Journal of Structural Engineering

Steel fibers in ultra-high performance concrete (UHPC) have been demonstrated to improve resistance to crack growth, decrease deflection, and increase fatigue life under cyclic loads. Using steel fibers as shear reinforcements, this paper investigates its effect on the postcracking shear fatigue behavior of nonstirrup UHPC beams. The study involved five UHPC beams comprising two static and three fatigue specimens to test the effects of the upper fatigue load, steel-fiber ratio, and stirrup ratio on the shear fatigue response. The test results showed that increasing the initial crack width in the nonstirrup UHPC beam from 0.05 to 0.2 mm (the equivalent of doubling the upper fatigue load) decreased the fatigue life from over two million to 24,875 cycles. The use of steel fibers in place of stirrups increased the residual-to-static strength and residual-to-initial stiffness ratios by 5% and 16%, respectively, after two million cycles. Compared with the specimen with stirrups, the steel-fiber-reinforced specimen had a lower midspan fatigue deflection-to-ultimate static deflection ratio owing to the relieved fatigue strain localization and increased tensile steel reinforcement-UHPC bond fatigue strength. The current codes predict the shear fatigue strength conservatively. A calculation model for accurately predicting fatigue deflection was proposed and compared with the existing test results. A future study on the wider parametric analysis and large-scale beam tests on the fatigue performance of these beams should be conducted.

Adaptive high-rise buildings can achieve significantly reduced resource consumption by incorporating actuators, sensor systems, and a control unit into the structure to compensate for the reduced stiffness. A vital step toward the wide real-world viability of this concept is ensuring the long-term operational reliability of adaptive buildings even in case of components malfunctioning. This article presents a comprehensive mechanical model of a large-scale adaptive high-rise structure incorporating various sensor systems as well as different actuation principles. As a basis for the simulative validation under critical load scenarios, a detailed model for wind disturbances is derived, accounting for time-variant velocity and direction. A parity space-based fault diagnosis scheme is employed for the reliable diagnosis of faulty sensors and actuators. Based upon the fault diagnosis, a reconfiguration strategy for estimation and control in adaptive structures is proposed. Assembling these components yields a fault-tolerant control loop that automatically adapts to the emergence of faults in sensors and actuators. In exemplary simulation scenarios, the proposed approach is demonstrated to be effective, largely mitigating the faults’ impact and recovering the nominal performance.

The limiting deformations of RC members used in seismic design and assessment are mainly based on experimental measurements, for which the lack of structural wall tests of large sizes restricts our knowledge on the deformations of large walls. Thus, experimental studies are compensated by numerical analyses to derive performance-based deformation limits for structural walls. In this study, 2,600 cantilever wall models with a wall length of 2–8 m were analyzed in a parametric study using a verified finite element procedure. The variables were selected as wall boundary-element confinement level and longitudinal reinforcement ratio, wall aspect ratio, axial load ratio, and steel and concrete material strengths. While most models exhibit flexural behavior, a sufficient number of wall models were also included to examine the shear-flexure interaction on walls. Complex localized compressive failure of concrete due to shear-compression interaction at the tip of the compression strut acting diagonally on the web or bending-compression action are the typical localized failure modes that are occasionally encountered in the boundary zones of structural walls. Its size dependence has often been disregarded in the literature for structural walls. This study demonstrates that as the wall length increases, the deformation capacity of structural walls decreases significantly due to localized compressive strains that are amplified by the tension shift effect. Similar to the size effect on strength, a size effect rule on the plastic rotation capacity is proposed. The plastic rotation capacity of structural walls was evaluated in the framework of ASCE/SEI 41. It was found that current damage limits are far from being safe for large walls, and even the response is flexural. Refined plastic rotation limits were proposed for the design and assessment of structural walls.

Displacement-based design (DBD) methods were originally developed in the 1990s for concrete structures and later modified for light-frame wood buildings and mass timber buildings. DBD methods use displacement as the design criterion to minimize economic losses, control component damage, and enable rapid return to functionality for increased resilience. This paper introduces a performance-based design check procedure that uses the direct displacement-based design (DDD) method and numerical pushover analysis for the design of cross-laminated timber and light-frame shear (CLT-LiFS) buildings. Prestressing CLT panels with embedded posttensioning high-strength steel cables or external high-strength steel rods enable them to provide robust lateral load resistance and self-centering capacity. LiFS walls offer additional lateral load resistance and energy dissipation through the slip of nail connections. A six-story CLT-LiFS building was used as a design example meeting three performance levels (damage limitation, life safety, and collapse prevention) based on ASCE/SEI 41-17 criteria. Each performance level corresponds to a set of three criteria: interstory drift limits, cable stress limits, and CLT compressive strain limits at the rocking interface. The design was checked using a series of nonlinear time history (NLTH) analyses with a suite of ground motion records. The results from the design check procedure and the statistical distributions of performance criteria from NLTH analysis confirmed that the six-story CLT-LiFS building met performance expectations. Additionally, a collapse fragility curve was developed to provide a comprehensive understanding of the building’s behavior.

Glued-in rods are proficient and high-performance connectors, finding application in the construction of new timber edifices as well as retrofitting of pre-existing timber structures. Given the prevalence of high-rise timber constructions facilitated by the advent of cross-laminated timber (CLT), there exists an opportunity for the incorporation of glued-in rods in the connectivity solutions for these load-bearing structural elements. This study examines the empirical behavior of CLT connections employing multiple glued-in rods. To achieve this objective, comprehensive experiments were conducted, involving full-scale tests on specimens subjected to both monotonic and cyclic loading conditions. The investigation included variations in the embedment lengths of the rods (250 and 350 mm) as well as their orientations with respect to the wood grain (parallel, perpendicular, and interposed amid layers). The monotonic test results showed that glued-in rods embedded on the edge of the CLT could provide high-capacity connections. The angle at which the rod was oriented relative to the wood grain had an impact on both the failure mode and the shape of the force-displacement curve in the connections. To calculate the pull-out strength of the glued-in rods anchored in CLT, a design equation is proposed based on the shear strength of the bondline. Additionally, a comparative investigation was performed to study the effect of the reinforcement by screws on the connections strength under monotonic loading in tension and in compression. Moreover, it was observed that glued-in rod connections keep their elastic properties until failure under cyclic loading. The results obtained from this research exemplified that this specific connection type could present a reliable and sturdy option for use in CLT construction projects.

Abstract

This paper presents a design approach, the direct stiffness–strength method (DSSM), for the strength prediction of sheathed cold-formed steel (CFS) structural members. The DSSM considers each sheathing–fastener connection as a bracing point and incorporates its stiffness and strength in the CFS wall panel design. The DSSM is formulated and validated based on large experimental test results. The motivation for this work is to address research gaps in the latest American Iron and Steel Institute (AISI) standards for the sheathing braced design of CFS structural members. A total of 41 individual sheathing–fastener connection tests were carried out, including parameters such as four different sheathing board types (to account for the performance of different sheathing) and seven different dimensions of the CFS Z-sections (to account for the slenderness). A new set of expressions was formulated to predict the stiffness and strength of the individual sheathing–fastener connections (bracing points) based on the observations from tests. A design-oriented assessment (comparing the design results with full-scale wall panel test results) indicated that the expressions formulated for DSSM are appropriate for the use of sheathed CFS Z-section member design. Further, this study used a design example to demonstrate the appropriateness of the DSSM approach.

Practical Applications

Since the introduction of the sheathing braced design concept in conjunction with the direct strength method (DSM) established by the American Iron and Steel Institute in RP13-1, this approach has gained traction among researchers to further enhance it for the various failure modes of the CFS structural members. However, there is an inconsistency in the sheathing bracing design concepts of the AISI design standards. The proposed DSSM is a simple one; it considers each sheathing fastener connection as a bracing point and incorporates its stiffness and strength in the CFS wall panel design. This proposed DSSM can be completed in two steps by checking the bracing ability of the provided sheathing. The DSSM method can be used with any design specification for determining the moment capacity of the CFS member with the contribution of the sheathing board. The design steps involved in DSSM are demonstrated with flowcharts and a design example. The DSSM proved that use of sheathing boards can eliminate the additional steel bracings at the intermediate length and breadth of the CFS wall panels, but steel bracings are required at the building corners for stability. This design method can realize up to a 15% reduction in steel consumption in the CFS building construction.

The first step in current methodologies for the (probabilistic) performance-based fire safety design and assessment of structures is characterizing fire hazard scenario(s). Next, structural response analysis is performed to estimate hazard consequences in terms of damage or loss metrics of interest. These metrics are eventually appraised to verify whether various performance objectives are achieved and design iterations are needed. Nevertheless, such approaches rely on preliminary assumptions on the structural configuration and features characterizing scenarios used as thermal inputs. Consequently, they do not fully exploit the fire–structure coupling effect, where fire affects a structure, and the characteristics of the structure also affect the combustion process and fire dynamics. Indeed, the structural design choices define the fire scenarios that could potentially affect the structural and nonstructural performance. This paper introduces a consequence-oriented fire intensity optimization (CFO) approach to the fire safety design of structures to address such limitations. The proposed approach considers fire scenarios as additional design variables and delivers them as procedure outputs, optimizing the balance between increasing structural capacity and decreasing fire intensity. Furthermore, it evaluates the effect of input uncertainties through Monte Carlo sampling. A single-span bridge subject to a car fire is used to showcase the proposed approach. For this case study, it is shown that fire scenario characteristics (fuel bed and traffic load positions, heat release rate history) maximizing consequences are strongly correlated to the structural features and cannot be set a priori. Finally, design decisions exploiting the described coupling effect to achieve various performance objectives are discussed. Overall, the proposed approach highlights the benefits of enhancing the fire and heat transfer model’s capability to capture the fire–structure coupling effect for achieving more optimized design solutions.

Abstract

Limited research has been conducted on the fire resistance of high-strength steel (HSS) T-joints after an earthquake. The aim of this study is to assess postearthquake fire resistance and determine the optimal design dimensions of HSS-circular hollow sections-square hollow sections (CHS-SHS) T-joints considering the nonuniform temperature field effect through parameter analysis. The computational models of HSS T-joints developed in this study were validated against existing test results. Twenty four computational models related to HSS-CHS-SHS T-joints were established using the finite element software ANSYS; further, a sequential temperature-displacement analysis method was used to perform a parameter study. Subsequently, parameter analysis was conducted on HSS-CHS-SHS T-joints by varying parameters such as chord-width-to-thickness ratio ($\gamma$), brace-to-chord-thickness radio ($\tau$), brace-to-chord-width ratio ($\alpha$), chord-length-to-height ratio ($\beta$), and damage variable ($\tilde{D}$). The results indicate that the failure of HSS-CHS-SHS T-joints is attributed to plasticization of the chord surface, and that parameters $\alpha$ and $\tilde{D}$ have a significant influence on the postearthquake failure mechanism and catastrophic process. Moreover, an empirical theoretical model for determining the bearing capacity of HSS-CHS-SHS T-joints after exposure to fire is proposed and validated.

Practical Applications

The HSS-CHS-SHS T-joint is widely used in offshore platforms, building frames, transmission towers, etc. Secondary fires occur frequently after an earthquake, which typically cause huge casualties and economic losses. Since earthquakes may produce residual stress and deformation in HSS-CHS-SHS T-joints, the failure mechanism of HSS-CHS-SHS T-joints under fire cannot be used to accurately evaluate their reliability and safety under a postearthquake fire. Thereby, the investigation of this study can provide valuable reference for design, repair, and reinforcement of HSS-CHS-SHS T-joints.

Ensuring the robust performance of damping devices during intense vibrations, such as seismic excitation, is crucial for maintaining structural protection. Hazard analysis serves as an effective method for evaluating the reliability of vibration-damping devices installed on structures subjected to vibrations. This study aimed to conduct a seismic hazard analysis comparing the rotary eddy current damper (R-ECD) and the widely used fluid viscous damper (FVD). To facilitate analysis and engineering applications, this study introduces innovative contributions by proposing a simplified formula for estimating the nonlinear damping force of the R-ECD under specific conditions. This formula incorporates two physically significant parameters: maximum damping force ($F_{\max }$) and critical velocity ($v_{cr}$). By doing so, it offers a swift and efficient tool for the intricate calculation of damping force. The accuracy of this estimation formula is validated by testing a large-scale R-ECD with a maximum damping force exceeding 1,500 kN, revealing a remarkable agreement of $\pm 6.8\%$ between the estimated and experimental results. Subsequently, employing stochastic linearization, the equivalent linear damping ratio of single-degree-of-freedom (SDOF) structures with R-ECD and FVD were derived. Leveraging this stochastic linearization, seismic hazard analysis of these two dampers was carried out. The results indicate that R-ECDs exhibit superior reliability in terms of self-limited output force and their ability to handle large strokes, outperforming FVDs. Moreover, the findings confirm that ECDs are particularly well-suited for long-span structures such as bridges, which are prone to temperature-related influences.

The dynamic characteristics of large-scale civil structures are generally evaluated by performing operational modal analysis based on ambient vibration records measured by contact sensors, e.g., accelerometers. However, owing to accessibility restrictions, it is sometimes inconvenient to conduct dynamic response measurements by contact sensors. Therefore, there is a need to adopt noncontact measurement technologies for monitoring the dynamic performance of civil structures. On the other hand, only response records are available for operational modal analysis on account of the unknown nature of ambient excitations, which may lead to uncertainties in modal estimates, particularly damping estimates. In this regard, this paper adopts a combined method consisting of modal decoupling, natural excitation technology, and an eigensystem realization algorithm to perform high-accuracy modal identification with uncertainty quantification. Through numerical simulation study of a frame structure, the accuracy and effectiveness of the combined method for identifying structural modal parameters are verified. Then, taking advantage of noncontact simultaneous multipoint measurements by an interferometric radar system and the combined modal estimation scheme, the noncontact operational modal analysis strategy is utilized to evaluate structural dynamic characteristics of a supertall building after experiencing a sudden vibration event that attracted considerable concern from the public. The purpose of this study is to provide a practical and reliable means of conducting modal estimation with uncertainty quantification from a single measurement and suggest a noncontact modal analysis strategy for structures with difficulty installing contact sensors.

Motivated by the magnitude of discrepancies that are typically encountered in blind analysis contests between numerical model predictions and test data, a methodology is presented to incorporate modeling uncertainties in the assessment of capacity-designed steel moment-resisting frames (MRFs) under earthquake loading. Sources of modeling uncertainties are identified in order to define a set of variables that control the seismic response of steel MRFs. For each variable, statistical distributions that rely on experimental databases are deduced. Special attention is paid to the truncation limits to enable the generation of individual parameter samples that have an actual physical meaning. Besides strength modification factors for various steel grades, distributions are offered for the parameters that are used to model wide-flange (composite) beams, steel hollow structural section columns and damping. Both intracomponent and intercomponent interdependencies are explicitly discussed in an attempt to propose correlation coefficients that comply with the current design philosophy and construction sequence. Although the focus is strictly on collapse of capacity-designed steel MRFs, the proposed methodology can be utilized to examine less severe limit states and/or existing structures where capacity-design principles do not necessarily apply. A 4-story steel MRF, tested full-scale at the E-Defense facility to collapse, is employed as a case study to demonstrate the applicability of the proposed methodology. It is shown that although the steel MRF examined is insensitive to modeling uncertainties regarding both collapse and story mechanism prediction, local response parameters can vary considerably versus the ones observed during the test. Through a parametric investigation on the strong-column-weak-beam (SCWB) ratio, the parameters that have an impact on the associated predictions are identified. Implications for capacity design, collapse capacity and residual drift are also discussed, highlighting the benefits that higher SCWB ratios can have on seismic response.

Abstract

The quality of being adaptable/deployable of tensegrity structures provides intriguing challenges for researchers and designers. In this work, the shape-change capabilities of a V-Expander tensegrity column are analyzed: a numerical investigation is carried out for the study of the cable-actuation process for a column composed of elementary V-Expander tensegrity cells sharing horizontal cables. The actuation is performed by considering suitable sets of active and passive cables among the whole set of cables of the column. The attention is focused on the following kinds of shape-changes: stretching/shrinking, flexure, shear and torsion. Moreover, a strategy for predicting the nonlinear response of the tensegrity column in the cable-actuation process is proposed. This strategy allows for estimating quickly, among all the possible choices of active and passive cable sets, those leading to maximum displacements or rotations and to the minimum ratio between the normalized work of active cables and the deformation of the column. Further numerical analyses are performed by varying the aspect ratio of the V-Expander cell, to evaluate the effects of changes in the geometrical parameters on the nonlinear actuation process of the column.

Practical Applications

Tensegrity structures have inspired artists, biologists, engineers, and architects. Many studies have been conducted in recent years, especially related to geometry, form finding, static analysis, and dynamic response. Their use is very encouraging in projects requiring adaptable and/or deployable structures. Indeed, a tensegrity system can provide the minimal mass structure required to sustain a given external load, an intriguing feature for uses as lightweight deployable systems, such as satellites, antennas, or robots. Tensegrity systems are reticulated structures composed of bars in compression maintained in equilibrium by a network of cables in tension. This work focuses on the shape-change capabilities of V-Expander tensegrity columns, a promising type of tensegrity system in this context. Shape change can be obtained by shortening some cables using actuators, while other cables lengthen elastically keeping the system in equilibrium during the process. A fast linear prediction strategy is proposed for evaluating the efficiency of any choice of actuated elements. This strategy allows to prefigure the choices of lengthening and shortening cables which lead to the most efficient shape-change processes of V-Expander columns, here presented for the first time. Such a strategy can be applied with minor adjustments to the other types of tensegrity systems.

A viscoelastic damper (VED) reinforcement option focusing on recovering the seismic performance of earthquake-damaged historic timber structures on a slope, as regulated by a direct displacement-based design procedure, was proposed in an attempt to extend the service life of culture buildings while ensuring deformation control of the weak area. A timber frame with an unequal height of column feet (UF) was designed, and a timber frame with an equal height of column feet (EF) served as the control group. The two full-scale timber frames consist of two-story units, a roof, and four spans. A series of cyclic loading tests were conducted on Models EF and UF to investigate the seismic performance of timber frames before and after reinforcement. It was found that the pulling out of the tenon and the sliding of column feet was significant at the upper embedding end. The upper embedding story is the weak area of the timber structure on a slope. The damage patterns of the two damped frames were not significantly different from those of the corresponding bare frames, and the ductility coefficients of all tested frames were greater than 10. The test results also revealed that the designed VED restored the seismic performance of the UF to the undamaged level while effectively controlling the deformation of its weak area. In addition, the effect of uniformly arranged dampers on the seismic performance of timber frame UF was also investigated.

This study introduces a new type of shear panel damper that utilizes iron-based shape memory alloy (Fe-SMA) with enhanced low-cycle fatigue (LCF) resistance for structural damage control, with a particular focus on the effect of solution treatment on the properties of interest of Fe-SMA. Material-level investigation was conducted first, aimed at establishing a clear connection between microscopic structure and macroscopic properties, and to determine the most suitable solution treatment method for Fe-SMAs for structural damping purposes. This was followed by laboratory tests on a Fe-SMA based shear panel damper receiving an appropriate solution treatment, along with conventional metal shear panel dampers made of low yield point steel LYP225 and mild steel Q235. Results reveal that among the considered solution treatments, 1,100°C for 1 h leads to the most desired mechanical properties, such as lower yield stress, increased ductility, and longer LCF life, of Fe-SMA for seismic damping applications. The Fe-SMA shear panel damper demonstrates a significant improvement in LCF life and total energy dissipation capacity compared to traditional steel dampers. Finally, detailed numerical investigations were carried out to analyze the seismic response. A calibrated micromechanics-based model, i.e., cyclic void growth model, was employed to predict the fracture initiation of the Fe-SMA shear panel damper under seismic loading. This approach achieved reasonable accuracy, assisting in understanding the distinctive behavior of Fe-SMA shear panel dampers.