AISC DESIGN GUIDE 12-1999 Modification of Existing Welded Steel Moment Frame Connections for Seismic Resistance.pdf

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1、Steel Design Guide SeriesModification of ExistingWelded Steel Moment FrameConnections for Seismic ResistanceModification of ExistingWelded Steel Moment FrameConnections for Seismic ResistanceJohn L. GrossNational Institute of Standard and TechnologyGaithersburg, MDMichael D. EngelhardtUniversity of

2、Texas at AustinAustin, TXChia-Ming UangUniversity of California, San DiegoSan Diego, CAKazuhiko KasaiTokyo Institute of TechnologyYokohama, JAPANNestor R. IwankiwAmerican Institute of Steel ConstructionChicago, ILAMERICAN INSTITUTE OF STEEL CONSTRUCTIONSteel Design Guide Series 2003 by American Inst

3、itute of Steel Construction, Inc. All rights reserved.This publication or any part thereof must not be reproduced in any form without permission of the publisher.Copyright 1999byAmerican Institute of Steel Construction, Inc.All rights reserved. This book or any part thereofmust not be reproduced in

4、any form without thewritten permission of the publisher.The information presented in this publication has been prepared in accordance with rec-ognized engineering principles and is for general information only. While it is believedto be accurate, this information should not be used or relied upon fo

5、r any specific appli-cation without competent professional examination and verification of its accuracy,suitablility, and applicability by a licensed professional engineer, designer, or architect.The publication of the material contained herein is not intended as a representationor warranty on the p

6、art of the American Institute of Steel Construction or of any otherperson named herein, that this information is suitable for any general or particular useor of freedom from infringement of any patent or patents. Anyone making use of thisinformation assumes all liability arising from such use.Cautio

7、n must be exercised when relying upon other specifications and codes developedby other bodies and incorporated by reference herein since such material may be mod-ified or amended from time to time subsequent to the printing of this edition. TheInstitute bears no responsibility for such material othe

8、r than to refer to it and incorporateit by reference at the time of the initial publication of this edition.Printed in the United States of AmericaRevision: October 2003 2003 by American Institute of Steel Construction, Inc. All rights reserved.This publication or any part thereof must not be reprod

9、uced in any form without permission of the publisher.TABLE OF CONTENTSPreface1. Introduction 11.1 Background . 11.2 Factors Contributing to Connection Failures . 21.3 Repair and Modification . . 31.4 Objective of Design Guide. . 42. Achieving Improved Seismic Performance . 52.1 Reduced Beam Section

10、52.2 Welded Haunch . 62.3 Bolted Bracket 73. Experimental Results 93.1 Related Research 93.1.1 Reduced Beam Section. 93.1.2 Welded Haunch . 153.1.3 Bolted Bracket. . 153.2 NIST/AISC Experimental Program. 203.2.1 Reduced Beam Section. . 223.2.2 Welded Haunch . 243.2.3 Bolted Bracket. . 274. Design Ba

11、sis For Connection Modification . . 294.1 Material Strength . 304.2 Critical Plastic Section . 304.3 Design Forces . 324.3.1 Plastic Moment . 324.3.2 Beam Shear. . 334.3.3 Column-Beam Moment Ratio . 334.4 Connection Modification PerformanceObjectives. . . . . . . . . . . . . . . . . . . . . . . 355.

12、 Design of Reduced Beam SectionModification. . 375.1 Recommended Design Provisions. . 375.1.1 Minimum Recommended RBSModifications. . 375.1.2 Size and Shape of RBS Cut. 375.1.3 Flange Weld Modifications . 425.1.4 Techniques to Further EnhanceConnection Performance 435.2 Additional Design Considerati

13、ons. . 465.3 Design Example. . 466. Design of Welded Haunch Modification. 496.1 Recommended Design Procedure 496.1.1 Structural Behavior and DesignConsiderations. 496.1.2 Simplified Haunch Connection Modeland Determination of Haunch FlangeForce . 516.1.3 Haunch Web Shear. . . . . . . . . . . . . 546

14、.1.4 Design Procedure. . 556.2 Recommended Detailing Provisions 556.2.1 Design Weld. 556.2.2 Design Stiffeners. 556.2.3 Continuity Plates 566.3 Design Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 567. Design of Bolted Bracket Modification . 597.1 Minimum Recommended Bracket Desi

15、gnProvisions . 607.1.1 Proportioning of Bolted HaunchBracket. . 607.1.2 Beam Ultimate Forces . . . . . . 627.1.3 Haunch Bracket Forces at BeamInterface. . . . . . . . . . . . . . . . . . . 627.1.4 Haunch Bracket Bolts. 637.1.5 Haunch Bracket Stiffener Check . . . 647.1.6 Angle Bracket Design. 667.2

16、Design Example. . 698. Considerations for Practical Implementation 738.1 Disruption or Relocation ofBuilding Tenants. . 738.2 Removal and Restoration of CollateralBuilding Finishes 738.3 Health and Safety of Workers and Tenants . . 738.4 Other Issues. 749. References. . 75Symbols 77Abbreviations. .

17、79APPENDIX A . 814.5 Selection of Modification Method . . . . . . . 367.1.7 Requirements for Bolt Hole and WeldSize . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 697.1.8 Column Panel Zone Check . . . . . . . . . . . 697.1.9 Column Continuity Plate Check . . . . . . 69Rev.3/1/03 2003

18、by American Institute of Steel Construction, Inc. All rights reserved.This publication or any part thereof must not be reproduced in any form without permission of the publisher.PREFACEThe Congressional emergency appropriation resultingfrom the January 17, 1994, Northridge earthquake pro-vided the B

19、uilding and Fire Research Laboratory (BFRL)at the National Institute of Standards and Technology(NIST) an opportunity to expand its activities in earth-quake engineering under the National Earthquake HazardReduction Program (NEHRP). In addition to the post-earthquake reconnaissance, BFRL focused its

20、 effortsprimarily on post-earthquake fire and lifelines and onmoment-resisting steel frames.In the area of moment-resisting steel frames damagedin the Northridge earthquake, BFRL, working with prac-ticing engineers, conducted a survey and assessment ofdamaged steel buildings and jointly funded the S

21、AC(Structural Engineers Association of California, AppliedTechnology Council, and California Universities for Re-search in Earthquake Engineering) Invitational Workshopon Steel Seismic Issues in September 1994. Forming ajoint university, industry, and government partnership,BFRL initiated an effort

22、to address the problem of therehabilitation of existing buildings to improve their seis-mic resistance in future earthquakes. This design guide-line is a result of that joint effort.BFRL is the national laboratory dedicated to enhanc-ing the competitiveness of U.S. industry and public safetyby devel

23、oping performance prediction methods, measure-ment technologies, and technical advances needed to as-sure the life cycle quality and economy of constructedfacilities. The research conducted as part of this industry,university, and government partnership and the resultingrecommendations provided here

24、in are intended to fulfill,in part, this mission.This design guide has undergone extensive review bythe AISC Committee on Manuals and Textbooks; theAISC Committee on Specifications, TC 9Seismic De-sign; the AISC Committee on Research; the SAC ProjectOversight Committee; and the SAC Project Managemen

25、tCommittee. The input and suggestions from all those whocontributed are greatly appreciated. 2003 by American Institute of Steel Construction, Inc. All rights reserved.This publication or any part thereof must not be reproduced in any form without permission of the publisher.Chapter 1INTRODUCTIONThe

26、 January 17, 1994 Northridge Earthquake caused brit-tle fractures in the beam-to-column connections of certainwelded steel moment frame (WSMF) structures (Youssefet al. 1995). No members or buildings collapsed as a re-sult of the connection failures and no lives were lost.Nevertheless, the occurrenc

27、e of these connection fractureshas resulted in changes to the design and constructionof steel moment frames. Existing structures incorporat-ing pre-Northridge1practices may warrant re-evaluationin light of the fractures referenced above.The work described herein addresses possible designmodification

28、s to the WSMF connections utilized in pre-Northridge structures to enhance seismic performance.1.1 BackgroundSeismic design of WSMF construction is based on theassumption that, in a severe earthquake, frame memberswill be stressed beyond the elastic limit. Inelastic action1The term “pre-Northridge“

29、is used to indicate design, detailing or con-struction practices in common use prior to the Northridge Earthquake.is permitted in frame members (normally beams or gird-ers) because it is presumed that they will behave in a duc-tile manner thereby dissipating energy. It is intended thatwelds and bolt

30、s, being considerably less ductile, will notfracture. Thus, the design philosophy requires that suffi-cient strength be provided in the connection to allow thebeam and/or column panel zones to yield and deform in-elastically (SEAOC 1990). The beam-to-column momentconnections should be designed, ther

31、efore, for either thestrength of the beam in flexure or the moment correspond-ing to the joint panel zone shear strength.The Uniform Building Code, or UBC (ICBO 1994) isadopted by nearly all California jurisdictions as the stan-dard for seismic design. From 1988 to 1994 the UBC pre-scribed a beam-to

32、-column connection that was deemed tosatisfy the above strength requirements. This “prescribed“detail requires the beam flanges to be welded to the columnusing complete joint penetration (CJP) groove welds. Thebeam web connection may be made by either welding di-rectly to the column or by bolting to

33、 a shear tab which inturn is welded to the column. A version of this prescribeddetail is shown in Figure 1.1. Although this connectionFigure 1.1 Prescribed Welded Beam-to-Column MomentConnection (Pre-Northridge)1 2003 by American Institute of Steel Construction, Inc. All rights reserved.This publica

34、tion or any part thereof must not be reproduced in any form without permission of the publisher.detail was first prescribed by the UBC in 1988, it has beenwidely used since the early 1970s.The fractures of “prescribed“ moment connections inthe Northridge Earthquake exhibited a variety of originsand

35、paths. In general, fracture was found to initiate at theroot of the beam flange CJP weld and propagate througheither the beam flange, the column flange, or the weld it-self. In some instances, fracture extended through the col-umn flange and into the column web. The steel backing,which was generally

36、 left in place, produced a mechani-cal notch at the weld root. Fractures often initiated fromweld defects (incomplete fusion) in the root pass whichwere contiguous with the notch introduced by the weldbacking. A schematic of a typical fracture path is shownin Figure 1.2. Brittle fracture in steel de

37、pends upon thefracture toughness of the material, the applied stress, andsize and shape of an initiating defect. A fracture analysis,based upon measured fracture toughness and measuredweld defect sizes (Kaufmann et al. 1997), revealed thatbrittle fracture would occur at a stress level roughly in the

38、range of the nominal yield stress of the beam.The poor performance of pre-Northridge moment con-nections was verified in laboratory testing conductedunder SAC2Program to Reduce Earthquake Hazards inSteel Moment-Resisting Frame Structures (Phase 1)(SAC 1996). Cyclic loading tests were conducted on12

39、specimens constructed with W30X99 and W36x150beams. These specimens used connection details andwelding practices in common use prior to the Northridge2SAC is a Joint Venture formed by the Structural Engineers Associ-ation of California (SEAOC), the Applied Technology Council (ATC),and the California

40、 Universities for Research in Earthquake Engineering(CUREe).Figure 1.2 Typical Fracture PathEarthquake. Most of the 12 specimens failed in a brittlemanner with little or no ductility. The average beam plas-tic rotation developed by these 12 specimens was approxi-mately 0.005 radian. A number of spec

41、imens failed at zeroplastic rotation, and at a moment well below the plasticmoment of the beam. Figure 1.3 shows the results of oneof these tests conducted on a W36x 150 beam.1.2 Factors Contributing to Connection FailuresBrittle fracture will occur when the applied stress inten-sity, which can be c

42、omputed from the applied stress andthe size and character of the initiating defect, exceeds thecritical stress intensity for the material. The critical stressintensity is in turn a function of the fracture toughness ofthe material. In the fractures that occurred in WSMF con-struction as a result of

43、the Northridge Earthquake, sev-eral contributing factors were observed which relate to thefracture toughness of the materials, size and location of de-fects, and magnitude of applied stress. These factors arediscussed here.The self-shielded flux cored arc welding (FCAW) pro-cess is widely used for t

44、he CJP flange welds in WSMFconstruction. Electrodes in common use prior to theNorthridge earthquake are not rated for notch toughness.Testing of welds samples removed from several buildingsthat experienced fractures in the Northridge earthquakerevealed Charpy V-notch (CVN) toughness frequently onthe

45、 order of 5 ft-lb to 10 ft-lb at 70F (Kaufmann 1997).Additionally, weld toughness may have been adverselyaffected by such practices as running the weld “hot“ toachieve higher deposition rates, a practice which is not inconformance with the weld wire manufacturers recom-mendations.The practice of lea

46、ving the steel backing in place intro-duces a mechanical notch at the root of the flange weldjoint as shown in Figure 1.2. Also, weld defects in the rootpass, being difficult to detect using ultrasonic inspection,may not have been characterized as “rejectable“ and there-fore were not repaired. Furth

47、er, the use of “end dams“ inlieu of weld tabs was widespread.The weld joining the beam flange to the face of therelatively thick column flanges is highly restrained. Thisrestraint inhibits yielding and results in somewhat morebrittle behavior. Further, the stress across the beam flangeconnected to a

48、 wide flange column section is not uni-form but rather is higher at the center of the flange andlower at the flange tips. Also, when the beam web con-nection is bolted rather than welded, the beam web doesnot participate substantially in resisting the moment;instead the beam flanges carry most of th

49、e moment. Simi-larly, much of the shear force at the connection is trans-ferred through the flanges rather than through the web.These factors serve to substantially increase the stress on2 2003 by American Institute of Steel Construction, Inc. All rights reserved.This publication or any part thereof must not be reproduced in any form without permission of the publisher.(a) Connection Detail(b) Moment-Plastic Rotation Responseof Test SpecimenFigure 1.3 Laboratory Response of W36x150 Beam withpre-Northridge Connectionthe beam flange groove welds and surrounding base metalregio