Journal of Structural Engineering

During an earthquake, buildings are simultaneously excited by multiple (horizontal and vertical) ground motion components. In the design process, the combinatorial effects of multicomponent horizontal loading are commonly determined using simplified methods such as the percentage (e.g., 100%–30%, 100%–40%) and square root sum of squares (SRSS) rules. In this paper, the state of the art in assessing the effectiveness of combination rules for orthogonal seismic effects is first reviewed. Prior studies are categorized based on the adopted analysis method (e.g., response spectrum versus response history), type of structural system, presence of structural irregularities, and considered demand parameters. A recent attempt to evaluate the combination studies using strong motion data is also discussed. Subsequently, the adoption of combination rules in popular codes and standards is presented and discussed including the conditions that trigger their implementation. Based on the review of prior research on the topic, it is concluded that the structural engineering community has not reached a consensus on the relative effectiveness of the various orthogonal combination methods. The paper concludes with a comprehensive discussion that covers specific challenges, knowledge gaps, and suggestions for future research.

Structural insulated panels, commonly known as SIPs, constitute a panelized building system composed of external facer panels, such as oriented strandboard (OSB) sheets, bonded to a lightweight foam core. As the demand for SIPs increases as an alternative to light-frame construction in residential and light-commercial buildings, so too does the need for proper design requirements to satisfy regulatory agencies and building officials. This paper describes a combined experimental and analytical study whose objective was to investigate the structural behavior of OSB-faced SIPs subject to short-term axial loading. A total of 53 panels with varying types of foam core, thickness, and other construction details were subjected to concentric and eccentric loading. The test results indicated that the strength of SIPs was primarily influenced by the panel slenderness and the type of foam core. Reliability-based design expressions were developed for the ultimate limit state of SIPs subjected to short-duration concentric and eccentric axial loading. The results were also compared to current allowable stress design practices. In addition to presenting important test data for researchers, this paper presents a number of practical design recommendations to improve the performance of SIPs.

The deterioration of structural systems is unavoidable after their long time in service. To ensure the safety of these systems, an effective inspection strategy with a minimal cost is essential. This paper presents an analytical approach for determining the optimal inspection plan using a cost-based criterion in which structural damage and failure are modeled as stochastic processes. This allows the consideration of the correlation of damages at different times in the reliability-based inspection plan. An example is provided to illustrate the application of the proposed methodology, taking the number of inspection actions and the critical damage level at which repair is performed as the optimization variables. The proposed procedure provides a fast solution for the optimization of an inspection plan with superior performance over those based on simulation. In addition, correlation has a big impact on the result of an optimal inspection plan matching that of the cost of failure.

This paper describes the design, construction, and experimental results of a series of full-scale 2-story mass-timber building shake table tests conducted at the Natural Hazards Engineering Research Infrastructure (NHERI) at the University of California, San Diego large outdoor shake table facility. The building specimen utilized a lateral force-resisting system consisting of two post-tensioned rocking walls made of cross-laminated timber (CLT) panels. The structural system was designed to be resilient with the ability to undergo repetitive testing under strong ground motions without significant damage. The test building had an open floor plan suitable for mixed commercial and residential applications. The CLT floor and roof diaphragm had large cantilevered portions that represented realistic aspect ratios. The building was subjected to a series of 14 earthquake ground motions and pushed to a maximum roof drift of 5%. After completion of the dynamic tests, which included several ground motions at the maximum considered earthquake hazard level, the building was able to recenter with no unintended structural damage, highlighting the resilience of the mass-timber rocking-wall structural system.

Field measurements of wind speeds, wind directions and acceleration responses on top of the Shanghai World Financial Center building (SWFC), which is 492 m high, were obtained from 2011 to 2016 when the adjacent Shanghai Tower, which is 632 m high, was under construction. This paper utilizes these data to study the amplitude dependencies of the dynamic properties of the SWFC and the effect of the adjacent and newly built Shanghai Tower on the dynamic properties of the SWFC. The findings demonstrate that the natural frequencies of the two fundamental modes of the SWFC decreased with the vibration amplitude and construction phase of the Shanghai Tower. The opposite finding was observed for the damping ratios.

Severe tropical cyclones or typhoons (hurricanes) cause significant damage in the coastal regions of China, Japan, and the United States. Estimating the wind speed accurately and efficiently is important for hazard mitigation and risk management. This study developed a refined empirical model of a typhoon wind field. The gradient wind field is obtained based on the gradient equilibrium equation and the variable pressure field. Then the relation between the gradient speed and surface speed is established using Arya’s boundary layer model, which is integrated in the planetary boundary layer (PBL) wind field model. To facilitate engineering practice, a set of closed-form formulas was developed to simplify the original complex relation between the gradient speed and surface speed. Instead of solving the dynamic equation numerically as does the PBL model, the surface wind speed is derived directly from the gradient wind speed using the proposed model, which makes wind speed prediction much more efficient. Additionally, the proposed model is more accurate than previous empirical models, which use the simple reduction factor. The comparison of the measured wind speed and simulated counterpart validated the refined empirical model. Based on the proposed model, the typhoon hazard analysis for nine cities in China was performed. The circular subregion method was adopted to estimate the extreme wind speed for return periods of 50 and 100 years. Results show that the refined model has good performance in terms of the accuracy and efficiency.

To achieve light weight, high load-carrying capacity, and corrosion resistance, a novel hybrid FRP-aluminum space truss system is proposed in detail. An advanced pretightened teeth connection (PTTC) and its combined joint–aluminum bolt–ball connecting system (ABCS) were designed to interconnect pultruded FRP elements. In view of the significant effect of the ABCS on structural deformation, the axial stiffness properties of the ABCS were first investigated through experimental tests and numerical simulations. Then, in establishing a structural calculation model for the hybrid space truss, a new combined-line-elements solution was proposed to simulate the complex axial stiffness variation in the ABCS, to which the traditional Timoshenko beam element cannot be applied. Beam elements with different cross sections were adopted to simulate the FRP truss member and the PTTC. The structural calculation model was verified with a previously published case. The design of a hangar structure using this hybrid space truss system is discussed here. The preliminary design was made using the traditional simple-link-system model, and a comparative calculation was made with the new combined-line-elements model. The results indicated that the structural design of this unique hybrid system under a flexural moment is stiffness-driven rather than strength-driven owing to the low elastic modulus of the materials. The conventional simple-link-system model ignores actual differences in component stiffness and thus is dangerous for structural design. Based on the discussion, a design strategy for the hybrid space truss system is proposed.

One of the most commonly used types of isolator is the steel-reinforced elastomeric isolator (SREI), but recent research has been completed on the use of fibers as a replacement to the steel reinforcement layers, reducing weight and potentially costs associated with isolation systems. These fiber-reinforced elastomeric isolators (FREI) can be used in bridges in an unbonded application, where they are placed between the bridge deck and bent cap beams. The use of isolators with bridges requires that the isolators are able to withstand the same negative temperatures that bridges located in cold climates will experience. This study investigates the behavior of unbonded fiber-reinforced elastomeric isolators (U-FREI) subjected to low temperature conditioning, representative of conditions that can be experienced in various regions throughout Canada. The response under lateral deformations expected during seismic events and vertical and rotational loading expected during regular operation were evaluated. The experimental results, in terms of effective stiffness and energy dissipated, indicate that the influence of temperature on the lateral, vertical, and rotational response of U-FREI is acceptable.

A cross-laminated timber (CLT) wall is a composite structural assembly with high strength and stiffness properties. In this study, the beneficial properties of this wall were used in designing a new CLT-reinforced concrete (RC) hybrid system building. The hybrid system consisted of an RC moment-resisting frame with CLT infill. The energy dissipation capacity of this system was further enhanced by using steel slit dampers as connectors between the CLT and RC beam. To facilitate the adoption of new technology in earthquake engineering applications, direct displacement-based design was used. The utility of the proposed system was illustrated on a six-story CLT-RC frame dual system. Performance of the proposed design method was shown by conducting nonlinear time history analyses with consideration of seismicity of Vancouver, Canada. The results indicated that the proposed CLT-RC hybrid system is capable of withstanding lateral forces due to intense seismic excitations.

The implementation of fatigue-damage models into the governing equations of the disturbed stress field model algorithm for fatigue analysis of reinforced concrete structures is presented in this paper. The models account for concrete deterioration, localized reinforcement crack growth, and accumulation of irreversible compressive strain in conventional reinforced concrete and steel-fiber reinforced concrete due to fatigue loading. As such, analyses involving fatigue damage can be expressed in terms of the deformation evolution and residual capacity. These concepts overcome the well-known limitations of stress-life models for fatigue analysis of reinforced concrete structures. The implementation concepts using robust models from the literature are described. As a means of further illustration, the solutions to the deformation of a shear element under pure shear fatigue loading are presented. The validation of the modified algorithm using finite-element analysis with experimental results for fatigue life and residual strength prediction gave good correlation.

The increase in the number of deliberate and accidental explosion events over the past two decades has necessitated evaluating the performance of blast-vulnerable structural components and developing subsequent risk mitigation strategies. In this context, several studies have focused on the out-of-plane behavior of either unreinforced masonry walls or non-load bearing reinforced masonry shear walls (RMSWs). However, to date, few studies have focused on the interaction between the axial load and the out-of-plane (e.g., when shear walls are subjected to blast loads) demands on such walls. As such, the current study focuses on evaluating the out-of-plane behavior of seismically-detailed RMSWs with different design parameters. In this respect, the experimental results of seven scaled RMSWs, with different in-plane ductility seismic classifications and axial stress levels, subjected to out-of-plane loading are first presented. Such results include the wall damage sequence, load-displacement response, stiffness degradation, and energy dissipation. Subsequently, the resistance function predictions based on the Unified Facilities Criteria guidelines are compared with the experimental results. Furthermore, an experimentally validated analytical resistance function is developed considering the postpeak behavior of RMSWs, including second-order effects. The current study extends the database of experimental and analytical results pertaining to load-bearing RMSWs, to facilitate the development of relevant provisions within the next generation of blast design standards.

The use of concrete-filled hollow steel columns is increasing in construction. These columns with a high load-bearing capacity allow the use of smaller cross sections. However, the use of slender elements in buildings makes necessary new construction solutions that ensure the required performance in case of fire. This paper presents the results of experimental research on the performance of thermal restrained concrete-filled double-skin and double-tube hollow steel columns subjected to fire. The influence of several parameters on the behavior of these columns in the case of fire was tested, such as the type of concrete infill (normal, high-strength, and light-weight concrete), loading level, and axial and rotational restraint level. This type of column has two concentric tubes in which the outer tube has a diameter twice that of the inner tube. The double-tube columns were completely filled with concrete; the double-skin columns had concrete only between the tubes, and the inner tube was empty. The results of this research show that the loading level had a large effect on the reduction of the fire resistance (critical time) of the tested columns. However, the type of concrete and the thermal restraint level had a minor effect. The double-tube columns did not have much better fire performance compared with the double-skin tubes.

The experimental results of a research program on the response of $1/3$-scale reinforced concrete infilled frames are presented and evaluated. The main objective was to assess the seismic performance of RC frames infilled with a cast-in-place RC wall designed according to the Greek code for retrofitting existing buildings. Seven infilled RC frames with a height-to-length wall aspect ratio $h_w/{\mathcal{l}}_w=0.58$ were subjected to cyclic horizontal excitation. The parameters of the experimental study included the infill-to-frame method of connection along the horizontal and/or vertical interfaces and the anchorage length of the shear connectors placed along the interfaces. The columns of two of the infilled frame specimens were strengthened by a closed RC jacket. The presence of the RC wall resulted in a significant increase of the ultimate strength, in-plane stiffness, energy dissipation, and, to a certain extent, higher ductility. Specimens with jacketed columns appeared to have the best response.

Reinforced concrete columns confined by steel tubes, also known as steel tube–confined reinforced concrete (STCRC) columns, are a kind of composite column in which the outer steel tube acts predominantly as hoop reinforcement. This is achieved by the provision of breaks to the longitudinal continuity of the steel tube. The compressive behavior and seismic performance of STCRC columns have been extensively studied in the last few decades. However, knowledge of the fire behavior of STCRC columns is very limited. Hence, experimental and numerical studies to investigate the response of STCRC columns under combined thermal (fire) and structural loading are presented herein. Four full-scale STCRC columns and one concrete-filled steel tubular (CFST) column were first axially loaded and then subjected to fire until failure. The measured furnace temperatures, specimen temperatures, axial displacement versus time curves, and fire resistance of the columns are presented and discussed. A nonlinear finite-element model employing a sequentially coupled thermal-stress analysis was then developed and validated against recent fire tests on STCRC and CFST columns reported in the literature. Following extensive parametric studies, a simplified method is proposed for predicting the temperatures of the steel tube, reinforcing bars, and concrete. Design rules are then proposed for predicting the load-bearing capacity of STCRC columns exposed to fire, which are consistent with the design method for STCRC columns at ambient temperature.

The results of the first experimental study on the behavior of axial cyclically loaded rubberized concrete under active confinement are presented. Four different mixes of concrete with rubber replacement ratios of 0%–18% were considered to investigate the influence of the rubber content on the cyclic compressive behavior of concrete under different levels of lateral confining pressure ranging from 5 to 25 MPa. The results show that trends of the axial stress-axial strain and lateral strain-axial strain relationships of rubberized concrete under cyclic loading closely follow those of the corresponding monotonically loaded concrete. At a given confining pressure and rubber content, concrete exhibits a higher ductility under cyclic loading than under monotonic loading. Furthermore, cyclically loaded rubberized concrete specimens exhibit higher peak and residual stress ratios, but slightly lower peak and residual strain ratios compared with those of their monotonically loaded counterparts. Results also show that the unloading and reloading tangent modulus of rubberized concrete increase but axial plastic strain of rubberized concrete decreases with an increase in the confining pressure, following a trend similar to that of natural aggregate concrete. Owing to the lower unloading tangent modulus of the concrete with a higher rubber content, for a given confining pressure, the axial plastic strain of cyclically loaded rubberized concrete slightly decreases with increasing rubber replacement ratio. Based on the test results, simple model expressions were developed to predict the axial stress-strain relationship of actively confined rubberized concrete under cyclic axial compression. Promising findings of this study suggest that confined rubberized concrete can be used in column applications to reduce the environmental impact of waste tires and conserve nonrenewable natural resources.

Existing reinforced concrete buildings constructed in low-seismic zones are most vulnerable to disproportionate collapse due to low reserve strength and presence of reinforcement discontinuity. This research aims to experimentally observe the collapse behavior of such ordinary structures, particularly the beneficial development of compressive arch and catenary actions, and evaluate the effects of various reinforcement detailing on building collapse resistance. Three $1/3$-scale beam-slab-column assemblies representing parts of a prototype building structure were monotonically loaded until complete failure through a 12-point loading system simulating uniformly distributed loads. The test units with beam and slab longitudinal reinforcement discontinuity exhibited relatively ductile collapse modes with a beam chord rotation angle of 12%. Catenary action developed well in beam and slab top reinforcement, whereas the mobilization of compressive arch action was insignificant. Providing longitudinal reinforcement continuity could increase both load and deformation capacities of test structures up to 160% and 140%, respectively. The yield-line approach can conservatively predict the collapse load of beam-slab structures and can be used as a simple but reliable check for the potential collapse.

When subject to earthquakes, some objects and structures, such as statues, obelisks, storage systems, and transformers, show a dynamic behavior that can be modeled considering the object/structure as a rigid block. To reduce the likelihood of the failure of these kinds of elements, they can be paired with safety devices that are designed to avert the overturning of the blocks. Although safety devices have proven to be effective, their effectiveness changes substantially as a function of the seismic input (which is generally uncertain) and the parameters that characterize the system. Furthermore, there is a need to quantify probabilistically the effectiveness of safety devices. This paper proposes probabilistic models to evaluate the failure probability of block-like elements coupled with a safety device. The paper considers two candidate safety devices: an isolating base and a pendulum mass damper. The proposed models are then used to compare the seismic responses of coupled block-device systems with one of stand-alone rigid block-like elements. To account for the relevant uncertainties, expressions for the probability of failure are developed using a logistic regression model calibrated with a Bayesian approach. The expressions of the probability of failure are then used to construct fragility curves that give estimates of the conditional probability of overturning occurrence as a function of some characteristics of the blocks (i.e., the slenderness of the rigid body) and the safety devices (i.e., the characteristic period) for a given seismic excitation (i.e., the peak ground acceleration). The data needed to develop the probabilistic model are obtained integrating the nonlinear equations of motion of the two systems subject to selected ground motions. The proposed models and fragility estimates can be used to quantify the likelihood of failure of rigid block-like elements due to seismic excitations as well as the effectiveness of two common safety devices. It is found that, for the adopted ratio between the mass of the block-like element and the mass of the safety devices, base isolation works better than a pendulum mass damper.

There is a lack of rapidly constructible, modular, and lightweight structural components and systems used for building construction. Such structures will in the future be able to sustainably and cost effectively meet new, changing demands for structures such as changing occupancies and extreme events. In an effort toward making structures more efficient, a lightweight, rapidly constructible and reconfigurable, modular steel floor (RCRMSF) system has been developed using cold-formed steel components. Current design guidelines for vibrations are written for conventional structural systems, but the suitability of the lightweight RCRMSF to resist vibrations due to human activity is unclear. To assess the dynamic behavior a design assessment has been adopted and high-fidelity finite-element models have been created. A parametric study was conducted to investigate the effect of important design parameters on the vibration response and serviceability of the RCRMSF for walking and rhythmic loading. The parametric study found that many RCRMSF configurations could be classified as high-frequency floors and that the RCRMSF can meet serviceability limits with adequate design parameters.

A seismic risk assessment of a 42-story reinforced concrete dual-system building considering mainshock and aftershock hazard is presented. Sequential nonlinear response history analyses are performed using as-recorded mainshock-aftershock sequences. Aftershock assessment describes the case where the mainshock has occurred, and the associated damaged state of the building is known. A Markov process model is used to integrate the increase in vulnerability of the mainshock-damaged building with the time-dependent aftershock hazard. Aftershock risk is quantified as the probability of exceeding the structural response demand limits used in performance-based seismic design of tall buildings at different instants in time following the mainshock. The same metric is used to quantify mainshock-aftershock risk; however, for this type of assessment, the uncertainty in the intensity and damage caused by both the mainshock and aftershock is considered. The results of mainshock-only, aftershock, and mainshock-aftershock assessment show that the coupling beam rotation demand limit generally has the highest probability of exceedance compared with the peak transient and residual story drift ratios, frame beam rotations, and compressive and tensile strain in boundary element concrete and steel, respectively. The implied 50-year exceedance probabilities of the demand limits currently used for the collapse prevention performance level were found to vary significantly across the various response parameters.

Increasingly, seismic isolation (SI) has been adopted in buildings and highway bridges to mitigate damage from earthquakes. However, its appropriateness in high-speed rail (HSR) bridges remains a topic of major concern for engineers and stakeholders. The effects of SI on the response of HSR bridge systems during future earthquakes need to be evaluated probabilistically accounting explicitly for all uncertainties associated with the seismic loading. To this end, this paper compares the probabilistic seismic responses of a California high-speed rail (CHSR) prototype bridge with and without seismic isolation. The uncertainties in the seismic responses for the isolated bridge and nonisolated bridge are quantified and compared to explore the effects of SI when accounting for the uncertainty of the seismic input. The probabilistic seismic response is evaluated using two alternative approaches, and the less restrictive—but computationally more expensive—approach is selected to quantify the beneficial and detrimental effects of SI for the CHSR prototype bridge. The pros and cons of SI identified in probabilistic terms require a trade-off design in applying SI to HSR bridges.

The use of mobile ground and aerial robotics presents a powerful means to augment current visual inspection practice by overcoming some common weaknesses related to accessibility, repeatability, hidden defect detection, and quantification. In this paper, a novel ground robotic bridge inspection platform consisting of a rugged mobile platform equipped with an onboard computer and several calibrated and time-synchronized sensors is introduced. This platform, along with custom localization and mapping software, is shown to produce high-quality 3D point cloud maps for the underside of typical concrete bridges, which are often the most challenging to inspect. The quality of these maps is compared to ground truth using terrestrial laser scanner measurements and the maps are shown to have an overall scale error of just 1.3%. The maps from the proposed system can be generated in real time, while continuously scanning the bridge, resulting in a significant reduction in the inspection time compared to using a terrestrial laser scanner (TLS). Additionally, this work presents a novel procedure for fully automated point cloud colorization and semiautomated defect localization and quantification. The former allows for realistic visual renderings of the bridge for remote inspection by offsite inspectors. The latter allows for increased accuracy of defect quantification compared to traditional inspections, while eliminating subjectivity between inspections and inspectors.

Novel mechanics-based closed-form solutions for long- and short-term serviceability deflections and crack widths of fiber-reinforced concrete (FRC) and ultra-high-performance fiber-reinforced concrete (UHPFRC) beams are presented. These solutions incorporate the bond properties from bond tests directly and as such obviate the need for constant bond stress simplification and consequently the need for member calibration as is commonly required in code approaches. The closed-form solutions are validated on 12 simply supported and 4 continuous UHPFRC beams as well as 10 normal-strength FRC beams with good correlation. A design example is also included for a UHPFRC T-beam demonstrating application of the solutions.

A comprehensive experimental investigation into the axial compressive response of concrete-filled double skin tubular (CFDST) sections with stainless steel square and rectangular outer tubes is presented. A total of 28 tests was carried out. The experimental setup and procedures are described, and the test observations are fully reported. The test results are employed to assess the applicability of the current European and North American design provisions for composite carbon steel members to the design of the studied CFDST cross sections. Modifications to the current design codes are also considered—a higher buckling coefficient $k$ of 10.67 to consider the beneficial restraining effect of the concrete on the local buckling of the stainless steel outer tubes and a reduction factor $\eta$ to account for the effective compressive strength of high-strength concrete. Overall, the comparisons revealed that the existing design rules may generally be safely applied to the prediction of the compressive resistance of CFDST cross sections with stainless steel outer tubes, although the modified design rules offered greater accuracy and consistency.

A simplified analysis method is proposed to predict the response of segmental elastic spine–braced frames (SESBFs) and select the appropriate truss-segment configuration for a given frame. The method relies on a simplified structure model that can reproduce both the elastic flexural response and inelastic shear response of the braced-frame system. The proposed simplified model is described, and a flowchart is presented to illustrate the steps leading to the frame properties required to achieve the optimum seismic drift response for a given truss-segment configuration. In the design, the process is repeated for different potential truss-segment configurations, and their seismic responses are compared to select a suitable configuration for the structure. The application of the proposed procedure is illustrated for a 24-story building structure located in Vancouver, British Columbia, Canada. Five different truss-segment arrangements were investigated, and two configurations were identified as appropriate for the structure. Final design of the four most promising candidates was performed to confirm the findings from the preliminary design, and the comparison confirmed that the proposed method and simplified analysis model are suitable tools for the preliminary design of SESBFs.

Engineers inspecting damaged buildings after an earthquake are tasked with determining whether the lateral stiffness and strength of the building has been compromised, including whether the displacement associated with peak shear force of the building has been reached or exceeded. For buildings constructed with low-aspect-ratio reinforced concrete shear walls, a structural repair may not be necessary if the earthquake-induced displacement of the wall is less than the displacement associated with peak shear force. However, there are no quantitative data in the archival literature or technical reports that enable an engineer to determine whether the displacement associated with peak shear force has been reached based on postearthquake observations of damage, including crack widths and lengths, and spalled concrete. This paper documents damage to low-aspect-ratio shear walls at peak transient displacements and subsequent displacements at zero lateral loading and enables decisions to be made regarding whether peak shear force has been attained. Ratios of crack width at peak transient displacement and subsequent displacement at zero lateral loading (the postearthquake condition) are provided to assist with the postearthquake evaluation of damaged buildings.

The present paper summarizes recent research on timber–concrete composite (TCC) systems exposed to fire conditions. Experimental work on these systems in fire conditions is not as extensive as in normal temperature conditions. Therefore, only a few types of TCC systems are covered herein. The secondary objective of this paper is to present the principles of existing simplified procedures for TCC systems in fire. These procedures were developed and/or validated with regard to available fire test data, and to discuss the possibilities for the development of new enhanced, simplified calculation methods. A summary is then presented of design recommendations based on previously discussed experiments. A short discussion follows on their further development, and the possibility of the recommendations being included in future versions of European standards. Design criteria for screwed connections are presented, while for notched and glued-in rod connections, further research is required.