BUCKLING RESTRAINED BRACES AN OVERVIEW. By Kimberley Robinson, S.E. & Angus W. Stocking, L.S.

Transcription

1 BUCKLING RESTRAINED BRACES AN OVERVIEW By Kimberley Robinson, S.E. & Angus W. Stocking, L.S. October 2013

2 BUCKLING RESTRAINED BRACES AN OVERVIEW By Kimberley Robinson, S.E. & Angus W. Stocking, L.S. Buckling Restrained Braces (BRBs) are a structural component useful when providing bracing for seismic or other loads. BRBs have a large ductility capacity and are designed to yield under loads without buckling. They offer robust cyclic performance and significant cost savings, compared to conventional bracing systems. BRBs have been fully codified since 2005 by the AISC and the ASCE. Specification and design of BRBs and BRBFs (BRBs placed in concentrically braced frames) is relatively straightforward. A technology introduced in the late 1990s, the Buckling Restrained Braced Frame (BRBF) represents the state-of-theart in braced frame design. Over the last decade or so, the technology has reached a significant level of maturity through research, codification, and practice. BRBFs lateralload resisting systems that incorporate a Buckling Restrained Brace (BRBs) have been codified since 2005 and are covered by both the AISC Seismic Provisions (ANSI/ AISC and the upcoming release of ) and ASCE/SEI BRBFs have been incorporated into over 600 buildings to date, from 200-squarefoot sheds to high-rise structures. The system s critical element, the BRB, is a brace that does not buckle and harnesses the inherent ductility of steel to provide stable, predictable dissipation of seismic energy. Although the brace can be used wherever buckling of the brace is undesirable or where higher ductility and energy dissipation is desired (such as bridge, outrigger, or blast designs), they are typically incorporated as part of a BRBF concentrically braced frame system. The rapid expansion of the use of the BRB in all types of projects has occurred due to the clear cost savings of the overall system and the simplicity of design and erection. A study presented at the NASCC steel conference found a cost savings of $2.40/ sf. for a six-story structure over a similar Special Concentric Braced Frame design. The majority of the savings found was due to the smaller, simpler gusset plates, but there were also significant savings on beams, columns, and foundations. Anatomy of a BRB The main characteristic of a BRB is its ability to yield both in compression and tension without buckling. A BRB is able to yield in compression because it is detailed and fabricated such that its two main components perform distinct tasks while remaining de-coupled. The loadresisting component of a BRB, the steel core, is restrained against overall buckling by the stability component or restraining mechanism, the outer casing filled with concrete (see Figure 1). Bonding of the steel core to the concrete is prevented during manufacturing to ensure that the BRB components remain separate and composite action not allowed to take place. The BRB brace is placed in a concentric braced frame and becomes a BRBF system. This lateral-load system is used most often for structures in seismic demand categories D, E, or F, regardless of whether wind or seismic loads govern the design of the structure. BRBF systems provide cost savings over conventional bracing systems because engineers are better able to estimate applicable seismic demands, and then size the connections and foundations accordingly. The use of BRBF systems has also been explored for bridge, blast, and lower seismic applications where the highly-ductile, non-buckling attributes of the BRB might still provide a significant benefit. BRBF systems exhibit robust cyclic performance and have large ductility capacity, which is reflected in the seismic response factors, R. When the beams in the Instructions The Professional Development Series is a unique opportunity to earn continuing education credit at no cost to you by reading specially focused, sponsored articles. After reviewing the learning objectives below, read the Professional Development Series article and complete the quiz online at continuingeducation.zweigwhite.com. Quiz answers will be graded automatically and, if you answer at least 80 percent of the questions correctly, you can immediately download a certificate of completion and will be awarded 1.0 professional development hour (equivalent to 0.1 continuing education unit in most states). Note: ZweigWhite is an Approved Provider by the American Institute of Architects Continuing Education System (AIA/CES). However, it is the responsibility of the licensee to determine if this method of continuing education meets his or her governing board(s) of registration s requirements. Learning Objectives After reading this article you should be able to: Know the basics of how BRBs are used. Know the basics of BRB construction. Know the basics of designing and specifying BRBs for particular projects. Be able to provide examples of successful BRB applications. Professional Development Series Sponsor 2 PDH Professional Development Advertising Section Bently Systems, Incorporated

3 Bucking Restrained Braces Engineering Overview Restraining mechanism (steel HSS shell and concrete fill shown) Engineered gap allowing for elongation and contraction without imposing loads on the restraining mechanism Yielding steel core (Typically KSI yield strength) Stiffer non-yeilding end sections Star Seismic WildCat brace shown Nonyielding end Yielding core Nonyielding end Figure 1 lateral force resisting frame are moment connected to the columns, R = 8. Testing performed on BRBs to date suggests that they are capable of withstanding multiple seismic events without failure or loss of strength. The approximate fundamental period of the building, T a, is given by the equation: T a = 0.3h 0.75 Where h is the height of the structure in feet. Typical BRB Design Process The design approach of applications using BRB s typically incorporates the coordination of a BRB manufacturer. This is because some of the factors needed for design with BRB s are dependent on the design of the brace itself and may differ from manufacturer to manufacturer and even from brace connection type provided by an individual manufacturer. It is essential for the design engineer to incorporate design attributes of a BRB brace into their design for a brace that is possible to manufacture. Otherwise, an uncomfortable discussion awaits the design team during the bidding process or after the project has been awarded, when redesign to achievable brace design parameters may be necessary. Figure 1 shows the typical design process for a BRBF project, demonstrating the flow of information back and forth between the design engineer and the BRB manufacturer. The domestic brace manufacturers do not charge for this service, nor do they need to be under contract or obligation to provide it. Though the input from the brace manufacturer may include a variety of important contributions to the design, there are three critical design items that are contributed by the BRB manufacturer: brace stiffnesses, brace overstrength factors, and verification of testing coverage for the proposed project braces. Professional Development Advertising Section Bently Systems, Incorporated PDH 3

4 BUCKLING RESTRAINED BRACES AN OVERVIEW Brace Stiffness and Modeling For an Ordinary or Special Concentric Braced Frame, brace stiffness is tied to the brace area and is determined using the simple equation below. AE Where L wp-wp is the workpoint-to-workpoint distance along the axis of the brace. This analysis is automatically done as part of most structural design software packages. However, a buckling restrained brace is non-prismatic and consists of the yielding core segment, with the minimum cross-sectional area of the brace, and the outer portions of the brace that are designed to stay elastic and therefore include a greater cross-sectional area. Brace strength is controlled by brace core area, but the use of this core area in the structural model without any adjustment will not correctly capture the stiffness of the brace. This stiffness is usually captured in the model through the use of a stiffness modification factor (KF). The modeled brace stiffness would then be represented by the equation below. K model = KF(A sc )E L wp-wp Where A SC is the steel core area of the brace. The modeled brace stiffness can also be represented as a spring with a defined stiffness K model. The stiffness factor or modeled brace stiffness is unique to each brace manufacturers design, although it may be similar between manufacturers. It is also dependent on brace capacity, bay geometry and connection details. The design engineer will need to assume an initial value for this factor for early estimation of required brace capacity and preliminary beam, and column sizes and send this information to a brace manufacturer for early coordination to obtain the recommended stiffness factors for the braces. If brace capacities are adjusted, final values should also be confirmed with the manufacturer prior to finalizing contract documents. Brace Overstrength Factors For the BRBF system, the brace is designated as the fuse element and all other parts of the frame and connections are designed to remain elastic. As the BRB brace engages in a seismic event, the steel core is designed to yield and then to strain harden. This process will require the beams, columns, and connections to be designed for these higher, strain-hardened brace forces. The increase in the brace force in tension is represented by the factor ω, while the increase in compression is represented by the factor βω. These factors are determined from the results of the AISC 341 required testing. Again, these factors vary by brace manufacturers and even by brace connection type. TYPICAL BRB DESIGN PROCESS FLOWCHART FOR EQUIVELENT LATERAL FORCE METHOD, IBC Figure 2 START Calc. V using R, I e & T a Lay out braces 30%/70% C/T brace ratio does not apply Assume brace Stiffness (K=1.5 A sc E/L) Distribute lateral forces Estimate brace over-strength factors βω and β (1.5 & 1.1) Size beam/column sizes and brace core areas Calc. actual bldg period T and refine base shear V Redistribute lateral forces Did member YES sizes change? NO EOR Sends BRB MFR 1. Bay sizes/brace configuration 2. Approx. beam/column sizes 3. Approx. brace cap./areas. 4. R/C d, I, ρ, and assumed F y-min value and stiffness factor KF 5. Code forces P u and bldg drifts Proceed with design documents. Send manufacturer final info for coordination. Approximate brace and connection design Approximate brace lengths Calc. brace stiffness/ stiffness factors KF Calculate brace over-strength factors ω and βω. Confirm project braces do not exceed tested assemblies. BRB MFR sends EOR 1. Rec. F y range for core material. 2. Brace stiffnesses/kf factors. 3. Over-strength factors ω / βω. 4. MFR may make design recommendations NOTE: DESIGN ENGINEER PROCESS NOTED IN BLUE, BRB MANUFACTURER PROCESS NOTED IN TAN. END 4 PDH Professional Development Advertising Section Bently Systems, Incorporated

5 Standard and Innovative Uses for BRBs BRBs have been used on many types of structures as part of a standard BRBF Frame. They are enjoying widespread usage in building structures such as office buildings, hospitals, retail, car parks, multi-story residential, schools, religious, stadiums and arenas, as well as non-building and industrial structures. However, many projects use the Buckling Restrained Brace in unique ways that differ from the standard BRBF concentrically braced frame. The braces have been used in or proposed for a variety of applications, including bridges, civil structures, horizontal diaphragm elements, high-rise outrigger frames, externally anchored braces, wind towers, and many other unique applications. The following projects show a sampling of some of the most innovative applications. Metal Buildings The Seahawks Practice Facility is a large facility where the Seattle Seahawks NFL Football team practices, outside of the frequent rain and inclement weather that is a frequent companion of the city of Seattle. The project was high profile, and many teams offered numerous methods for building the structure, including several metal building approaches. The winning bid incorporated the team from HCI Steel Building Systems. The design incorporated BRBFs instead of the SCBF options proposed by the other teams. Since that structure was built in 2007, numerous metal buildings from a variety of manufacturers have incorporated the technology. Single Brace Retrofit Rutherford & Chekene, a structural engineering consulting firm in San Francisco, was presented with a unique challenge in the seismic evaluation and retrofitting of a historic steel and concrete structure. This two-story electrical substation was built in the early 1920s and remains an important link in the region s electrical power network. Renovations performed over the years had removed the lower portion of one of the concrete walls. The resulting structure was not adequate to meet the owner s seismic performance objectives. Retrofit options were limited. Replacement of the concrete wall that had been removed was not an option, as critical communications equipment that could not be moved had been placed in that area. Bracing on the exterior of the structure was not possible because of the historic character of the building and the presence of high-voltage buried conduits. A single brace could be allowed in the high-bay room adjacent to the area where the wall had been removed. A buckling restrained brace was selected as it was able to support both tension and compression loads while maintaining the required strength and ductility (see Figure 1). In addition, the brace strength could be tuned to avoid overloading collectors and floor diaphragms, and to match the strength of the remaining walls and reduce the possible plan-torsion of the structure under strong earthquake shaking. A new collector and foundation were provided to complete this portion of the retrofit. High-rise Outrigger System The One Rincon Hill South Tower is a 56-story, 578-foot tall residential structure. It is located next to the western approach of the San Francisco-Oakland Bay Bridge. While at the heart of one of the most seismically active regions in the U.S., the design was also governed by design considerations from powerful Pacific winds due to its prominent location on the skyline. The design of the structure includes a rectangular concrete core for the seismic and wind forces. The length of the core in one direction was sufficient to resist overturning demands but the other was too narrow to adequately control building sway. The design team at Seattle- JWA Parking Str C, Orange, CA Single Brace Retrofit Professional Development Advertising Section Bently Systems, Incorporated PDH 5

6 BUCKLING RESTRAINED BRACES AN OVERVIEW based Magnusson Klemencic Associates decided to incorporate an outrigger system into the structure to bolster the stiffness in the core s narrow dimension, much the same as the use of ski poles can stabilize a skier. The outrigger system served to reach out to the large concrete outrigger columns to engage them for resistance to overturning at four levels of the structure (see Figure 2). Buckling restrained braces allowed the design team to limit the amount of load that would be delivered to the outrigger columns while controlling the stiffness and response of the building. In addition, a large tank at the top of the building holding up to 50,000 gallons of water is used for two purposes: As a tuned liquid damper to counter the sway from wind forces and as a reservoir for firefighting purposes. Civil Structure Casad Dam is a concrete gravity arch dam built in the 1950s that includes an integral intake tower located on the upstream face at the center of the dam. The intake tower was not adequate to support the anticipated seismic demands, where the peak ground acceleration was increased due to the proximity of the Seattle fault and new research into the magnitude of potential events. A retrofit scheme was needed for the intake tower that would have minimal impact on the normal operation of the dam, would have minimal underwater work, and could be done with minimal expense. The design team at Hatch Associates Consultants, Inc. in Seattle investigated several options and found that bracing the tower back to the dam best met their key objectives for the retrofit, rather than strengthening the tower at its base. However, the arch dam required protection by limiting the brace forces. Viscous dampers and buckling restrained braces were considered and, after detailed simulations, stainless steel buckling restrained braces with a yielding steel core were selected. The project successfully met diverse functional objectives that included preventing tower collapse under a maximum credible earthquake with a 0.78g peak ground acceleration, meeting low maintenance requirements while providing high reliability, and ensuring that there were no environmental or water quality impacts. Bridge Foresthill Road Bridge, the tallest in California and the fourth tallest in the U.S., needed seismic retrofitting. This famous truss bridge, built in the early 1970s, is 2,428 ft. long and the deck is more than 730 ft. above the American River. After seismic evaluation completed with the help of the Placer County Department of Public Works, and after review of multiple alternatives, designers at Sacramento s Quincy Engineering proposed a BRB-based system that achieved performance objectives and allowed for repairable damage after a seismic event. Designers wanted to limit seismic forces on the bridge s longitudinal abutment anchors in order to protect the anchors and surrounding truss members. So, inelastic One Rincon Core Rendering, Hill Tower, San Francisco, CA demands were confined to replaceable sacrificial link plates that are designed to fail at prescribed strains, allowing BRBs to engage. The final retrofit configuration featured BRBs located longitudinally at the junctures of truss bottom chord connections and abutments. Design objectives were met effectively and economically. The projects listed above provide only a small sampling of unique uses for buckling restrained braces. As the brace usage expands, functions requiring symmetrical capacity between tension and compression, calibrated stiffness of elements, limiting of force transfer through an element, the incorporation of ductility and energy absorption and other features of the brace will continue to be found. The applications found truly demonstrate the abundant creativity of the engineering designers using the technology. Kimberley Robinson, S.E. is the chief engineer at Star Seismic, Park City, Utah. The company designs and builds buckling restrained brace frames (BRBFs) for all types of structures. Angus W. Stocking, L.S., has been writing about construction and infrastructure since PDH Professional Development Advertising Section Bently Systems, Incorporated

7 BUCKLING RESTRAINED BRACES AN OVERVIEW Go to to take the following quiz online. Quiz answers will be graded automatically and, if you answer at least 80 percent of the questions correctly, you can immediately download a certificate of completion. 1. While effective, BRBFs are a new technology whose use is not yet covered by applicable codes. a) True b) False 2. Which of the following is true? a) BRBs are intended solely for use in civil structures. b) BRBs have been in use since the 1990s, and have been installed in 500-plus structures c) BRBs should be used as a sacrificial truss element, and must be replaced after most seismic events. d) All of the above. e) None of the above. 3. BRBs are typically made of: a) Steel and concrete b) Steel and polymers c) Steel and various alloys d) Steel and aluminum e) None of the above 4. True or False: BRBs are essentially a commodity, and manufacturers do not need to be consulted when specifying BRBs in building projects? a) True b) False 5. When should a BRB manufacturer first be consulted? a) Before or during the modeling and design process b) After the permit submittal c) During the bidding phase of the project 6. The stiffness factor or modeled brace stiffness for BRBs: a) Adheres to a standard followed by all manufacturers. b) Is specified by several codes. c) Varies by manufacturer. d) Can be ignored in most projects. e) None of the above. 7. BRBs are not suitable for metal buildings: a) True b) False 8. BRBF systems are typically used for seismic demand category(s): a) D b) E c) F d) All of the above e) None of the above 9. In addition to the standard load combinations, beams, columns, and connections in a buckling restrained braced frame must be designed to what loads? a) W o Overstrength Factor Loads b) BRB specific overstrength factors, ω and βω c) The load determined by the stiffness of the brace d) All of the above 10. Which of the following statements are true? a) It is essential for the design engineer to incorporate design attributes of a BRB brace into their design for a brace that is possible to manufacture. b) It is not essential for the design engineer to incorporate design attributes of a BRB brace into their design for a brace that is possible to manufacture. c) The manufacturer is not an important consideration when manufacturing specifying BRBs. d) Design of BRBs is always the responsibility of the manufacturer, working from designer-provided specs. Professional Development Advertising Section Bently Systems, Incorporated PDH 7

8