Superior Seismic Performance of Buckling Restrained Braces Nathaniel R. Gant Abstract: A seismic load is a critical component in building design. Not only do following specified codes prevent building collapse, but techniques used to construct buildings prevent collapse as well. This report examines the use of buckling restrained braces that are commonly used in structural engineering today and the aspects of its seismic performance. This report includes experiments performed by engineers who have researched buckling restrained braces. These experiments reveal that buckling restrained braces have excellent seismic performances and can be applied to new structures or retrofitted structures. Keywords: Seismic design, Buckling Restrained Brace, Earthquake engineering
Introduction Structures battle various loads and forces in the world daily. These forces can derive from several sources. A dead load is a load that comes from the weight of the structure because of gravity. This load will always have an effect on any structure as long as it is standing. A live load is a load caused by weight that moves from one location to another on a structure. For example, a vehicle rolling across a bridge is a live load because the vehicles weight is moving throughout the bridge, causing a force to occur at different sections of the bridge structure. Most engineers have designed buildings, especially ones-built centuries ago, to support dead and live loads solely. Chandra and Waritichai (2011) referred to buildings built to mainly support dead and live loads as gravity load designed (GLD) buildings; the effects of dead load and live loads on GLD buildings are shown in Figure 1. Though buildings designed to support dead and live loads are structurally sound when the ground is static, most of these structures fail when strong earthquakes or hurricanes occur.
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Figure 1. Illustration of (a) dead load and (b) live loads effects on a building (Munach 2010). Seismic loads are those caused by earthquakes. “An earthquake generates several types of waves, which travel at different speeds. In particular, non destructive P waves (P for primary) propagate out from a ground rupture at about six or seven kilometers per second, while stronger S (secondary) wave that cause most of the damage travel only at about three or four kilometers per second� (Kumagai 2007). Earthquakes shake a structure back and forth causing a oscillating force on an entire structure. As a structure undergoes extreme loads, beams, columns, joints and other various members in the structure undergo rapid tensile and compressive changes that lead to buckling and ultimately failure. Engineers analyze seismic performance based on hysteresis, the phenomenon displayed by a system in which the reaction of the system to changes is dependent upon its past reactions to change. By using shake tables or hydraulic actuators as shown in Figure 2, engineers are able to discover if a brace performance effectively for hysteretic behavior. This loading can cause story drift. Story drift is a vital factor in seismic design because it is the difference in lateral deflection between two adjacent stories as shown in Figure 3. As the drift increases, secondary loads are introduced. These loads can be very severe and ultimately lead to failure.
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Figure 2. Illustration of a shake table used for seismic evaluation (Williams and Nerurkar 2012).
Figure 3. Illustration of story drift on a structure due to a seismic load (FEMA). GLD buildings perform poorly during seismic activities because the non-ductile reinforcement detailing and inappropriate proportioning of beam and column stiffnesses resulting in the beam being strong and column being weak. Therefore, if a strong beam and column connection could not support a load, the column would buckle and the building would collapse. This applies to soft story buildings, multi-story buildings in which one or more floors have windows, wide doors, large unobstructed spaces, or other openings in places where a shear wall would normally be required. Another reason why GLD buildings have very poor seismic performances is that the designs are not based on modern seismic build codes. Williams and Nerurkar (2012) noted that the 1994 Northridge Earthquake that occurred in Reseda, California was one of the costliest seismic disasters in North American history. In addition, the great earthquake in San Francisco, 3
California in 1906 causes significant damage to the entire city because the buildings were built without seismic considerations. Engineers that designed buildings before these incidents did not know that earthquake of these magnitudes would cause such destruction. They used insufficient design loads to construct many buildings. Two sets of ground motion used for seismic analysis today taken from Pacific Earthquake Engineering Research Center (PEER) Strong Motion Database Next Generation Attenuation (NGA) Project are the San Fernando and Northridge Earthquakes (Chandra and Wainitchai 2011). Literature by Badoux and Jirsa (1990) stated that typical buildings of reinforced concrete built in California about 30 years ago need seismic retrofitting. The lateral resistance in these typical buildings are inadequate because there perimeter frames consist of weak short column that are likely to fail in an undesirable mode. The seismic code provisions have change several times since their construction. In many cases, the current seismic values are more than twice the original values of these typical buildings (Badoux and Jirsa1990). Many earthquake and structural engineers have developed retrofitting techniques to improve seismic performance of buildings. These include jacketing with concrete, steel, or fiber reinforced polymer; column strengthening; adding concrete or masonry walls; and adding steel braces. Though an engineer can take many different approaches to tackling seismic strengthening, there is not a single solution to satisfy all design considerations (Chandra and Warnitchai 2011). Finding the better solution involves a great deal of research into issues such as time, cost, and availability. This report reviews the applications of the Buckling Restrained Brace (BRB) and the aspects of its seismic performance with intention to reveal the importance of this retrofitting technique for seismic stability.
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Buckling Restrained Brace (BRB) In recent years, most earthquake engineering principles have sought to design “damage controlled structures” (Chandra and Warnitchai 2011). The idea of a damage-controlled structure is the entire structure consists of a primary structure and an auxiliary structure. While under a strong earthquake, the primary structure will remain in an elastic state while the auxiliary structure takes all the seismic force. The majority of damaged due to the earthquake will occur on the auxiliary structure that would be able to be replaced after the earthquake. This allows the entire structure to be operational even under a strong earthquake (Chandra and Warnitchai 2011). Buckling restrained braces implement this “damage control” technique. An early attempt to create a brace that dissipates energy but does not buckle consisted of a conventional brace encased in a steel pipe filled with mortar (Black et al 2004). Stable hysteretic characteristics were noticed when the brace was tested. Engineers performed similar tests on BRB braces using reinforced concrete instead of mortar. Modern BRB braces are made up of two vital parts: the central steel core and the surrounding outer casing assembly as shown in Figure 4 (Robinson 2012). An outer assembly confines the central core to a point that it will yield in compression (Robinson 2012). When the brace encounters a load, the ductile steel core carries the axial forces; the core casing restrains the buckling; and adequate clearance and unbond material between the steel core limits shear transfer (Hikino et al 2012). This allows the brace to have a strength that is the same in both compression and tension, which is a major advantage to a structure. Robinson (2012) commented that the BRB brace could control the yield load, expected stain-hardened capacity, and elastic stiffness. Engineers commonly incorporate the BRB brace into frames. The design aim of a simple-connected steel frame with buckling
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restrained braces is to let the beams and columns remain elastic even under severe earthquakes, so that the structure will not collapse (Sun et al 2011).
Figure 4. Buckling restrained brace as manufactured by Nippon Steel (Carden et al 2006).
Applications of the BRB Engineers have used or proposed buckling restrain 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. In San Francisco, a structural engineering firm had a challenge in the seismic evaluation and retrofitting of a historic steel and concrete structure. Because a portion of the concrete walls were removed, the structure had not met seismic performance objectives. Bracing could not be done because of the buildings historic character and the presence of high-voltage wires. Their decision was to use a buckling restrained brace in the previously renovated room as shown in Figure 5. Robinson (2012) noted that the brace was able to support tension and compression loads in the building while maintain strength and ductility requirements.
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Figure 5. Picture of single BRB brace retrofit in previously renovated room (Robinson 2012). The One Rincon Hill South Tower also located in San Francisco is a 56-story, 578-foot tall residential structure. The design team of this structure aimed to strengthen the stiffness in the structure’s rectangular concrete core by using buckling restrained braces as shown in Figure 6. The braces allowed the design team to lower the design load affecting the columns because the braces controlled the stiffness and response of the building. The outriggers system’s purpose was to engage the large concrete outrigger columns, which resisted overturning at four levels of the structure; the braces on the outrigger system functioned similar to ski poles that stabilize a skier (Robinson 2012).
Figure 6. Schematic of One Rincon Outrigger System and placement of BRBs (Robinson 2012).
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The tallest bridge in California, the Foresthill Road Bridge, was retrofitted using BRB braces. The bridge spans the North Fork Canyon measuring 2,428 feet across and 728 feet above the river as shown in Figure 7. Engineers constructed the bridge in the early 1970s and at that time, it was the fourth highest bridge in the United States and ninth highest bridge in the world (Robinson 2012). The design team chose buckling restrained braces for this design because it achieved performance objectives and it would allow easy repairs. Link plates would fail at a certain train and then the BRB braces would engage which would cause large energy dissipation (Robinson 2012). This solution proved to be the most effective and economical.
Figure 7. The Foresthill Road Bridge (Robinson 2012). The Casad Dam, built in the 1950s, is a concrete gravity arch dam. The intake tower of the dam required seismic support. The design team decided to brace the tower to the damn rather than strengthen the base of the tower as shown in Figure 8. Stainless steel buckling restrained braces were used the brace the tower. The braces prevented the tower’s collapse under a maximum credible earthquake with 0.78g peak ground acceleration (Robinson 2012). The tower required low maintenance and provided high reliability with no impact of the environment or water quality (Robinson 2012).
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Figure 8. Visual of the Casad Dam and intake tower retrofitted with BRBs (Robinson 2012). Buckling restrained braces were also used as external anchors. Because the owner could not shut down the facility during the retrofitting process, engineers configured external braced frames using BRBs. The BRBs braced the external foundation to the first and second levels of the structure as shown in Figure 9 (Robinson 2012).
Figure 9. BRBs used as external anchor braces (Robinson 2012).
BRB Seismic Analysis Engineers have performed several experiments and computational methods to analyze seismic performance on buckling restrained braces. Black et al (2004) performed an experiment where test specimens were load via hydraulic actuator on two reaction frames anchored to the laboratory floor as shown in Figure 10. Each brace was subjected to a standard seismic design load agreed on by engineers and additional tests on large deformations, low-cyclic fatigue, and
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simulated earthquake displacement. The brace exhibited stable hysteretic behavior for the entire test of 31 cycles (Black et al 2004). Only 31 cycles were performed because the loading could not continue without damaging the testing equipment. The maximum displacement was 47.2 mm and the brace did not fail.
Figure 10. Black et al experiment setup. Some engineers use the finite element approach to analyze buckling restrained braces. For this approach, a program subdivides a brace longitudinally into a series of segments, which the program subdivides into a number of fibers. A large displacement analysis of the complete system is performed as shown in Figure 11 (Soroushian et al 1988). This method is applicable to many types of structural system and requires member geometry and material properties (Souroushian et al 1988). However, since this method is so expensive, engineers consider this method impractical for nonlinear analysis of large structures.
Experiments
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Figure 11. Finite element analysis of a connector-brace assembly (Oliveira). Chandra and Warnitchai (2011) performed an experiment using the software platform OpenSees, which engineers modernly use as a tool for numerical simulation of nonlinear systems. Their objectives were to improve the performance of a GLD building by preventing soft story mechanism, to reduce building’s global displacements, interstory drifts, and building’s damages. Using OpenSees, they performed a seismic test on an original GLD build and the GLD building retrofitted with BRB braces thus making it a BRB building. The loading conditions used were those from Northridge earthquake and San Fernando earthquake. The seismic performance for the GLD building was very poor due to the soft story that existed on the first floor (Chandra and Warnitchai 2011). However, the BRB building had some minor damages in the BRBs and the infill walls when simulating the Northridge earthquake. For the San Fernando earthquake, the BRB building suffered some minor damage in the BRB but a soft story mechanism existed. In addition, the drift remained below 2% for all cases as shown in Figure 12. Chandra and Warnitchai’s (2011) experiment proved that BRBs could significantly improve seismic performance of the GLD building.
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Figure 12. Maximum story drift of the BRB building resulting from the moderate San Fernando (top left), the strong San Fernando (bottom left), the moderate Northridge (top right), and the strong Northridge (bottom right) (Chandra and Warnitchai 2011). Fahnstock et al (2007) conducted four hybrid pseudodynamic earthquake simulations and a quasi-static cyclic test on a buckling restrained braced frame as shown in Figure 13. The frame withstood significant drifts and had little damage. Overall, Fahnstock et al (2007) noted that the BRBs exhibited excellent performance using the code-based equivalent-lateral-force procedure.
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Figure 13. Photograph of Fahnestock et al test frame. An experiment by Hikinio et al (2012) focused on the out-of-plane stability of buckling restrained braces. Test beds applied ground shaking to the buckling restrained braced frames as shown in Figure 14. A test bed is a multi-purpose devices that supplies horizontal mass to a specimen while adding minimal lateral force resistance. These results from the test beds were scaled using specific scaling factors. Hikinio et al (2012) used several instruments to measure the affects of the seismic loading. Load cells were used measure the shear in each story. Displacement transducers measured the story drift and the out-of-plane deformation of the beam and BRBs. Strain gauges measured the force distributions in beams, columns, and BRBs. Elongation of the BRB steel core was the change in relative distance between end of the steel casing and the core projection of the BRB (Hikinio et al 2012). The results revealed that one specimen did not buckle where as the second specimen did end up buckling due to out-of-plane buckling. However, the buckled specimen exceeded predicted critical strengths by 56 to 68% and the resiliency of the BRB enabled stable energy dissipation even as the buckling deformation progressed to an extreme extent (Hikinio et al 2012).
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Figure 14. Hinkino et al specimen 2 (a) side view and (b) close-up view during test.
Conclusions This report explained several applications and aspects of the buckling restrained braces used to improve seismic performance. Engineers use BRBs in a variety of retrofitting projects aimed to improve structural strength, stiffness, and ductility. Engineers use BRBs in a single brace retrofit, high-rise outrigger system, bridge, dam tower, and external anchor brace. The use of the BRB in all the systems allowed support for both tension and compression loads. Experiments performed by various engineers reveal the superior performance of buckling restrained braces. The buckling restrained brace keeps story drift below the 2% regulation. In addition, the brace exhibits excellent hysteretic behaviors. Since many old structures have fatigued or been built under old provisions, they are in need or will be in need for retrofitting. BRB braces will be an excellent retrofitting technique to consider. They are also excellent for new construction and show much promise for the future.
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Appendix I. – References
Badoux, M., and Jirsa, J. (1990). "Steel Bracing of RC Freams for Seismic Retrofitting." J. Struct. Eng., 116(1), 55-74. Black, C., Makris, N., and Alken, I. (2004). "Component Testing, Seismic Evaluation and Charaterization of Buckling-Restrained Braces." J. Struct. Eng., 880-894. Chandra, J., and Warnitchai, P. (2011). "Application of Buckling Restrained Braces for Seismic Strengthening of Irregular Gravity Designed Reinforced Concrete Frame Buildings." Civil Engineering Dimension, 13(2), 65-74. Fahnestock, L., Ricles, J., and Sause, R. (2007). "Experimental Evaluation of a Large-Scale Buckling Restrianed Braced Frame." J. Struct. Eng., 133, 1205-1214. Hikino, T., Okazaki, T., Kajiwara, K., and Nakashima, M. (2012). "Out-of-Plane Stability of Buckling-Restrained Braces Placed in a Chevron Arrangement." J. Struct. Eng., 1-20. Kumagai, J. (2007). "How to Master a Seismic Disaster." IEEE Spectrum, 48-51. Robinson, K. (2012). "Novel Uses for the Buckling Restrained Brace." Structure Magazine, 810. Soroushian, P., and Alawa, M. (1988). "Efficient Formulation of Physical Theory Brace Models." J. Struct. Eng., 114, 2457-2473. Williams, B., and Nerurkar, A. (2012). "Seismic retrofit for a Pre-Northridge Moment Frame Building." Structure Magazine, 36-38.
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