Ductile Structures Research Articles

Some Considerations on the Behaviour of Bolted Stainless-Steel Beam-to-Column Connections: A Simplified Analytical Approach

Stainless-steel has proven to be a first-class material with unique mechanical properties for a variety of applications in the building and construction industry. High ductility, strain hardening, durability and aesthetic appeal are only a few of them. From a specific point of view, its nonlinear stress–strain behaviour appears capable of providing a significant increase in the rotational capacity of stainless-steel connections. This, in turn, may provide significant benefits for the overall response of a structure in terms of capacity and ductility. However, the bulk of the research on stainless-steel that has been published so far has mostly ignored the analysis of the deformation capabilities of the stainless-steel connections and has mostly focused on the structural response of individual members, such as beams or columns. For such a reason, the present study aims to contribute to the general understanding of the behaviour of stainless-steel connections from a conceptual, numerical and design standpoint. After a brief review of the available literature, the influence of the use of stainless-steel for column–beam connections is discussed from a theoretical standpoint. As a novel contribution, a different approach to compute the pseudo-plastic moment resistance that takes into account the post-elastic secant stiffness of the stainless-steel is proposed. Successively, a refined finite element model is employed to study the failure of stainless-steel column–beam connections. Finally, a critical assessment of the employment of carbon-steel-based design guidelines for stainless-steel connections provided by the Eurocode 3 design (EN 1993-1-8) is performed. The findings prove the need for the development of novel design approaches and more precise capacity models capable of capturing the actual stainless-steel joint response and their impact on the overall ductility and capacity of the whole structure.

Analytical approach for force-displacement behavior of deformed reinforcing bars embedded in concrete

Component-based joint model suggested by Eurocode is a simple yet efficient approach to evaluating the joint behavior of composite beam-column subassemblies. The reinforcing bars in this approach play an instrumental role, especially for progressive collapse analysis where the ultimate resistances and structural ductility of beam-column subassemblies are significantly influenced and in some extreme cases directly governed by the reinforcing bars. This paper describes an analytical approach for predicting the force-displacement characteristics of deformed reinforcing bars that are embedded in concrete blocks. In contrast to previous studies that roughly predefined the profiles of bar and bond stresses in tension, this paper utilizes an iteration method to provide a more accurate and simpler description of such non-uniform local behavior. This approach has greater flexibility to account for the variations in the embedment length of reinforcing bars, lateral restraints and failure modes. The proposed approach is verified using four groups of published reinforcing bar pull-out tests. The predicted force-displacement relationships, distributions of local slips, bond stresses, and bar stresses along the length are shown in reasonable agreement with the test results. Finally, component-based joint models are established for composite beam-column subassemblies under progressive collapse scenarios, in which the force-displacement relationships of the reinforcing bar components are determined through the proposed approach.

Ductility evaluation and flexural failure mode recognition of reinforced Ultra-High performance concrete flexural members

Although ultra-high performance concrete (UHPC) exhibits material tensile ductility far in excess of that expected from conventional or high-strength concretes, structural UHPC members typically fail in flexure due to reinforcement rupture after crack localization, which may result in a low structural ductility. In the flexural design of reinforced UHPC (R-UHPC) members, attention should be paid to addressing the issue related to the low structural ductility due to crack localization. This study firstly develops a two-dimensional numerical model to simulate the flexural response of R-UHPC beams, and the numerical results agree well with the experimental results of the load–deflection curves as well as the failure modes. Parametric analyses are then carried out to examine the effect of the flexural reinforcement ratio, tensile strength of steel reinforcement, tensile behavior of UHPC and compressive strength of UHPC on the flexural failure mode and ductility behavior of R-UHPC members. The results indicate that when the failure is controlled by fracture of tensile steel bars after crack localization, higher reinforcing ratio, higher steel post-yield hardening strength, and lower steel fiber volume lead to more ductile behavior. When the failure is controlled by crushing of UHPC, however, higher reinforcing ratio and lower UHPC compressive strength lead to a lower ductility. In addition, a flexural failure mode recognition method is proposed for R-UHPC flexural members, whose accuracy is verified based on the available experimental results of 105 R-UHPC flexural members. Finally, a minimum flexural reinforcement ratio is proposed to avoid low structural ductility in R-UHPC flexural members.

Seismic Risk of Weak First-Story RC Structures with Inerter Dampers Subjected to Narrow-Band Seismic Excitations

This study evaluates the seismic risk of weak first-story reinforced concrete (RC) structures retrofitted with inerter dampers at their ground level when subjected to narrow-band seismic excitations. The main advantages brought about by the ground-level inerters are the reductions in seismic demands (e.g., drifts, floor accelerations). This study shows that structures with inerters are reliable systems in terms of peak story drifts for large ground motions. For moderate ground-motion intensities, the opposite could occur, mainly for soft soil sites. The reliabilities of structures with inerter dampers at their ground level are in general higher for buildings under seismic intensities associated to limit state of incipient collapse, especially for low-height buildings. This could be reversed for intensities associated to the limit state of damage limitation. The findings of this study could guide practicing engineers to use inerter-based dampers in retrofitting ductile structures consisting of moment-resisting reinforced concrete (RC) frames subjected to narrow-band earthquake excitations in regions such as Mexico City.

REVIEW ON CONVENTIONAL CONCRETE AND NYLON FIBER REINFORCED CONCRETE BEHAVIOR

The addition of fibers to reinforced cement concrete enhances the suitability of structures under impacts and earthquake loads by boosting firmness or energy-consuming proficiency of the hardened concrete material. During cracking of concrete, one of the most relevant behaviors of fiber like “bridge effect” starts functioning. As a result of this, fiber blocks crack formation and propagation, and hence strength and ductility of concrete structure improve. The goal of this study is to compare the mechanical properties of reinforced cement concrete by mixing nylon fibers and without mixing nylon fibers. In this study, data from existing article are assessed to compare the strengths of nylon fiber reinforced concrete and conventional concrete by using concrete strength measuring test results such as compressive strength test, flexural strength test, and split tensile strength test. Adding fiber with concrete mix reduces the moisture content, thereby lessening the workability of concrete which produces fiber ball. Due to the obvious lack of homogeneity caused by fiber balling, the efficiency of fiber reinforced concrete is reduced. During the mixing of small portions of fibers with dry concrete the difficulty of fiber separation can prevent by providing good distribution and scattering within the mixtures which enhance concrete strength and thus the balling incident can escape. The dynamic transmission of stress between concrete mix and the fiber achieves by considering significant factors like aspect ratio, volume, and orientation of the fiber. Nylon fiber provides mechanical improvement of reinforced concrete by enhancing impact resistance and increasing flexural toughness, as a result the load-bearing capacity of concrete develops effectively. Introducing nylon fibers with concrete mixture promotes strong tensile comportment of concrete outstandingly after the post peak stage.

Effects of Wall Thickness Variation on Hydrogen Embrittlement Susceptibility of Additively Manufactured 316L Stainless Steel with Lattice Auxetic Structures

In the present study, the hydrogen embrittlement (HE) susceptibility of an additively manufactured (AM) 316L stainless steel (SS) was investigated. The materials were fabricated in the form of a lattice auxetic structure with three different strut thicknesses, 0.6, 1, and 1.4 mm, by the laser powder bed fusion technique at a volumetric energy of 70 J·mm-3. The effect of H charging on the strength and ductility of the lattice structures was evaluated by conducting tensile testing of the H-charged specimens at a slow strain rate of 4 × 10-5 s-1. Hydrogen was introduced to the specimens via electrochemical charging in an NaOH aqueous solution for 24 h at 80 °C before the tensile testing. The microstructure evolution of the H-charged materials was studied using the electron backscattered diffraction (EBSD) technique. The study revealed that the auxetic structures of the AM 316L-SS exhibited a slight reduction in mechanical properties after H charging. The tensile strength was slightly decreased regardless of the thickness. However, the ductility was significantly reduced with increasing thickness. For instance, the strength and uniform elongation of the auxetic structure of the 0.6 mm thick strut were 340 MPa and 17.4% before H charging, and 320 MPa and 16.7% after H charging, respectively. The corresponding values of the counterpart's 1.4 mm thick strut were 550 MPa and 29% before H charging, and 523 MPa and 23.9% after H charging, respectively. The fractography of the fracture surfaces showed the impact of H charging, as cleavage fracture was a striking feature in H-charged materials. Furthermore, the mechanical twins were enhanced during tensile straining of the H-charged high-thickness material.

Structural failure assessment system exemplified by dumbbell-shaped CFST arch

The study of structural failure law is the basis of structural safety assessment, but engineering structures present complex nonlinearities and diversity, which make it difficult to analyze the failure laws of structures. However, the failure of ductile structure is a process of quantitative cumulation to qualitative change, and this turning point is the key to structural failure analysis. Based on this view, we propose a new system for structural failure assessment, which includes the identification of mutation locations, strain data visualization, structural internal force estimation, and finite element stochastic simulation, etc. Moreover, every new method is compared and validated. Experimental data of a dumbbell CFST arch model are used throughout the paper for illustration. Finally, the time-varying failure probability of the whole structure is evaluated. In summary, this paper organically combines experiment, simulation and engineering practice to provide a new approach to structural safety analysis, structural health monitoring and structural design.

Study on Shear Resistance Property of a New PBL Connector with Steel–Rubber Tenon

In order to improve the shear resistance and structural ductility of the perfobond rib (PBL) connector, a new PBL connector with steel-rubber tenon is proposed in this study, which aims to increase the shear load capacity of the connector while improving the ductility of the connector. First, models of new PBLs are established based on the validated finite element method, and their mechanical properties are compared with other shear connectors. The results show that the stiffness and shear load capacity of the proposed connector can be effectively improved when the steel ring is added, where the shear stiffness can be reduced, and the deformation capacity of the specimen can be improved when the rubber ring is added. When a steel ring with a thickness of 5 mm and a rubber ring with a thickness of 5 mm are involved, the shear load capacity of the connector with steel-rubber tenon is increased by 13.7%, and the shear stiffness is reduced by 37.3% compared to the conventional concrete tenon connector, while the ductility is increased by 75.1% compared to the connector with steel ring tenon. Subsequently, as for the connectors with steel-rubber tenon, the effects of the thickness of the steel ring, the thickness of the rubber ring, the diameter of perforated rebar, the strength of concrete and the strength of perforated steel plate are analyzed based on the finite element model of a PBL. The results show that an increase in the thickness of the steel ring, the diameter of the perforated rebar and the strength of the concrete will cause an increase in the shear stiffness and shear load capacity of the connector; however, an increase in the thickness of the rubber ring can cause a decrease in the shear stiffness and shear load capacity of the connector, while a change in the strength of perforated steel plate has little effect on the shear stiffness and shear load-carrying capacity. Finally, based on the finite element parametric analysis results and the damage mechanism of the proposed connector, a calculation equation applicable to the PBL connector with steel-rubber tenon is presented to predict the shear load capacity of the connector.

Experimental and finite element analysis: Out-of-plane mechanical performance of infill walls with flexible connection

Flexible connection between the wall and the frame can improve the seismic performance of the structure, thus minimizing the pushing effect of the infill wall on the main frame. Masonry infill walls with flexible connection have lower out-of-plane stability than walls with rigid connection. However, local collapses are more likely to occur. A comparative study was conducted on the load-carrying capacity, initial stiffness, deformation and ductility of different wall structures based on a monotonous static load test, and the out-of-plane mechanical properties exhibited by flexible masonry infill walls were investigated. A simplified separated finite element model of masonry infill wall was built in accordance with the test results, and then the monotone out-of-plane static loading was performed. The structural configuration, the wall-frame connection method, and the spacing between structural columns were found to have significant effects on the out-of-plane mechanical behavior of the infill wall. The wall structural configuration was found to have the strongest effect, followed by the wall-frame connection method. Under the conditions of flexible connection, when the wall structure was in the “grid-beam” form, and the spacing between structuring columns was 2.5–3.0 m, the frame-infill wall was found with the best restraint effect, so the optimal out-of-plane comprehensive mechanical performance would be achieved.

Seismic behavior of prestressed concrete filled steel tubular lattice tower subjected to combined compression-bending-torsion

Prestressed concrete filled steel tubular (CFST) lattice tower is an innovative application to combine prestress and CFST composite structure to onshore tall wind turbines. In order to explore the seismic behavior of prestressed CFST lattice tower under a complex load state, there was pseudo-static testing carried out for prestressed four-chord CFST lattice columns under compression-bending-torsion combined load. The hysteretic performance of the specimens was analyzed by obtaining the failure mode, load-displacement hysteretic curve, energy dissipation, stiffness degradation, variation of strands stress and steel strain. The influence of prestress on seismic behavior of CFST lattice chords was investigated from prestress loss rate. Results showed that: the inclined battens of CFST lattice columns were seriously damaged with or without strands, while the inclined battens of non-prestressed specimen were damaged first and then the CFST chords. The existence of inclined battens had a great influence on the structural stiffness, while the application of strands significantly affected the initial stiffness and structural ductility of the prestressed specimen, but did not have any significant effect on ultimate bearing capacity. Post-tensioned strands could protect the main bearing chords and reduced the cracking of concrete in the tension area under the coupling action of compression, bending, and torsion.

Development and testing of knife-plate connected steel panel dampers

Steel panel dampers (SPDs), consisting of the inelastic panel and elastic connecting segments, have been widely used in steel building structures commonly with brace- or stub-column-type configurations, especially in high seismic regions. The elastic connecting segments of stub-column-type SPDs were usually significantly enlarged to ensure their elasticity under the associated extreme loads of the inelastic panel. With the aim of improving the structural efficiency and ductility capability of SPDs, a novel design, named as knife-plate-connected SPD (KSPD), was proposed in the study. The proposed KSPD consisted of one steel panel and two boundary-columns on the horizontal sides with pinned column ends, which were achieved by knife-plate details, connecting to the framing systems. The designed configuration of KSPDs potentially reduced the aspect ratio of the steel panels for easily achieving shear yielding while remaining the stud-column appearance, and mitigated the deformation amplification of the steel panels usually occurred in conventional stub-column-type SPDs. Five full-scale KSPD specimens with various configurations of the panel stiffeners have been experimentally examined and compared under cyclic loadings including fatigue cycles. The feasibility of the KSPD design was experimentally validated, and the developed KSPDs were verified to achieve stable energy dissipation and significant ductility of approximate ten and cumulative plastic deformation (CPD) value up to 796. For the design purpose of the members, the width-to-thickness limits of the steel panel were established to prevent panel buckling and the sub-panel edge fracture upon the test results, respectively, and a design inequality of the knife plates upon strengths was developed to prevent the fracture of the knife plate. Finally, the design suggestions of the proposed KSPD were summarized upon the experimental results and findings of the study.

Seismic characterization of symmetrical structure damage and ductility obligation

Seismic characterization of symmetrical structure damage and ductility obligation

Diagrid Core-tube Structure Seismic Performance Based on Equivalent Stiffness Ratio of Inner and Outer Tubes

To study the relationship between the stiffness ratio of the inner and outer tube of the diagrid core-tube structure of high-rise buildings and the redistribution of the storey shear force, the structural ductility, overstrength factor and the seismic performance, a total of 12 kinds of diagrid core-tube structures of equivalent stiffness ratios were established at 71°, 77° and 80° diagonal angles. Based on the static pushover analysis method, combined with the principle of capacity spectrum and demand spectrum, the characteristics and laws of the position change of the weak layer, the redistribution of the storey shear force, the shear lag effect of the bottom compression flange and the overall ductility of the structure are analyzed under the action of the frequent earthquake (70 gal), fortification earthquake (200 gal), rare earthquake (400 gal), extremely rare earthquake (510 gal) and huge earthquake (620 gal). The results show that under different earthquake actions, there is an obvious redistribution of floor shear force in the diagrid core-tube structure, and the storey shear coefficient decreases with the increase of earthquake level and equivalent stiffness ratio. The diagonal angle is the main factor affecting the location of the weak floor and the shear lag effect. The shear lag effect of the structure increases with the increase of the seismic fortification level. The structural ductility with a diagonal angle of 77° is generally optimal. When the equivalent stiffness ratio is 0.64, the structural ductility reaches the optimal value of 1.46, which is 1.1 times that of 71° and 80° diagonal angles. In the structure with the same diagonal angle, the overstrength factor increases with the increase of the equivalent stiffness ratio.

Inelastic cyclic response of RBS connections with jumbo sections

This paper examines the cyclic performance of reduced beam section (RBS) moment connections incorporating larger member sizes than those allowed in the current seismic provisions for prequalified steel connections, through experimentally validated three-dimensional nonlinear numerical assessments. Validations of the adopted nonlinear finite element procedures are carried out against experimental results from two test series, including four full-scale RBS connections comprising large structural members, outside the prequalification limits. After gaining confidence in the ability of the numerical models to predict closely the full inelastic response and failure modes, parametric investigations are undertaken. Particular attention is given to assessing the influence of the RBS-to-column capacity ratio as well as the RBS geometry and location on the overall response. The numerical results and test observations provide a detailed insight into the structural behavior, including strength, ductility, and failure modes of large RBS connections. It is shown that connections which consider sections beyond the code limits, by up to two times the weight or beam depth limits, developed a stable inelastic response characterized by beam flexural yielding and inelastic local buckling. However, connections with very large beam sections, up to three-times the typically prescribed limits, exhibited significant hardening resulting in severe demands at the welds, hence increasing susceptibility to weld fracture and propagation through the column. The findings from this study point to the need, in jumbo sections with thick flanges, for a deeper RBS cut than currently specified in design, to about 66% of the total beam width. This modification would be required to promote a response governed by extensive yielding at the RBS while reducing the excessive strain demands at the beam-to-column welds. Moreover, for connections incorporating relatively deep columns, it is shown that more stringent design requirements need to be followed, combined with appropriate bracing outside the RBS, to avoid out-of-plane rotation.

A self - Consistent higher-order strain gradient plasticity analysis of micropolar dual-phase crystalline FGM made of Titanium-boride/ Titanium

A self-consistent higher order couple stress model is based on Fleck - Hutchinson Strain Gradient Plasticity to consider flexural behavior of brittle - ductile microscale structures. The dual phasic functionally graded material made of Titanium-boride/Titanium structure is modeled by defining a material flow stress which reflects a parameter of combined effective length scale, dependent on Geometrically Necessary Dislocations. The dual phasic crystalline micro-beam is built with the properties having continuously and non-linearly variation along the thickness. The perfectly plastic behavior of micropolar functionally graded material is described by incorporating a ductile –like mixture law, so-called TTO model, possessing a simple possibility of transition through defining a graded phase acting as an equilibrium mixed phase. Further, the effects of the elastic foundation and temperature rise on the plastic deformations are investigated. While an energy method is used to derive governing equations, the obtained results are verified individually in three steps.

23rd International Colloquium on Stability and Ductility of Steel Structures, University of Aveiro

Steel ConstructionVolume 16, Issue 1 p. 1-1 EditorialFree Access 23rd International Colloquium on Stability and Ductility of Steel Structures, University of Aveiro Paulo Vila Real, Paulo Vila Real SDSS 2022 ChairmanSearch for more papers by this authorNuno Lopes, Nuno Lopes SDSS 2022 Co-chairmanSearch for more papers by this author Paulo Vila Real, Paulo Vila Real SDSS 2022 ChairmanSearch for more papers by this authorNuno Lopes, Nuno Lopes SDSS 2022 Co-chairmanSearch for more papers by this author First published: 09 February 2023 https://doi.org/10.1002/stco.202370103AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume16, Issue1February 2023Pages 1-1 RelatedInformation

Novel FRP interlocking multi-spiral reinforced-seawater sea-sand concrete square columns with longitudinal hybrid FRP–steel bars: Monotonic and cyclic axial compressive behaviours

• Combined uses of FRP interlocking multi-spirals, SSC and hybrid FRP–steel bars are explored. • Hybrid FRP–steel bars as longitudinal reinforcement endure structural controllability. • Reducing spacings of middle or/and corner spirals improves post-peak deformability. • Unloading/reloading cycles foster plastic deformation and load degradation in FMSSCs. • Design-oriented load-strain model based on full-section confining stress is proposed. Fibre-reinforced polymer (FRP) interlocking multi-spiral reinforced-seawater sea-sand concrete (SSC) square columns (FMSSCs) can optimize the merits of FRP and SSC while providing excellent confinement to the concrete in square columns. However, existing works on FMSSCs are limited to columns with longitudinal FRP bars, whose complex compressive properties and natural brittleness challenge the structural ductility. To this end, this paper develops a variant of FMSSCs by employing hybrid FRP–steel bars (HFSBs) as longitudinal reinforcements to break out of existing limitations. The HFSBs with the inner steel bar and outer FRP layer possess distinctive post-yield stiffness and remarkable corrosion-resistant performance. A series of monotonic and cyclic axial compression tests are conducted to investigate effects of concrete types, reinforcement configurations, and loading schemes on behaviours of FMSSCs. Test results demonstrated that the use of SSC had a negligible effect on the short-term structural performance, and the presence of longitudinal HFSBs endured FMSSCs to introduce damage controllability, especially under cyclic loading. Additionally, repeated loading/reloading cycles exerted cumulative impacts on the plastic deformation and load degradation of FMSSCs but affected cyclic envelope behaviours little. Finally, a design-oriented load–strain model was proposed to predict monotonic and cyclic axial responses of FMSSCs with longitudinal HFSBs.

Behavior of lap splices in reinforced concrete beams after bar yielding

AbstractStructural ductility in the case of plastic hinges forming close to a lap splice has been an open issue with the revision process of the next version of Eurocode EC2 concerning reinforced‐concrete structures. This research has provided experimental evidence by the large experimental campaign for the background for revision of the Eurocode EC2. Within this context, 38 specimens were tested in four‐point bending, with the longitudinal bars either lap‐spliced (partly or fully), or un‐spliced (for reference). The tests were carried out involving the variation of the bar diameter (12, 16, 20, and 25 mm), and the lap length (20–60 times the bar diameter). The main result is the sizeable enhancement of the structural ductility brought in by the extension of the lap length in excess of the length required by the yielding of the longitudinal bars. Based on these findings, a preliminary model is developed to describe the bond‐stress profiles in lap splices, after the yielding of the bars.

Study on the load-carrying capacity of an innovative cold-formed steel floor system

To account for the higher structural ductility requirement and higher load-bearing capacity and stability requirements of floor-to-wall connections of mid-rise cold-formed steel (CFS) structures, an innovative CFS floor system with gridded beams and filler blocks (BB-based CFS floor system, where BB stands for “beams” and “blocks”, respectively) was first proposed. Then, experimental and refined numerical simulation studies were performed to investigate the flexural behaviour of the floor system. Afterwards, a simplified calculation method of the load-carrying capacity of the floor system was deduced based on its mechanical mechanism. The results show that (1) the deformation of the floorboard-to-beam screw connections and the relative buckling of the compression beam flange and adjacent web are the primary reasons for the nonlinear load-deflection relationship and bending failure of the floor system. (2) Compared with the existing CFS composite floor systems, the combination effect of each component in the BB-based CFS floor system prevented the beams from unstable failure after losing the restraint of floorboards, thereby effectively improving the load-carrying capacity and nondeformability of the floor system. (3) After decomposing the BB-based CFS floor system into composite floor beams, the load-carrying capacities of the floor system can be determined simply according to the cumulative flexural bearing capacities of these composite floor beams based on plasticity theory, the relative errors between the deduced load-carrying capacities and the test results and simulated results of different BB-based CFS floor systems are within 10%, and the calculated results are relatively safe.

Additive manufacturing of polyaniline blends for lightweight structures with tunable conductivity

Printable feedstocks that can produce lightweight, robust, and ductile structures with tunable and switchable conductivity are of considerable interest for numerous application spaces.