Unbonded Brace Research Articles

Structures with Added Buckling Restrained Brace Elements

The number of high rise building has increased significantly as supporting infrastructure in many big cities in Indonesia. Most of the high rise buildings in Indonesia are constructed using reinforced concrete (r/c) moment resisting frame systems. However, due to the high seismicity of many of the regions of Indonesia, the use of moment resisting frames as single system, especially for tall buildings, are restricted. This structural system is in general quite flexible, and therefore excessive lateral drifts are experienced under seismic forces. In practice, adding buckling restrained brace (BRB) can be used to limit the lateral drift in high rise buildings. The objective of this paper is to present a study on the use of BRB elements in high rise r/c frame buildings to evaluate their seismic performance. In this paper, a 20 story office building located in a high seismicity region is modeled into two basic distinctions, i.e. building with BRB and without BRB. The adopted system of BRB is UBB (Unbonded Brace) type. The seismic response of the buildings is obtained by conducting non-linear time history analysis with seven scaled ground motions. Then, the performance of these buildings are evaluated and compared. Based on the findings, some recommendations are proposed for the use of BRB in the design of high rise r/c frame buildings located in regions with high seismicity.

Structural vibration passive control and economic analysis of a high-rise building in Beijing

Performance analysis of the Pangu Plaza under earthquake and wind loads is described in this paper. The plaza is a 39-story steel high-rise building, 191 m high, located in Beijing close to the 2008 Olympic main stadium. It has both fluid viscous dampers (FVDs) and buckling restrained braces or unbonded brace (BRB or UBB) installed. A repeated iteration procedure in its design and analysis was adopted for optimization. Results from the seismic response analysis in the horizontal and vertical directions show that the FVDs are highly effective in reducing the response of both the main structure and the secondary system. A comparative analysis of structural seismic performance and economic impact was conducted using traditional methods, i.e., increased size of steel columns and beams and/or use of an increased number of seismic braces versus using FVD. Both the structural response and economic analysis show that using FVD to absorb seismic energy not only satisfies the Chinese seismic design code for a “rare” earthquake, but is also the most economical way to improve seismic performance both for one-time direct investment and long term maintenance.

Confined Steel Brace for Earthquake Resistant Design

An experimental study of a passive energy dissipation tension-compression yielding brace or buckling-restrained brace has been conducted. The Confined Yielding Brace (CYB) consists of a steel yielding core confined within a tube filled with non-cohesive material. The external tube and confined non-cohesive material provide lateral stability to the core enabling the device to yield in compression as well as tension. This device is similar to the Unbonded Brace developed by Nippon Steel of Japan. Fourteen large-scale CYBs were tested to determine the effect of varied confining material, yielding core perforation blocking configurations, and displacement histories. Based on experimental results, Confined Yielding Braces produce relatively stable and symmetric hysteretic damping.

Component Testing, Seismic Evaluation and Characterization of Buckling-Restrained Braces

This paper reports on the results from a comprehensive component testing program on a type of buckling-restrained brace known as the Unbonded Brace™. The experimental data are used to: ~1! verify the results of theoretical predictions on the structural stability of the braces; ~2! validate the inelasitc capacity of the braces under severe earthquake demands; and ~3! calibrate a macroscopic hysteretic model that is found to predict, with fidelity, the brace force-displacement behavior. The study concludes that the unbonded brace is a reliable and practical alternative to conventional framing systems to enhance the earthquake resistance of new and existing structures; capable of providing both the rigidity needed to satisfy structural drift limits, while delivering a substantial and repeatable energy absorption capability.

Application of low yield strength steel on controlled plastification ductile concentrically braced frames

The hysteretic behavior of buckling inhibited braces (BIBs) (or unbonded braces) made of low yield strength steel is investigated experimentally. Test results from four large-scale specimens showed that the BIB is able to prevent early buckling and cracking that occur in conventional braces and develop to the full capacity the strength, ductility, and energy-dissipation capacity of the steel used. In addition, the use of low yield strength steel results in small yield deformation and dramatic strain hardening of the BIB. Earthquake simulation tests of a 0.4-scale three-storey ductile concentrically braced frame (DCBF), which employed low yield strength steel BIBs as concentric braces, were carried out. Test results verified the applicability of the BIB and the high seismic performance of the DCBF. The idea of controlled plastification worked well. In addition, both the 70% lateral-force distribution requirement of the Uniform Building Code and the 30% lateral-force distribution requirement of CAN/CSA-S16.1-94 seem unnecessary for a DCBF system.Key words: concentrically braced frames, earthquake-resistant structures, braces, steel structures.

Stiffening Requirements for Unbonded Braces Encased in Concrete Panels

This paper addresses the design and behavior of a composite brace system comprised of unbonded steel flat-bar brace embedded in a precast reinforced concrete panel. Debonding material is painted onto the steel flat-bar brace to remove the bond stress between the steel flat-bar brace and the precast concrete panel, so that axial load is applied only on the steel flat-bar brace. The precast concrete panel of a stiffening member is designed to prevent the steel flat-bar brace from buckling in order to effectively mobilize the compressive brace together with the tensile brace during seismic loading. The stiffening requirements to restrain the buckling of the steel flat-bar brace are derived from equilibrium considerations, and are verified by tests. Two types of tests were performed: (1) fundamental buckling tests; and (2) tests of frame with the stiffened brace subjected to the earthquake type of loading. By using the stiffened brace as a hysteretic damper, a designer does not have to consider the complicated postbuckling behavior of steel braces, and can easily control the horizontal stiffness, strength, and energy absorption capacity of steel buildings.

アンボンドブレースの動的性能に関する実験的研究

In applying dampers on structures for reducing response against seismic forces, confirming dynamic performances of such dampers are essential for guaranteeing safety of whole structural system. In this paper, axial-type elasto-plastic dampers (Unbonded Braces) are examined on full-scale dynamic test-system using shaking table with real-time response against recorded earthquake shake. As a result, dynamic performances of whole system of this type of dampers, including effect of restraint encasement, welding, local buckling, local bending etc., are confirmed and reported. The test covers maximum axial stress up to 7.2%, and cumulative plastic-ratio up to 1,400.

Active/passive vibration control systems for tall buildings

Three examples of vibration control systems are described. The first is a hybrid mass damper system, which is one type of active vibration control system, as installed on the top floor of a complex triangular building of forty-three stories in order to reduce the response of the building to strong winds and moderate earthquakes. The second is an unbonded brace damper, which is a kind of elasto-plastic damper using low-yield-point steel. It has been installed in a fifteen-story building as an energy absorption member to control severe earthquake motion. The last is a rotational variable damper using an electrorheological fluid. The feasibility of applying this type of damper to a real scale structure as a semi-active control device has been investigated.

フレームに組み込まれたアンボンドブレースの弾塑性挙動解析

Numerical analyses on buckling-restrained unbonded braces fixed in a frame were made using the non-linear finite element method and the following conclusions were obtained. 1) The hysterisis charachteristics and strain distributions of core steels on "frame model" are almost same as those on "basic model (non-eccentricity and no-moment model)." 2) The hysterisis charachteristics on "obliquely loaded model" are almost same as those of "basic model" although the strain distributions of core steels on "obliquely loaded model" are slightly larger at the end parts than those on "basic model." 3) Locally concentrated cumulative plastic strains of core steels due to earthquake are much small in comparison with cumulative plastic strain of steel to low cycle fatigue failure. 4) The hysterisis characteristics and strain distribution of extremely long unbonded braces are almost same as those of "basic model."

実大アンボンドブレースに関する実験的研究

The practical - scale "unbonded braces" were tested and the following conclusions were obtained. 1) Basic model : In the practical - scale test, it was confirmed that the stable hysteresis characteristics was obtained up to an axial displacement of ± 50 mm (axial strain : approximate 1/80). 2) Thin-walled pipe model : Safety from buckling under the present design standard is sufficiently high. 3) Model with different unbonding material thickness : In the elastic region, there is no difference in initial stiffness. It was, however, found that if the thickness of the unbonding material is increased, plasticization proceeds at a higher rate due to delicate undulation of the core steel plate. 4) Model using different grades of steel as the core : There is possibility of producing a brace which can freely control the hysteresis characteristics. 5) Eccentric model : In the "elastic region" corresponding to the behavior of the brace which is set in the frame, a sufficiently high buckling constraining effect can be obtaind, regardless of the magnitude of eccentric moment. Lateral deformation and rotation proceed in the plastic region and maximum strength decreases in the compressive region.