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Buckling Restrained Braced Frame (BRBF) Structures: Analysis, Design and Approvals Issues

Saif Hussain, S.E. Paul Van Benschoten, S.E. Mohamed Al Satari, Ph.D. Silian Lin, Ph.D. Coffman Engineers, Inc. Los Angeles, CA

Abstract

Due to a number of structural performance advantages over conventional braced frames, the BRBF system appears to be gaining in popularity. This paper provides an overall understanding of this system along with a case study of a recent project in the City of Los Angeles. The paper includes material on BRBF background and development, the various issues related to code provisions, agency approvals, analysis, design, detailing as well as construction and erection challenges. The latter points are especially pertinent due to the authors' recent experience with the above mentioned project. The project is a three story 211,000 square foot steel framed building located in Southern California. The design of the structural system was completed in 2005 generally following the provisions of the 2003 NEHRP Recommended Provisions for New Buildings and Other Structures (FEMA 450) and the 2002 Los Angeles Building Code. Pre-manufactured Buckling Restrained Braces were approved under the guideline of ICC AC238-1003-R1 Acceptance Criteria for Buckling Restrained Braced Frame Subassemblages in conformance with FEMA 450. The structure is the first BRBF structure approved for new construction in the City of Los Angeles. "One-time" approval was granted through the Los Angeles Department of Building and Safety's ordinance 98.0403 LAMC Request for Modification of Building Ordinances as an "Undefined Structural System." The building is almost complete and is scheduled to go into operation later this year. Current building codes in California do not address BRBF systems. Recent documents including the 2005 AISC Seismic Provisions for Structural Steel Buildings and the 2006 International Building Code (IBC) include complete design provisions for BRBF systems. The California Division of the State Architect (DSA), the Office of Statewide Heath Planning (OSHPD) as well as numerous municipalities have approved in excess of 40 projects with BRBF systems in the western United States. Extensive testing of Buckling Restrained Brace subassemblages and full scale BRBF models has demonstrated the high ductility, toughness and predictable behavior of BRBF systems. BRBF systems are currently used as primary lateral force resisting elements both in new construction and seismic retrofit projects. However, future applications may include use of BRBFs as supplemental hysteretic dampers in seismic retrofitting, which was the original motivation behind the initial development of the BRBF system. Careful analysis and brace sizing can result in a considerable increase in damping without an intolerable decrease in building period. The robust nature of these systems resulting from the elimination of the buckling mode of the BRB makes these braced frames good candidates for a variety of applications in high seismic regions.

BRBF Introduction and Background

The concept of eliminating the compression buckling failure mode in intermediate and slender compression elements has long been a subject of discussion. The theoretical solution for eliminating the buckling failure mode is very simple: laterally brace a compression element, at close regular intervals, so that the compression element's un-braced length effectively approaches zero.

performed for a proposed project at University of California at Davis. The first Buckling Restrained Brace was installed in the United States at UC Davis on January 17th 2000. The original test results at UC Berkeley demonstrated good performance under various loading histories specified by protocols from the joint venture of SEAOC1, ATC2, and CUREE3 (SAC). The BRBF delivered ductile, stable and repeatable hysteretic behavior. The plastic deformation capacity exceeded performance requirements, both in terms of ultimate deformation and cumulative plastic strain.

Figure 1. Idealized Buckling Control Mechanism

In the 1980's Professor Akira Wada, of the Tokyo Institute of Technology, began a collaborative effort with Nippon Steel Corporation in developing a compression element that eliminated the bucking failure mode of slender elements. Dr. Wada's inspiration for the Buckling Restrained Brace (BRB or Un-Bonded BraceTM) came from the collarbone of the human body. It was through this rational thought process that Dr. Wada envisioned the "damage control" BRB element for use as a seismic protection element. Dr. Wada's BRB design resembled a typical human bone: bigger at the ends and a reduced section in the middle. The original building design scheme for Buckling Restrained Braces was for use as a "hysteretic damper" in conjunction with moment resisting frames. Relatively "soft" braces used as hysteretic dampers reduced overall steel tonnage by 5 to 10% for the entire structure. In early 1988 the first BRBF/Moment Resisting Frame lateral resisting system was used in Japan. The first tests of BRBF (Nippon Steel Corporation: Un-Bonded BraceTM) were performed in the United States at the University of California at Berkeley in 1999. The tests were conducted under the supervision of Professor E. Popov and Professor N. Makris. The testing was

Figure 2. Sample Test Results of Hysteretic Performance of a BRBF BRBF System Overview

BRBF systems are unique due to the configuration of the brace elements. They consist of two major components: the steel core resists axial stresses and the outer concrete filled steel casing resists buckling stresses, and the casing restrains the steel core from buckling thereby developing almost uniform axial strains in tension and compression. BRBF have full, balanced hysteresis loops with compression yielding similar to tension yielding behavior. This is achieved through the decoupling of the stress resisting and flexural buckling resisting aspects of the compression strength. Plastic hinges associated with buckling cannot form in a BRBF. The near equal tension and compression capacities of BRBF's eliminate the post1 2

Structural Engineers Association of California Applied Technology Council 3 Consortium of Universities for Research in Earthquake Engineering

Steel Core Concrete Fill Steel Casing

A

Debonded Gap

a) Section A

b) Buckling Restrained Brace (BRB)

Figure 3. Typical BRBF Element

bucking load imbalance inherent in the conventional braced frames such as the building code designated Special Concentric Braced Frames (SCBF) systems. the demand. Recent full scale testing of a SCBF by the University of California at Berkeley (Uriz, et al, 2004) illuminated the lack of ductility and overall poor inelastic seismic performance due to the inherent buckling behavior. The effects of out-of-plane buckling and subsequent failure of the braces dominated the overall behavior of the tested specimen.

Code Jurisdiction, Approvals and Applications

The design of BRBF systems is currently governed by the 2003 NEHRP Recommended Provisions for New Buildings and Other Structures (FEMA 450) and the 2005 AISC Seismic Provisions for Structural Steel Buildings. However, both FEMA 450 and 2005 AISC Seismic Provisions have not been incorporated or adopted into current building codes; the 2001 California Building Code (CBC) and the 2003 IBC. The recent release of the 2006 IBC references the 2005 ASCE-7 and the 2005 AISC Seismic Provisions which recognize BRBF systems. Currently, two BRBF systems are defined in 2005 ASCE-7, 2005 AISC Seismic Provisions and FEMA 450. The first system utilizes moment-resisting beam­to-column connections, which corresponds to a Response Reduction Factor (R) of 8. The R factor is assigned a value of 7 for the second system which utilizes pinned beam column connections. BRBF systems have been previously approved by DSA, OSHPD, University of California and many other local and state agencies on a case by case basis. As of this writing, over 40 buildings have been constructed in the western United States using BRBF systems both in new and seismic retrofit building designs.

a) BRBF Frame Testing Configuration

b) BRBF Uni-Axial Testing Configuration

Figure 4. Test Configurations

The Berkeley test of a full-scale SCBF frame with HSS braces was cycled per the loading history protocol of AISC/SEAOC for BRBF's (Figure 5). In these tests,

SCBF vs. BRBF

Conventional SCBF systems have inherent problems due to the vastly different compression and tension capacities of the braces. During a major seismic event the compression brace will most likely buckle resulting in the companion tension brace resisting the majority of

Figure 5. Example of Loading History

increasing cycles of inelastic demands on the HSS brace created small partial thickness fractures at half the target roof displacement. The small fractures formed in the cold worked radius of the HSS due to local buckling from inelastic compression buckling and subsequent restraightening of the buckled brace when cycled into tension. Continued increase of demand cycles resulted in the complete rupture of both braces at first cycle at the target roof displacement. The tests revealed that compression bucking of the brace dominants the poor inelastic cyclic behavior of SCBF systems. Figure 6 below shows an overlay of both BRBF and SCBF behaviors. The laboratory­tested, superior performance of BRBF over the conventional SCBF is clearly apparent.

Figure 7. Typical SCBF Gusset Plates Availability/Competitors

The concept of the BRBF is not proprietary. However, the configuration and details of the brace assembly is usually subject to exclusive rights under US patent laws (i.e. connections, de-bonding of core or gap between concrete fill and core, and assembly processes). In the last 10 years the competition in BRB manufacturers has increased, which provides owners with competitive bidding.

Figure 6. Hysteretic Loops for SCBF and BRBF from UC Berkeley Tests

Another significant advantage to BRBF design is that single diagonal braces oriented in the same direction may be utilized. Unlike SCBF the BRBF system does not require to have brace layouts that provide a maximum of seventy percent of the horizontal component of member forces in tension or compression for a given line of resistance (CBC 2213A.8.2.3). Stable BRB behavior, elimination of the compression buckling mode, and comparable capacities in both tension and compression allow the designer more flexibility in the layout of BRB members to accommodate a particular architectural scheme. Furthermore, BRB's need considerably smaller gusset plates than SCBF's (Figure 7), do not need zipper columns in chevron configurations and require lighter beams.

The original BRBF was produced by Nippon Steel Corporation under the product name Unbonded BraceTM. However, currently there are at least three manufacturers: Nippon Steel Corporation: UnbondedBraceTM, Star Seismic LLC: PowerCatTM, and CoreBraceTM. Each manufacturer has been rapidly developing new brace connections, manufacturing improvements and aggressively promoting their products to increase their market share of the expanding BRBF applications in new and retrofit building design. Currently there are three configurations for BRB end connections. Nippon Steel and CoreBrace have a typical bolted connection. CoreBrace has developed a modified bolted connection that uses significantly fewer bolts than the typical bolted connection. Star Seismic has a unique true pin connection consisting of a large drift pin with retaining plates. Each connection has specific advantages and disadvantages as listed below:

Standard Bolted Connection Advantages: · Oversized (OVS) holes allow for more erection tolerance than single pin connection. · Multiple bolts provide more connection redundancy and distribute potential gusset plate inelastic bearing deformations when compared to single pin connection. Disadvantages: · Larger gusset plates and shorter BRB yield length when compared to a single pin connection. · A large quantity of splice plates and bolts is significantly more labor intensive than single pin connection. · Not a true pinned connection. Frame drift results in secondary moments in connection and brace.

Modified Bolted Connection Advantages: · Same as Standard Bolted Connection listed above. · Significantly fewer bolts and no splice plates to install resulting in labor reduction. Disadvantages: · Same as Standard Bolted Connection listed above.

Figure 9. Modified Bolted Connection

True Pin Connection

Figure 8. Standard Bolted Connection

Figure 10. Star Seismic LLC BRB with Restraining Collar & True Pin Connection

Advantages: · Longer BRB yield length results in smaller strains for a given demand. · A true pinned connection that eliminates in-plane secondary moments due to drift. · Single pin reduces installation costs. Disadvantages: · Erection tolerance is very small (1/32"). All connections meet or exceed the testing criteria of FEMA 450 and the 2005 AISC Seismic Provisions.

counter to intuition, by eliminating the compression buckling mode BRBF's have higher axial capacities in compression than in tension. Tests show that this difference could reach 10%. This is due to the different Poisson's effects on the true strains in compression and tension. When carrying out linear elastic analyses in ETABS, for example, a non-prismatic frame element may be utilized to model a BRB fairly accurately and account for the length adjustments. On the other hand, performing nonlinear inelastic analyses is not as simple. A nonprismatic element may still be utilized, but only if accompanied by an axial nonlinear hinge. The nonlinear hinge location and properties can be readily assigned in ETABS. These properties could reflect the initial stiffness, yield strength, as well as post-yield stiffness. Different input values for tension and compression properties could be incorporated into the hinge element to match the tested nonlinear behavior.

Analysis Issues

Modeling of conventional Concentric Braced Frames (CBF) is relatively straight forward using their tabulated section properties from standard section tabulations. BRBF manufacturers on the other hand, have multiple pre-tested braces with well defined behavior, design strengths, and material properties. These BRB's normally have yield capacities that range from 100 kip to 1000 kip. Incorporating such elements into the building's computer model should be handled with some caution. Geometry, actual core yield lengths, and connections should be accurately considered and accounted for in the mathematical representation. Typical actual core yield lengths range between one half to two thirds of the work-point to work-point lengths depending on the type of connection detail and thus a BRB is physically shorter than its analytical model. This fact makes the BRB significantly stiffer and forces it to undergo higher strains than accounted for by the design engineer. Furthermore, differences between analytical and physical lengths are not eliminated even when member offsets are automatically assigned to centerline dimensions by the analysis software. Such considerable differences need to be accounted for in the analysis and design of BRB's and the connections. Different manufacturers have different connection details as described earlier. After competitive bidding is considered and a vendor has been selected, final revisions to the brace designs are required from the design engineer. Overlooking this could potentially compromise original sound designs. Furthermore, and

BRBF's as Potential Yielding Dampers

BRBF's can offer supplemental damping through stable cyclic yielding of the steel core. A significant amount of energy is dissipated through this phenomenon. Considerable equivalent viscous damping can be added to the structural system. This puts the BRBF's in the same class as the Added Damping And Stiffness (ADAS) devices. Being the weakest structural elements and due to their stable yielding potentials, BRBF's can also serve as effective damage fuses during the Design Basis Earthquake (DBE). Limiting and containing the inelastic behavior in BRBF's allows the conventional frames to remain essentially elastic. Furthermore, yielding and softening of the BRBs reduces the effective period of the structure and thus effectively reduces the total base shear. Proper sizing of the steel core is an important and iterative step in designing BRBF's, in order to attain desired performance. Too large of a core steel area may limit or even prevent brace yielding at the DBE event which increases the design base shear. On the other hand, too small of a core steel area may not provide enough stiffness and toughness for the structure which increases the drifts. Therefore, successive iterations of

Figure 11. BRB Different Failure Types

modified stiffnesses may be needed to fine tune the BRBF system for optimum response. To ensure stable and consistent behavior, the core steel needs continuous confinement and lateral support. Consequently, BRBF's cannot be spliced. Welded or bolted splices of braces are not allowed in situations where the braces are likely to be subjected to inelastic demands because that would probably result in undesirable behavior leading to possible brittle fracture. Therefore, retrofit installation using long and heavy BRBF braces in functioning buildings is likely to be problematic. connection beyond the concrete filled HSS confinement occurred in one test. In another test a brittle tear developed in the beam flange and web propagating from the gusset plate. In one test of a fully restrained beam column connection the complete joint penetration weld fractured completely. Most failures were located in supplementary elements of the BRB frame (beams and columns). Out-of-plane buckling of the connection was a direct consequence of the configuration of the BRB assembly (Figure 11 above). Star Seismic LLC was the first to develop a proprietary connection design that essentially eliminates this potential problem; a centering HSS collar slides over the end of the concrete filled HSS confinement assembly. When loaded, the centering collar restricts buckling of the BRB connection. Star Seismic LLC has shown in a number of tests that their BRB assembly does not buckle out-of-plane (Merritt, et al, 2003).

BRBF: Some Questions and Challenges

The UC Berkeley tests illuminated a number of failures in BRBF assemblies. Out-of-plane buckling of the

Future research is needed to determine improved design and detailing of gusset plates to avoid stress concentrations that cause fractures in adjacent beams or columns. Beam column rotation due to frame drift places great demands on the gusset plate. The gusset plate demands result in significant stress concentrations on the adjacent beams and columns at the termination end of the gusset plate. Welding design and quality control may mitigate brittle fractures in the beams or columns. Further gusset plate research would benefit the performance of BRBF systems in the future.

Earthquake of 1994. The existing Nordstrom Topanga store is located approximately five miles from the epicenter of the Northridge earthquake. In the owner's view this event illustrated the value of properly designed and constructed structures. Immediately after the 1994 earthquake the Nordstrom store experienced some relatively minor structural and non-structural damage that was repaired in a few weeks. Whereas the adjacent mall structure and surrounding major retail stores had significant damage that required extensive repairs lasting a number of months. Based on the above lessons it was the owners desire to replace the existing store with a structure that would perform well in subsequent earthquakes without being cost prohibitive. There was a willingness to spend a little more on the initial cost of the facility to insure better earthquake performance and consequently less post earthquake business interruption (loss of sales and possibly staff), lower risk to life and limb for building occupants and lower repair costs.

Case Study: Nordstrom Topanga Mall

The following case study is a new retail store which replaces an existing operating store as part of a major shopping mall expansion. Building Description Footprint: 283 ft by 248 ft Three Story (211,000 sq ft) Typical Floor Height is 18'-0" Typical Bay Width 30'-0" up to 35'-0" 2nd and 3rd Floors: 3-1/4" Lt Wt Concrete Over W3 Roof Diaphragm: Verco PLB-36 1-1/2" Metal Deck Foundation: Pre-cast Driven Concrete Piles Exterior Pre-Cast Wall Panels (85 psf) Geometric Irregularities Along one line of lateral resistance a vertical discontinuous frame line from the roof to the third floor occurs due to a large mechanical well. Structural System Selection The basic structural system was selected with the following factors in mind: speed and ease of construction, overall cost, ease of future modification or remodeling, longevity and serviceability. Consequently, structural steel construction was chosen as the best system for this application. The owners had developed a keen awareness of the direct and indirect consequences of major earthquake hazards on their business operations and their bottom line based on their experience in the Northridge

Figure 12. Isometric View of ETABS Lateral System Model

Several lateral system schemes were discussed and evaluated. In the past many of the Nordstrom stores located in highly seismic areas used moment resisting frame lateral systems. This system is now being used less and less due to various factors which include the presence of a relatively stiff, brittle and heavy pre-cast concrete panel cladding system which poses challenges when used in conjunction with flexible moment frame systems. Furthermore, Special Moment Resisting Frame (SMRF) system, in most cases, are not cost competitive

with other systems given the various factors that are part of the typical building design for this owner. Braced frame systems and structural precast shear panel systems have also been used for other projects belonging to this owner. Braced frames were judged to be the most appropriate option for this particular building. The engineer presented to the owner some background information about the poor performance of conventional braced frames (OCBF and SCBF) from recent full scale testing. Eccentric braced frames (EBF) were considered but abandoned due to the system's use of inelastic deformation in the beam and the extensive detailing required for the link beams. The prospect of repairing EBF link beams permanently deformed by strong ground motion did not set well with the owner. The logical choice based on performance and cost for this particular project and project owner was the BRBF system. The recommendation of a BRBF system did not come without unique challenges. Since the BRBF is not recognized or defined in the 2002 Los Angeles Building Code (LABC), which is based on the 1997 UBC, Los Angeles Department of Building and Safety (LADBS) considered the BRBF system as an "undefined system". The process of approvals involved a process called a "Request for Modification." Not only did this involve review and approval of the project-specific design criteria by the plan review section of the LADBS, the pre-manufactured BRB assemblies also required LADBS Engineering Research Section's "product approval" (LARR). Over a four month period consisting of numerous meetings, letters, submittals, emails and conversations a final "one-time" project specific approval was obtained for this project. The design criteria were developed in conformance with 2003 NEHRP Provisions (FEMA 450) that was reviewed and approved by LADBS. Both FEMA 450 and the 2002 LABC were the design guidelines for the structure (Note: The 2005 AISC Seismic Provisions were still in draft form and not used for the design criteria. However, the more strict design criteria of the draft version of the 2005 AISC Seismic Provisions were used in the design). The approach for obtaining LADBS approval was to clearly illustrate that a BRBF system is essentially a CBF system without the undesirable behavior of compression buckling. Another convincing

argument was that BRBF systems are designed using actual tested limit state capacities as opposed to expected or nominal values as traditionally used in conventional CBF design. Additionally, BRB manufacturer product submittals were developed in accordance with acceptance criteria of ICC-ES AC238. Three different manufacturer's quality control procedures, Los Angeles steel fabrication certifications and prototype testing results were submitted to the Engineering Research Section. Three manufacturers were chosen to provide competitive bidding to the owner. Final "one-time" approval was obtained with a set of conditions that in some cases exceeded the requirements of FEMA 450. LADBS Conditions for Approval 1. Maximum inelastic inter-story drift ratio 0.013. 2. One-time LARR approval for BRB premanufactured assemblies based on ICC-AC238. 3. BRBF lateral system design based on 2002 LABC and FEMA 450. 4. Braces shall be fabricated in a LADBS licensed shop. 5. R=7.0 for proposed connections. Seismic Design Criteria The following seismic design criteria were determined by a consulting geotechnical engineer: SE Soil Type (Soft Clay) Ca = 0.36, Cv = 0.96 Na = 1.0, Nv = 1.0 Near Field: 13km from Santa Susana Fault (Type B) ETABS Modeling Criteria & Assumptions · · LDP ­ Linear Dynamic Procedure (Response Spectrum). Use 9 modes to obtain 100% mass participation. pinned beam/column

· · · · · ·

Scale response spectrum to 100% CBC static base shear using an R = 7.0. Pinned boundary conditions at column base. All diaphragms modeled as rigid. Roof diaphragm later hand checked as flexible. Model only lateral resisting system. Bounded ETABS modeling to capture 3 different manufacturer's BRB properties: manufacturer's material properties & stiffness (yield length and connection stiffness).

in area of steel from one size to the next. The incremental jumps in design capacity increase more when considering slenderness and compact section requirements of conventional braces. Conventional brace design can lead to over-designed connections, beams and columns when a slightly overstressed brace must be changed to the next larger size. BRB design allows the designer to optimize the design without over sizing the brace. Careful sizing of BRB braces can result in the desirable behavior of all braces in a floor in a given direction yielding simultaneously. On the other hand, simultaneous yielding of conventional CBF systems is extremely difficult. Maximum Adjusted Brace Force in BRB

Layout of BRB: 3rd to Roof: 16 BRB's, 8 Braced Bays (2 each side) 2nd to 3rd: 24 BRB's, 12 Braced Bays (3 each side) 1st to 2nd: 32 BRB's, 16 Braced Bays (4 each side) Seismic Design Summary: Total Building Weight: 15,180 kips VBASE = 0.129 W = 1,952 kips T1 = 0.52s (ETABS) Analysis Results: Interstory Drift M @ Roof = 3.7" (0.57% of Building Height) 3rd to Roof ­ 0.75% 2nd to 3rd ­ 0.52% 1st to 2nd ­ 0.29% Final BRB Required Yield Core Areas Redundancy Factor ( = 1.08): Core Steel ASTM A36 Plate Fy = 42 ksi verified by coupon tests 3rd to Roof ­ 5.5 to 6.5 in2 2nd to 3rd ­ 8.3 to 10.8 in2 1st to 2nd ­ 8.6 to 10.8 in2 One significant advantage of BRB design is the ability to "fine-tune" the brace core. The core is cut from plate so that just about any core area or design capacity can be specified. HSS or other shapes jump incrementally

Py = 312 to 658 kips (@ 2bm)

(Note: and are strain hardening and compression over-strength factors from actual tested prototypes and 2bm is the elastic brace deformation at twice the maximum calculated inelastic drift M) = 0.7% to 1.0% (BRB core strain @ 2bm) Construction Issues The fabrication and erection of a BRBF system is generally the same as conventional CBF systems. Close coordination must be maintained between the steel fabricator/erector and the BRB manufacturer. Early communication, correspondence and preconstruction meetings can minimize detailing and fabrication errors. The BRB assembly cannot be modified after production. Therefore, all dimensions and details must be coordinated prior to BRB production. The BRB contract at Nordstrom Topanga was awarded to Star Seismic LLC. Star Seismic was a subcontractor to the steel fabricator. Combining both steel fabrication and BRB vendor into one contract made the fabricator responsible for the entire steel structure. There was only one significant erection issue that caused some minor delays. The Star Seismic connection is a single pin with a 1/32" tolerance. This tolerance is almost impossible to obtain in steel fabrication. Some of the gusset plates needed some modification to fit up the pin connection. Star Seismic provided a prompt response to fit-up problems and had

reasonable and effective solutions based on previous project experience.

Cost Considerations BRB assemblies are costlier than standard HSS or other structural shapes. However, an R = 7.0 combined with a higher fundamental mode period results in a reduced design base shear when compared to OCBF and some reduction compared to SCBF (based on the current CBC). The base shear reduction reduces the cost of collectors, drags, cords, diaphragms and foundations. This project with its pile foundation benefited from increased savings due to the BRBF system. Overall, the bottom line cost for a BRBF system, in this case study, was essentially the same as a conventional CBF system, but obviously with a clear performance advantage for the BRBF.

Conclusions Figure 13. Typical Gusset Plate Sizes

The BRBF system is an elegant and potentially cost effective solution which avoids the poor buckling behavior of conventional CBF systems. The system provides superior ductility, consistent and repeatable behavior, and has the tested capacity to sustain multiple major seismic events without significant degradation. The analysis and design of BRBF systems involves a few complexities but these are not overly burdensome. The few cautions, concerns and drawbacks with BRBF's listed above should not be deterrents for the usage of these ductile braces in most projects. Once the IBC 2006 has been adopted by the various jurisdictions, the BRBF system will become just one more code-accepted lateral force resisting system to be considered for use in buildings without the need to resort to special approvals processes. It is expected that this will spur the much more widespread use of this braced frame system, which effectively avoids most of the known drawbacks of conventional braced frames. In the meantime the engineer has the choice of using established precedents to obtain special approval of the BRBF system as an "undefined system" for use in their building.

Acknowledgments Figure 14. Chevron & Two-Story X Brace Configurations

This work would not have been possible without the assistance of Dennis Firth, S.E. The authors would like to thank him for his efforts. Special thanks are also due to Jennifer Van Vleet for proof reading this manuscript.

References

AISC, 2005, Seismic Provisions for Steel Buildings, American Institute of Steel Construction, Inc., Chicago. AISC, 2001, Load and Resistance Factor Design Manual of Steel Construction, 3rd Ed., American Institute of Steel Construction, Inc., Chicago. FEMA 450, 2003, NEHRP Recommended Provisions for New Buildings and Other Structures, Building Seismic Safety Council for The Federal Emergency Management Agency, Washington D.C. AISC/SEAOC, 2001, Recommended Provisions for BRB, Structural Engineers Association of California: Seismology and Structural Standards Committee and American Institute of Steel Construction, Inc. ICC, 2003, Acceptance Criteria for Buckling-restrained Braced Frame Subassemblages, Subject AC238-1003R1 (BNG/BG), International Code Council, Whittier, California. SEAOC, October 2004, BRBF Seminar Structural Engineers Association of California. Notes,

SSEC, July 2004, Steel Tips: Seismic Design of Buckling Restrained Braced Frames, Lopez, W.A., Sabelli, R. for the Structural Steel Education Council Uriz, P., Mahin S., "Summary of Test Results for UC Berkeley Special Concentric Braced Frame Specimen No.1 (SCBF-1)," 2004, University of California at Berkeley. Merritt, S., Uang, C.M., and Benzoni, G., "Subassemblage testing of Star Seismic Buckling Restrained Braces," 2003, Report No. TR-2003/04, University of California, San Diego, La Jolla. Nippon Steel News, No. 333, September 2005

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