ISSUES ON USING WELDED BUILT-UP BOX COLUMNS IN STEEL SPECIAL MOMENT FRAMES

Transcription

1 10NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska ISSUES ON USING WELDED BUILT-UP BOX COLUMNS IN STEEL SPECIAL MOMENT FRAMES P. Lee 1, R. Garai 2, G. Ozkula 3, C. M. Uang 4 and M. Sarkisian 5 ABSTRACT This paper summarizes key issues resulting from the cyclic testing of steel moment frame specimens using large built-up box column and reduced beam section (RBS) welded moment connections. The test program was initiated in the design development of the new 24-story San Diego Central Courthouse facility to investigate extending the moment frame connection prequalification limits of AISC 358 for effective, economical and reliable application of steel special moment frames with large built-up box columns in regions of high seismicity. Primary issues investigated in the testing program included evaluating connection response with respect to: 1) the use of electro-slag welding (ESW) process for making the continuity plate completejoint-penetration groove weld; 2) RBS geometry and beam-to-column force transfer mechanism; and, 3) what modifications, if any, to the AISC and AWS detailing requirements can be introduced to further improve connection response. A total of three full-scale moment connection specimens were fabricated and cyclically tested in accordance with AISC 341. Testing of the first two specimens revealed the detrimental notch effect built into the ESW process, although one specimen passed the AISC acceptance criteria. Measures were then made to improve not only the RBS dimensioning, beam top flange steel backing treatment, weld access hole geometry, and especially the ESW weld details in an attempt to mitigate the notch effect inside the column. With these improvements, the cyclic performance of the third specimen was significantly improved. 1 SE, Associate Director, Skidmore, Owings & Merrill, LLP, San Francisco, CA PE, Associate, Skidmore, Owings & Merrill, LLP, San Francisco, CA Graduate Student Researcher, Dept of Structural Engineering, University of California, San Diego, CA PhD, Professor, Dept. of Structural Engineering, University of California, San Diego, CA SE, Partner, Skidmore, Owings & Merrill, LLP, San Francisco, CA Lee P, Garai R, Ozkula G, Uang C M, Sarkisian M. Issues for using welded built-up box columns in steel special moment frames. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

2 6 10NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska Issues On Using Welded Built-Up Box Columns In Steel Special Moment Frames P. Lee 1, R. Garai 2, G. Ozkula 3, C. M. Uang 4, and M. Sarkisian 5 ABSTRACT This paper summarizes key issues resulting from the cyclic testing of steel moment frame specimens using large built-up box column and reduced beam section (RBS) welded moment connections. The test program was initiated in the design development of the new 24-story San Diego Central Courthouse facility to investigate extending the moment frame connection prequalification limits of AISC 358 for effective, economical and reliable application of steel special moment frames with large built-up box columns in regions of high seismicity. Primary issues investigated in the testing program included evaluating connection response with respect to: 1) The use of electro-slag welding (ESW) process for making the continuity plate complete-jointpenetration groove weld; 2) RBS geometry and beam-to-column force transfer mechanism; and, 3) What modifications, if any, to the AISC and AWS detailing requirements can be introduced to further improve connection response. A total of three full-scale moment connection specimens were fabricated and cyclically tested in accordance with AISC 341. Testing of the first two specimens revealed the detrimental notch effect built-into the ESW process, although one specimen passed the AISC acceptance criteria. Measures were then made to improve not only the RBS dimensioning, beam top flange steel backing treatment, weld access hole geometry, and especially the ESW weld details in an attempt to mitigate the notch effect inside the column. With these improvements, the cyclic performance of the third specimen was significantly improved. Introduction Welded built-up box columns are frequently used in the construction of steel Special Moment Frames (SMF) to provide effective bi-axial column strength and stiffness, especially in taller SMF structures where seismic drift demands and impact on architectural layout, as well as efficient and economical structures are particularly important. The AISC 358 [1] standard Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications provides several prequalified moment connection types. These design requirements are based mainly on cyclic testing of moment connections with W-shaped columns, although built-up columns with some dimensional limitations (up to 24-in width and depth) are also allowed. 1 SE, Associate Director, Skidmore, Owings & Merrill, LLP, San Francisco, CA PE, Associate, Skidmore, Owings & Merrill, LLP, San Francisco, CA Graduate Student Researcher, Dept of Civil Engineering, University of California, San Diego, CA PhD, Professor, Dept. of Civil Engineering, University of California, San Diego, CA SE, Partner, Skidmore, Owings & Merrill, LLP, San Francisco, CA Uang CM, Lee P, Ozkula G, Garai R, Sarkisian M. Issues for using welded built-up box columns in steel special moment frames. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

3 Several issues arise when using built-up box columns, yet these are not addressed in the AISC or AWS [3, 5] design provisions. Consider fabrication first. Continuity plates at the beam top and bottom flange levels are almost always required. Since these plates are placed inside the box section, they can only be welded to three sides when one side of the column is still open, while the last weld is performed using the electro-slag welding (ESW) process when the closing plate of the built-up column is in position. ESW is prone to creating a notch condition inside the column, yet there is no way to get access to it for insepction once welding is completed. This notch condition is further prone to initiate brittle and premature fracture under cyclic loading. Next consider the design issue. Unlike that of when a W-shaped column is used where column web is aligned with beam web, the two webs in a box column constitute a very strong panel zone, which forces the beam to deliver all the inelastic deformation. Additionally, the stress flow between the beam and column is also altered because the stiffer web is not located at the midwidth of the beam flange as in the W-shape column case. During the design development and construction document phases of the new San Diego Central Courthouse, a total of three full-scale RBS moment connection subassemblies were cyclically tested. The box column consisted of A572 Gr. 50 steel with dimensions of 24-in. wide and 36-in deep with 2-in. thick plate material. The beam consisted of a W rolled section of A992 steel. The first two specimens incorporated a 32% total flange width cut in the RBS region. While tests demonstrated that only one of the first two specimens reached the target story drift angle of 0.04 rad, both specimens failed in nearly identical brittle failure modes. The undesirable brittle fracture in the welded connection was neither desirable nor typical of RBS moment connection behavior even though the first specimen did pass AISC 341 [2] acceptance criteria. A detailed analysis of the fractured welded joints showed that a notch condition was created, during fabrication, inside the column at the ESW of the continuity plate, which caused the brittle fracture of the connection. Since the first two specimen tests demonstrated that the current practice of fabricating built-up box columns needs to be improved, two major steps were undertaken for the construction of the third specimen which retained identical built-up box column and beam sections as for the first two specimens. Finite element analysis indicated that reducing the RBS beam flange width dimension up to about 50% was helpful in reducing the stress demand at the welded joints. More significantly, a modified detail for ESW welding was incorporated in an attempt to minimize the notch condition. The third specimen testing successfully reached a story drift angle of 2 cycles at 0.05 rad and exhibited RBS ductile behavior as was expected based on AISC design equations and predicted finite element modeling of the test specimen. Based on additional post-test analysis and observations, and to further assure a positive outcome of the testing program for the project, two additional detailing improvements had been incorporated in the third specimen testing, which included the use of alternate AWS D1.8 [3] beam weld access hole geometry, and the removal of steel backing for the beam top flange CJP weld. Project Description The design of the new San Diego Central Courthouse facility for the California Administrative Office of the Courts (AOC) consists of a 24-story superstructure above grade with two below grade basement levels, encompassing a full city block with a total of 704,000 gross sq. ft. of building area is shown in Fig. 1. The superstructure lateral force resisting system consists of

4 two-way steel SMF. Typical SMF consists of W24 and W36 beams with RBS connections, as well as two-w30 cruciform columns and built-up box columns typically 33 in. square range from 20 to 36 in. dimension. Additional energy dissipation and drift control is provided by supplemental viscous damping devices in the transverse SMF direction. Fig. 2 shows the test set-up and specimen for cyclic testing [4]. Figure 1. New San Diego Central Courthouse Figure 2. Test Set-up at UCSD Laboratory Use of Welded Built-Up Box Columns in Steel SMF AISC 358 SMF RBS Connection Limitations AISC 358 [2] provides several prequalified moment connection types. These design requirements are based mainly on cyclic testing of moment connections with W-shaped columns, although cruciform columns and built-up columns with some dimensional limitations up to 24 in. in width and depth are also allowed. If the design is tied to these limitations of AISC 358, it results in heavy thick plates for columns, which is not an efficient use of material. Thus, the designers herein are of the opinion that for a high-rise structure, where the design is governed by the lateral stiffness, using a larger box column while maintaining the b/t ratio limitation is the most cost-effective solution and, hence, the need for further testing and research is warranted to extend the AISC 358 limits. Additional limitations include the depth and weight of rolled shape beams to 36 in. and 300 lbs/ft, respectively, with flange thickness limited to 1¾ in. Fabrication of Built-Up Box Columns Figure 3 shows typical moment connection detailing for SMF built-up box column. The member sizes and RBS dimensions are listed in Table 1. The beam was framed into the narrow 24-in. wide face of the built-up column. Complete-joint penetration (CJP) groove welds were used to connect 1¾ in. continuity plates to the column. Three sides of each continuity plate were connected to the column by flux-cored arc welding (FCAW) [Figure 4 (a)] while the electro-slag welding (ESW) process was used to complete the fourth side [Figure 4 (b)]. The ESW joint used a 7/8-in. root with conventional AWS D1.1 [5] containment plates [Figure 11(a)] were used for Specimens 1 and 2, which were subsequently improved for Specimen 3. The ESW was

5 purposely configured to align opposite the beam top and bottom flange CJP welds. (a) Elevation (b) Plan Figure 3. Box Column Connection Detail Table 1. Member Sizes and RBS Dimensions Detail (a) FCAW Joint (b) ESW Joint Overview (c ) ESW Joint (Detail) Figure 4. Continuity Plate Welded Joints Notch Condition Continuity Plates, Electro-Slag Weld at Closure, ESW Prone to Notch condition Based on standard U.S. building practice, the ESW backing consists of two to three consumable containment plates on either side of the electro-slag weld in conformance with AWS D1.1 [5]

6 procedures. Due to the fabrication process, a notch effect or gap is created at the juncture of the innermost containment plate and inside face of the column plate which is easily identified in the macro-etch analysis. Under cyclic load, the notch condition is prone to initiate premature crack propagation and fracture through the column flange plate at each side of the weld. ESW Design Limitations of AWS D1.1 and AWS D1.8 While the ESW is necessary to connect at least one edge of the continuity plate to the column, it is generally recognized as the weaker weld when compared with the FCAW process and it is therefore generally recommended that the beam moment connection be located on the FCAW side of the column. However, this would add complexity to the on-site coordination. Moreover, for conditions where SMF beam occurs on all four sides subjected to biaxial frame action, moment connections on the ESW face of the column are inevitable and desirable. Welding provisions in AISC 341 [2] and AWS D1.8 [3] do not explicitly address ESW requirements as a demand critical weld in the SMF. AWS D1.1 [5] refers only the Procedure Qualification Record (PQR) essential variable changes in requiring Welding Procedure Specification (WPS) requalification. Even though the ESW is an essential or demand critical weld in the moment connection load path, AWS D1.8 does not include any discussion on the processes, procedures and weld preparation for ESW such as requirements for electrodes, guide tubes, type of flux, Charpy V-Notch toughness and hardness requirements for weld metal and heat affected zone, etc., whereas, the AWS D1.5 [6] Bridge Welding Code does provide some information regarding the ESW. It should be noted that for bridge construction, ESW is primarily used for groove welds in butt joints, which is different from the T-joint application in the case of continuity plate welding to column flange. As a result of significant findings during the test, a well documented design and acceptance criteria was established for the moment connection to be used for this project. Specific WPS with supporting PQR were developed for the ESW used in the test specimens in conformance with the demand critical filler and weld metal criteria of AWS D1.8 [3] 6.3 and Tables 6.1 and 6.2. Full-Scale Cyclic Testing of RBS Moment Connection Subassemblies Test Set-up and Loading Protocol The overall geometry of the test setup is shown in Figure 5. Each test specimen was tested in an upright position so that simulated field welding of the moment connection could be performed properly in the testing laboratory. Two servo-controlled ±450-kip actuators with a ±24 in. stroke were used to apply cyclic loading at the end of the beam. The beam was laterally braced at the beam end near the actuators. The AISC 341 [2] loading sequence expressed in terms of the story drift angle for beam-to-column moment connection test was used as illustrated in Figure 6. Displacement was applied to the end of the beam. The loading began with six cycles each at 0.375%, 0.5% and 0.75% drift. The next four cycles in the loading sequence were at 1% drift, followed by two cycles each at 1.5%, 2%, 3%, 4%, 5%, 6%, or until the specimen failed. According to Section E3.6b of AISC 341 [2], beam-to-column connections used in the seismic load-resisting system shall satisfy the following requirements: 1) the connection must be capable of sustaining a story drift angle of at least 0.04 radians for one cycle, and 2) the required flexural strength of the connection, determine at the column face must be equal at least 80 percent of the nominal plastic moment of the connected beam at a story drift angle of 0.04 radians.

7 Figure 5. Test Set-up Figure 6. Test Protocol AISC 341 K3.4c [2] Specimen 1 Testing Summary Results Minor yielding in the beam flanges was observed at 1% drift. Minor beam web local buckling started at 2% drift. The yielding deformations at 4% drift and hysteretic response are shown in Figure 7(a) and (b). Although Specimen 1 testing met AISC 341 [2] qualification test acceptance criteria of one cycle at 4% drift, both flange local buckling and web local buckling was limited, exhibiting no strength reduction as might be expected for an RBS connection [7]. After successfully completing two cycles at 4% drift, the sudden and brittle mode of failure was bottom flange rupture at the start of the 5% drift cycle. Contributing to the poor RBS connection behavior was: 1) the beam width-thickness ratios were low, and 2) the amount of RBS cut (c dimension) of 0.16b f was relatively low compared to the maximum allowed by AISC 358 [1] of 0.10b f min, and 0.25b f max. (a) Yielding and Buckling Patterns (b) Figure 7. Specimen 1 at 4% Drift (2 Cycles) Global Hysteretic Response Additionally, significant cracking at one edge of the beam top flange was observed after 2nd cycle at 4% drift at one-half cycle prior to beam bottom flange rupture failure mode. Two factors contributed to this crack propagation: 1) strain and stress concentrations are higher at both ends across the beam flange width when the beam is connected to a box column, and 2) CJP

8 steel backing was left in place, although a reinforcing fillet weld was placed underneath the steel backing connecting directly to face of column as permitted by AISC 341 [2]. This detail creates a discontinuity of weld material between the flange CJP weld and backing fillet weld. Specimen 2 Testing Summary Results Specimen 2 experienced premature brittle fracture similar to Specimen 1, but at a drift of 3.9% during the first positive excursion cycle to 4% drift as shown in Figure 8(a) and (b). Unlike Specimen 1, Specimen 2 did not meet the AISC 341 [2] acceptance criteria of achieving one complete cycle at 4% drift. Additionally, as was observed in Specimen 1, a crack at the top flange edge condition was observed prior to failure mode at lower drift levels and propagated further at subsequent increasing drift levels. Again, the crack initiated at the interface of the top of the backing bar and extended into the CJP weld root. It is believed that this discontinuity between the CJP root and the backing bar fillet weld may have contributed to increased levels of strain prior to the brittle bottom flange rupture. (a) Fracture at Beam Bottom Flange (b) Global Hysteretic Response Figure 8. Specimen 2 at 3.9% Drift (1 st Cycle) Post-test analysis and investigation of Specimen 1 and 2 Testing In addition to the two continuity plates that were installed at beam flange levels, the continuity plate at the top end of the column where the column connected to the reaction wall had also been detailed in the same manner which provided an opportunity to cut pieces from that location to examine the condition of the similarly welded joints but outside the moment connection region. This sampling of Specimen 1 allowed for baseline comparisons with samples taken at Specimen 2 top and bottom flange welded joints as discussed below. Results from hardness testing of macro-etched sections indicate that the hardness of the ESW weld was not significantly higher than that of the base metal and similar at both locations. Although brittle fracture did not occur at the beam top flange level, Figure 9(a) and (b) illustrate that distinct and sharp notches are created during the ESW welding. The notch condition initiates at the non-welded juncture of the ESW containment plates and inside column face. From Figure 9(a), it is further noted that the ESW joint was shifted during fabrication creating a fully fused but a somewhat eccentric ESW condition and beam flange to continuity plate load path.

9 (a) Overall Macro-etch View (b) Detail Illustrating Notch Condition Figure 9. Specimen 2 ESW Welded Joint at Beam Top Flange Level The macro-etch cross sections taken at the beam bottom flange are shown in Figure 10(a) and (b). It is observed that the formation of a crack initiating at the ESW notch condition also triggered a crack that propagated into and through the column flange plate. Therefore, the notches that were created by the ESW process are responsible for the brittle fracture at the bottom flange level. (a) East Section (b) West Section Figure 10. Specimen 2 ESW Welded Joint at Beam Bottom Flange Level Fabrication of Built-Up Box Column ESW to Mitigate Notch Condition Fabrication of Specimens 1 and 2 utilized conventional ESW containment plates made up of three ½-in plates at each side of the weld as shown in Figure 11(a). To mitigate the notch effect, the Specimen 3 detail was made up with two ¾-in containment plates on each face of the weld with a 45-degree 3/8-in transition bevel at the innermost containment plates at the juncture of the containment plate and inside column face as shown on Figure 11(b). Additionally, 1/8-in shims were introduced between the innermost containment plate and the continuity plate. This modified containment plate configuration results in a larger diameter and smoother transition of the electro-slag weld bulb. The effect is to postpone the potential of cracking from the notch effect, if not eliminate it completely, by increasing the length of the weld in the fused column plate heat effected zone. Also, the larger electro-slag weld bulb helps to provide some redundancy for offset fabrication tolerances between the beam flange and continuity plate alignment. More work can be done to further improve the ESW fit-up and process to address the containment plate notch effect. Prior to the fabrication of Specimen 3, a new ESW weld procedure specification (WPS) with supporting procedure qualification (PQR) testing was conducted in conformance with both AWS D1.1/D1.8 and AWS D1.5 specifications.

10 (a) Specimen 1 and 2 (b) Specimen 3 Specimen 3 Testing Summary Results Figure 11. ESW Joint Details In addition to ESW beveled weld backing plates, three other improvements were made to Specimen 3 including (1) Increasing the RBS cut c dimension from in (16%) to in (25%); (2) Instead of using AISC 360 [8] Alternate 1 Fig. C-J1.2, the modified beam weld access hole geometry per AWS D1.8 Fig. 6.2 was used; and (3) At beam top flange CJP, the backing bar was removed, back gouged, and reinforcing fillet was provided similar to AISC 341 [2] requirement for the bottom flange CJP. Specimen 3 successfully underwent two cycles at 4% and 5% drifts each exhibiting very good RBS ductile behavior and exceeded AISC acceptance criteria with no beam flange welded joint failures as had been seen in Specimens 1 and 2. Figure 12(a) illustrates that the beam experienced significantly more local flange, web and lateral-torsional buckling as compared to Specimens 1 and 2. Additionally, by increasing the c dimension of the RBS section, beam flange weld demands were reduced. As indicated by earlier research by Ricles, et al [9], use of an improved weld access hole geometry and removing steel backing in the top flange also eliminated the development of top flange edge condition cracks as had been witnessed in Specimens 1 and 2. The beam eventually fractured in the beam top flange RBS region due to low cycle fatigue during the first cycle at 6% drift shown in Fig. 12(b). The test specimen response is illustrated in Figure 12(c), demonstrating overall ductility and RBS behavior with distinct levels of stiffness degradation while meeting AISC acceptance criteria. (a) Specimen 3 at 6% Drift (b) Fracture RBS Top Flange (c) Global Hysteretic Response Figure 12. Specimen 3 RBS Results

11 Conclusions The results of the testing program described herein amount to a significant contribution to available knowledge on the performance of welded built-up box columns in steel special moment frames (SMF). This is particularly important given the very limited amount of testing conducted in the U.S. using these connections, notwithstanding, the widespread use of the built-up box column in regions of high seismicity for use in complex structures such as tall buildings. This testing program identified a notch effect condition inherent in the fabrication process of ESW electro-slag welding that when used as a closer weld in a built-up box column continuity plate moment connection, is prone to initiate a crack that propagates through the weld heat effected zones and column plate material leading to brittle premature fracture under cyclic seismic loading. Additionally, this testing program developed several measures to significantly improve performance and reliability of the built-up box column RBS moment connection. Most significantly, improvements included the beveled transition of the ESW containment plates at the continuity plate to inside column plate face and thereby mitigating the ESW notch effect, whereby, prolonging the premature onset of brittle fracture allowing predicted ductile moment connection behavior. The application of the SMF RBS qualification testing herein is limited to the San Diego Central Courthouse project, however, significant care was employed in configuring the test specimens in the interest of contributing to extending the AISC 358 qualification limits. Beyond the scope of this paper, additional finite element modeling of this test specimen has demonstrated that good ductile behavior of the built-up box column RBS connection is achieved, and therefore, is limited primarily by the reliability of the ESW. Two potential contributions beyond the scope of this project can be achieved as a consequence of this moment frame connection qualification testing program: (1) Extend limits of AISC 358 prequalification for large built-up box columns with reduced beam section (RBS) moment connections; and, (2) Address the reliability of the ESW in moment connections as a demand critical weld and integral component of the load path under peak cyclic seismic loads to achieve intended ductile performance of the qualified connection. An AWS ESW Task Group has been formed as a result of this testing to address brittle failure mode issues and identify further research and/or testing to reduce or eliminate brittle behavior. Acknowledgments The authors are very grateful to the support provided by the Judicial Council of California and Administrative Office of the Courts (AOC) in pursuing this testing effort, thus contributing to furthering the state-of-practice and research. References 1. AISC 358, Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications, Including Supplement No. 1, ANSI/AISC , ANSI/AISC 358s1-11, American Institute of Steel Construction, Chicago, IL. 2. AISC 341, Seismic Provisions for Structural Steel Buildings, ANSI/AISC , American Institute of Steel Construction,, Chicago, IL. 3. AWS D1.8, American Welding Society, AWS D1.8/D1.8M: 2009, Structural Welding Code Seismic

12 Supplement, 2 nd Edition, Miami, FL. 4. Ozkula, G. and Uang, C. M. (2013), Cyclic Testing of Steel RBS Moment Connections with Built-up Box Column for the San Diego Central Courthouse, Report No. TR-13/01, Department of Structural Engineering, University of California, San Diego, La Jolla, CA. 5. AWS D1.1, American Welding Society, AWS D1.1/D1.1M: 2011, Structural Welding Code Steel, 22nd Edition, Miami, FL. 6. AWS D1.5, American Welding Society, AASHTO/AWS D1.5M/D1.5: 2008, Bridge Welding Code, 5th Edition, A Joint Publication of American Association of State Highway and Transportation Officials and American Welding Society. 7. Zhang, X., Ricles, J.M., (2006), Experimental Evaluation of Reduced Beam Section Connections to Deep Columns, Journal of Structural Engineering, American Society of Civil Engineers. 8. AISC 360, Specification for Structural Steel Buildings, ANSI/AISC , American Institute of Steel Construction, Chicago, IL. 9. Ricles, J.M., Fisher, J.W., Lu, L.-W. and Kaufmann, E.J. (2002), Development of Improved Welded Moment Connections for Earthquake-Resistant Design, Journal of Constructional Steel Research. Vol. 58, pp