Beam Elongation Research Articles

The fiber hinge model for unbonded post‐tensioned beam‐column connections

AbstractThe precast concrete frame with unbonded post‐tensioned beam‐column connections (UPBC) has been emphasized in recent years for it is capable of sustaining a design‐basis earthquake with minor damage. The fiber hinge model (FHM), which is a simple and effective approach, is proposed in this paper for the simulation of UPBC. In the FHM, the mild steels and prestressed tendons crossing the interface are integrated into one element. The mechanism and behavior of the FHM are presented in this paper. The proposed model, which is developed in OpenSees, has been validated by tests, and the FHM is able to accurately describe the shift of compressive center induced by the opening of the interface between the precast beam and column, as well as “beam elongation effects.”

Co-rotational 3D beam element using quaternion algebra to account for large rotations: Formulation theory and static applications

This paper presents a co-rotational beam element based on quaternion algebra as a means of parameterizing large rotations. This co-rotational framework is based on a decomposition of beam kinematics into a rigid element frame, which follows the element and its pure deformation. Once the decomposition between rigid body motion and deformation is obtained, the principle of virtual work allows calculating the element response projected onto large displacements and rotations. This 3D co-rotational element lies within the framework of incremental formulations. The special feature of this formulation pertains to the decomposition of kinematics to extract the rigid body motion and pure deformations through use of quaternion algebra by solving an internal nonlinear kinematic equation. Thus, from the 2 quaternions defining global rotations and the 2 displacement vectors at the ends of the beam, this method is able to extract: 1 quaternion and 1 displacement vector parameterizing the rigid body motion, plus 2 other quaternions defining the internal rotations and beam elongation. The proposed formulation based on quaternion algebra therefore constitutes an alternative to the literature’s treatment of large-rotation kinematics using a quaternion algebra. The operators are also linearized in order to obtain the algorithmic stiffness operator. Eleven distinct static numerical applications are presented, along with comparisons from the literature in the aim of demonstrating the efficiency of this proposed element.

Effect of beam elongation phenomenon on lateral load resistance of RC frame

In this study, a quasi-static cyclic loading test was conducted on a two-span reinforced concrete frame to investigate the beam elongation phenomenon. The load–drift ratio responses and failure modes of the test specimens were examined, and the local response of the column members was analyzed to calculate the force and displacement caused by beam elongation. Based on the results, constitutive equations representing the beam elongation phenomenon were derived, and the design implications were proposed for the criteria of the beam-column strength ratio and joint shear strength presented in the ACI 318–19 code. The proposed design implications were found to accurately simulate the experimental results.

Single-side yielding precast concrete slotted beam-column connection with replaceable energy dissipation bars: Experimental investigation and finite element analysis

A new type of rebar coupler, non-slipping combined threaded sleeve (NCTS), is proposed, and based on this, replaceable energy dissipation (ED) connector is assembled. An innovative single-side yielding slotted beam-column connection with replaceable ED bars (SYSC-REDB) was developed by placing the ED connector in the beam end bottom of prefabricated concrete frame. Four quasi-static tests were conducted to evaluate the seismic behavior, such as the damage distribution, failure characteristics, hysteretic response, ED capacity, and deformation pattern. The hysteretic curves were stable and full without pinching, indicating excellent seismic performance. The deformation pattern of single-side yielding can enhance the ED capacity of connectors, concentrate damage and failure on the ED bars, and significantly reduce the deformation of the top beam and beam elongation. The joint could be repaired by replacing the ED bars via the inner space of the NCTS. The seismic behavior of the repaired specimens was similar to those of the original specimens. In addition, the finite element models of ED connector and SYSC-REDB joint were established by ABAQUS respectively, and their accuracy were verified by comparing with the test phenomena and results. Furthermore, parametric finite element analysis was executed for some key factors, including the top-hinge area depth, ED bar area, and top longitudinal reinforcement bar area. Theoretical and parametric study of bending moment capacity is also performed.

Evaluation of seismic response of coupled wall structure with self-centering and viscous damping composite coupling beams

In order to reduce the residual displacement and floor acceleration of coupled wall structures, a novel self-centering and viscous damping composite coupling beam is proposed. The proposed coupling beam consists of the elastic beam segments and the fuse part, which incorporates self-centering friction dampers and viscous dampers. The self-centering friction damper can provide strength and stiffness for the proposed coupling beam and reduce the residual displacement of the coupled wall structure. The viscous damper can add supplemental damping to the system, and this can control the peak lateral displacement demands and floor accelerations of the structure. Moreover, the proposed coupling beam can eliminate the beam elongation effect of the self-centering coupling beam with a rocking behavior. Design methodology and nonlinear numerical model of the proposed coupling beam are developed. The seismic behaviors of the coupled wall structure with the self-centering and viscous damping composite coupling beams (CW-SCVCCB) are assessed and compared with the structure with reinforced concrete coupling beams (CW-RCCB), the structure with steel coupling beams (CW-SCB) and the structure with self-centering coupling beams (CW-SCCB). Analysis results showed that the supplemental viscous damping can effectively reduce the peak interstory drift ratio and floor accelerations of the structure. The residual displacement of the novel coupled wall system is larger than that of the CW-SCCB, however, it is significantly reduced compared with the CW-SCB and the CW-RCCB.

Manipulating geometric and optical properties of laser-inscribed nanogratings with a conical phase front.

The formation of volumetric nanogratings in fused silica by femtosecond laser pulses are shown to afford new opportunities for manipulating the physical shape and tailoring the optical properties of the modification zone by harnessing unconventional beam shapes. The nanograting assembly was observed to rigorously follow the beam elongation effects induced with conical-shaped phase fronts, permitting a scaling up of the writing volume. Detailed optical characterization of birefringence, dichroism, and scattering loss pointed to flexible new ways to tune the macroscopic optical properties, with advantages in decoupling the induced phase retardation from the modification thickness by controlling the conical phase front angle. Further insights into an unexpected asymmetric response from Gaussian beams modified with concave and convex phase fronts have been provided by nonlinear propagation simulations of the shaped-laser light.

Effects of Axial Restraints on Beam Flexural and Joint Shear Behaviors in Reinforced Concrete Frames under Seismic Loading

ABSTRACT The elongation of beams in reinforced concrete frames due to flexural cracking and yielding is restrained by the surrounding structural components, thereby leading to axial force in the beams. To study the effects of beam axial restraint on the beam flexural and joint shear behaviors, cyclic lateral loading experiments were conducted on four beam-column substructures, two of which were restrained from beam elongation. The experiments indicated that the passively developed axial force significantly increases the beam flexural capacity. Moreover, the beam axial restraint increased shear demand on beam-column joints; as a result, joint shear failure occurred in one test specimen.

Experimental investigation on hysteretic performance and deformation patterns of single-side yielding precast concrete beam–column connection with energy dissipation bars

With the aim of achieving high construction efficiency, satisfactory seismic performance, and rapid restoration of structural functionality after an earthquake, this study proposes an innovative single-side yielding beam–column connection with replaceable energy dissipation bars (REDB-SYBC) for precast concrete moment-resisting frames (PCMF). During assembly, the concrete beam is attached to a column via the connection function of a non-slipping threaded sleeve assembly (NSTSA) and the support action of the shear keys (SKs) in the REDB-SYBC. This reduces the need for temporary supports for the precast beams on site. Five large-scale tests on precast concrete connections considering the partial flange effect of a slab subjected to quasi-static cyclic loading were conducted to investigate the mechanical behavior, deformation patterns, and energy dissipation (ED) capacity. The primary experimental variables included different loading protocols, restrainer forms, ED segment areas, and whether to replace the ED bars. This paper presents the assembly construction and load-carrying mechanism, and the analysis of the experimental results, including the observations, hysteretic performance, and deformation patterns. The single-side yielding deformation pattern of the REDB-SYBC connection minimized the deformation of the adjacent floor slabs and magnified the deformation of the ED bars at the bottom of a beam, thus facilitating deformation compatibility with the floor system and causing the damage to be focused on the ED bars. Consequently, the five specimens exhibited similar failure patterns via the fracture of the ED bars due to low-cycle fatigue failure, which was attributed to extensive yielding in both tension and compression. The REDB-SYBC connections exhibited a distinct beam end yielding mechanism, which demonstrated a failure pattern that was consistent with the desired “strong column and weak beam” design philosophy. The test results indicated that the proposed REDB-SYBC connection exhibited a dependable decoupled moment and shear load transfer mechanism, stable hysteretic behavior, negligible beam elongation, and reduced floor damage. The performance of a repaired specimen was nearly the same as its counterpart before repair, indicating that replacement was effective. The impact on the REDB-SYBC connection in response to a strong aftershock was also estimated by reloading an unrepaired specimen.

Experimental investigation of a single-yielding precast concrete beam–column connection with replaceable energy-dissipation connector

A single-yielding mechanism can effectively control the bending moment of a connection, and reduce the damage to the connection and column. In this study, a single-yielding precast concrete (SYPC) beam–column connection with a replaceable energy-dissipation connector (REDC) is proposed. The force transfer mechanism of the SYPC connection is discussed based on theoretical research. To verify the force transfer mechanism and repair ability of the SYPC connection and investigate the seismic performance of the SYPC connection, five low-cycle quasi-static loading tests are conducted on a single full-scaled specimen using five REDCs with different sizes. According to the test results, the seismic performance of the SYPC connection is discussed, including the bearing capacity, energy dissipation capacity, and stiffness. In addition, the single-yielding mechanism, neutral axis depth, beam elongation, and REDC strain in the SYPC connection are studied. The analytical and test results show that the structure of the SYPC connection is simple and reasonable, and that the path of the force transmission is direct and efficient. The SYPC connection provides excellent seismic performance, and is easy to repair after an earthquake. The single-yielding mechanism allows for control of the bending moment of the SYPC connection; this is beneficial to avoiding damage to the connection, e.g., damage owing to the overstrength of the beam strength and the alteration of the strength hierarchy in the structural system. Thus, it is conducive to realizing the design concept of “strong-column and weak-beam."

Development of novel self-centering steel coupling beams without beam elongation for earthquake resilience

Reinforced concrete (RC) coupled walls are extensively used as the seismic-resistant structural system of tall buildings in high seismic zones. In RC coupled wall systems, the coupling beams are typically designed and detailed to dissipate energy through large inelastic responses to meet the expected seismic performance under moderate-to-strong earthquakes. However, costly excessive repair or even complete demolition caused by the considerable damage in conventional RC coupling beams is usually inevitable. Such disadvantage of RC coupling beams has increased the interest on replaceable steel coupling beams, which can isolate the damage concentrated in the fuses; thus, these beams are expected to be replaced after a strong earthquake. However, replacing these beams in practice is difficult if considerable residual deformation exists in these replaceable fuses. Therefore, seismic resilience cannot be explicitly guaranteed. For this reason, this study proposes a novel self-centering (SC) steel coupling beam that incorporates superelastic shape memory alloy (SMA) bolts and steel angles. The SC steel coupling beam is composed of two elastic beam segments and one rocking segment. Elastic beam segments are designed by steel beams that connect to the RC walls at both ends of the coupling beam, whereas the rocking segment located in the middle of the coupling beam is controlled by the SMA bolts and steel angles. Two ingenious shear keys are designed between the elastic beam segments and the rocking segment; these shear keys help the SC coupling beam to exhibit centerline-rocking behavior that is free from beam elongation. The working principle of the novel SC steel coupling beam is described first. Then, the cyclic responses of the SMA bolt and the steel angle, which are the two core components in the SC steel coupling beam, are investigated. The seismic performance of the SC steel coupling beam is computationally investigated in consideration of the effects of steel angles and the prestrain in the SMA bolts. Results show that the proposed SC steel coupling beams exhibit excellent SC capability and energy dissipation. Most damage is isolated in the steel angles, which can be easily and rapidly inspected or replaced after earthquakes without operation interruption. Importantly, beam elongation is almost eliminated under cyclic loading. With these advantages, the SC steel coupling beam can provide a promising solution for high-performance coupled wall systems.

Revisiting flexural overstrength in RC beam-and-slab floor systems for seismic design and evaluation

Beam flexural overstrength is an important parameter in seismic design and evaluation of reinforced concrete (RC) moment resisting frames. It affects the column-to-beam flexural strength ratio (CBSR) which is a key parameter to avoid unfavorable column-driven mechanisms. In RC beam-and-slab floors, interaction between beam growth and slab panels is of a very complex nature which significantly alters the beam flexural overstrength. Enhancement of beam flexural capacity in slab floor systems depends on several parameters that should be considered on a case-by-case basis. To include some of the effects that were not considered in previous studies due to physical limitations of experimental works and lack of appropriate computational tools, a generic two-by-two bay archetype story model with a beam-and-slab floor system were considered for further investigations by means of numerical modelling. In order to consider the spatial effects of the floor system, several computational models were assembled by changing the span and cross section of beams in terms of depth, width and reinforcement ratio as well as the thickness of slabs to perform a parametric study on beam flexural overstrength ratio. Multilayer nonlinear shell elements were used for finite element modelling of slab panels to account for the kinematic effects of beam elongation along with the restraining effects of the floor slab. According to the results of analyses, variation of positive and negative flexural overstrength ratios of beams at different performance levels were presented for different parameters. Results demonstrate that the flexural overstrength ratio is greatly affected by the level of chord rotation (beam lateral drift or performance level) reinforcement ratio and the type of connection. However, beam-to-slab flexural stiffness ratio as well as beam length are not overly important parameters for flexural overstrength evaluation. This study reveals that, despite code prescriptions, CBSR should not be considered identical for all cases; hence, it should be revisited based on the desired performance level, type of connection and the beam reinforcement ratio.

Damage and deformation assessment of earthquake-resistant RC slotted beam-column connections using digital image correlation technique

Slotted-beam concrete connections with relocated plastic hinges mitigate yield penetration into the joint, result in minimal beam elongation, and achieve a non-tearing action to the attached slab when subjected to seismic excitation. Slotted-beam connections can have a single slot made at the bottom face of the beam member or double slots made at the top and bottom faces of the beam member. The former configuration is referred to as the Single Slotted Beam system (SSB), while the latter configuration is referred to as the Double Slotted Beam system (DSB). In this research, the damage evolution and deformation mechanisms of large-scale experimentally tested SSB and DSB connections with and without relocated vertical slots were investigated. The damage evolution was studied using ductility-based damage indices, energy–based damage indices, and other forms of damage assessment methods. The contribution of the beam deformation, column deformation, and joint deformation to the overall drift was assessed using the Digital Image Correlation Technique (DICT). The damage assessment indicated that relocating the vertical slots away from the face of the column resulted in decreasing the ductility capacity of the SSB system while it resulted in insignificant change up to a relocation distance equivalent to the beam effective shear depth in the DSB system. The deformation assessment indicated that increasing the relocation distance in the SSB system results in reducing the effectiveness of the system in relocating the center of rotation, while the relocation distance of the vertical slot has negligible influence on the effectiveness of relocating the center of rotation in the DSB system.

Spatial Expansion and Speeds of Type III Electron Beam Sources in the Solar Corona

Abstract A component of space weather, electron beams are routinely accelerated in the solar atmosphere and propagate through interplanetary space. Electron beams interact with Langmuir waves resulting in type III radio bursts. They expand along the trajectory and, using kinetic simulations, we explore the expansion as the electrons propagate away from the Sun. Specifically, we investigate the front, peak, and back of the electron beam in space from derived radio brightness temperatures of fundamental type III emission. The front of the electron beam travels at speeds from 0.2c to 0.7c, significantly faster than the back of the beam, which travels at speeds between 0.12c and 0.35c. The difference in speed between the front and the back elongates the electron beam in time. The rate of beam elongation has a 0.98 correlation coefficient with the peak velocity, in line with predictions from type III observations. The inferred speeds of electron beams initially increase close to the acceleration region and then decrease through the solar corona. Larger starting densities and harder initial spectral indices result in longer and faster type III sources. Faster electron beams have higher beam energy densities, and produce type IIIs with higher peak brightness temperatures and shorter FWHM durations. Higher background plasma temperatures also increase speed, particularly at the back of the beam. We show how our predictions of electron beam evolution influences type III bandwidth and drift rates. Our radial predictions of electron beam speed and expansion can be tested by the upcoming in situ electron beam measurements made by Solar Orbiter and Parker Solar Probe.

Femtosecond laser-induced circular dichroism in silica: Dependence on energy and focusing depth

Abstract The paper addresses the creation of strong circular dichroism from a femtosecond laser light with a linear polarization strongly focused in silica glass and perpendicular incidence. All aspects of the experiment are therefore achiral and should not give rise to chiral property creation if taken themselves alone. We investigate the laser-induced circular dichroism according to orientation of the laser polarization, pulse energy and focusing depth. It appears that deep focusing under high numerical aperture conditions leads to higher ellipticity when compared to shallow focusing or low numerical aperture focusing. Strong circular dichroism can be imprinted when polarization is at ±45° of the horizontal scanning direction in reference to the laser plan. In addition the generation of circular dichroism appears to be closely related to the spatial aberration, spatial chirp and beam elongation under strong focusing conditions.

Damage to concrete buildings with precast floors during the 2016 Kaikoura earthquake

The 2016 Kaikoura earthquake resulted in shaking in excess of design level demands for buildings with periods of 1-2s at some locations in Wellington. This period range correlated to concrete moment frame buildings of 5-15 storeys, many of which had been built in Wellington since the early 1980s, and often with precast concrete floor units. The critical damage states used to assess buildings during the Wellington City Council Targeted Assessment Programme are described and examples of observed damage correlating to these damage states are presented. Varying degrees of beam hinging were observed, most of which are not expected to reduce the frame capacity significantly. Buildings exhibiting varying degrees of residual beam elongation were observed. Cases of significant beam elongation and associated support beam rotation resulted in damage to precast floor unit supports; in one case leading to loss of support for double-tee units. The deformation demands also resulted in damage to floor diaphragms, especially those with hollowcore floor units. Cracking in floor diaphragms was commonly concentrated in the corners of the building, but hollowcore damage was observed both at the corners and in other locations throughout several buildings. Transverse cracking of hollowcore floor units was identified as a particular concern. In some cases, transverse cracks occurred close to the support, as is consistent with previous research on hollowcore floor unit failure modes. However, transverse cracks were also observed further away from the support, which is more difficult to assess in terms of severity and residual capacity. Following the identification of typical damage, attention has shifted to assessment, repair, and retrofit strategies. Additional research may be required to determine the reduced capacity of cracked hollowcore floor units and verify commonly adopted repair and retrofit strategies.

Seismic Performance of Modified Single-Slotted-Beam Concrete Connection

The slotted-beam connection system experiences minimal beam elongation and achieves a non-tearing action under seismic loading. The system is modified in this research in order to overcome the main challenges in the design process; the yield penetration into the joint, the joint cracking, and the complex joint shear deformation mechanism. These challenges are overcame by moving the vertical slot away from the face of the column, and hence, relocating the plastic hinge. Based on the experimental results of large-scale specimens (hysteretic response, cracking pattern, beam rotation, etc.), the optimum relocation distance is equivalent to the shear depth of the beam member.

Experimental study on a self‐centering coupling beam eliminating the beam elongation effect

SummaryTraditional self‐centering coupling beams display a beam elongation effect under large shear deformation. The beam elongation effect conflicts with the rigid slab assumption and may induce harmful compressive stress between the coupling beam and the walls. This paper proposes a novel self‐centering coupling beam, aimed at eliminating the beam elongation effect. The proposed self‐centering coupling beam consists of two connecting parts and one central rocking part. The main feature of the self‐centering coupling beam is the gap between the central rocking part and the one connecting part. This gap provides sufficient space for the beam elongation effect. Friction dampers are installed at the corners of the coupling beam to provide energy dissipation capacity. An experimental study, including three specimens, was carried out. The test results demonstrate the satisfactory self‐centering and energy dissipation capacities of the novel coupling beam. Based on these test results, we quantitatively discuss the pretension force in the pretensioning strands, the friction force of the damper and the gap width in the coupling beam. A finite element analysis is performed, showing that the coupling beam gap protects the coupling beam and adjacent shear wall from damage caused by the beam elongation effect. Copyright © 2015 John Wiley & Sons, Ltd.

Recent Developments in UV Optics for Ultra-Short, Ultra-Intense Coherent Light Sources

With the advent of Free Electron Lasers and general UV ultra-short, ultra-intense sources, optics needed to transport such radiation have evolved significantly to standard UV optics. Problems like surface damage, wavefront preservation, beam splitting, beam shaping, beam elongation (temporal stretching) pose new challenges for the design of beam transport systems. These problems lead to a new way to specify optics, a new way to use diffraction gratings, a search for new optical coatings, to tighter and tighter polishing requirements for mirrors, and to an increased use of adaptive optics. All these topics will be described in this review article, to show how optics could really be the limiting factor for future development of these new light sources.

Optical efficiency enhancement methods for terahertz receiving photoconductive switches

Abstract We improve the efficiency of THz receiving photoconductive switches by improving the conversion of the optical pump to signal current. This is achieved by both optimizing the incident excitation beam polarization and spatial profile of excitation. Due to boundary conditions of the electric field at the electrode edge, a nanometer-sized polarization-dependent shadow is created in the substrate at the electrode edge where most picosecond lifetime photocarriers are collected. This edge effect is further harnessed by elongating the excitation beam next to the stripline electrode. The effects of excitation beam polarization and spatial profile were experimented with InGaAs/InAlAs quantum-well-based photoconductive switch. In both cases notable enhancement in the signal is observed—30% with polarization optimization and up to 100% with beam elongation. Both techniques preserve the pulse quality and are applicable with readily available optical elements.

The Coordinated Radio and Infrared Survey for High-Mass Star Formation (The CORNISH Survey). I. Survey Design

ABSTRACTWe describe the motivation, design, and implementation of the CORNISH survey, an arcsecond-resolution radio continuum survey of the inner galactic plane at 5 GHz using the Very Large Array (VLA). It is a blind survey coordinated with the northern Spitzer GLIMPSE I region covering 10° < l < 65° and |b| < 1° at similar resolution. We discuss in detail the strategy that we employed to control the shape of the synthesised beam across this survey, which covers a wide range of fairly low declinations. Two snapshots separated by 4h kept the beam elongation to less that 1.5 over 75% of the survey area and less than 2 over 98% of the survey. The prime scientific motivation is to provide an unbiased survey for ultra-compact H II regions to study this key phase in massive star formation. A sensitivity around 2 mJy will allow the automatic distinction between radio-loud and radio-quiet mid-IR sources found in the Spitzer surveys. This survey has many legacy applications beyond star formation, including evolved stars, active stars and binaries, and extragalactic sources. The CORNISH survey for compact ionized sources complements other Galactic plane surveys that target diffuse and nonthermal sources, as well as atomic and molecular phases to build up a complete picture of the interstellar medium in the Galaxy.