Journal of Structural Engineering

Rigorous assessment of structural response to seismic excitation poses challenges for researchers in their bid to account for the effects of (1) biaxial interaction; and (2) incidence angles of a ground motion relative to the structure. Keeping these two major issues in view, this paper presents a comprehensive review of the progress of research in the related areas. Studies since the 1970s clearly bring out the significance of interaction effect especially for stiff systems, and also the sensitivity of response to incidence angles. Further, a scrutiny of the design guidelines embodied in modern codes shows that, to account for the interaction effect, the codes primarily rely on the approximate combination rules evolved in the mid-1970s. In addition, both research findings and codified guidelines clearly reveal an apparent lack of consensus on the choice of appropriate incidence angle. With reference to bidirectional analysis, modern views of researchers on important aspects including quantification of damage and selection of ground motions are also discussed. Based on a critical review of the recent works and the codes, the paper recommends the use of an improved yet simple methodology for practical design. Directions for future research are also brought out.

This paper investigates the seismic response analysis of the 9-story SAC building equipped with pressurized sand dampers, a new type of low-cost energy dissipation device where the material enclosed within the damper housing is pressurized sand. The strength of the pressurized sand damper is proportional to the externally exerted pressure on the sand via prestressed steel rods; therefore, the energy dissipation characteristics of a given pressurized sand damper can be adjusted according to a specific application. The strong pinching behavior of pressurized sand dampers was characterized with a previously developed three-parameter Bouc-Wen hysteretic model that for this study was implemented in the open source code OpenSees with a C++ algorithm, and it was used to analyze the seismic response of the 9-story SAC building subjected to six strong ground motions that exceed the design response spectrum for all soil categories. The paper shows that for the family of strong ground motions used in this study, pressurized sand dampers with strength of the order of 5%–10% of the weights of their corresponding floors were able to keep the interstory drifts of the 9-story SAC building at or below 1%, while base shears and peak plastic hinge rotations were reduced in the damped configuration. Supplemental damping produced mixed results on floor accelerations; nevertheless, in most floors, peak accelerations were reduced.

Experiments on post-Northridge welded unreinforced flange bolted web (WUF-B) connections demonstrated a new low-cycle fatigue (LCF) crack initiation mechanism with final rupture occurring either near the weld access hole or the weld regions. Post-Northridge connection research reports and related commentary indicated that the weld and heat-affected zone (HAZ) conditions might be contributing factors in LCF-initiated failures of the modified WUF-B connections. The experimental study reported herein investigated the influence of weld sequence on the fatigue failure of the WUF-B connections. Two exterior WUF-B connections were fabricated using different weld sequences in laying the complete joint penetration welding between the beam and column flanges. These connections were tested under a constant-amplitude displacement-controlled loading protocol until crack initiation. Both specimens failed by cracking at the weld access hole, one in a brittle manner and the other in a ductile manner. Analysis of the recorded data demonstrated the influence of weld sequence on strain responses at the weld toe and weld access hole regions. Accumulation of strain with cycles, which is a phenomenon known as strain ratcheting, was observed near these locations in both tests. Recorded strain responses near the crack locations indicated the cause of earlier failure of one specimen compared with the other. Finally, future research needs in mitigating the influence of welding sequence on fatigue failure of welded steel moment connections are discussed.

This paper presents analysis of fracture in structural steel subjected to cyclic loads, which is of paramount importance for evaluating the safety of steel buildings exposed to extreme events such as strong earthquakes. The analysis relies on the phase-field concept to describe the formation and propagation of cracks in a solid body, in conjunction with a large-deformation description of the motion and an elastoplastic constitutive law having nonlinear isotropic and kinematic hardening to account for the material hysteretic response. The evolution of the phase-field is driven by a low-cycle fatigue criterion, aimed to phenomenologically describe the effect of irreversible processes ultimately leading to ductile fracture. The proposed modeling scheme is used in the simulation of experimental tests on circumferentially notched steel coupons tested by the authors, and its capability to capture fracture occurrence, as well as cyclic crack opening and closing, is demonstrated.

The compatibility strut-and-tie method (C-STM) is a well-established nonlinear modeling tool to efficiently and accurately predict the overall load–deformation response of shear-critical RC members subjected to combined bending and shear. Such a high-performance tool, however, is computationally demanding. In this paper, a simplified limit analysis model based on C-STM is presented to predict the load-carrying capacity of RC beams. Amenable for hand methods of analysis, the approach is called the truss-arch model unified (TAMU). The TAMU approach accounts for the failure mechanism of the diagonal concrete strut that is softened because of the transverse tensile strain. Instead of providing overall force–deformation behavior, this model focuses on evaluating the ultimate load-carrying capacity by assuming the failure mechanism occurs when the principal diagonal strut reaches its softened strength in shear-critical beams. An explicit equation for the principal strain ratio is derived to evaluate the softened concrete strength and is used to develop formulas for the ultimate load-carrying capacity. The validity of the formulas is verified through a comparison of the predicted ultimate load-carrying capacities with maximum measured strengths from previous experimental results on large-scale physical tests representing bridge piers. The TAMU approach is then compared with other code-based strength analysis methods and shows better predictions of the maximum load-carrying capacity.

Performance-based design and system-level assessment methods are becoming the preferred approaches for evaluating the safety of structures. This is particularly important for highway bridges where, because of their exposure to long-term deterioration as well as sudden localized failures, the generally conservative traditional member-oriented approach does not necessarily lead to an accurate evaluation of the actual structural system’s safety levels nor, consequently, to the efficient allocation of the limited resources available for infrastructure management. The objective of this paper is to quantify the effect of damage size and location on bridge elements and how this affects the performance of the entire superstructure system. The paper also presents a simplified equation for estimating the structural robustness of typical highway girder bridge superstructures as a function of the damage type. A numerical example is presented to illustrate alternative approaches for how these concepts could be implemented during the design and safety assessment of highway bridges. In particular, the analysis showed that the occurrence of damage directly under the live load reduced the ultimate capacity of the system in the range of 70%–95%. This reduction was between 40% and 70% when the damage was located away from the loaded zone.

This paper presents the results of an investigation of the dynamic behavior of a reinforced concrete frame that was strengthened by a buckling-restrained brace (BRB). The behavior of this frame was benchmarked against a control frame without strengthening. Both frames were tested concurrently on a shake table using the acceleration time-history record from the 1995 Kobe earthquake. The BRB-strengthened frame was additionally subjected to two levels of a synthetic time-history record to observe its behavior under severe ground shaking. The stiffness of the BRB-strengthened frame increased by 70% relative to the control frame. Furthermore, the cracked BRB-strengthened frame retained approximately 80% higher stiffness compared with the cracked control frame at an applied peak ground acceleration (PGA) of $0.679g$. Conversely, the lateral story drift of the BRB-strengthened frame was 46% less compared with the control frame at this level of acceleration. The control frame failed by concrete crushing at base of the columns at PGA of $0.679g$ after yielding of the longitudinal reinforcing bars. The dissipation of hysteretic energy for the control frame reached a maximum at $0.435g$, whereas the BRB-strengthened frame exhibited high strength capacity without any marked change in stiffness at $0.679g$.

There is a growing trend toward utilizing high steel-reinforcement ratios (as high as 5%) in ultrahigh-performance concrete (UHPC) beams. This tendency is driven by the desire to take full advantage of the unique mechanical properties of UHPC, but results in construction that is expensive given the high cost of both steel reinforcement and UHPC. This paper focuses on UHPC beams with low reinforcement ratios and relatively low levels of fiber reinforcement in an attempt to reduce the cost of UHPC construction and hence broaden its appeal. Four-point bending tests were conducted on UHPC beams with steel-reinforcement ratios of $\rho =0.85\%$ and 1.54% and fiber volume fractions $V_f=1.0\%$, 1.5%, and 2.0%. The experimental results showed that increasing the fiber volume fraction, which increases material tensile ductility, did not necessarily lead to enhanced structural ductility. This observation was confirmed through validated computational modeling using a hybrid fixed/rotating crack model. Synthesizing the test and computational results, this paper advocates for an underreinforced design philosophy and use of a fiber volume fraction that is just enough to ensure that crack localization does not occur under working conditions. This philosophy minimizes the cost of UHPC beams, making them more attractive for broader usage. The latter consideration ensures that steel rebars will have enhanced protection against corrosion under working conditions, which, coupled with the high durability of UHPC, ensures highly durable construction with low long-term maintenance costs.

Deformations in dam structures can have a critical impact on dam safety and life. Accurate methods for dam deformation prediction and safety evaluation are thus highly needed. Dam deformations can be predicted based on many factors. The analysis of these influences on the deformation of the dam reveals a problem that deserves further attention: dam deformation lags behind environmental factors of the water level and temperature as well as the time lag of the temporal dam deformation data. In this paper, a hybrid deep learning model is proposed to enhance the accuracy of dam deformation forecasting based on lag indices of these factors. In particular, dam deformations are predicted using deep networks based on gated recurrent units (GRUs), which can effectively capture the temporal characteristics of dam deformation. In addition, an improved particle swarm optimization (IPSO) algorithm is used for optimizing the GRU hyperparameters. Furthermore, the complete ensemble empirical mode decomposition with adaptive noise algorithm (CEEMDAN) and the partial autocorrelation function (PACF) are exploited to select the lag factor indices. The accuracy and effectiveness of the proposed CEEMDAN–PACF–IPSO–GRU hybrid model were evaluated and compared with those of other existing models in terms of four different evaluation indices (MAE, MSE, $R^2$, and RMSE) and using 9-year historical data for the case of a pulp-masonry arch dam in China. The experimental results show that our model outperforms other models in terms of the deformation prediction accuracy ($R^2$ increased by 0.16%–9.74%, while the other indices increased by 14.55% to reach 96.69%), and hence represents a promising framework for general analysis of dam deformations and other types of structural behavior.

The expanding height of multistory mass timber buildings makes the development of guidance to prevent disproportionate collapse an increasingly important area of engineering design; however, there is no guidance available in building codes or design standards on how to prevent disproportionate collapse of multistory mass timber buildings. In this paper, practical project-specific solutions applied in constructed prominent multistory mass timber buildings to prevent disproportionate collapse are presented. Various approaches have been successfully used for different construction types of multistory mass timber buildings to decrease the potential risk of disproportionate collapse, such as using walls above supports as deep beams and using columns to carry tension forces and hold the floor below in case a supporting column or wall is damaged. Then, recent and ongoing research in the field of prevention of disproportionate collapse of multistory timber buildings is reviewed. Such research is mostly based on numerical simulations of element removal scenarios and pushdown experiments on floor systems and connections and provides valuable insights and guidance to designers on important structural aspects regarding the prevention of disproportionate collapse in multistory mass timber buildings. Finally, the draft provisions for an upcoming standard related to the prevention of disproportionate collapse are reviewed, and the conclusions regarding the state of the art are presented.

Current design codes specify the need for two-way shear strength analysis for prestressed slab-to-column connections. This paper is an investigative study of said interaction for post-tensioned (PT) transfer slabs. Two-way shear strength and failure mode of the tested PT slab specimens were evaluated. To analyze the interaction quantitatively, concentric compression tests were performed on recent PT transfer slabs having small shear span-to-depth ratios. Test parameters included the number and layout of post-tensioning reinforcement. The test results demonstrated that the two-way shear strength of the PT transfer slab specimens was 53%–87% greater than that of conventional RC transfer slab specimen. Increase was tied to concentrated layout of post-tensioning tendons rather than distributed layout. The two-way shear strengths of the test specimens were compared with predictions under current design methods for PT flat plates and the strut-tie model.

This paper proposes an analytical solution based on a strut-and-tie model (STM) for RC deep and short beams to predict the web-shear capacity of precast prestressed concrete hollow-core (PCHC) slabs. The proposed solution is applicable to PCHC slabs within a wide range of slab thicknesses and shear-span-to-effective-depth ($a/d$) ratios. Additionally, the model accounts for different cross-sectional properties of this type of slabs, geometric properties of longitudinal voids, and diameters and transmission length of prestressing strands. The proposed STM was validated against a database of 46 tests on PCHC slabs, whose capacity was also predicted using shear equations from current standards. In addition, this research investigates effects of $a/d$ ratio and assumed depth of compression zone on shear-strength predictions for PCHC slabs. In summary, based on comparison between experimental data and analytical results, it can be concluded that the proposed STM approach is capable of providing predictions for PCHC slabs that are as reliable as current code methodologies. In addition, a direct noniterative STM approach is proposed as well to avoid challenges of iterations and refinements.

Windblown sand action affects civil structures and infrastructures in sandy environments, such as deserts and coasts. The wind interacts with human built structures of any kind leading to harmful effects, endangering their serviceability and users’ safety. To counter it, a number of sand mitigation measures (SMM) have been proposed, primarily through the trial-and-error empirical approach. As such, innovative approaches to properly quantify windblown sand action and to design SMM are needed in the current state-of-art and practice. In this study, the authors propose a novel hybrid approach to derive the life-cycle performance of SMM based on the combination of reliable wind-sand tunnel tests and innovative wind-sand computational simulations. Wind-sand tunnel tests are carried out to characterize the incoming sand flux in open field conditions. In a hybrid approach perspective, wind-sand tunnel measurements allow to properly tuned cheaper wind-sand computational simulations of the full-scale SMM performance. A probabilistic approach for determining windblown sand action and frequencies of sand removal maintenance is applied to a case study on a desert railway. Finally, a life-cycle cost analysis is carried out to assess extra-costs and savings derived from the implementation of the SMM. The proposed approach paves the way toward a comprehensive hybrid approach to the performance assessment of SMM.

Earlier studies found that acceptable probabilities of collapse for seismically isolated buildings could be achieved by an appropriate combination of strength in the superstructure, isolator displacement capacity, and stiffening behavior. The results were based on the study of braced and moment frames in which the story drift ratio at collapse was predetermined on the basis of the design details of the frames (special or ordinary in steel). This paper further investigates the effects the value of the peak story drift limit at which collapse occurs, in combination with the displacement capacity of the isolation system, have on the collapse probability of these structures. The results provide information on the utilization of superstructures with fewer ductile design details in seismically isolated buildings. This is important because buildings with fewer ductile design details have lower construction costs and can be built more quickly.

Undesirable cracking is commonly observed in existing bridge bents with inverted-T bent caps at the web-to-ledge interface. Although the cracking is unexpected, such cracks may decrease service life and are possible signs of structural deficiencies. In this study, concepts of retrofit solutions for improving the performance of such bent caps were developed and experimentally investigated. The solutions were designed to provide supplemental ledge and hanger load paths by attaching additional steel components: a post-tensioning (PT) threadbar with a “catcher” channel or end-region stiffeners. Experimental test results demonstrated the retrofitted inverted-T bent caps remained in a good (operational) condition to levels above the service limit state and had load carrying capacity up to 60% above the specified ultimate load demands. These experimental observations for both as-built and strengthened bent caps were compared to the calculated strength capacities, and the code-based strength capacities were shown to be overconservative. Repair and retrofit design recommendations are provided, including limitations, based on the experimental investigation.

This paper introduces an innovative three-piece boltless connection for assembly of volumetric building units or modules in multistory modular buildings. The proposed connection comprises a vertical connector and an independent horizontal connector. Overall development was targeted for steel and steel-hybrid modules of the corner-supported or selective bearing type, and on satisfying certain structural, constructional, and manufacturing performance requirements. The vertical connector makes use of a standard mechanism to enable fast, simple, safe, and remote operability, whereas the proposed horizontal connector uses a simple transfer plate. Details regarding the design of the connector and the experimental verification of its structural and functional performances are presented in this paper. Structural performance tests were conducted to verify key axial and shear characteristics, whereas functional performance tests were performed to assess the overall functionality after connector fabrication, test specimen fabrication, specific stages of loading, and during remote operation. The results highlight that the proposed connection satisfactorily meets its key expectations and can provide quantum improvements to the current modular construction industry for realizing fully modular buildings.

The extremum probability density evolution method (EPDEM) was introduced for the life-cycle risk analysis of the roofs of low-rise buildings subjected to typhoons. This method can use small sample sizes to account for the uncertainties of winds during the entire track of a typhoon and the changes in resistance of structures over time. This study generated typhoons in Shanghai, China using the Yan Meng wind field model. The EPDEM-based wind load analysis combined with a wind tunnel test was used to generate the probability density curve of typhoon wind loads. To improve the computational accuracy, a modified $k$-means clustering algorithm was introduced in representative point selection. This approach was compared with the Monte Carlo method to show its effectiveness and accuracy for practical applications. Furthermore, risk analysis based on the EPDEM, considering changes in roof resistance within the lifetime of the building, was carried out. The results show that the risk is time-variant and is influenced greatly by the resistance of a structure, and the reliability of the structure declines rapidly after 50 years. A methodology is proposed to predict the vulnerability curves for a new structure at different service times.

This paper aims to investigate the inclusion efficiency of straight microsteel fibers to improve the structural ductility and deformability of reinforced concrete frames to achieve progressive collapse resistance. Four RC frames’ behavior has been tested at one-third scale in the removal scenario of the middle column. To compare, two frames without steel fiber were prepared to be the control samples. Frames of 0.5% steel fiber fraction volume were also prepared. The study aims to analyze mechanical properties, crack pattern, failure mode, structural ductility, and load-displacement behavior of the frames of normal concrete (NC), together with steel fiber–reinforced concrete (SFC). Based on the findings, an enhancement was observed in the structural ductility (the increase ratio reached up to 26%), stimulated by incorporating steel fiber in the normal concrete. The frames of SFC demonstrated more deflection by 19% compared with NC frames. Steel fiber, therefore, can be utilized to enhance RC elements’ ductility as an innovative design strategy in inhibiting progressive collapse.

During the vulcanization process of steel-laminated elastomeric bridge bearings, unavoidable flow of elastomer occurs inside the mold under high pressure and nonuniform heating. This may cause the interior steel laminates to misalign or distort in the finished bearing. There has been limited research to understand and quantify the effects of manufacturing imperfections, such as vertically shifted, rotated, and bent steel laminates, on the mechanical behavior of elastomeric bearings. This paper presents a finite element investigation on the effects of manufacturing imperfections on the performance of elastomeric bridge bearings subjected to service-level loads. It is observed that such imperfections have a negligible effect on the maximum shear strain induced in the elastomer. The strain hardening behavior of the elastomer under relatively large shear strains limits the maximum shear strains to values lower than those predicted by the pressure solution. In one cycle of loading, the vertical deflection of bearings is affected slightly by these imperfections. However, manufacturing imperfections can increase the tensile stresses in the steel laminates causing plastic damage. The spread of plastic damage in the steel laminates with repeated cycling may affect the long-term performance of the bearings.

A shake-table test of a 2-story full-scale low-damage concrete wall building was conducted on the multifunctional shake-table array at Tongji University to validate the system-level dynamic response of this type of building. The test building implemented state-of-art low-damage structural systems, including post-tensioned (PT) precast concrete walls, precast concrete frames with slotted beam connections, and a variety of energy-dissipating devices. The building was subjected to 39 separate earthquake tests, consisting of a range of structural design configurations and ground motions including different intensity records, far-field and near-fault records, and short- and long-duration records. The test data and documentation recorded throughout the project have provided a significant high-quality data set for improving understanding both global and local response of post-tensioning concrete wall buildings. The data set has been published and is publicly available. The methodology of the test documentation and data collection for each stage of the project are described, and a road map for navigating the archived data set is provided to support its future use.

Transportation networks are critical components of lifeline systems. They can experience disruptions due to seismic hazards that could lead to severe emergency response and recovery problems. Finding an efficient and effective method to evaluate the seismic reliability of bridge networks is crucial for risk managers. This study proposes a method that can compute the seismic reliability of bridge networks using machine learning techniques. The proposed method is computationally less expensive than existing methods and can be implemented easily in emergency risk management systems. Moreover, it includes information on ranking bridges and prioritizing retrofit plans.