Survey and Testing of Pre-1988 Braced Frame Structures From The West Coast of the United States

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1 Survey and Testing of Pre-1988 Braced Frame Structures From The West Coast of the United States Dan Sloat 1, Charles W. Roeder 2, Dawn E. Lehman 3, and Jeffrey W. Berman 4 1 Graduate Student, Dept. of Civil Engineering, University of Washington, Seattle, WA dansloat@uw.edu 2 Professor, Dept. of Civil Engineering, University of Washington, Seattle, WA, croeder@uw.edu 3 Associate Professor, Dept. of Civil Engineering, University of Washington, Seattle, WA, delehman@uw.edu 4 Associate Professor, Dept. of Civil Engineering, University of Washington, Seattle, WA, jwberman@uw.edu ABSTRACT Concentrically braced frames are one of the most common lateral-load resisting systems in the US. Modern design in seismic regions uses special concentrically braced frames (SCBFs) that have detailing to ensure ductility. Prior to the development of modern codes, detailing for a ductile response was not required. These older systems are termed non-seismic braced frames (NCBFs). There is significant uncertainty regarding the performance of NCBFs in seismic events, generating concern about their vulnerability. This paper presents the results of an extensive survey and evaluation of NCBF systems in the United States, as well as the results of the first three tests of braced frames representative of typical NCBFs. The survey investigated pre buildings in high seismic regions that used braced frames as the lateral load resisting system. Frames in these structures were evaluated using modern requirements. The results demonstrated that most NCBFs cannot meet modern SCBF design criteria. The experimental study consists of three completed tests, plus additional planned tests, at the University of Washington. These tests investigate brace and connection performance using details developed to reflect the results of the building survey. The results of these tests demonstrate that NCBFs perform better than modern design checks suggest they would, but they lack the level of ductility required for modern design. KEYWORDS: Braced Frame, Repair, Retrofit, Connections, Seismic 1. INTRODUCTION Concentrically braced frames (CBFs) are commonly used for the lateral load resisting system in steel buildings. Typical CBFs are composed of diagonal braces connected to the beams and columns through gusset plate connections. CBFs provide considerable strength and stiffness with smaller steel weights and simpler connection details than most other steel lateral systems, which make them an economical and efficient design choice. Special concentrically braced frames (SCBFs) are typically employed in high seismic regions in modern design. They must meet the requirements associated with reduced seismic design loads and have special detailing to promote ductile behavior according to current AISC Seismic Provisions [1]. Brace tensile yielding and compressive buckling are the primary yielding mechanisms that help the frame sustain large inelastic deformations. The gusset plate connections are designed to withstand the deformation and strength resulting from the brace through both capacity and geometric clearance limits. Prior to the 1988 Uniform Building Code (UBC) [2] CBFs were designed with no consideration of overstrength demand and ductile detailing requirements of the connection, which may lead to uncertainty in the balance between the connection and brace strength. In some cases, the connections will be stronger than the brace while others it may be weaker. As a result, these older CBFs have uncertain failure modes and are less likely to exhibit ductile seismic

2 response. These frames are referred to herein as non-seismic concentrically braced frames (NCBFs). It is obvious that the older NCBF design philosophy and detailing requirements have deficiencies compared to modern SCBF systems. However, many of the NCBFs are still in service throughout the United States and in other countries, so the evaluation of these systems is urgent. Due to a lack of previous research, the seismic response of these existing frames is still not well understood and the NCBFs have become one of the most vulnerable yet understudied seismic resisting systems. The research presented herein was conducted to evaluate and quantify the seismic performance of NCBFs. First, a survey of existing structures containing NCBFs was conducted to evaluate the ability of these systems to meet current design standards and give direction to the design of test specimens. In addition to potentially non-ductile connection details, it was found that many of the existing buildings contained framing members considered inadequate by today s capacity design standards. Second, an experimental study of three single-story, single bay NCBF frames (including one pilot test [3]) was conducted at the University of Washington. The results showed that these frames behave in a more ductile manner than suggested by design criteria, but they are vulnerable to rapid loss of stiffness and exhibit much lower ductility. Additional tests are planned to examine different connections and braces. 2. INFRASTRUCTURE REVIEW A survey of structures on the West coast of the United States designed prior to 1988 was conducted to determine the types of connections and the associated deficiencies common in the area, which is prone to large seismic events. The survey included 14 buildings and, while limited, is believed to represent a reasonable sample of older braced frame configurations. From each of these structures, a seismic load resisting frame was selected and analyzed to determine the expected seismic performance under modern design criteria. 2.1 Characteristics of surveyed buildings A survey of existing structures containing CBFs that were designed prior to 1988 was conducted in order to evaluate the ability of these systems to develop the full brace capacity, as required by current SCBF design. The selected structures were chosen to give a variety of building characteristics representative of what exists today. A wide variety of building locations, heights, uses, and construction years were considered. The most common type of brace was square Hollow Structural Section (HSS), which were found in over 70% of buildings. Wide flange sections were the next most common and some pipe and angle braces were also observed. Many buildings used multiple brace configurations. The most common bracing configuration was chevron (also called inverted-v), which were present in 70% of buildings. Single diagonal braces with a matching brace in an adjacent bay were present in half of the analyzed buildings, and were the predominant system in taller buildings. X- bracing was less common, though it was also more frequently observed in taller structures. Figure 2.1 shows selected connection details to highlight their variability and commonalities. As shown, brace to gusset plate connections were fairly uniform. HSS and pipe braces were slotted and fillet welded to the gusset plate in all instances. Net section reinforcement was not provided on any of the braces, as these requirements were not included in the design codes at that time [1]. Angle sections were all bolted via single lines of bolts to the gusset plate. The most common gusset plate to beam connection was fillet or complete joint penetration welds between the gusset plate and the beam flange. In some instances, the beam flange was coped on one side, and the gusset plate was bolted to the beam web. Other connection methods were observed, but they were uncommon and thus not considered good candidates for future study.

3 Many gusset plates were welded directly to either the column flange or, for weak axis column orientations, the web. Others were bolted or welded to a shear tab, which was then welded to the column. Most of the shear tabs were also bolted or welded to the beam and served as the beam to column connection in addition to the gusset plate to column connection (see figure 2.1a,c). Some gusset plates were welded to end plates, which were either bolted or welded to the column flange. As with the shear tabs, these plates typically also served as the beam to column connection (see figure 2.1b). Where the beam and column were not connected by a shared system with the gusset, the connection was generally welded, occasionally including a shear tab for erection purposes. (a) (b) (c) (d) Figure 2.1 Samples of Existing Connections from Survey 2.2 Connection and frame analysis The capacities of selected connections from the surveyed buildings were evaluated and compared to the brace tensile and buckling demands. For this analysis, R y and R t, the ratios between nominal and expected yield and tensile strengths, were applied to establish brace yield and buckling demand loads and to other brace failure modes to permit a capacity based evaluation of the connection and framing member designs. Resistance factors were not applied for any capacity calculations in order to better simulate the expected performance of the system. Thus, if the connection was evaluated per current AISC design requirements [4], the resulting demand-capacity ratios would be slightly higher (depending on the resistance factor for each mode). Additionally, out-of-plane displacements and moments were not considered. The CBFs were assumed to act as trusses, so for most connections, it was reasonable to assume that the vertical brace force was transmitted directly to the column, while the horizontal brace force was transmitted directly to the beam. The minimum buckling capacity (0.3R y F cr A g ) was also used evaluate unbalance loads on beams and other members for chevron brace configurations. 2.3 Observed deficiencies All frames failed the capacity design check, since they were not capable of developing the expected yield and/or buckling demands of the brace. Connections typically had a large number of limit states that could not develop this expected demand (see figure 2.2). Thin gusset plates and short brace to gusset plate splice lengths were common. As a result, Whitmore yielding was a limit state of concern in over 75% of the connections. Due to the absence of cover plates on braces, net section fracture was also a frequent concern. Figure 2.3 shows that beams in chevron (V- or Inverted V-bracing) consistently failed to have adequate beam resistance to resist brace demands during postbucking deformation. Figure 2.3 also shows that in many cases the columns were also not adequately sized to develop brace yielding over the height of the braced frames.

4 Frame Limit State Connection Limit State 5 th International Conference on Advances in Experimental Structural Engineering Brace Net Section Fracture Whitmore Yielding Weld Fracture at Beam-Gusset Plate Weld Fracture at Brace-Gusset Plate Gusset Plate Shear at Beam-Gusset Gusset Plate Buckling Gusset Plate Shear at Column-Gusset Block Shear of Gusset Plate at Brace Bolt Shear at Brace-Gusset Whitmore Fracture Brace Block Shear Weld Fracture at Column-Gusset Plate Bolt Shear at Beam-Gusset Block Shear at Column-Gusset Plate Bolt Shear at Colum-Gusset Plate Block Shear at Beam-Gusset Plate Bolt Bearing at Column-Gusset Plate Bolt Bearing at Beam-Gusset Plate 1.5<DCR 1.2<DCR< <DCR< Percentage of Frames Failing Figure 2.2 Demand-Capacity Ratios for Connection Limit States Column Compression Beam Bending Percentage of Frames Failing 1.5<DCR 1.2<DCR< <DCR<1.2 Figure 2.3 Demand-Capacity Ratios for Frame Limit States 3. EXPERIMENTS AT UNIVERSITY OF WASHINGTON The results of the infrastructure review have informed an ongoing series of experiments at the University of Washington. These tests investigate commonly observed deficiencies in brace and connection design from the infrastructure review. The tests are single bay frames with a single diagonal brace. The connections were designed to imitate the types and severities of deficiencies commonly observed in the infrastructure review, rather than being designed to develop the expected capacity of the brace per modern design requirements. Special detailing of the gusset plates was not included for any tests, resulting in low end clearance for the brace that reduces the capacity for brace end-rotation during out-of-plane buckling. Also, braces used typically did not meet modern width-to-thickness requirements, as was common in older structures.

5 3.1 NCBF 0 The first NCBF test [3] was conducted in 2012 as a pilot to demonstrate the concerns with performance of older braced frames prior to the completion of the infrastructure review. The connection used a field-bolted double angle connection. A pair of double angles was used to connect each the gusset plate and the beam web to the column, as shown in Figure 3.1. The welds for the specimen complied with AWS E71T-8, which is used for demand critical welds. However, most older frames use less tough materials. The design of the connections conformed with the 1988 UBC [2]. a) b) Figure 3.1Dimensions of NCBF 0 (a) Frame and (b) Connection Figure 3.2a shows the applied load history, and Figure 3.2b shows the hysteretic response with annotations of significant points, which are described below. Initial buckling of the brace was observed at 0.36% story drift (Figure 3.2c), with a compressive load of 182 kips. At 0.44% story drift, cracking initiated in the welds between the gusset plate and the brace. The crack length increased in subsequent cycles, resulting in fracture of the welds at 0.52% story drift (Figure 3.2d). Brittle connection fractures in braced frames are undesirable, as they are difficult to predict and result in a rapid loss of lateral resistance. The fracture in this test confirmed the need for design to the expected yield capacity of the brace, rather than the design capacity, in order to ensure ductility. The frame also reached a very small drift level before fracture occurred, meaning the system was not very ductile. After the brace fractured, additional cycles were run at larger drift levels to determine the residual lateral resistance of the frame. The angles connecting the gusset plate to the column fractured at approximately 3% drift after sustaining significant deformations (Figure 3.2e). A crack also initiated in the gusset plate to beam weld in the opposing gusset plate (Figure 3.2f), though this did not result in complete connection failure prior to angle fracture. These cycles demonstrated that the frame retained some lateral resistance after the fracture of the brace connection.

6 a) c) d) b) e) f) Figure 3.2 NCBF 0 (a) Loading Protocol and (b) Hysteretic Responses; the Photos of (c) Local Buckling, (d) Connection Fracture, (e) Fracture of Gusset Connecting Angle, and (f) Weld Failure in Opposing Gusset. 3.2 NCBF 1 NCBF 1 was the first test designed based on the results of the survey of existing buildings. It used a less compact HSS 7x7x1/4, which was found to have a width-to-thickness ratio typical of many of the observed structures. It had a slightly longer brace to gusset splice length, and large fillet welds connecting the brace. Large welds were used to reduce the likeliness of weld fracture. NCBF 0 sufficiently demonstrated concerns with brace to gusset plate weld fracture limit state, so NCBF 1 was designed with larger brace to gusset welds in order to investigate other limit states. The connection was designed to have demand-to-capacity ratios for the connection that were representative of what was observed in the building survey and is shown in figure 3.3. A thin, 3/8 gusset plate was used, and only 1 of clearance was provided for the end of the brace. The beam web and gusset plate were welded to a shear tab, which was welded to the column flange. One flange of the beam was coped to allow the shear tab to be continuous between the gusset plate and the beam. Figure 3.3 NCBF1 Connection Detail

7 Figure 3.4 shows the base shear versus story drift hysteresis and photos from the test. Initial buckling of the brace was observed at 0.22% drift. During subsequent cycles, the brace buckling became more extensive. At 0.51% drift, the brace hinged 1 foot northeast of the brace center. The local deformations were very large, in part due to the slenderness of the brace walls. These local deformations accelerated brace fracture, which occurred at 0.71% drift in tension. As the brace buckled, the gusset plate and shear tab bent upward to accommodate the rotation of the brace end. Despite the lack of clearance for the end of the brace, the connection was able to sustain out-of-plane brace deflections as large as 10 inches. The connection also sustained minimal damage aside from the yielding associated with the bending of the plate, despite the design expectation that the connection would not be able to develop the brace capacity. After brace fracture, the frame was cycled to determine residual capacity in a similar method to NCBF 0. (a) (b) (c) Figure 3.4 NCBF1 (a) Load-Displacement History (b) Gusset Plate Bending (c) Brace Hinge Formation 3.3 NCBF 2 Because NCBF 1 sustained minimal connection and frame damage aside from the brace fracture, the frame was reused for a similar test. The old brace was cut off of the gusset plates, minor weld damage was repaired, and the gusset plates were heat-straightened. The brace was replaced with a more compact HSS 5x5x3/8, which meets modern requirements for with-to-thickness ratio. The minor damage to the gusset plates was expected to have little impact on the performance of the frame. Figure 3.5 shows photos from the experiment. Unfortunately, a data acquisition error prevented capture of the hysteretic response of the specimen. However, observations were made throughout the cyclic loading which detail the frame behavior. (a) (b) Figure 3.5 NCBF2 (a) Initial Weld Cracking (b) Connection Fracture

8 NCBF 2 buckled at the same drift level as NCBF 1. However, in subsequent cycles, the brace retained a much larger portion of its compressive capacity and did not hinge. This was due to the lower width-thickness ratio of this brace walls compared to NCBF1. A lower width-thickness ratio provides more resistance against local buckling. The lack of hinging in the brace placed larger axial and rotational demands on the connection. This caused weld tearing to initiate at the ends of the gusset plate to beam welds. The weld tear lengthened over subsequent cycles. The weld tore completely during the first tensile cycle at 1.6% applied drift. The tear propagated along the gusset plate to shear tab weld, causing complete connection fracture. The additional demands put on the connection by the more compact brace, combined with the lack of clearance of the end of the brace, caused connection failure. As with previous tests, the frame was cycled after fracture. 4. CONCLUSIONS AND FUTURE TESTING The first three NCBF tests have demonstrated that braced frames designed with pre-1988 detailing requirements exhibit lower lateral resistance and drift capacity than modern SCBFs. However, the connections performed better than suggested by modern design checks, indicating that extensive retrofit might not be necessary to ensure acceptable performance in some cases. Future tests will explore retrofit options used in practice that can increase drift capacity. Tests will also be conducted on other connection types commonly observed in the survey of existing buildings. This will extend the range of understanding of the seismic performance or older braced frames, as well as their possible retrofit strategies. In addition, a set of parallel tests are being conducted at National Center for Research in Earthquake Engineering in Taiwan. These tests involve 2-story chevron configuration braced frames using pre-1988 detailing. The larger configuration investigates system-level performance issues associated with NCBFs, including brace compactness and weak beams in chevron bracing [5]. The results of these tests will allow engineers to make informed decisions about retrofit of existing buildings, improving the safety of older structures and offering potential cost-effective retrofit solutions. REFERENCES 1. AISC (2010a). Seismic Provisions for Structural Steel Buildings. American Institute of Steel Construction, Chicago, IL. 2. ICBO (1988) Uniform Building Code. International Conference of Building Officials, Whittier, CA. 3. Hsiao, P. C., Lehman, D. E., Berman, J. W., Roeder, C. W., and Powell, J. (2012). Seismic Vulnerability of Older Braced Frames. Journal of Performance of Constructed Facilities, DOI: AISC (2010b). Manual of Steel Construction, Load and Resistance Factor Design. 14th Edition, American Institute of Steel Construction, Chicago, IL. 5. Sen, A.D., Sloat, D., Pan, L., Roeder, C.W., Lehman, D.E., Berman, J.W. (2013). Evaluation of the Seismic Performance of Two-Story Concentrically Braced Frames with Weak Beams. 5th International Conference on Advances in Experimental Structural Engineering. Taipei, Taiwan.