Journal of Structural Engineering

The engineering properties of concrete with recycled aggregates (RAC) can vary more widely than normal aggregate concretes (NAC). Because concrete-filled steel tubes do not rely solely on the concrete in compression, using RAC in a concrete-filled steel tube (RACFST) is a sustainable structural solution. Most work has focused on using only coarse recycled aggregate (CRA), but with virgin sand supplies decreasing, there is a need to use fine recycled aggregate (FRA). Meanwhile, the long-term deformation of RACFST may be notably affected by FRA. A research program was undertaken to investigate the impact of FRA on the time-dependent behavior for the RACFST. Study parameters included CRA replacement ratio ($r_C$), FRA replacement ratio ($r_F$), and concrete strength ($f_{cm}$). The experimental results showed that the FRA and CRA had a modest compounding effect (i.e., the FRA effect depends on $r_C$) in RACFST, but this effect can be more pronounced for lower-strength concretes. Using RAC increases the scatter in the deformability but does not affect the rate of creep development. A creep model for NAC was then modified to predict the long-term deformation of RACFST. A parametric study was carried out to evaluate the time-dependent effects of the RACFST with other untested parameters.

This paper presents a test campaign concerning the fatigue behavior of the bird-beak square hollow section (SHS) gap K-joints subjected to brace in-plane force. A total of four bird-beak SHS gap K-joint specimens were tested. Prior to the high-cycle fatigue tests, static in-plane force was applied to the braces of K-joint to determine the hot-line strain distributions and the stress concentration factors (SCFs) via extrapolation. Based on the results of static loading tests, the amplitude of cyclic load can be further determined. Subsequently, high-cycle fatigue tests were performed on the square and diamond bird-beak SHS gap K-joints to investigate the fatigue behavior from different perspectives, such as failure mode, fatigue life, crack propagation, and stiffness degradation. The test results demonstrate that the brace crown region always possessed the most severe stress concentration, which coincided with the location of crack initiation. This implies that the experimentally obtained SCFs can accurately predict the crack initiation location. The test fatigue lives of bird-beak gap K-joints were compared with the predictions by the International Institute of Welding (IIW) $S_{\mathrm{rhs}}–N$ design curves originally developed for the conventional joints.

To investigate the performance of self-centering RC frame structures subjected to debris-flow impacts, a series of debris-flow impact tests of a traditional RC frame structure (TF) and a self-centering RC frame structure (SCF) was designed and carried out to determine the damage mechanism and the mechanical responses. The test results indicated that in the same loading cases, the residual displacement of the SCF was 68% that of the TF, the peak acceleration was 75% that of the TF, and the peak strain of the SCF was 71% that of the TF. In addition, the test phenomena showed that the damage extent and the crack propagation of the SCF were significantly lower than those of the TF. Finite-element analysis (FEA) of the tests was carried out. The numerical results were in good agreement with the test results, based on which the impact force and the reaction force were discussed. The numerical results and the experimental results revealed that the self-centering frame structure had the prominent impact-resistance performance under debris-flow impacts, which provides some reference for self-centering structure design in regions in which debris flow frequently occurs.

Reinforced concrete pile caps are important structural elements transmitting vertical loads from the structure to the ground. The performance of pile caps is complex, with a wide range of structural parameters in play, such as geometry, tension reinforcement, pile arrangement, and so on. Their complexity and criticality demand a good degree of reliability in strength estimations. Current design approaches are devoid of contribution from key parameters, resulting in inaccurate strength estimations. Moreover, there are interpretation difficulties when using the existing code provisions for members with pile arrangements that are not too simple. A general approach considering the influence of critical parameters for estimating the strength of pile caps in shear and flexure is proposed in this paper. The proposed approach can capture the accurate contribution of nondominant load paths, typically observed in pile caps with nonsymmetric pile arrangements. The model predictions for 21 specimens with varying parameters and failure modes are compared with test strengths and American design code (ACI) code estimations. Results showed an excellent correlation with test results, better than code estimations, showing the accuracy and efficacy of the proposed model.

Lead rubber bearings (LRBs) have been widely used in seismic isolation systems for buildings and other structures to effectively mitigate the damaging effects of horizontal earthquake ground shaking. The horizontal flexibility of the LRBs, typically placed at the base of the structure, result in a concentration of displacements in the bearings while limiting deformations in the structure. Strong earthquake shaking can produce large displacement demands on the isolation system and are typically limited by a clearance or a moat around the perimeter of the structure. Recent research has evaluated the displacement capacity of isolated structures and the significant risk of failure from increased seismic demands on the isolators or from pounding to moat walls. This paper proposes a parallel nonlinear model to capture the complex behavior of LRBs at large strains in an effort to better predict isolator displacements and the potential for exceeding a limit state. The model captures strength degradation from lead core heating, stain hardening of the rubber, and unloading effects. Allowing the bearings to sustain large strains to induce hardening is explored as a means for reducing the impact velocity in the case that the clearance to the stop is exceeded. Numerical time history analyses are used to demonstrate the sensitivity of model selection for LRB especially when considering beyond design ground motions.

This study developed a special energy dissipation X-bracing for cold-formed steel (CFS) shear walls to improve the ductility of the corrugated steel sheathing. In previous research, CFS shear walls with corrugated sheathing demonstrated poor ductility under cyclic loading. The paper presents a potential solution by adding a special energy dissipation bracing (EDB) to improve the ductility of the corrugated sheet–sheathed shear walls. The EDB can be tuned according to the CFS shear wall base performance to achieve the highest postpeak deflection capacity, thereby significantly improving the seismic ductility. To prove the concept and analyze the feasibility of the new EDB, a test program of nine full-scale shear walls was conducted. The experimental results showed that with the tuned EDB, the corrugated steel sheet–sheathed shear walls had improved ductility with no considerable reduction in the initial stiffness and shear strength. Installing the proposed EDB can be an efficient approach to enhance the seismic performance of CFS-framed shear walls. Details of the test program and discussion are presented.

The strip model, which is currently the most commonly used simplified analytical model for steel plate shear walls (SPSWs), neglects the compressive stiffness and resistance of the infill plate, thus underestimating the lateral stiffness and ultimate strength. Therefore, in this study, a more general simplified analytical model, the uniform strip model, is developed for SPSWs with different web plate thicknesses. The new model incorporates the compressive stiffness and resistance of the infill plate by adjusting the cross-sectional area and yield stress of the strips of the original strip model. The numerical simulations of experimental specimens demonstrate the validity of the new model. Parametric studies were performed to investigate the effects of the aspect ratio, plate thickness, and story number on the accuracy of the proposed model relative to a detailed shell element model. The results demonstrate that the uniform strip model agrees well with the shell element model. In comparison, the original strip model is most suitable only for what is classified as thin SPSWs, and the proposed model is a feasible tool for modeling buckling-restrained SPSWs.

RC shear walls are commonly used as lateral load-resisting elements in seismic regions, and the estimation of their shear strengths can become simultaneously design-critical and complex when they have so-called squat geometries, i.e., height-to-length ratios less than two. This paper presents a study on the training and interpretation of an advanced machine-learning model that strategically combines two algorithms for the said purpose. To train the model, a comprehensive shear strength database of 434 samples of squat RC walls is utilized. First, the eXtreme Gradient Boosting (XGBoost) algorithm is used to establish a predictive model for estimating the shear strength, wherein 70% and 30% of the data are respectively used for training and validation. This effort resulted in an approximately 97% validation accuracy, which well exceeds current mechanics-based/semiempirical models. Second, the SHapley Additive exPlanations (SHAP) algorithm is used to estimate the relative importance of the factors affecting XGBoost’s shear strength estimates. This step thus enabled physical and quantitative interpretations of the input-output dependencies, which are nominally hidden in conventional machine-learning approaches. Through this setup, several squat wall attributes are identified as being critical in shear strength estimates.

This study presents an energy-based procedure for the design of RC frames under earthquake strong motions. For this purpose, three RC frames having 4, 8, and 12 stories that were used in FEMA P695 were selected. The code-based designed frames were analyzed and redesigned employing the proposed energy-based procedure. A comparison was conducted between the performance of the code-based and energy-based designed models. Results reveal that the energy-based models appeared to perform more effectively under strong ground motions in terms of more optimized use of beam capacity for dissipation of energy, higher ductility values, and more uniform distribution of plastic hinges in height of structure. Comparison of pushover curves showed that ductility value in energy-based frames increases nearly 1.5–2.0 times the ductility of code-based frames. Calculated hysteretic energy demand in energy-based frames is also proved to decline by 50% in lower stories and, instead, the contribution of upper stories is increased. The same pattern for distribution of interstory drift ratio is also observed.

Dry and ice galloping of power transmission lines (conductors) that occur at moderate to large wind speeds cause large-amplitude motion in these long-suspended cables. This phenomenon can cause catastrophic damages such as flashover, wire burning, tripping, transmission line tower collapse, accident, interphase short circuit, and structural or fatigue failure of transmission towers or conductors. Wind-induced cable vibration, which has been extensively studied, can be classified based on its sources, such as rain-wind-induced vibration (RWIV), vortex-induced vibration (VIV), wake galloping, and dry/ice galloping. This study primarily focuses on the predictions of time-domain response and onset of dry- and ice-conductor galloping by measuring the self-excited and buffeting load parameters of the bare conductors and conductors with ice formation in normal and yawed wind. In this regard, a series of static and dynamic wind tunnel experiments were performed to fundamentally study the conductor vibration in dry and ice conditions. Surface pressure distribution and aerodynamic forces were measured for stationary section models of nonyawed and yawed dry conductors in a smooth flow. Additionally, the dynamic response of dry and iced conductors using a one-degree-of-freedom system was recorded by employing a free vibration setup to extract self-excited load parameters. Buffeting load parameters were measured by generating a sinusoidal-oscillating wind upstream of dry and iced conductors for different yaw angles. The experiments resulted in the identification of the Strouhal number ($St$), aerodynamic load coefficients ($C_D$ and $C_L$), buffeting indicial derivative functions, aerodynamic stiffness, and aerodynamic damping of a conductor for yaw angles ($\beta$) ranging from 0° to 45°. Dynamic tests led to the proposing of several empirical equations to determine the critical reduced velocity (${\mathrm{RV}}_{cr}$) or critical wind speed for dry and ice galloping of conductors at a given Scruton number (${\mathrm{Sc}}_h$) and yaw angle. Finally, a procedure was proposed to calculate the least damping required to suppress the conductor galloping under dry or iced conditions up to the design wind speed. The wind load parameters identified in this study can be used to numerically simulate the dynamic load and response in the time domain of dry and iced conductors in turbulent wind.

Steel fiber–reinforced concrete (SFRC) is extensively used in coupling beams due to its superior mechanical properties. In this study, the two-parameter kinematic theory is modified by introducing the consideration of the critical crack angle and longitudinal reinforcement strain. The modified method is validated by 64 sets of experimental coupling beams in the literatures, and the predicted shear capacities are found to have good agreement with the test results. Moreover, the decomposition of the shear capacity provided by steel fibers, stirrups, the critical loading zone, aggregate interlock, and the dowel action of the longitudinal reinforcement can be evaluated separately. Thus, the effects of the span-to-depth ratio, volume fractions of fibers, and stirrup ratios on the shear capacity are investigated. Additionally, the equivalent substitution relationship between stirrups and steel fibers is quantified, which provides a solution to the reduction of the density of stirrups in coupling beams and similar components.

The lateral strength and stiffness requirements due to wind loads usually govern the design of tall buildings. The current building codes in the US, Canada, and Europe recognize the first significant yield point as an ultimate limit state. Consequently, the current design practices ignore the plastic capacity of structural systems in the nonlinear range, which could result in overdesigned buildings. Thus, the classical linear-elastic design arguments shall be reexamined with consideration of performance-based wind design (PBWD) approaches, innovative technologies, and materials. We are presenting two companion papers to demonstrate the benefits of considering the nonlinear capacity of structural systems in the design of wind-excited buildings. In this paper, Part I, we have postulated and then proved the capability of self-centering systems in controlling the possible damage accumulation in structural systems subjected to along-wind loads. Our arguments are based on an extensive parametric study through nonlinear time history analyses considering peak and residual ductility-demands, normalized hysteretic energy dissipation, and the rate of damage accumulation as performance indicators. Overall, the results of the parametric study revealed that self-centering systems could benefit the most from the ductility-based design due to their inherent recentering capability, higher energy dissipation, and lower sensitivity to wind duration. Consistent with the notion of PBWD, for self-centering systems, the companion Part II paper demonstrates the benefits of the ductility-based wind design in terms of economics and safety through structural reliability analysis.

Part I of the two companion papers postulated and proved the capability of self-centering systems in controlling the wind-induced damage accumulations due to long-duration along-wind loads. The present Part II paper demonstrates the benefits of ductility-based wind design in terms of economics and safety through structural reliability analysis. Initially, for self-centering systems, the ductility demands are estimated for various levels of force reduction factors, structural damping, postyield stiffness ratio, natural frequency, and energy dissipation capacity. To reduce the computational cost of structural reliability analysis, empirical equations of the mean of peak ductility demands are derived in terms of the force reduction factor and natural frequency. In the reliability estimations, two limit states, the first significant yield and incipient collapse, are considered. Both analytical and simulation techniques are used to compute the failure probabilities by considering uncertainties in both the wind load effects and capacity. Overall, the results indicate that ductile self-centering systems could be designed for reduced along-wind loads and still achieve the minimum required safety level. The results also reveal that self-centering systems designed using the linear-elastic approach but additionally detailed for ductility have a significant reserve of safety against incipient collapse.

Steel corrugated shear walls (SCSWs) are an alternative to ordinary steel plate shear walls (SPSWs) with flat infill plates. This paper deals with the shear resistance design of corrugated panels in SCSWs under monotonic loading. Based on the validated finite-element analysis (FEA), key parameters affecting the ultimate and postultimate strengths of corrugated shear panels were identified and the proper values were recommended. By introducing the normalized slenderness ratio, design formulas were established for the ultimate strength and residual strength of corrugated panels. Recommended corrugation profiles were tabulated for convenience of practical use to ensure high and stable shear resistance. It was found that, compared with the ultimate strength, variation of the residual strength against the normalized slenderness ratio is much more complicated, and the corrugation dimensions must be chosen properly according to the proposed design equations and recommendations to avoid unstable postbuckling behavior.

External installation of steel braces is an effective approach to increase the lateral load resistance of steel moment-resisting frames. However, the effects of existence of steel braces on the robustness of steel moment-resisting frames to resist progressive collapse is still not clear because little study has been carried out. To fill this gap, in this paper, six multistory steel moment-resisting subframes (three bare frames and three braced frames) were fabricated and tested. Test results indicated that the specimen with reduced beam section in the connection zone performed best among three types of connections due to the guaranteed formation of plastic hinges at the location of reduced section and avoiding brittle fracture of weld at the connection. Experimental results proved that steel braces could increase the load-resisting capacity by 45.1% and 83.9% of the frame with weld connection and end plate connection, respectively. The gusset plate restricted the rotation of the plastic hinges in the second story of the braced frames with V-shaped bracing, which decreased its deformation capacity and degraded its catenary action capacity. Actually, the ultimate load of the braced frames with V-shaped bracing is only 87.5% of that of the counterpart without any braces. Because the compressive braces were severely buckled before the displacement reached 0.4% of the beam span, it had little effect on yield load but increased the initial stiffness of the bare frames. Thus, a majority of the benefits of the bracing system were attributed to the tensile braces. Moreover, the analytical results evaluated the differences in load resistance and development of load-resisting mechanisms in different stories. Furthermore, the contribution of compressive and tensile braces was decomposed individually by analytical analysis.

Due to the excellent mechanical properties of ultrahigh-performance concrete (UHPC), the shear strength of stud–UHPC composite connections is significantly enhanced. Nevertheless, the underlying mechanism remains not fully understood. This study discusses the process of failure of shear studs embedded in UHPC and develops a concrete wedge block model in an attempt to explain such improvement. The procedure to determine the key characteristics of the wedge block is established. Based on the wedge block model, a modified compression-dispersion model and plastic deformation theory were utilized to derive a shear strength prediction formula for stud–UHPC composite connections. Push-out test results combined with relevant data collected from available literature were used to validate the proposed formula. The deviation of predicted shear strength from experimental results was typically within 25%. The contribution from the UHPC wedge block to the shear strength of the stud–UHPC composite connection was found to be approximately 13%.

This paper aims to investigate the seismic performance of a steel frame with an external diaphragm joint between a concrete-filled steel tube (CFST) column and H-shaped steel beam through experiments and finite-element (FE) analysis. A quasistatic test was conducted on five steel frame specimens. The parameters involved in the test were joint stiffness, beam-to-column stiffness ratio and concrete strength. Based on the experimental results, failure mode, lateral load versus displacement hysteretic curve, skeleton curve, strength and stiffness degradation effects, ductility, and energy dissipation capacity were analyzed. The experimental results show the hysteretic curves have a plump shuttle shape; the strength degradation effect and stiffness degradation effect are relatively feeble; and the ductility and energy dissipation capacity are excellent. Therefore, the tested specimens have good seismic performance. Also, this suggests that larger joint stiffness brings about superior seismic performance; a stronger column leads to higher bearing capacity as well as inferior ductility and energy dissipation capacity; and a CFST frame performs better than a hollow steel tube column frame under cyclic loads. On the basis of the experimental results, the ABAQUS software version 6.14 was adopted to establish the FE models. The applicability and accuracy of FE simulation were first verified by experimental results. Then, quantitative parametric analysis was carried out to study the effects of different parameters on seismic performance of the CFST frame with an external diaphragm joint.

The Imperial College Robustness Assessment Framework is a methodology—based upon application of the alternate load path concept—for quantitatively comparing the performance of different structural arrangements to a threat-independent triggering event (removal of a single column) with the potential to induce a progressive collapse of the structure. Initially, it depended upon the numerical analysis of a representative beam. More recently, the application of more basic structural mechanics has been found to yield almost identical results at a fraction of the cost, thereby making use of the approach by nonspecialists far simpler. Although this study was mainly restricted to the coverage of steel and composite frame structures, the principles apply to any construction type. Important aspects of these simplifications, including demonstrating both the accuracy and rigor of the resulting approach and the ease with which many variants of a proposed scheme may be compared in such a way that the precise influence of key variables may readily be assessed, are collected together herein.

Experimental, numerical, and theoretical analyses were conducted of the axial load resistance of a novel ultrahigh-performance fiber-reinforced concrete (UHPFRC) grouted square hollow section (SHS) tube sleeve connection. The experimental study tested 10 full-scale specimens with varying shear key spacings, grout thicknesses, grout lengths, and volume proportions of steel fiber in the UHPFRC. Two types of failure modes were observed: (1) for the connection with high strength of the grouted part, the failure mode was fracture of the inner tube; and (2) for the connection with lower strength of the grouted part, the failure mode was grout shear crushing with significant bond-slip between grout and steel tube. To understand further the load transfer mechanism of the connection, an advanced three-dimensional (3D) nonlinear finite-element (FE) model was built to simulate the axial load–displacement behavior, state of stress and strain, and crack development of the grout. Based on the test and FE results, a new theoretical model was derived to predict the axial-load resistance of the connection. The proposed model considers the effect of section shape and material parameters, and is applicable to UHPFRC grouted SHS tube sleeve connection with different corner radii. Validation versus the test results showed that the new model can provide reasonably effective and accurate predictions of the axial-load resistance of the novel grouted sleeve connection subjected to tension.

Hurricanes are devastating natural hazards that often cause damage to the built environment as a result of their loadings, which include storm surge, waves, and wind, often in combination. Modeling these hazards individually and their effects on buildings is a complex process because each loading component within the hazard behaves differently, affecting either the building envelope, the structural system, or the interior contents. Realistic modeling of hurricane effects requires a multihazard approach that considers the combined effects of wind, surge, and waves. Previous studies focused primarily on modeling these hazards individually, with less focus on the multihazard impact on the whole building system made up of the combination of the structure and its interior contents. The analysis resolution used in previous studies did not fully enable hurricane risk assessment through a detailed investigation of the vulnerability at the component-level or subassembly-level (a group of components such as interior contents, structural components, or nonstructural components). To address these research gaps, a robust multihazard hurricane risk analysis model that uses high-resolution hazard, exposure, and vulnerability models was developed. This model uses a novel approach to combine the storm surge and wave fragility functions with a suite of existing wind fragilities to account for structural damage and then combines them with another suite of flood-based fragilities to account for interior content damage. The proposed vulnerability model was applied to the state of North Carolina as an example of a regional-scale assessment to demonstrate the ability of the method to predict damage at the building level across this large spatial domain. This model enables better understanding of the damages caused by hurricanes in coastal regions, thereby setting initial postimpact conditions for community resilience assessment and investigation of recovery policy alternatives.

Buckling-restrained braces (BRBs) are seismic devices that provide structures such as buildings and bridges with lateral support, dissipating more energy than traditional bracing. Large-scale laboratory testing to assess every buckling-restrained braced frame (BRBF) is desirable but cost prohibitive. Computer simulation that incorporates realistic BRB mechanical behavior is an attractive option to supplement such testing. Predicting the cyclic response and ensuring stability of BRBFs during severe earthquake events is of particular interest. A finite-element analysis (FEA) strategy that can model the testing of BRBs was developed using Abaqus software. The development of nonlinear material and contact models are important aspects that affect accuracy and convergence in each model. The Chaboche method, using six back-stress curves, is used to characterize the combined kinematic and isotropic hardening exhibited in the steel cores of BRBs. A simplified approach was developed to capture the contact interaction between the restrainer and the core of each BRB design modelled. Each model captures important frictional dissipation as well as lateral motion and bending associated with higher-order constrained buckling of the core in both the strong and weak axis. At the same time, the methodology sought to minimize computational expense for this highly nonlinear system. The strategy was validated by comparing cyclic axial force versus displacement predictions to experimental data for three different BRB designs. This modeling strategy could be helpful for simulating the performance of other generic BRB designs and subassemblages.

Corrugated steel tube (CST) confined reinforced concrete (RC) sections, which consists of galvanized corrugated steel tubes infilled with concrete and steel reinforcements, provide a solution for their practical application in the outdoor and offshore corrosive environment. To investigate their compression-bending, cross-sectional behavior, a fiber model is established by treating it as a confined reinforced concrete section. The model considers the nonuniformity of concrete confinement pressure across the cross-sectional depth and tensile axial-strain concentration at the lock seam. The ultimate strength and lateral deflection of the CST confined RC sections are investigated. Further in-depth analyses are carried out to study the influence of the eccentricity ratio, geometric parameters (thickness, helical angle and corrugation profile of tubes, and reinforcing bars ratio), as well as materials strength (concrete and tube). Finally, a design method is proposed to predict the compression-bending strength of CST-confined RC sections, and its precision is verified by comparison with tests and numerical results.

The vast majority of current structural design codes and standards do not separately consider the wind and turbulence profiles for different storm types. Based on the 356-m-high Shenzhen Meteorological Gradient Tower, which is the tallest of its kind in Asia, this paper investigates the characteristics and vertical profiles of mean wind and turbulence for different types of strong wind events, including typhoons, monsoons, and thunderstorms, from a comparative perspective. Commonalities and differences among the vertical profiles of mean wind speed, turbulence intensity, and turbulence integral length scale for these storm events are comprehensively investigated. It is observed that the wind and turbulence profiles in typhoons and monsoons can be reasonably described by design codes and standards, while those in thunderstorms substantially deviate from typhoons and monsoons as well as code stipulations. The outcome of this study is expected to facilitate the different forms of wind and turbulence profiles for typhoons, monsoons, and thunderstorms to be taken into consideration in design codes and standards and improve the wind-resistant design of high-rise structures in regions that are prone to these storm events.

A reduced-order modeling approach is proposed for the linear and nonlinear analysis of automated rack-supported warehouse (ARSW) structures under seismic loads. Steel ARSWs are massive structures that employ traditional racking configurations to support both the stored pallets and the external cladding shell. Capturing all the structural details of an ARSW requires tens or hundreds of thousands of elements, leading to complex numerical models that are analyzed with difficulty even in the linear range and are clearly unsuitable for nonlinear analysis. Thus, despite being prolific in earthquake-prone areas, their true seismic behavior remains largely untested. As a viable compromise between fidelity and low computational cost, we offer instead a reduced-order model that relies on the substitution of the one or more built-up columns (i.e., upright frames) with equivalent Timoshenko beam-column elements and two-node link elements. The resulting model can support both two-dimensional (2D) and three-dimensional (3D) elastic and inelastic analyses, offering a powerful tool for the seismic assessment of ARSWs and of complex structures comprising built-up columns in general.

Explosives installed on a commercial unmanned aerial vehicle (UAV) may be carried and detonated close to a structural element. Although the explosive mass may not be large, the standoff distance is expected to be very small, and therefore, a potentially high-level of damage can be developed. A popular method to improve the performance of RC elements is by adding fibers to the concrete mix. In the current research, an experimental study is conducted to investigate the performance of RC slabs with and without steel fibers to close-in detonations, which aim to simulate an explosive installed on a UAV. The local damage is recorded and measured in terms of the crater diameters and depths. The structural damage is compared with available numerical tools. In addition, concrete debris from the rear face of the slab was collected and weighed, and their velocities were captured by a high-speed camera. The mass distribution of the debris, together with their velocities, led to a study on the damage hazards to potential occupants in the building.

Façade overturning is a frequently observed collapse mechanism occurring in unreinforced masonry (URM) buildings during high-intensity, earthquake-induced shaking. Following complete separation from a building, the rocking motion of a URM façade and the associated impact against the return walls are the factors that continue to contribute to the façade out-of-plane capacity. Seismic vulnerability studies of URM façades have historically neglected the interaction between building earthquake response and the rocking response of the façade, whereas in the study reported herein this interaction was analyzed using the discrete element modeling (DEM) approach, resulting in a façade out-of-plane capacity reduction. The increment in the dynamic rocking capacity caused by the frictional connection between the URM façade and the building was also analyzed and is reported.

The propensity of buildings for progressive collapse can be assessed using the alternative path method. Generally, there are three scenarios to be considered: internal column, side column, and corner column removals. Many researchers have conducted robustness analysis of two- and three-dimensional substructures subjected to internal column removal, but only limited studies have investigated side and corner column removals. Because the side and corner columns are exposed directly to the external environment, they are more susceptible to damage caused by extreme events occurring outside the building. This paper presents experimental and numerical investigations of progressive collapse behavior of a 1:3-scale six-column subframe composite floor system subjected to the removal of a side column. A specially designed six-point loading system was adopted to apply an equivalent uniform load to the composite floor slab. The load was applied in a displacement-controlled manner to trace the complete load–displacement behavior at the point of column removal. The experiments also captured the stress distributions, failure patterns and load-transferring mechanisms. A reduced nonlinear finite-element (FE) model was developed to capture the load–displacement behavior of the tested frames, and the predicted results were compared with those from the tests. It was found that the collapse of the composite frame system is governed by the failure of primary beam–column connection. The resistance to progressive collapse is contributed predominantly by the flexural action (FA). Contradicting observations by other researchers, enhanced resistance due to catenary action (CA) in the beam and tensile membrane action (TMA) in the slab were not observed in the tests. In addition, the dynamic increase factor (DIF) obtained from the energy balance method was found to be in reasonable agreement with the factors proposed by Department of Defense recommendations.

Having a sufficient degree of robustness is generally believed to be an essential property of a safe structure. However, there is a lot of confusion about the exact meaning of the word as well as the way how this criterion should be interpreted in various circumstances. Of course, the notion has been translated into quite a number of codified rules that can be prescribed by authorities and can be dealt with by designers. Although there is some basic logic in the rules, the background is quite weak. Many studies focus on the structural behavior under extreme conditions, but relatively little research is done on the underlying effectivity and cost efficiency of the applied safety measures. Given the large epistemic uncertainties involved, the reluctance to deal with such issues is understandable. Nevertheless this paper aims at discussing these aspects and making them more prominent on the research agendas.

Crack inspection is a crucial but labor-intensive work of maintenance for in-service bridges. Recently, the development of fully convolutional network (FCN) provides pixel-wise semantic segmentation, which is promising as a means of automatic crack detection. However, the demand for numerous training images with pixel-wise labels poses challenges. In this study, a benchmark data set called a bridge crack library (BCL) containing 11,000 pixel-wise labeled images with $256\times 256$ resolution was established, which has 5,769 nonsteel crack images, 2,036 steel crack images, 3,195 noise images, and their labels. It is aimed at crack detection on multiple structural materials including masonry, concrete, and steel. The raw images were collected by multiple cameras from more than 50 in-service bridges during a period of 2 years. Various crack images with numerous crack forms and noise motifs in different scenarios were collected. Quality control measures were carried out during the raw image collection, subimage cropping, and subimage annotation steps. The established BCL was used to train three deep neural networks (DNNs) for applicability validation. The results indicate that the BCL could be applied to effectively train DNNs for crack detection and serve as a benchmark data set for performance evaluation of DNN models.

Real-time hybrid simulation (RTHS) results from the integration of numerical modeling with experimental testing of structural systems. In substructured RTHS tests, a reference structure may be partitioned into two or more substructures to save cost and space. Researchers may choose to model the elastic and less critical structural components numerically, while physically testing the components where appropriate numerical models are lacking. The dynamic characteristics of the numerical and physical substructures are combined by imposing the boundary conditions calculated from the numerical model in the physical substructure and returning measured physical forces back to the numerical model. To date, the majority of RTHS have been focused on substructuring with a single boundary point, physical substructure, and actuator. A multiaxial real-time hybrid simulation (maRTHS) framework was recently proposed, also using a single boundary point and physical substructure. However, for many practical engineering and research applications, more than one boundary point and physical substructure is necessary. Challenges to direct extension of the previously proposed maRTHS strategy to multiple boundary points include incorporation of a larger number of degrees of freedom in the physical experiment, robustness to coupling through the hybrid simulation when using multiple boundary points, and problems introduced by out-of-plane 3D motion. After presenting the analytical constructs of the proposed framework, these issues are explored, and a validation study is introduced involving a multispan curved bridge. This RTHS experiment employs two load and boundary condition boxes (LBCBs) with 12 actuators to assess the scalability of the proposed maRTHS framework to accommodate multiple LBCBs at multiple boundary points. Out-of-plane behaviors of this RTHS experiment are intrinsic. Further, both mechanical coupling present between the actuators for motions in Cartesian coordinates and the coupling introduced through the numerical structure in the RTHS are present. Nonetheless, the decoupled model-based control strategy performed well for both the linear and nonlinear structural responses. These results demonstrate the promising nature of the proposed maRTHS framework for investigating complex and nonlinear structural systems.

Superelastic shape memory alloys (SMAs) introduce supplementary energy dissipation with minimal residual deformation, which is desired in earthquake engineering–related applications (e.g., SMA bracing systems or SMA-based isolation systems). An appropriate phenomenological model of SMA flag-shaped hysteresis is essential to simulate the seismic response of structures equipped with such devices. In the structural analysis and design software SAP2000, no element is available specifically for modeling SMAs. The CSi knowledge base webpage suggests a parallel combination of elementary multilinear-elastic (MLE) and multilinear-plastic (MLP) link elements. This modeling strategy, widely used in the literature, can be affected by the cumulative damage in the MLP link elements after the first plastic excursion. As demonstrated in this technical note, the cumulative damage in MLP elements leads to incorrect modeling of the SMA flag-shaped hysteresis in nonlinear time-history analysis (NLTHA). This issue encountered in the model suggested in the CSi knowledge base webpage is resolved by an alternative modeling strategy proposed in this work, which makes use of parallel MLE and MLP link elements along with series gap and hook spring elements available in SAP2000. The proposed approach not only eliminates the cumulative damage of MLP links over repeated cycles, but also lends itself to a more general representation of the flag-shaped hysteresis of SMAs than do other formulations proposed in the literature, thus proving to be a simple and versatile modeling strategy.