Journal of Structural Engineering

Structural health monitoring of civil infrastructures is a difficult task, often impeded by the geometrical size of the monitored systems. Recent advances in conducting polymers enabled the fabrication of flexible sensors capable of covering large areas, a possible solution to the monitoring challenge of mesoscale systems. The authors have previously proposed a novel sensor consisting of a soft elastomeric capacitor (SEC) acting as a strain gauge. Arranged in a network configuration, the SECs have the potential to cover very large surfaces. In this paper, understanding of the proposed sensor is furthered by evaluating its performance at vibration-based monitoring of large-scale structures. The dynamic behavior of the SEC is characterized by subjecting the sensor to a frequency sweep, and detecting vibration modes of a full-scale steel beam. Results show that the sensor can be used to detect fundamental modes and dynamic input. Also, a network of SECs is used for output-only modal identification of a full-scale concrete beam, and results are benchmarked against off-the-shelf accelerometers. The SEC network performs well at estimating both natural frequencies and mode shapes. The resolution of the sensor is currently limited by the available electronics to measure small changes in capacitance, which reduces its accuracy with increasing frequencies in both the time and frequency domain.

This paper investigates the impact of seismic load history on longitudinal bar buckling in reinforced concrete (RC) bridge columns. Previous research has shown that reinforcing bars are prone to buckling upon reversal from tensile strain. To quantify this effect, a hybrid analysis method using both fiber and solid elements is developed and implemented to assess the impact of seismic load history on reinforcing bar buckling. Forty earthquake ground motions are utilized to conduct nonlinear time history analysis of bridge columns using a fiber-based model. The longitudinal bar strain history from the fiber-based model is then utilized as the input to the finite element model. A parametric study is conducted for the purpose of developing design equations that provide strain limits prior to the onset of bar buckling. Simple design approaches are proposed based on the design equations.

Flexural yielding in plastic hinge regions of reinforced concrete (RC) beams results in a complex strain state and axial elongation of the member. This elongation has a significant effect on the seismic response of the structure. The shear strength and plastic deformation capacity of the hinge region are affected. This paper presents a strain based model to predict the axial elongation of RC hinge regions subjected to reversed cyclic loading, with the help of the tension chord model and the cracked membrane model. Based on these models, the member’s flexural and shear deflection components are considered separately. Further, the model predicts the distribution of average axial strains along the axis of RC beams and represents a new tool for evaluating the general behavior of plastic hinges. The proposed method is applied to test results taken from the literature. A comparison of measured elongations to predictions proves the applicability of the model because reasonable accuracy is achieved.

In this study, a three-dimensional finite-element analysis (FEA) was conducted to study the parameters that affect the maximum flexural fatigue capacity of RC beams strengthened with fiber-reinforced polymer (FRP) sheets. Forty-seven specimens were designed and analyzed by using FEA. Additionally, a fatigue capacity prediction model was developed to reflect the influences of the major parameters, including the fatigue behavior of steel reinforcement, FRP sheets, and FRP-to-concrete bonding; and the influences of minor parameters, such as the yield strength of steel reinforcement, concrete strength, width and thickness of the FRP sheet, and other parameters. The results of experiments on 181 beams reported in the literature were analyzed to verify the accuracy of the proposed model. The mean values of 1.05 and 1.02 and the corresponding coefficients of variation of 17.12 and 16.06% were determined by comparing the calculation results from the proposed model with the experimental data. These results reflect the superior accuracy of the proposed model in predicting the fatigue capacity of RC beams with and without FRP strengthening.

Concrete-filled steel tubular (CFST) arches are often used in engineering structures, particularly in bridge construction. In some instances, to expedite their construction and transport, curvilinear steel tube segments are fabricated and delivered to the construction site and then joined together at the crown to create a pin, so the arch becomes a crown-pinned arch. Wet concrete is then pumped into the steel tubes to form the concrete core after curing, which experiences time-dependent shrinkage and creep. This paper investigates the effects of geometric nonlinearity on the long-term in-plane behavior of crown-pinned circular CFST arches under a sustained central concentrated load, and derives analytical solutions for their nonlinear response and buckling loads. It is found that the geometric nonlinearity influences the long-term behavior of crown-pinned CFST arches significantly. The long-term deformations predicted by the nonlinear analysis are larger than those predicted by linear analysis, and they may be so large that the reserve of the serviceability limit state of crown-pinned CFST arches may be reduced significantly. The nonlinear analysis also predicts significant long-term increases of the axial forces and bending moments in crown-pinned CFST arches, which is quite different from linear analysis that predicts no long-term changes of the axial force and bending moment for thee-pinned CFST arches and only small long-term changes for singly pinned CFST arches. It is also found that the nonlinear long-term in-plane buckling loads of crown-pinned CFST arches are smaller than their counterpart values determined from linear analysis. Hence, the geometric nonlinearity together with the shrinkage and creep of the concrete core may reduce the reserve of the stability limit state of crown-pinned CFST arches in the long term. To determine the long-term structural response and buckling of crown-pinned CFST arches correctly, nonlinear analysis is required.

Extensive research has shown that properly detailed and closely spaced transverse reinforcement in reinforced concrete columns can ensure ductile behavior during earthquakes. However, in regions of high seismicity, detailing requirements can result in heavily congested sections; the use of self-consolidating concrete (SCC) can facilitate construction in these situations. Although extensive research exists on the axial load behavior of traditional concrete columns, only limited research exists on the behavior of columns constructed with SCC. Research over the past two decades has also shown that use of steel fiber-reinforced concrete (SFRC) can improve the strength and ductility of columns by delaying cover spalling and improving core confinement. Recent research has also shown that the combined use of SCC and steel fibers can ease problems associated with the workability of traditional fiber-reinforced concrete. This paper presents the results from an experimental program that was conducted to study the axial behavior of reinforced concrete columns constructed with SCC and SFRC. Full-scale columns having rectangular cross sections were tested under pure axial compressive loading. The columns were detailed with varying amounts of transverse reinforcement in accordance with the requirements of the Canadian Standards Association. The results confirm that increasing confinement with closely spaced transverse reinforcement in rectangular SCC columns results in improvements in behavior and ductility. In addition, the results demonstrate that the use of SFRC in rectangular columns results in improvements in performance and ductility.

Experimental studies were conducted to examine the behavior of concrete-filled double-skin tube (CFDST) columns exposed to fire after being subjected to simulated seismic loads. The experiments were conducted in two separate phases, consisting of (1) the quasi-static cyclic tests, followed by (2) fire tests. Three nominally identical column specimens were constructed for these studies. One of the specimens was directly tested under fire to quantify its resistance in an undamaged condition. The other two specimens were first subjected to quasi-static cyclic lateral loads, imposing varying degrees of lateral drift to simulate two different seismic events with moderate and high damage levels before being exposed to fire. Both of the specimens were pushed to the maximum drift of 6–6.5% with different residual drifts of 1.4 and 3.9% for moderate and high damage levels, respectively. The undamaged and damaged columns were then subjected to the same fire tests in accordance with a standard temperature-time curve while sustaining an axial load until the column failed due to global buckling. Local buckling of the tubes was also observed in the specimens due to the thermal expansion and separation from the concrete. Overall the results showed marginal differences in the fire resistance of the three specimens, providing evidence for the resilient performance of these columns under post-earthquake fire scenarios. An additional quasi-static cyclic loading test was conducted on the specimen that had been exposed to fire without any prior damage to investigate the behavior of the column subjected to seismic loads after the fire test. Differences in behavior were modest, except for a 5.7% drop in strength attributed to permanent degradation in material properties due to the fire test.

The self-centering energy-dissipative (SCED) brace is an innovative cross brace for buildings that provides a nonlinear response with good energy dissipation and postyield stiffness while minimizing residual drift after an earthquake. This provides a high level of seismic performance by allowing structures to remain operational even after major seismic events. Recently, the SCED brace has been improved through the design and experimental evaluation of a high-capacity SCED (HC-SCED) that has an axial capacity similar to some of the largest available conventional cross braces for buildings. This prototype HC-SCED satisfied testing protocols for buckling-restrained braces and exhibited full self-centering behavior during cycles up to 1.5% drift. To characterize the hysteretic response of the brace in detail, a new analytical approach is developed. This new approach is necessary because simplified stiffness estimates do not provide good predictions of the low-amplitude displacement response and initial effective stiffness that was measured in the full-scale experiments. The proposed analytical approach includes the effects of fabrication tolerances, which have been identified as the main reason for incorrect low-amplitude displacement predictions that result from the simplified stiffness estimates. Using the results from the HC-SCED tests, the new analytical approach provided good estimates of the initial stiffness of the braces and also was able to predict the behavior of the brace well under a larger fabrication tolerance scenario. These improved predictions may be used to improve the characterization of the effective hysteretic behavior of actual SCED braces for use in nonlinear time history analyses.

This paper describes an experimental program investigating the local–global interaction buckling of stainless steel I-section beams. Three alloys were included: austenitic S30401, ferritic S44330, and lean duplex S32101. Extensive coupon tests were conducted to determine the specific properties of stainless steel sheets, including nonlinearity, anisotropy, and strain hardening. The test program encompassed six laterally braced tests and 24 unbraced tests with spans ranging between 1.9 and 4.0 m. The unbraced specimens were tested in a specially designed setup featuring clearly defined loading and support conditions. Interaction buckling was successfully achieved in the tests. The geometric imperfections, bearing capacity, and sectional and global deformations of each specimen were carefully recorded before or during the tests. Key factors affecting the test conditions were identified. The research addressed the lack of experimental data for local–global interaction buckling of stainless steel beams with open cross section and establishes the foundation for further theoretical study, as described in a companion paper.

The paper presents a detailed finite-element model used to study the local–global interaction buckling of stainless steel I-section beams. The model was constructed with the commercial software package ABAQUS v.6.11 and was verified against the experimental data presented in a companion paper, yielding accurate predictions of interaction buckling behavior and ultimate capacity. Parametric studies were conducted by using the calibrated models to extend the experimental database. The accuracy of the Australia/New Zealand, American, and European standards for stainless steel structures was evaluated by using the available data. It was found that the codes were incapable of capturing the interaction buckling effect, thus affording overly optimistic strength predictions for beams of high section slenderness. Existing direct strength method formulas, as proposed for carbon steel beams and stainless steel columns, were also evaluated to assess their applicability to the stainless steel beams.

The elastic response of a class of nonlinear oscillators that represent structural systems with a relatively rigid superstructure on a soft ground story is evaluated under lateral base excitation. The governing equations of motion are developed, accounting for both lateral translation and overturning rocking of the rigid superstructure relative to the flexible ground story. In this work, the system parameters are identified initially for linear elastic response and, subsequently, in a companion publication, for the case of the base exhibiting nonlinear resistance–deformation characteristics. The dynamic coupling between lateral and rocking modes is evaluated under a base excitation for different system period and resistance parameters, and the deformation and force demands are compared with the single-degree-of-freedom lateral-response-only predictions, as typically considered in seismic design. It is shown that the presence of rocking of the rigid superstructure causes an increase in the transverse deformation demands compared with conventional spectral estimates using the single-degree-of-freedom approach.

Superelastic (SE) shape memory alloy (SMA) is an advanced material that may be used as an alternative to conventional reinforcing steel in civil engineering structures to control residual deformations. The most common SE SMA type is an alloy of nickel and titanium (NiTi), which has been used in medical instruments and aerospace industries. A state-of-the-art review was conducted and factors that affect stress-strain behavior of SE SMA were identified. Subsequently, a simple constitutive stress-strain model was adopted from the literature, and mechanical properties for SE SMA that are of interest to structural engineering were defined. A procedure was proposed to extract mechanical properties of reinforcing SE SMA from test data obtained according to a standard test, and the range for each property was determined. Comprehensive seismic analyses of a bridge column were carried out to determine the effect of each mechanical property on the moment-curvature and force-displacement relationships. Then, a design specification was proposed for reinforcing SMA bars. The proposed SMA model was used in a finite-element computer program to simulate the seismic behavior of a $1/3$-scale SMA-reinforced bridge column experiment. The simulated responses were in reasonable agreement with the test data.

An experimental investigation has been conducted to examine the seismic behavior of seven shear-critical, high-strength concrete columns and one normal-strength concrete column. The effects of compressive strength of concrete, shear span-to-depth ratio, axial-load ratio, and transverse reinforcement amount on shear capacity, ductility capacity, energy dissipation, and stiffness degradation were critically examined. Test results indicate that increase in shear span-to-depth ratio and confinement effectiveness, and reduction in axial load, resulted in increase of energy dissipation capacity and ductility. Compared with the test results, predictions of shear strength from three available models may not be directly applicable to the shear-critical, high-strength concrete columns, whereas the calculated values by the truss-arch model considering the condition of deformation compatibility are shown to be consistent with the test data.

High-frequency steel and concrete floors are often used to support sensitive equipment to minimize vibration response to walking. Equipment vibration tolerance limits are sometimes expressed as waveform peak acceleration, and are more often expressed as narrowband spectral acceleration, or one-third octave spectral velocity. Current methods predict the waveform peak response after a footstep. However, postprocessing beyond what is practical for typical design office usage is often required to predict responses directly comparable to spectral tolerance limits. Also, current methods are not calibrated to provide a specific level of conservatism. This paper presents new methods for predicting the waveform peak acceleration, narrowband spectral acceleration maximum magnitude, and one-third octave spectral velocity maximum magnitude. A total of 89 walking vibration tests were performed on five high-frequency floor bays. The measurements are used to assess the precision of the proposed methods and to calibrate the prediction methods to provide a specific probability that the actual response will exceed the predicted response. The measurements are compared to predictions by the proposed method and five established methods.

When an eccentrically braced frame configuration requires that the links be connected to the columns, standard seismic guidelines offer little guidance on the link-to-column connection design. Recent testing showed that several welded moment connection details that have been developed after the 1994 Northridge earthquake for special moment frames still cannot prevent brittle fracture in the link flange groove welds. In support of an experimental study on the development of reliable link-to-column connection details, the supplemental web doubler connection is proposed. The connection uses a pair of partial-height doubler plates that are parallel to and offset from the web in the link panel next to the column. Finite-element analyses were conducted to guide the development of design requirements, including the minimum reinforced length, minimum supplemental doubler thickness, and weld design requirement. A closed-form solution, with some minor adjustment based on the finite-element analysis results, for the calculation of the required shear force in the supplemental doublers is also proposed. A step-by-step design procedure is presented.

Link-to-column connections in steel eccentrically braced frames must be capable of sustaining large cyclic forces and inelastic rotations. United States code provisions require that satisfactory performance of link-to-column connections be verified by testing under a prescribed loading protocol. However, there are few link-to-column connection details that have satisfied these testing criteria. This paper describes a pilot investigation of two promising link-to-column connection details, as follows: (1) the fillet welded connection, which uses fillet welds or a combination of fillet and groove welds between the link and the column flange; and (2) the supplemental web doubler connection, which reinforces the connection region using a pair of doubler plates in the first link web panel next to the column, parallel to but offset from the link web. These two details showed excellent performance in a series of eight tests. This paper describes these link-to-column connection details, summarizes the results of cyclic loading tests, and discusses design issues and approaches for these connections.

A response spectrum analysis (RSA) procedure, which estimates the peak response directly from the earthquake design spectrum, is available for the preliminary phase of design and safety evaluation of concrete gravity dams. This analysis procedure includes the effects of dam-water foundation interaction, known to be important in the earthquake response of dams. This paper presents a comprehensive evaluation of the accuracy of this RSA procedure by comparing its results with those obtained from response history analysis (RHA) of the dam modeled as a finite-element system, including dam-water-foundation interaction. The earthquake response of an actual dam to an ensemble of 58 ground motions, selected and scaled to be consistent with a target spectrum determined from a probabilistic seismic hazard analysis for the dam site, was determined by the RHA procedure. The median of the peak responses of the dam to 58 ground motions provided the benchmark result. The peak response was also estimated by the RSA procedure directly from the median response spectrum. Comparison of the two sets of results demonstrated that the RSA procedure estimates stresses to a degree of accuracy that is satisfactory for the preliminary phase in the design of new dams and in the safety evaluation of existing dams. The level of accuracy achieved in the RSA procedure is noteworthy, especially considering the complicated effects of dam-water-foundation interaction and reservoir bottom absorption on the dynamics of the system, and the number of approximations necessary to develop the procedure.

It is often assumed that, at a given lateral strain, the axial compressive stress and strain of fiber reinforced polymer (FRP)–confined concrete are the same as those of the same concrete when it is actively confined under a confining pressure equal to that supplied by the FRP jacket. An experimental program was undertaken to assess the validity of this assumption, in which 63 actively confined and FRP-confined normal-strength (NSC) and high-strength concrete (HSC) specimens were tested under axial compression. The axial stress-strain and lateral strain-axial strain curves obtained from the two different confinement systems were assessed. The results indicate that, at a given axial strain, lateral strains of actively confined and FRP-confined concretes correspond when they are subjected to the same lateral confining pressure. At the points of intersection on the lateral strain-axial strain curves, FRP-confined NSC exhibits only slightly lower axial compressive stresses compared with those of actively confined NSC. Conversely, the difference between the axial stresses of actively confined and FRP-confined HSC is found to be significant, indicating that the compressive behavior of confined HSC is more sensitive to the application path of confining pressure than the behavior of confined NSC. By using the combined results of the present study and two comprehensive experimental databases of actively confined and FRP-confined concretes, an expression has been developed for the prediction of the difference in the confining pressures that results in differences in the axial stresses between actively confined and FRP-confined concretes.

Analytical models are proposed to predict the compressive resistance of discontinuous bracing members in steel X-braced frames when governed by flexure of the connecting plates at the mid-connections. The connections are bolted single shear splice connections commonly used for HSS bracing members. An incremental analysis procedure is developed to predict the axial load-deformation response of the brace-connection assembly including geometric and material nonlinearities. The buckling strength depends on the thickness of the connecting plates, the length of the connection, the clear distance between the continuous and discontinuous bracing members at the brace intersection, and the length of the discontinuous brace segments. The method is validated against test results from full-scale X-bracing specimens. It is also used to verify the design procedure proposed in the AISC Design Guide 24 for Hollow Structural Section Connections for a series of 17 X-braces covering a wide range of properties. The AISC design procedure generally over-predicts the buckling resistance. A new design procedure is proposed that better reflects the buckling response of discontinuous braces in X-bracing.

A finite-element (FE) model to simulate nonlinear structural behavior of posttensioned composite beams with partial shear connectors is introduced in this paper. Stress-strain material models for concrete and steel are implemented in the FE formulations and the nonlinear load-slip relation of shear connectors is also considered. With the load-slip relation, a nonlinear numerical formulation to consider the slip effect along the interface of posttensioned composite beams is derived. A solution procedure, which includes modeling of the section in prestressed concrete (PSC) structures, tendon behavior for consideration of sequential loading conditions, and a unique section analysis methodology to consider the partial shear connection of posttensioned composite beams, is also proposed. Numerical results yielded by the proposed model are compared with experimental results for verification of the model’s performance. The results show satisfactory structural behavior of partially composite posttensioned beams, and thus the applicability of the proposed FE model is validated.

Metallic dampers have been used to dissipate large amounts of seismic energy through inelastic deformation in order to minimize damages to the main structural components. However, cumulative inelastic deformation of the metallic dampers results in fatigue cracks, so the life of the device may be reduced significantly. In this paper, an experimental study was carried out to investigate the low-cycle fatigue damage for the proposed hourglass-shaped strip damper (HSD). Four different types of cyclic loadings were applied in this test. The microstructures of the fracture surfaces and the crack distribution patterns were also observed. From the results, it was found that the shape of HSD was well designed for excellent fatigue performance, and the low-cycle fatigue characteristics of the steel strip damper could be well defined by the Manson-Coffin relationship. To consider the mean deformation and the loading sequences, a modified fatigue damage prediction model was proposed that incorporates the effective stiffness. Finally, it may be said that the newly proposed model can predict well the remaining life of the damper damaged by cumulative inelastic deformation.

This paper describes an integrated topology optimization framework using discrete and continuum elements for buckling and stiffness optimization of high-rise buildings. The discrete (beam/truss) elements are optimized based on their cross-sectional areas, whereas the continuum (polygonal) elements are concurrently optimized using the commonly known density method. Emphasis is placed on linearized buckling and stability as objectives. Several practical examples are given to establish benchmarks and illustrate the proposed methodology for high-rise building design.

The wind directionality factor, $K_d$, is a nondimensional quantity smaller than unity that reflects the fact that the climatologically and aerodynamically or dynamically most unfavorable wind directions typically do not coincide. The ASCE 7-10 standard specifies for buildings a directionality factor $K_d=0.85$, although no justification appears to have been adduced for this value in the literature. This paper presents a more informed discussion of this topic by the engineering and codification communities. The results of this work show that the directional factor $K_d$ varies as a function of type of wind storm, geographical location within an area with a given type of wind climate, type of wind effect (e.g., local pressures, internal forces in members of wind force resisting systems, global shear or torsion), and position of the wind effect being considered in the structure. These results suggest that, in most if not all cases, the dependence of the directionality factor on mean recurrence interval of wind effects is weak, and they confirm a recent finding that $K_d$ values are larger for hurricane-prone than for nonhurricane regions. For pressures at individual points on building envelopes, the value $K_d=0.85$ is typically found to be adequate for hurricane-prone regions and conservative for nonhurricane regions. For internal forces in wind force resisting systems (e.g., frames), the value $K_d=0.85$ is typically adequate for nonhurricane regions while $K_d=0.90$ is required for hurricane-prone regions, in spite of its conservatism in some situations. Finally, for global effects, such as building base shears and global building torsion, the values $K_d=0.90$ and $K_d=0.95$ are appropriate for nonhurricane and hurricane-prone regions, respectively.

Similar to the definition of the moment-independent importance measure on model failure probability, the authors define the moment-independent importance measure concerning the importance sampling method. After combining the importance sampling procedure, the sampling efficiency is improved and the cost of computation is cheaper. After the moment-independent importance measures of all inputs are obtained, a new method called conditional importance sampling is proposed to calculate the failure probability and those inputs with high importance measures are chosen to be conditional variables. And state-dependent parameter method is employed in the new method. A numerical example and two structural engineering examples are used to demonstrate that the failure probability estimated by the proposed conditional importance sampling method is accurate and converges faster than those estimated by the traditional importance sampling method.