Lee et al. -- AISC Final Report on Column Reinforcement

464 Pages·2005·19.4 MB·English

by  Jerome F. Hajjar

Structural Engineering Report No. ST-01-3.2

COLUMN STIFFENER DETAILING
AND PANEL ZONE BEHAVIOR OF
STEEL MOMENT FRAME CONNECTIONS

Daeyong Lee, Sean C. Cotton, Robert J. Dexter,
Jerome F. Hajjar, Yanqun Ye, and Sara D. Ojard

June 2002

Department of Civil Engineering
Institute of Technology
University of Minnesota
Minneapolis, Minnesota 55455
Abstract

Extensive damage to steel moment connections has been reported since the 1994
Northridge earthquake. While low-toughness welds and notch-like discontinuities
created by the bottom flange backing bars have been identified as primary causes of the
fractures that occurred, other factors have also been linked to the failures. Column
stiffening design practices, resulting in the design of weak panel zones and a lack of or
insufficient use of continuity plates, have been speculated as potential contributors to the
fractures. The concern associated with the role of column stiffener detailing in the
Northridge damage has subsequently led to a tendency towards over-conservatism in
stiffener design. However, the additional expense of larger stiffeners coupled with the
potential for fabrication cracking due to larger, highly restrained welds required to attach
such stiffeners is an undesirable consequence of this increased conservatism.
To study these column stiffener design issues, a research project was launched to
reassess the AISC design criteria related to the limit states of Local Flange Bending
(LFB), Local Web Yielding (LWY), and Panel Zone (PZ) strength, and to develop new,
economical alternatives for the detailing of such stiffeners. The project contains three
components: finite element analyses to investigate the performance of various column
stiffener details and to corroborate the results of the experiments, monotonically-loaded
pull-plate tests to evaluate the non-seismic LFB and LWY design criteria and to
investigate non-seismic behavior of the alternative stiffener details, and cyclically-loaded
cruciform tests to evaluate the seismic LFB, LWY, and PZ design criteria and to
investigate the seismic behavior of the alternative stiffener details. This report focuses
primarily on reporting the results of the cyclically-loaded cruciform tests.
A total of six cruciform girder-to-column joint subassemblages were fabricated
and tested in this project. Five joint subassemblages were originally designed and
fabricated for the investigation into the provisions for detailing of column stiffening.
However, due to premature brittle girder fractures occurring in one of the five test
i
specimens, an additional cruciform specimen with similar detailing and improved notch-
toughness in the weld metal was fabricated and tested. The column sizes selected in this
project ranged from a W14x145 to a W14x283. The range of these column sizes
permitted several stiffener details to be included in the test matrix, and for the limits of
the LFB and PZ limit states to be explored. A W24x94 was used for all girders. The
panel zones of the specimens were designed to be relatively weak in most specimens,
such that the stiffening details would be thoroughly tested at large strains. The
alternative stiffener details included two fillet-welded doubler plate details, one groove-
welded offset doubler plate detail, and one fillet-welded continuity plate detail with a
thickness of the continuity plate being approximately half the thickness of the girder
flange. These details avoid placing highly restrained Complete Joint Penetration (CJP)
groove welds in the potentially low-toughness k-area of the columns.
The connection detail tested in this project may be classified as a Welded
Unreinforced Flange-Welded Web (WUF-W) connection detail, which is a prequalified
connection within FEMA 350. This connection consisted of a CJP-welded girder web-to-
column flange, an improved weld access hole detail in the girder, and girder flange-to-
column flange CJP weld details outlined in FEMA 350 and FEMA 353. Welding
included the use of E70T-6 consumables for the girder flange-to-column flange CJP
welds and E71T-8 for the girder web-to-column flange CJP welds and the shear tab-to-
girder web fillet welds. Shear tab-to-column flange and all stiffeners-to-column flange
welds used E70T-1 consumables.
From the cruciform tests, the performance of the alternative stiffening details as
well as the limit states of LFB and PZ strength were assessed. The five specimens,
excluding the prematurely fractured specimen, completed the SAC loading history, each
with several cycles at 4.0% interstory drift without noticeable strength degradation. The
primary failure mode was Low-Cycle Fatigue (LCF) fracturing in one or more girder
flanges. The test results showed that, when properly detailed and welded with notch-
tough filler metal, the WUF-W moment connections can perform adequately under large
quasi-static cyclic loads even though relatively weak panel zone strengths are chosen. In
addition, the test results confirmed that satisfactory cyclic connection performance may
ii
be achieved without continuity plates if the column flanges are sufficiently thick. The
alternative column stiffener details in steel moment-resisting connections were also
successfully verified.
The test results showed that the LFB strength equation included in the 1999 AISC
LRFD Specification is adequate, if slightly conservative, for non-seismic detailing (in
addition, related research on this project proposed alternative LFB and LWY strength
equations that better match the test data). For seismic detailing, when coupled with the
application of the seismic demand specified in the 1992 AISC Seismic Provisions and
FEMA 350, the LFB strength equation is more clearly conservative. The panel zone
strength equation included in the 1997 AISC Seismic Provisions was also evaluated
based on the five successful test results, as well as on a comparison with the 44 past
experimental tests. This equation was found to significantly overpredict the panel zone
strength in many cases, particularly for specimens having larger columns. An alternate
model (i.e., a modified Fielding and Huang model) estimating the panel zone post-elastic
stiffness was developed from a newly assumed panel zone behavior at its ultimate state,
and modified based on the five experimental results so as to more accurately capture the
post-yield panel zone strength. This modified Fielding and Huang model was found to be
more accurate in its prediction of the panel zone strength of W14 and larger columns, and
was shown to be somewhat conservative for smaller columns.
In addition, it was determined that in order to provide a more accurate assessment
of the panel zone strength corresponding to experimental results, the panel zone strength
equation of the 1997 AISC Seismic Provisions, or that proposed in this research based
upon the modified Fielding and Huang model, needs to be scaled. For this purpose, a
new methodology, which also properly scales the corresponding panel zone design
demand as well as the panel zone strength, is introduced. This methodology may be used
to evaluate the selected panel zone equation based on experimental results, and can
provide an appropriate demand for the selected panel zone strength equation.
The report concludes with a series of specific conclusions related to column
stiffener design of steel moment-resisting connections. Conditions under which no
stiffeners are required, both for non-seismic and seismic detailing, are clarified,
iii
assessments of the alternative stiffener details are summarized, and issues related to the
proposed design equations are highlighted.

iv
Acknowledgements

This research was sponsored by the American Institute of Steel Construction, Inc.
(AISC) and by the University of Minnesota. In-kind funding and materials were
provided by LeJeune Steel Company, Minneapolis, Minnesota, Danny’s Construction
Company, Minneapolis, Minnesota, Braun Intertec, Minneapolis, Minnesota, and Edison
Welding Institute, Columbus, Ohio. Special thanks are due to Lincoln Electric Company
for donating an Idealarc DC-600 power supply and an LN-10 wire feed unit to the
University of Minnesota Structures Laboratory. Supercomputing resources were
provided by the Minnesota Supercomputing Institute. This support is gratefully
acknowledged.
The authors wish to thank Professor Theodore V. Galambos of the University of
Minnesota, Mr. Lawrence A. Kloiber of LeJeune Steel Company, and Dr. John C. Nelson
of the University of Minnesota Institute of Technology Characterization Facility for their
valuable assistance with the design, fabrication, and characterization of the experimental
tests. A great deal of thanks is also extended to Mr. Paul M. Bergson of the University of
Minnesota for his many hours of assistance and advice related to the experimental
research. The authors also appreciate the efforts of a number of undergraduate and
graduate students who assisted at various times throughout this project. Finally, the
authors would like to thank the members of the external advisory group on this project
for their advice and guidance, as well as others who have provided assistance throughout
this research. These individuals include: T. Schlafly, AISC, C. Carter, AISC, W. Baker,
Skidmore, Owings & Merrill, J. Barsom, Pennsylvania, R. Bjorhovde, The Bjorhovde
Group, M. Engstrom, Nucor-Yamato Steel Company, J. Fisher, Lehigh University, M.
Johnson, Edison Welding Institute, J. Malley, Degenkolb Engineers, D. Miller, Lincoln
Electric Company, S. Rolfe, University of Kansas, and W. Thornton, Cives Engineering
Corporation.
v
Table of Contents

Abstract............................................................................................................................i

Acknowledgements.......................................................................................................v

List of Figures ................................................................................................................x

List of Tables...........................................................................................................xxvii

1. Introduction..............................................................................................................1
1.1 Research Objectives................................................................................................3
1.2 Organization of the Report......................................................................................5

2. Background of Design Provisions for Column Stiffening........................7
2.1 History of Panel Zone Design Provisions...............................................................7
2.1.1 Panel Zone Shear Strength.............................................................................8
2.1.2 Panel Zone Slenderness...............................................................................15
2.2 Opinions Regarding Panel Zone Design and Behavior........................................16
2.3 Background of LFB and LWY Provisions...........................................................21
2.4 Economic Considerations of Column Stiffening..................................................24

3. Specimen Selection and Design........................................................................28
3.1 Parametric Study of Panel Zone Stiffening Requirements...................................29
3.1.1 Definitions of Parameters............................................................................29
3.1.2 Results of Panel Zone Parameter Study.......................................................35
3.1.3 Conclusions..................................................................................................39
3.2 Specimen Selection...............................................................................................40
3.2.1 Cruciform Specimen Selection Procedure...................................................40
3.2.2 Specimen Justification.................................................................................55
v i
3.2.2.1 Specimen Parameter Discussion.........................................................55
3.2.2.2 Individual Specimen Discussion.........................................................59
3.3 Specimen Design..................................................................................................62
3.3.1 Dimensions of Cruciform Specimens..........................................................62
3.3.2 Girder-to-Column Moment Connection Design..........................................63
3.3.2.1 Welded Connection Details................................................................64
3.3.2.2 Access Hole Details............................................................................66
3.3.3 Panel Zone and Column Stiffener Design...................................................67
3.3.3.1 Panel Zone Design..............................................................................67
3.3.3.2 Fillet-Welded Doubler Plate Design...................................................69
3.3.3.3 Groove-Welded Offset (Box) Doubler Plate Design..........................72
3.3.3.4 Continuity Plate Design......................................................................72
3.4 Material Properties................................................................................................74
3.4.1 Steel Material Properties..............................................................................74
3.4.2 Weld Material Properties.............................................................................76

4. Test Setup and Instrumentation......................................................................92
4.1 Test Setup..............................................................................................................92
4.2 Loading History....................................................................................................94
4.3 Instrumentation.....................................................................................................95
4.3.1 Strain Gages.................................................................................................95
4.3.2 Linear Variable Differential Transformers..................................................97
4.3.3 Specimen Coordinate System and Gage Locations.....................................99

5. Summary of Test Results.................................................................................131
5.1 Applied Loading History....................................................................................131
5.1.1 Loading History of Specimen CR1............................................................131
5.1.2 Loading History of Specimen CR2............................................................132
5.1.3 Loading History of Specimen CR3............................................................132
5.1.4 Loading History of Specimen CR4............................................................132
vi i
5.1.5 Loading History of Specimen CR4R.........................................................132
5.1.6 Loading History of Specimen CR5............................................................133
5.2 Summary of Specimen Performance..................................................................133
5.2.1 Performance of Specimen CR1..................................................................133
5.2.2 Performance of Specimen CR2..................................................................135
5.2.3 Performance of Specimen CR3..................................................................137
5.2.4 Performance of Specimen CR4..................................................................139
5.2.5 Performance of Specimen CR4R...............................................................140
5.2.6 Performance of Specimen CR5..................................................................142

6. Analysis and Discussion of Test Results.....................................................198
6.1 Comparison of Experimental Behavior and Finite Element Analysis................199
6.2 Panel Zone Behavior...........................................................................................201
6.2.1 Progression of Panel Zone Yielding..........................................................201
6.2.2 Panel Zone Behavior in the Elastic Range.................................................203
6.2.3 Effects of Large Panel Zone Deformation.................................................205
6.2.4 AISC Panel Zone Provisions.....................................................................207
6.3 Alternate Model for Post-Yield Panel Zone Strength.........................................211
6.3.1 Modified Fielding and Huang Model........................................................211
6.3.2 Verification of Modified Fielding and Huang Model (Equation 6.10)......215
6.3.3 Comparison of Panel Zone Thicknesses....................................................218
6.3.4 Scaling of Panel Zone Design Capacity and Demand...............................222
6.4 Local Flange Bending.........................................................................................225
6.5 Stress Distribution in Girder Flange...................................................................233
6.6 Comparison of Results and Discussion of Continuity Plate and Doubler
Plate Detailing.....................................................................................................235

7. Summary and Conclusions..............................................................................323
7.1 Research Summary.............................................................................................323
7.2 Conclusions and Recommendations...................................................................329
vi ii
7.2.1 Conclusions................................................................................................329
7.2.2 Recommendations for Future Research.....................................................334

Appendix A. Calculation of Specimen Deformation and Rotation
Quantities...............................................................................................................336
A.1 Rotation and Deformation from Displacement Data.........................................336
A.2 Calculated Plastic Rotation and Deformation Quantities..................................338
A.3 Plastic Rotations Relative to the Column Centerline.........................................340

Appendix B. Failure Analysis of Specimen CR4..........................................401

References....................................................................................................................427

ix