Unbonded Materials Research Articles

Performance of novel perforated double-core steel-plate assembled buckling-restrained braces

This study proposed and evaluated a novel perforated double-core steel-plate assembled buckling-restrained brace (PDBRB). First, the mechanical properties of the PDBRB were theoretically derived to inform its design. Second, quasi-static tests were carried out on three PDBRB specimens with different configurations and one conventional perforated steel-plate assembled buckling-restrained brace (PBRB) specimen to comparatively evaluate the mechanical properties, energy dissipation performance, and optimal configuration of the PDBRB. Third, finite element analyses of the four test specimens were conducted to determine their stress distributions, equivalent plastic strains, and high-mode buckling deformation patterns. The following conclusions were drawn from the results of these investigations: 1) the novel PDBRB exhibited similar mechanical properties to those of the PBRB but with superior cumulative plastic deformation and energy dissipation capacity; 2) middle restraining plates were essential for the PDBRB, as when they were not present, the PDBRB exhibited a larger axial compressive force and compressive strength adjustment factor, fewer fatigue loading cycles, less total dissipated energy, and a smaller cumulative plastic deformation and equivalent visual damping ratio, especially when there was no gap between the two core plates; 3) The proposed design method can be used to accurately predict the mechanical properties of PDBRB; 4) the yielding process of the PDBRB began from the opening segments, neither the connecting nor transition segments yielded under a maximum displacement of 30 mm, and the maximum equivalent plastic strains in the PBRB and PDBRB with restraining plates were similar and considerably smaller than those in the PDBRBs without middle restraining plates; 5) under compression, a larger number of high-mode waves were generated in the core of the PDBRBs than in that of the PBRB. The deformed shapes of the two core plates of the PDBRB were mutually independent and dissimilar when middle restraining plates were provided but similar when these restraining plates were excluded regardless of whether a gap or unbonded material was provided between the core plates.

Experimental and numerical studies of the multi-tube assembled buckling-restrained braces

In this paper, the multi-tube assembled bolt-connected buckling-restrained brace (MT-BRB) with four internally restrained rectangular steel tubes and one externally restrained square steel tube is experimentally analyzed. This study conducted cyclic loading tests on five specimens with different parameters to investigate the effects of different loading directions of the core plate, the presence or absence of unbonded materials, the gap-to-width ratio and the width-to-thickness ratio on the mechanical properties, seismic performance and failure mode of MT-BRB. The experimental results indicated that the MT-BRB specimens made of unbonded materials had stable cyclic performance and energy dissipation capacity. Subsequently, the same numerical model as the experimental specimen was established, and the hysteresis and stress curves extracted from the finite element model were compared with the experimental results. The simulation had ideal agreement with the experiment result, demonstrating that the model was effective for performance predictions. Based on this, the working mechanism of MT-BRB was further explored through finite element analysis. Finally, parameter analysis was conducted on the constraint ratio and constraint length ratio to further investigate the energy dissipation performance of MT-BRB. This study is beneficial for promoting the application of MT-BRB in energy dissipation and vibration reduction engineering.

The Effects of Variable Thermal Conductivity in Thermoelastic Interactions in an Infinite Material with and without Kirchhoff’s Transformation

In this paper, the problem of an unbonded material under variable thermal conductivity with and without Kirchhoff’s transformations is investigated. The context of the problem is the generalized thermoelasticity model. The boundary plane of the medium is exposed to a thermal shock that is time-dependent and considered to be traction-free. Because nonlinear formulations are difficult, the finite element method is applied to solve the problem without Kirchhoff’s transformations. In a linear case, when using Kirchhoff’s transformations, the problem’s solution is derived using the Laplace transforms and the eigenvalue approach. The effect of variable thermal conductivity is discussed and compared with and without Kirchhoff’s transformations. The graphical representations of numerical results are shown for the distributions of temperature, displacement and stress.

Recent progress in buckling restrained braces: A review on material development and selection

Passive energy dissipation devices are a widely accepted solution used to improve the response of the structures under seismic conditions. While several types of passive energy dissipation elements have been proposed for replaceable elements, buckling restrained braces (BRB) was one of the most effective replaceable elements which has been used to retrofit frame structures in the past few years. A traditional BRB consists of a steel brace core with a cruciform cross section that is surrounded by a stiff steel tube (restraining unit) and unbonded materials to ensure the brace to move freely. The region between the tube and brace is filled with concrete-like materials. In recent years, a significant amount of research has been carried out on new types of BRBs which are more efficient and inexpensive. This paper presents an overview of recent advances in the development of BRB materials and buckling mechanism, with an emphasis on the material development for the restraining unit. The application of fibre-reinforced polymer (FRP) as restraining unit in BRB is highlighted.

A further study of the nesting biology of Leptochilus (Neoleptochilus) regulus (de Saussure, 1855) (Hymenoptera, Vespidae, Eumeninae)

Two females of Leptochilus regulus (de Saussure, 1855) were observed nesting in reed stalks of a Fabre’s hive serving as a block of trap nests in Crimea, with the timekeeping of all their nesting behaviours. The building material used by the females to separate the nesting cavity into the cells consisted of pellets of dry soil, gravel particles, and small fragments of tree bark, leaves, and thin stems. Females rapidly carried these items collected in 1.0–1.5 m from the nests. They bonded only the last portions of the building material in each “partition” with a very small amount of mud. An additional amount of unbonded building material items was deposited into the nest after sealing the last nest cell. Females hunted exclusively for small larvae of an anobiid beetle (Coleoptera, Anobiidae); they stored 19–37 prey items per cell. Hunting and provisioning occupied the largest portion of the females’ time budget. Duration of hunting flights was different between the females. The nests of both females contained three brood cells. Cocoons of Chrysis ragusae De Stefani, 1888 (Hymenoptera, Chrysididae) were found in the nests; females of this cuckoo wasp species were also recorded entering the host nests at the stage of provisioning, acting apparently as inquilines. The cocoon ultrastructure of L. regulus is illustrated and discussed.

Numerical study on undrained cone penetration in structured soil using G-PFEM

This paper presents a model for the numerical simulation of cone penetration in structured soil under undrained conditions based on the Particle Finite Element Method. An extended version of the Clay and Sand Model (CASM), allowing microstructural bonding and its degradation to be taken into account, was implemented into the application G-PFEM and compared to the analytical solution for undrained spherical cavity expansion. A robust model for cone penetration is obtained using a nonlocal stress-integration combined with the IMPLEX integration scheme. A parametric study on the shape of the yield surface – which is adaptable for the CASM – and the degree of initial bonding, as well as the rate of destructuration, was conducted to investigate the influence on the obtained normalized parameters Qt, BQ, U, and the cone factors Nkt, NΔu, Nke used for the determination of the undrained shear strength. It was found that the highest cone factors, leading to conservative estimations of the peak undrained shear strength, are obtained for unbonded material. Both, the shear strength and the tip resistance increase due to soil structure, however, Nkt and Nke are not significantly influenced as long as no destructuration takes place during penetration.

Initial studies on brace-type shear fuses

This paper proposes new brace-type shear fuses (BSFs) connected in series. The BSF consists of a core plate, two filling plates, two perforated cover plates, unbonding material and bolts. It is designed to dissipate seismic energy via yielding mainly under shear. Perforated cover plates make damage of the core plate visible. To analyze BSFs, seven specimens were designed with different number of shear fuses, tension-to-shear strength ratios and radii of shear fuses. In the experiments, it was found that the deformation capacity had positive correlation with number of shear fuses, tension-to-shear strength ratio, and radii of shear fuses. No buckling of the core plates occurred due to the short length of specimen. Fracture modes of all the specimens were shear rupture. Contact between the lateral sides of core plate and filling plates due to flexural deformation was observed, which led to significant increase of force in both tension and compression. In addition, initial stiffness and yield force of BSFs were derived based on the Principle of Virtual Work. A design criterion to avoid direct contact was also proposed for design of BSFs.

Experimental investigation on buckling‐restrained braces using mortar‐filled steel tubes with steel lining channels

SummaryA new type of buckling‐restrained braces (BRBs) using mortar‐filled steel tubes with steel lining channels is proposed to reduce the frictional force between the core member and the restraining member. After covering the core plate with the unbonding material, steel lining channels are set on the surface along the longitudinal direction, thus transforming the contact property between the core plate and the restraining member from steel–mortar interface to steel–steel interface. First, comparative cyclic tests are conducted on two new BRBs with steel lining channels and one conventional BRB. All the specimens exhibit stable hysteretic performance without visible global or local instability prior to failure. It is confirmed that the steel lining channels can improve the interface evenness between the core plate and the restraining member, reduce the compression strength adjustment factor, and improve the low‐cycle fatigue behavior and energy dissipation capacity of the BRB. Furthermore, the analytical model for the local stability of the restraining tube is proposed when the core plate buckles about the strong axis, and the corresponding practical design criteria are provided. The influence of various core plate layouts on the local stability of the restraining tube is finally investigated.

Experimental Tests for Pre-Qualification of a Set of Buckling-Restrained Braces

Buckling restrained braces are increasingly used as structural fuse elements due to their stable and quasi-symmetric cyclic behaviour and capacity to dissipate a large amount of energy. However, a wider adoption of buckling restrained braced frames is often precluded by the proprietary character of most buckling restrained braces, need for their experimental qualification and sometimes lack of experience of designers. To overcome these problems, a set of typical buckling restrained braces were developed in view of their pre-qualification. Both "conventional" and "dry" devices were considered, with capacities corresponding to typical steel multistorey buildings in Romania. Detailing of buckling restrained braces aimed at investigating the core aspect ratio, gap size, strength of the buckling restraining mechanism, and the unbonding material. The paper presents the results of the experimental program, and recommended design parameters of qualified specimens.

Seismic Response of Multi-Storey Building with Buckling Restrained Brace System

Buckling Restrained Braces (BRBs) are made up by a steel core surrounded by a buckling restraining system, coated with an unbonding material. The buckling restrained system prohibits steel core from buckling when in compression. Experimental studies have been carried out to study the behavior of BRBs. Experimental results showed that the BRB exhibit stable hysteresis response with substantial energy dissipation and ductility. Therefore, seismic performance of buildings or other structure can improve effectively by using BRBs. In this work, numerical studies of eleven models were conducted using ETABS software. The numerical tests were carried out in different scenarios, i.e. building the model with a conventional brace system, building models with partial BRBs system and building models with BRBs system. From both equivalent static analysis and response spectrum analysis main findings are the effects of yielding core length on partial replaced BRBs system and BRBs system buildings are significant in controlling or altering base shear, maximum storey displacement and storey drift, and time period. Also, BRBs are effective in reducing the axial force for column and footing.

Development of a novel high-performance all-steel fish-bone shaped buckling-restrained brace

Based on investigation of recent strong earthquakes, there is a potential that BRBs may rupture during a strong earthquake or subsequent repeated aftershocks. This study aims to propose a novel type of light-weighted all-steel dismountable BRB with fish-bone shaped core plate, which is termed FB-BRB in this paper. The FB-BRB consists of a core plate, two filling plates, two restraining plates and unbonding material. Deformation capacity of the proposed FB-BRB is to be maximized by generating several necking locations at the core plate, and details to avoid strain concentration at stoppers are also proposed. Experimental study is carried out using four scaled specimens with different configurations. Favorable seismic performance is obtained through comparison with that of a conventional BRB. The failure mechanisms of the newly proposed FB-BRBs are also further verified through numerical study using a combination of a ductile fracture model and a cyclic plasticity model, where further improvement is required to fully achieve the expected deformation mechanism.

Local buckling behavior of core plate in all-steel buckling restrained braces

This study presents the finite element analysis results of the proposed all-steel buckling restrained brace (BRB) models. The objective of the analysis is to conduct a parametric study to investigate the effect of interface detail of BRBs on local buckling behavior of the core plate. Moreover, the effects of magnitude of friction coefficient (¦I) between the core and the buckling restraining mechanism (BRM) contact, the size of gap between the core and the BRM, and the unbonding material thickness on local buckling behavior of core plate in BRBs are investigated through the finite element analysis method. Based on the results, the interface configuration, the magnitude of friction coefficient at the interface, and the size of gap could significantly affect the local buckling behavior of core plate in all-steel BRBs. However, the thickness change in unbonding material was not found to considerably affect the overall response of BRBs. Finally, an appropriate interface detail in BRBs was suggested for design purposes based on the finite element analyses results.

Cyclic tests for unbonded steel plate brace encased in reinforced concrete panel or light-weight assembled steel panel

Cyclic loading tests for the panel buckling-restrained brace (panel BRB) comprising an unbonded steel plate brace encased in a panel were carried out to investigate constructional details of a novel type light-weight assembled steel panel and to study reinforcement forms to improve the punching shear capacity of reinforced concrete panel. The effects of unbonded materials and gaps between the panel and the brace, reinforcements in concrete panels and weight of panels on the hysteretic behavior of panel BRBs, were mainly examined. Tests reveal that, compared with additional steel bars and ties, perforated steel channels used as reinforcements along the entire length of brace can prevent concrete panels from failure by punching shear. Residual flexural deformations appeared in some braces due to the gaps. The maximum axial compressive strength of each specimen significantly exceeds its yield strength due to strain hardening and frictional action. Lubricating greases in the steel panel BRBs are helpful to reduce the frictional action and to achieve satisfied hysteretic responses. All specimens achieved great ductility and energy dissipation capacity. The concrete panel in one specimen failed by punching shear and the other specimens failed due to tensile fracture of the braces. The weight of the assembled steel panel is about 30% that of the concrete panel and the hysteretic behavior of the steel panel BRBs matches that of the concrete panel BRBs. The panels that can be assembled and disassembled would be advantageous in inspecting and replacing the braces, as well as in reusing the panels.

An Experimental Study on Mortar-Filled Buckling-Restrained Mechanism

Buckling-restrained braces (BRBs) have been widely used in earthquake resistance structure systems, which are designed to yield in both tension and compression. Among most mortar filling BRBs, the bearing capacity in compression is higher than in tension particularly at large displacement. This leads the joints to suffer extra force and thus unfavorable for structures. This paper presents a set of experimental results of BRBs with different unbonding materials and specific nonyielding segments. Four models with different unbonding materials are tested in axial compressive with stationarily increased displacement amplitudes. These models are all featured with reinforced nonyielding segments to prevent lateral buckling. Both experimental and numerical results indicate that epoxy resin is the best unbonding material that can avoid transferring the axial force to the restrained part and the new reinforcement in nonyielding segments can effectively prevent lateral buckling.

Effect of filler material on local and global behaviour of buckling‐restrained braces

AbstractThe nonlinear response of buildings has attracted a tremendous amount of attention in recent years. Braces, as lateral force‐resisting elements of a structure, are designed to not only react in the elastic region, but also to exhibit nonlinear response beyond the elastic limit. However, buckling in compression drastically degrades the performance of braces under earthquake loading. Buckling restrained braces (BRBs) have been evolved into very effective systems for severe seismic applications. They prevent buckling in compression through the encasing of core steel into a steel tube and confining infill concrete.The effect of infill material is investigated in this research through the use of experimental tests. Filler material may be concrete, grout or mortar, as well as granular material such as compacted aggregate. Moreover, lightweight concrete or lean concrete may be utilized as filler to reduce the overall structural weight. Furthermore, the need for unbonding material may not arise when sand and gravel mixture is used. Nevertheless, the strength of the aggregate should be such that no buckling or strength deterioration is observed. Parametric studies on BRB characteristics are carried out in this research. Results of cyclic loading tests are then provided for individual cases to characterize the effect of response parameters of BRB assemblages. Copyright © 2009 John Wiley & Sons, Ltd.

Subassemblage tests and finite element analyses of sandwiched buckling-restrained braces

This study presents the experimental and finite element analysis results of a proposed steel buckling-restrained brace (BRB). The proposed BRB has two components: (1) a steel core plate that carries all axial forces during tension and compression, and (2) two identical restraining members that sandwich the core plate with fully tensioned high-strength A490 bolts to prevent core buckling. Instead of using unbonded material, a small air gap is provided between the core plate and the restraining members to allow for lateral expansion of the core plate under compression. Since two restraining members can be disassembled easily by removing the bolts, a damaged steel core can be replaced after a large earthquake. Thus, manufacturing new restraining members is not required. Four BRB subassemblages were tested to investigate the inelastic deformation capabilities and verify the stability predictions for the braces. Test results indicate that three BRBs with sufficient flexural rigidity of the restraining member develop (1) stable hysteretic responses up to core axial strains of 2.1%–2.6%, (2) maximum compressive loads of 1724–1951 kN (1.4–1.6 times the actual yield load), and (3) a cumulative plastic ductility that is much higher than that specified in AISC seismic provisions (2005). One BRB, intentionally designed with inadequate flexural rigidity of the restraining member, experienced global buckling as predicted. Nonlinear finite element analysis was conducted for each BRB for a correlation study. The objective of the analysis was to conduct a parametric study for different BRBs to further verify the effects of restraining member size, number of bolts, core plate length and cross-sectional area on buckling load evaluation. The design procedure for the sandwiched BRB was provided based on test and finite element analysis results.

Tests of hysteretic behavior for unbonded steel plate brace encased in reinforced concrete panel

Quasi-static tests for ten pieces of the unbonded steel plate brace encased in reinforced concrete panel, which is referred to as the panel buckling-restrained brace (panel BRB), have been carried out. The effects of some constructional details, such as unbonded material, clearance between the panel and the brace, configuration of the steel bar and the edge reinforcement, effective width of the panel, etc., on the hysteretic behavior of the panel BRBs are examined. The results indicate that the panel BRBs with evener unbonded materials, smaller clearance and additional steel bars and ties along the encased braces exhibit better ductility and energy dissipation capacity than the others. The brace under compression appears to exhibit small amplitude flexural buckling with multiple waves, and its ultimate axial force exceeds its yield load capacity significantly due to strain hardening and frictional action. All specimens of panel BRB exhibit a stable performance under the quasi-static loading until local failure of the panel occurs by either flexure or punching shear. The results also reveal that, with the same construction details mentioned above, the hysteretic behavior of the specimens with the effective width panel almost matches that of the specimens with normal weight concrete panel, however, the former kind of panel BRB would be advantageous in the aseismic performance of the buildings because of its lighter weight.

Segregation on shearing a granular material

Experimental and analytical studies have been made on the kinetic laws of segregation during shearing in unbonded granular materials in a state of plastic strain. The mechanism of inelastic spherical particle action gives an expression for the driving force of the segregation. A method has been developed for determining the segregation coefficient and flow characteristics. A mathematical description can be given of the particle segregation during shearing of a granular medium on the basis of the mechanism for shearing flow separation. A test has been made on the adequacy of a model for particle segregation dynamics in a two-dimensional steady-state shearing flow.

Buckling-restrained brace using steel mortar planks; performance evaluation as a hysteretic damper

The authors developed a buckling-restrained brace that enables increased design freedom at both ends of the core plate and strict quality control while providing stable hysteresis characteristics even under high strains. The buckling-restrained brace can be formed by welding a core plate covered with unbonded material to a pair of mortar-filled channel steels (steel mortar planks) as a restraining part. The use of this approach enables visual confirmation of the status of the mortar filling and also facilitates standardizing structural members and member-by-member quality control. Specimens of a buckling-restrained brace with different steel mortar plank heights are fabricated to adjust the restraining force, along with specimens with different core plate width-to-thickness ratios. The tests were conducted to reveal the hysteretic characteristics of the braces, as well as their cumulative plastic strain energy, elastoplastic properties, and stiffening properties. A performance evaluation formula as well as a buckling-restrained brace design method using the test results is proposed. Copyright © 2006 John Wiley & Sons, Ltd.

Novel piezoelectric actuators for tuning an optical fiber Bragg grating

A method of stretching optical fiber holds interest for measur- ing strain in smart structures where the physical displacement may be used to tune optical fiber lasers. A small, lightweight, tunable fiber laser is ideal for demodulating strain in optical fiber Bragg gratings (FBGs) at- tached to smart structures such as the reusable launch vehicle that is being developed by NASA. A method is presented for stretching optical fibers using the thin-layer composite unimorph ferroelectric driver and sensor (THUNDER) piezoelectric actuators invented at NASA Langley Research Center. THUNDER actuators use a piezoelectric layer bonded to a metal backing to enable the actuators to produce displacements larger than the unbonded piezoelectric material. The shift in reflected optical wavelength resulting from stretching the FBG is presented. Means of adapting THUNDER actuators for stretching optical fibers are discussed, including ferrules, ferrule clamp blocks, and plastic hinges made with stereo lithography. © 2002 Society of Photo-Optical Instrumentation En- gineers. (DOI: 10.1117/1.1504723)