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1、318 LOCAL FLANGE BENDING AND LOCAL WEB YIELDING LIMIT STATES IN STEEL MOMENT-RESISTING CONNECTIONS Sara D. PROCHNOW1, Yanqun YE1, Robert J. DEXTER1, Jerome F. HAJJAR1, and Sean C. COTTON1, ABSTRACT Nine pull- plate experiments were conducted to examine the effect of column stiffening on the limit st
2、ates of local flange bending and local web yielding. The results show that AISC provisions for these limit states are reasonable and slightly conservative. Weld fractures did not occur despite the fact that some of the specimens were significantly under- stiffened. The use of half- thickness continu
3、ity plates fillet welded to the column web and flanges was shown to be sufficient in comparison to full- thickness continuity plates with CJP welds. INTRODUCTION Beam- to- column flange welds fractured in some girder- to- column connections in steel moment frames during the 1994 Northridge earthquak
4、e. These welds fractured primarily because of low fracture toughness of weld metal combined with a backing bar forming a notch at the weld root and weld root defects, Fisher, et al. (1). Subsequently, there has been a tendency to be overly conservative in the design and detailing of these connection
5、s. For example, there has been a tendency to over- specify column stiffeners even though there is no definitive evidence that inadequate column stiffeners contributed to the Northridge weld fractures. Continuity plates and web doubler plates have been specified when they are unnecessary and, when th
6、ey are necessary, thicker stiffeners have often been specified than would be required and complete joint penetration (CJP) welds of the continuity plates have been specified when more economical fillet welds may have sufficed. The tendency to be overly conservative with column stiffeners is understa
7、ndable since they do have a significant effect on the stress and strain distribution in the connection and on connection performance. For example, Roeder (3) observed that girder- to- column joints with modest continuity plates and/or doubler plates performed better in cyclic loading tests than join
8、ts without such reinforcement. Also, it has been observed from finite element analyses of these joints that there is a decrease in stress concentration at the middle of the girder flange- to- column flange weld when continuity plates are used, e.g., Roeder (3), El- Tawil et al. (4). The limit states
9、 of local web yielding (LWY), local flange bending (LFB), local web crippling (LWC), and panel zone (PZ) shear yielding are mitigated by the use of column stiffening. The design criteria for these limit states are provided in Section K1 of Chapter K of the AISC LRFD Specification for Structural Stee
10、l Buildings (5). There were additional, more stringent provisions in the requirements for Special Moment Frames 1 Department of Civil Engineering, 500 Pillsbury Drive SE, University of Minnesota, Minneapolis, Minnesota 55455- 0116, E-mail: dextertc.umn.edu. 319 (SMF) in the 1992 AISC Seismic Provisi
11、ons for Structural Steel Buildings. However, the 1997 AISC Seismic Provisions (6) removed all design procedures related to continuity plates, requiring instead that they be proportioned to match those provided in the tests used to qualify the connection. As part of the SAC Joint Venture, interim gui
12、delines and an advisory were published, FEMA (7), (8), that pertained to these column reinforcements in seismic zones. For example, the guidelines call for continuity plates at least as thick as the beam flange that must be joined to the column flange in a way that fully develops the strength of the
13、 continuity plate, i.e., this encourages the use of CJP welds. However, the SAC 100% draft document, FEMA (9), has reestablished design equations to determine whether continuity plates are required and, if so, what thickness they need to be. The present provisions for local web yielding, local flang
14、e bending, and panel zone shear are largely based on limit- load analyses that were developed in conjunction with girder- to- column joint subassembly test data, Graham et al. (10), Krawinkler et al. (11), Bertero et al. (12). The criteria for panel zone shear strength have been refined by numerous
15、tests and analyses of girder- to- column subassemblies for seismic moment frames, e.g., Krawinkler (13) and Popov et al. (14). Recent research has revealed that excessively thick continuity plates are unnecessary. El- Tawil et al. (4) performed parametric finite element analyses of girder- to- colum
16、n joints. They found that continuity plates are increasingly effective as the thickness increases to about 60% of the girder flange. However, continuity plates more than 60% of the girder flange thickness brought diminishing returns. Furthermore, over- specification of column reinforcement may actua
17、lly be detrimental to the performance of connections. As continuity plates are made thicker and attached with highly restrained CJP welds, they are sometimes causing cracking during fabrication. CJP welds have also been specified for the attachment of continuity plates to the web, where fillet welds
18、 have traditionally been adequate. Yee et al. (15) performed finite element analyses comparing fillet welded and CJP welded continuity plates. Based on principal stresses extracted at the weld terminations, it was concluded that fillet welded continuity plates may be less susceptible to cracking dur
19、ing fabrication than if CJP welds are used. The research described in this paper is part of an ongoing project sponsored by AISC to reassess the design provisions for column stiffeners for non- seismic and seismic conditions, and to investigate new alternative column stiffener details. The project i
20、ncludes three components: monotonically- loaded pull- plate experiments to investigate the need for and behavior of transverse stiffeners, cyclically- loaded cruciform girder- to- column joint experiments to investigate panel zone behavior and local flange bending as well as innovative doubler plate
21、 and continuity plate details, and parametric finite element analyses to corroborate the experiments and assess the performance of various transverse stiffener and doubler plate details. The test matrices for this project were designed by examining all practical combinations of girder and column siz
22、es to identify which girder- to- column joints satisfy the limit states of LFB, LWY, web crippling, and panel zone yielding, as well as which combinations satisfy the strong- column/weak- beam (SCWB) provisions according to AISC (5,6). A parametric study was then conducted using three- dimensional n
23、onlinear continuum finite element analysis (FEM) to model the behavior of these connections and the performance of various transverse stiffener and doubler plate details. These analyses permitted the behavior of these connections to be characterized in detail. Criteria were 320 established to identi
24、fy the limit states of LWY, LFB and panel zone yielding flange for stiffened and unstiffened specimens. The results of the parametric study showed that web crippling did not control the need for column stiffening in any of the practical combinations of girder and column sizes, and therefore was not
25、further investigated in the research program. The results of the finite element analyses and the comprehensive investigation of the limit states for all beam and column combinations were then coupled with the results of past experiments to establish the test matrices for the present project. Nine la
26、boratory experiments were conducted with pull- plates (simulating a girder flange) attached to column sections for the study of localized flange bending and web yielding, investigating both common and new alternatives for detailing. These monotonic tests focus on the non- seismic behavior, with some
27、 consideration given to seismic design as well. Additional experiments are then being conducted on five full- scale cyclic girder- to- column joint subassemblies. These tests will focus on seismic behavior, although they will provide useful information for non- seismic design as well. This paper out
28、lines the results of the nine pull- plate experiments and corresponding finite- element analyses. The complete literature review, including extensive background information on the various limit states investigated, description of the details of the nine pull- plate and five cruciform experiments and
29、 testing procedure, and further detail of the background of this research and justification for specific specimen selection were presented in Dexter et al. (16). TESTING PROCEDURE Figure 1 shows the basic schematic drawings of the pull- plate specimens. The pull- plate specimens consisted of three-
30、foot- long sections of A992 columns between pull plates that represent the flanges of the girders in the actual connections. 3/8in. thick continuity plate 36in. 18in. 3/4 in. 3/4in. 5in. W14x132 column 18in. 3/4in. 16in. E70T- 1 25in. 10in. 3/8 x 5 x 12- 5/8 3/8in. gap 1 4 Figure 1 Typical pull- pla
31、te specimen with a half- thickness continuity plate, fillet welded to the column flange and web 321 The The CJP welds joining the pull plates to the column sections were made using the self- shielded FCAW process and E70T- 6 filler metal with a minimum Charpy V- Notch (CVN) energy of 20 ft- lb at 0?
32、 F. The E70T- 6 wire had a diameter of 0.068 in. Figure 2 shows the detail of the girder tension flange- to- column flange connection, including the weld type and access hole dimensions. Continuity plates and web doubler plates were fillet welded using the 100% carbon dioxide gas- shielded FCAW proc
33、ess and E70T- 1 filler metal with a 0.063 in. diameter. In one case, CJP welds were used to join the continuity plate to the column flanges and in another case CJP welds were used to join the web doubler plate to the column flanges. These CJP welds were also made with the gas- shielded FCAW process
34、and E70T- 1 filler metal. 5/16in. min E71T- 8 air- arc back- up bar back- gouge overlap flange cut girder flange column flange 3/8in. Remove backing bar E70T- 6 3/4in. dia. drilled hole 3/4in. 1- 1/2in. 3- 1/8in. 1in. tack weld to hold 90 deg. alignment grind out tack weld when all welding is comple
35、ted 3/8in. 30 +1/16in. - 0 in. Figure 2 Girder to column weld detail Table 1 is a comparison of the coupon test results and the mill reports. The reported coupon yield strength was defined by the 0.2% offset. All values given in Table 1 are averaged values and are in units of ksi. The pull- plates f
36、or all specimens were based on the size of a girder flange from a W27x94 section. The variations of the specimens were the type and size of stiffeners and the column size. Three different column sections were tested, W14x132, W14x145, and W14x159. The stiffener details varied between half thickness
37、and full thickness (relative to the pull- plate thickness) continuity plates and a doubler plate box detail. The nine specimens could be grouped into three categories - specimens used to evaluate local web yielding, specimens focused on local flange bending, and specimens aimed at investigated the e
38、ffects of stiffening details on the connections. 322 Table 1 Material properties W14x132 W14x145 W14x159 Pull- plateHCP* FCP* DP Box* DP* Coupon Yield 49.254.4 58.259.4 51.1 2.2 48.2 50.0 46.0 46.5 56.2 Mill Yield 53.0 57.0 53.5 51.2 61.3 61.3 61.3 57.0 Coupon Tensile 69.470.3 74.175.1 71.5- 71.8 72
39、.5 72.2 72.5 72.5 73.8 Mill Tensile 70.5 73.5 72 72.1 80.4 80.4 80.4 71.0 HCP = half- thickness continuity plate, FCP = full- thickness continuity plates, DP Box = doubler plate box detail, DP = doubler plate The nine pull- plate specimens were as follows: 1. Specimen 1- LFB: W14x132 without continu
40、ity plates, with doubler plates, examined LFB 2. Specimen 2- LFB: W14x145 without continuity plates, with doubler plates, examined LFB 3. Specimen 1- LWY: W14x132 without any stiffeners, examined LWY and LFB 4. Specimen 2- LWY: W14x145 without any stiffeners, examined LWY and LFB 5. Specimen 3- UNST
41、: W14x159, without any stiffeners, examined LWY and LFB 6. Specimen 1- FCP: W14x132, with full- thickness continuity plates and CJP welds 7. Specimen 1- HCP: W14x132, with half- thickness continuity plates and fillet welds 8. Specimen 1B- HCP: repeat of 1- HCP to verify results 9. Specimen 1- DP: W1
42、4x132, with doubler plate box detail Specimens 1 and 2 had beveled doubler plates fillet welded to the column flange to avoid welding in the column k- line. The doubler plates stiffened the web of the two specimens in order to isolate local flange bending as the governing limit state. Specimens 3 th
43、rough 5 were unstiffened connections that looked at the interaction between local web yielding and local flange bending. Figure 3 Box detail with doubler plates welded to column flange away from web with CJP welds W14x132 column 3/4 in. thick doubler plates 2in.2in. E70T- 1 30 3/8in. root opening 32
44、3 Specimens 6 through 8 tested connections either with full- thickness continuity plates and CJP welds, replicating details often seen in present practice, or half- thickness continuity plates with fillet welds. Specimen 9 included no continuity plate, but rather two doubler plates placed out toward
45、s the column flange tips, as shown in Figure 3. These plates thus act both as continuity and doubler plates. This detail, first investigated by Bertero et al. (12), is included in AISC (6) and provides an economical alternative to connections that require two- sided doubler plates plus four continui
46、ty plates. Testing of the pull- plate specimens followed the SAC protocol, SAC (17), where it was applicable. Since, the SAC protocol does not specify a strain rate for monotonic tensile tests, a high strain rate of 0.004 sec- 1 was used, which approximates the strain rate from seismic loading at ab
47、out a 2 second period. The high strain rate increases the yield strength of the materials and increases the probability for brittle fracture, thereby testing the specimens under the most severe conditions. There were three basic instrumentation plans, one for each of the three categories of specimen
48、s. All nine specimens had high- elongation strain gages on the pull- plates and LVDTs that measured the overall specimen elongation and the separation of the column flange tips. The data acquisition system collected 56 channels of data at 100 Hz. RESULTS AND ANALYSIS Before testing began, connection
49、 failure criteria were developed for the LWY and LFB limit states. The primary indicator of failure was whether the weld fractured prematurely. Brittle fracture was potentially still a possibility, because the fracture toughness of the E70T- 6 weld metal is only marginally better than the E70T- 4 weld metal that was used in the pre- Northridge connections, FEMA (18). If brittle fracture occurred in some cases but not in others, the influence of column stiffener details on the occurrence of brittle fracture could be investigated clearly. However, there may be other u