and failure modes

Variations in Design Models for Buckling of Gusset Plate Connection Most center about the Uniform Force Method Inconsistencies in method –Not focused on seismic –Geometric limitations often contradictory –Tensile capacity uses yield not ultimate –Out-of-plane bending clearance (2t p ) for SCBF –Buckling limits unclear –Moment-Rotational demand not considered

Previous Experimental Results Many past experiments on gusset plate connections –Typically don’t simulate complete boundary conditions or demands from brace –Provide evidence regarding to support identification of yield mechanisms and failure modes –Some existing models are good and some poor

Work to Date Includes Extensive Evaluation of Previous Experiments Brown Edge Buckling Model Modified Thornton Buckling

Limitations of Existing Design Methods Most seismic design methods for braced frame gusset plate connections today use variations of AISC Uniform Force Method

Reference Specimens Application of Uniform Force Method to Seismic Design Applications developed through discussions with west coast structural design engineers Brace, beam and column sized using AISC-LRFD First find plastic capacity of brace - P p = R y F y A g or P p = R y F cr A g For each individual failure mode with resistance R n - P p <  R n or 1.1 P p <  R n for brittle modes

Sequential Check of Connection Behaviors Note asterisks (***) as potential improvement issues Prevent net section fracture of brace *** –R n =  A ne F u –Recall  = 0.75 and F u and F y close together for most modern steels. A ne often reduced from A n –Frequently requires expensive reinforcement plates –Past research suggests may not be required

Sequential Check of Connection Behaviors (2) Size and number of bolts - shear and bearing Check yield of gusset at Whitmore Section*** –Frequently controls gusset plate thickness –Net section fracture of gusset may be more appropriate –Variability in past experimental results –Effect of cyclic load? Block Shear of gusset plate and brace –Sometimes controls gusset plate thickness

Sequential Check of Connection Behaviors (3) 2 t p requirement -- Gusset plate rotation for out-of-plane buckling ******* –Produces extremely large gusset plates some cases –Finite element analysis suggests that perhaps there is a better way Gusset plate buckling *** –Runs counterproductive to 2 t p requirement –Presently uses variations of Thorton method –Past experiments suggest improvements possible

Sequential Check of Connection Behaviors (4) –Free edge buckling also commonly employed Past experiments suggest may not be meaningful Gusset plate-beam-column interface forces *** –Several different methods (regular Uniform force, parallel force, truss force, etc.) - different results –Concerned about impact on BRCBFs and SCBFs with in-plane buckling since significant moment is induced –Severe geometric contstrain when paired with 2tp

Sequential Check of Connection Behaviors (5) Welded connections –Bolted plate to column joints not commonly used for seismic design –Because of large plates required for other issues, welds frequently controlled by minimum size –Not a lot of weld fractures noted, but could be increasingly important with changes note above Beam-to-column connection –Welded webs commonly required due to large forces developed at gusset plate-beam interface

Possible Modifications to Existing Design Methods

Issues of Concern and Possible Improvement Net Section Fracture of Brace Yield of Whitmore Section 2 t p requirement Gusset Plate Buckling Gusset plate-beam-column interface forces

Net Section Fracture of Brace oNet Section Fracture pf brace well predicted in past experiments - moderately ductile - Perhaps Pp <  Rn where  ≈ 0.85 is adequate for ductility oNet section fracture of gusset plate on other hand very severe and poorly predicted

Some insight regarding - Yield of Whitmore Section - 2 t p requirement has been provided through FEM analysis Preliminary results

FEM Analysis Linear and Nonlinear Local and Global Further evaluate: –Effects of geometry –Progression of yield –Potential for fracture –Evaluation and balancing of different yield mechanisms and failure modes Note distribution of stress with out-of-plane bending

FEM Analysis -- 2

Gusset Plate Buckling Need to consider that gusset plate and brace buckle in series Need to consider that buckling demands may be very different for BRCBF and SCBF

Gusset Plate Buckling - Past Experimental Results Brown Edge Buckling Model Modified Thornton Buckling

-- Current Status Continuing examination of past results, FEM analysis and simple models for prediction gusset plate connection performance are continuing. Different models and balance conditions may be required for BRCBFs and SCBFs

Experimental Program Will examine these critical design issues

Objectives of Experimental Program Establish response of braced frame connections designed using current design methods Evaluate proposed design modifications Verify proposed design method –Based upon experiments and analysis

Experimental Program Test Specimens to be tested horizontally on strong floor. They will react against strong wall of test bay as illustrated

Experimental Program Approx 25 test specimens –Include brace, gusset plate at each end, and beam and column framing surrounding the braced bay. –Full scale specimens 3 or 4 story building or upper stories of taller building –Propose to test specimens in 3 or 4 groups of approx 6 to 8 specimens -- approx 1.5 to 2 month intervals Group 1 - Basic SCBF Group 2 - Basic BRCBF Groups 3 and 4 - SCBF and BRCBF -- to learn from basic test results and refine and verify our proposed methods –Have funding to do tests -- Currently working with AISC, suppliers and fabricators to seek support to build specimens

Experimental Program -- 2 Approximately two thirds to SCBF designs –Likely parameters : Tapered or rectangular geometry -- Appear to be problems and advantages in switching from one to the other Effect of reduced or increased plate thickness -- Expect that some local yielding of gusset may be beneficial -- Present design limits appear to prevent all yield Gusset Plate buckling Edge stiffener effect and requirements Simpler, more economical and more practical way of assuring out-of-plane rotation -- particularly for rectangular gusset plates Range of different failure modes

Experimental Program -- 3 Approximately one third to be BRCBF designs –Expect to test two manufacturers –Have a commitment for participation by Nippon Steel Unbonded braces –BRCBFs bring out different concerns than SCBF Tapered or rectangular geometry -- Effect on rotational stiffness of the total joint may be more significant Gusset Plate buckling -- May be more critical to avoid than for SCBF and may require different slenderness estimates Overall connection stiffness and progression of yielding in gusset plate

Experimental Program -- 4 Test program to include –Double angle braces with bolted joints –Rectangular tubular braces with welded joints –Approximately one third to be BRCBF designs Have a commitment for participation by two manufacturers –Test a range of parameters such as noted earlier

Possible SCBF Test Specimen with Rectangular Tube Braces

Possible Details of Typical SCBF Connection with Tube Braces

Progress on Experimental Program to date

Test Frame and Apparatus Has Been Built

Test Frame and Apparatus

Repeat of Test Setup

Initial Specimen Fabricated in Early August

Initial Specimen Fabricated and Erected in Early August

Still awaiting delivery of the major steel needed for the test specimens.

Acknowledgements Funding for research provided by National Science Foundation, CMS , "Performance-Based Seismic Design of Concentrically Braced Frames.” Unbonded Braces to be provided by Nippon Steel. Steel for test specimens to be provided by by the US steel industry as coordinated by Tom Shlafly of AISC and Mike Engestrom of Nucor-Yamato through the AISC Steel Producers Committee.