Journal of Structural Engineering

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

This paper describes an innovative bridge column technology for application in seismic regions. The proposed technology combines a precast posttensioned composite steel-concrete hollow-core column, with supplemental energy dissipation, in a way to minimize postearthquake residual lateral displacements. The column consists of two steel cylindrical shells, with high-performance concrete cast in between. Both shells act as permanent formwork; the outer shell substitutes for the longitudinal and transverse reinforcement, because it works in composite action with the concrete, whereas the inner shell removes unnecessary concrete volume from the column, prevents concrete implosion, and prevents buckling of energy dissipating dowels when embedded in the concrete. Large inelastic rotations can be accommodated at the end joints with minimal structural damage, since gaps are allowed to open at these locations and to close upon load reversal. Longitudinal posttensioned high-strength steel threaded bars, designed to respond elastically, in combination with gravity forces ensure self-centering behavior. Internal or external steel devices provide energy dissipation by axial yielding. This paper describes the main requirements for the design of these columns and also discusses the experimental findings from two quasi-static tests.

With the North American seismic codes moving toward adopting performance-based seismic design (PBSD) approaches, there is a need to develop seismic probabilistic risk assessment (PRA) tools for different construction systems, including reinforced masonry (RM). The current study focuses on the development of analytical fragility curves based on the performance of RM walls tested under base excitation generated by a shake table. The study has two phases. In the first phase, the seismic response of the RM shear walls is modeled using a simplified analytical model that was calibrated using previously reported shake table experimental results. The second phase of the study focuses on the development of fragility curves, as an essential component of a PRA framework, for two-story lightly reinforced masonry walls. In this phase, relevant limit states are defined and the corresponding capacities of the walls are quantified based on the shake table test results. The wall seismic demand levels are then determined through a probabilistic seismic demand analysis (PSDA) approach by performing 120 nonlinear response history analyses using the analytical model developed in the first phase. Fragility curves are derived based on the experimental capacity data and the analytical PSDA results. The results indicate that the lightly reinforced masonry shear wall category considered in this study would experience acceptable seismic performance associated with moderate lateral wall top-roof drifts even when subjected to ground motion records representing some of the highest seismic regions in eastern and western Canada. The study is a part of a larger ongoing PRA research program to provide the needed seismic performance data to facilitate the inclusion of different RM construction categories within the next generation of PBSD codes in North America.

Structures such as bridges, airport runways, marine installations, and railway tracks are often subjected to cyclic loads. These loads can result in a steady decrease in the stiffness of the structure and cause damage at a micro level that eventually leads to a fatigue failure. With the advent of steel-fiber-reinforced concrete (SFRC), there exist the possibilities of enhancing fatigue performance at the structural level, compared to that of members constructed of conventional concrete. Experimental tests at the materials level reported in literature indicate that steel fibers improve resistance to crack growth, decrease deflections, and increase the fatigue life of plain concrete under cyclic loading. This paper reports the results of an experimental study on the performance of SFRC beams tested in fatigue. Twelve (out of 16) reinforced concrete beams with variable fiber contents were tested under constant amplitude cyclic loading. Four reference beams were tested under static conditions. The steel fibers prolonged the fatigue life in SFRC beams by reducing the stress level in the tensile reinforcement. The SFRC beams also demonstrated smaller deflections and smaller crack widths than that of control specimens.

Blockhaus systems represent a traditional construction technology in which structural resistance is obtained by direct contact between multiple timber surfaces obtained through carvings, notches, and ancient joints. Native of forested areas, this technology is currently used for the construction of log buildings also in earthquake-prone areas. Their seismic structural behavior hence represents an interesting and not well known research topic. In the paper, experimental results obtained by cyclic and monotonic tests recently performed on Blockhaus shear walls with three different types of joints are presented and discussed. At the same time, sophisticated finite-element numerical models able to take into account the complex structural behavior of log-walls subjected to in-plane seismic forces are described. Performed numerical simulations highlighted that in general multiple factors affect the global structural response of Blockhaus shear walls, including the presence of imperfections (small gaps) in the joints, the contribution of friction phenomena, the mechanical properties of timber, and the interaction between in-plane seismic forces and vertical permanent loads. However, careful calibration of timber mechanical properties and contact interactions between overlapping logs can provide accurate numerical predictions and good correlation with full-scale experimental results.

Hollowcore slabs are commonly used in different types of structures. They usually include a 50-mm concrete topping. Structural engineers can use this topping to increase the slab load-carrying capacity. North American design standards relate the horizontal shear strength at the interface between hollowcore slabs and the concrete topping to the slab surface roughness. This paper presents the results of four push-off tests on hollowcore slabs supplied by two manufacturers and roughened using a conventional steel broom. The tested slabs sustained higher horizontal shear stresses than those specified by the design standards. Utilizing the data from the push-off tests, an analytical model was applied to evaluate the shear and peel stiffnesses, $k_s$ and $k_p$, of the interface between hollowcore slabs and concrete topping. Structural engineers can utilize $k_s$ and $k_p$ values to model the composite action between hollowcore slabs from the two manufacturers and concrete topping. The analytical model was also used to evaluate the actual distribution of shear and peel stresses.

Risk-based performance measures integrate probability of component or system failure with the consequences of this event. In this paper, a novel risk-based maintenance optimization methodology for deteriorating bridges to find the optimum maintenance options and their timing is proposed. Finding the optimum maintenance types and schedule for different components of bridges is formulated as a multicriteria optimization in which the lifetime maximum value of expected losses associated with failure and the lifetime total expected maintenance cost are considered as the conflicting objectives. The risk-based maintenance approach accounts for different deterioration levels of bridge components and consequences of both component failure and system failure. The methodology is illustrated on a bridge superstructure. The effects of considered life span of the structure and availability of maintenance options on optimum solutions are also investigated.

This paper summarizes the results of an experimental study on the rotational behavior of bolted beam-to-column glulam connections reinforced using locally cross-laminated glulam members. Twenty-two full-scale connection specimens were tested through monotonic and reversed cyclic loading, from which the moment / rotational angle relationships were established. The results indicated that the cross-laminated technique leads to a more ductile failure mode with cross-aligned inner layers of laminas fractured, in contrast to the perpendicular-to-wood-grain tensile failure of unreinforced connections. The connection maximum moment was found to increase by 52% and 46% under monotonic and cyclic loading, respectively; moreover, the deformability ratio evaluated on the basis of the rotation angles corresponding to the yielding and maximum moments of the connections was increased by 94%. Locally cross-laminated connections were also found to dissipate more energy (25%) than unreinforced connections. A comparison of the cross-laminated technique with self-tapping screws indicated that the respective reinforcing effects were comparable with that of the screws being slightly higher. Because the locally cross-laminated technique is readily incorporated into the production of cross-laminated timber products, it may have broad applications in heavy timber-bolted connections.

To check the safety of thin-walled circular steel columns used for bridge piers of elevated-girder bridges, a versatile interaction equation is derived to specify the ultimate state of the columns under the coupling of bidirectional horizontal seismic accelerations. These bidirectional seismic accelerations generally cause biaxial horizontal force components and biaxial bending moment components to act at the top of the columns. Therefore, the interaction equation is derived in this paper in terms of these force and moment components. In this interaction equation, the bending moment components are converted to equivalent horizontal force components. The accuracy of the ultimate interaction equation is first investigated by carrying out a bidirectional shaking table test and an advanced dynamic analysis on single-column models in which a mass with translational and rotational inertias is fixed at the top of the columns. As an application to actual problems, an advanced dynamic response analysis is used to examine the validity of the interaction equation when applied to a column of a two-span simply supported elevated-girder bridge model under bidirectional seismic accelerations. In this model, the center of a superstructure is supported by a single circular steel column, whereas both ends of the superstructure are supported by abutments. The numerical analysis showed that the proposed ultimate interaction equation considering the effects of the biaxial horizontal force components and biaxial bending moment components accurately predicts the ultimate state of the column. The biaxial bending moment components have some large effect on the ultimate state of the column.

This paper proposes the use of an advanced one-dimensional (1D) variable kinematic model to analyze typical civil engineering structures. The model is referred to as component-wise (CW), and it was originally introduced for the analysis of multilayered plate and shell structures. The CW approach is based on the Carrera unified formulation (CUF). CUF can be used for the straightforward development of a large variety of classical and refined beam theories that are able to capture three-dimensional stress/strain states and nonclassical phenomena such as the in-plane warping of the cross section. CUF can be seen as a hierarchical formulation because it has variable kinematic features; 1D models with arbitrarily chosen accuracy can be obtained, including the classical beam theories. CUF models are formulated in terms of fundamental nuclei, whose expressions are formally independent of the adopted hierarchical scheme. Lagrange polynomials are used in the proposed CW models to expand the displacement field of the beam above the cross section. The finite-element method was used in this work to obtain numerical solutions. The conducted numerical investigation shows that CW models can be successfully applied to any geometries with no restrictions on the ratio between the cross-sectional dimensions and the length of the beam. A very short C-shaped beam was therefore used to analyze a classical portal frame. Similarly, analyses of truss structures and a full industrial construction were carried out. Classical beam/plate/solid finite elements as well as combinations of them were used to obtain solutions from a commercial code for comparison purposes. The results show the effectiveness of the CW approach both in terms of accuracy and computational efficiency.

The seismic response of buckling-restrained braces has often been modeled as elastic plastic with kinematic hardening or, even more simplistically, as elastic perfectly plastic. The use of these models has not raised particular criticism because of the lack of models able to represent isotropic hardening with acceptable accuracy and reliability. In recent years, the effect of isotropic hardening has been included in some more refined models of the seismic response of buckling-restrained braces. One of these refined models is considered in this paper to assess the effectiveness of some common models in which the effect of the isotropic hardening is not taken into account. A comparison is made on single-story and multistory structures subjected to natural and artificial accelerograms.

A new interface model to simulate the bond-slip behavior of reinforcing bars is presented. The model adopts a semiempirical law to predict the bond stress-versus-slip relations of bars, accounting for the bond deterioration caused by cyclic slip reversals, the tensile yielding of the bars, and the splitting of concrete. The wedging action of the ribs is represented by assuming that the normal stress of the interface is proportional to the bond stress. The model has been implemented in a finite-element analysis program and has been validated with laboratory experiments that include monotonic and cyclic bond-slip and anchorage tests of bars with different embedment lengths and a test on an RC column subjected to cyclic lateral loading. The model is easy to calibrate and computationally efficient, and it accurately predicts the bond-slip behavior of bars embedded in well-confined concrete. It also simulates bond failure attributable to the splitting of concrete in an approximate manner.

An understanding of the behavior of shear connections in steel gravity frames under the unique combinations of moment, shear, and axial force relevant to extreme loading conditions is necessary to assess the vulnerability of a structure to disproportionate collapse. However, such an understanding is currently limited by a deficiency of physical test data. To investigate the inherent robustness of commonly used steel shear connections, an experimental program consisting of 35 full-scale physical tests was completed. Specimens included shear-tab, welded-bolted single-angle, bolted-bolted single-angle, and bolted-bolted double-angle connections. A testing procedure was developed that imposed on a connection the force and deformation demands that would be expected in the bays immediately adjacent to a compromised column in a symmetrical multibay frame. Various geometric arrangements of each connection type were tested, and each arrangement was subjected to a range of loading histories representing different column-removal scenarios. The test results characterize the load-development history, deformation mechanisms, and expected failure modes following column removal for each type of connection. Connection strength and ductility limits under the effects of this unique type of combined loading are quantified.

This paper introduces a model for strengthening slender reinforced concrete columns. The proposed technique aims at controlling second-order lateral deflections using longitudinal high-modulus bonded reinforcement, thereby altering the loading path to intercept the axial load-bending moment (P-M) interaction curve at a higher axial capacity. With the availability of high and ultra-high-modulus carbon fiber–reinforced polymer (CFRP) plates, this approach should be quite efficient according to Euler’s buckling rule, in which column strength is stiffness-controlled. This approach is different from the classical transverse-wrapping method for confinement, a technique that achieves strengthening by enlarging the (P-M) diagram in the compression-controlled region. The proposed model accounts for concrete nonlinearity in compression, cracking in tension, steel rebar plasticity, and certainly geometric nonlinearity, in addition to the possibility of premature CFRP-debonding failure in tension and the lower CFRP strength in compression than tension. The model is validated against experimental results and used in a parametric study to assess the effects of slenderness ratio $\lambda$, axial load initial eccentricity ratio $e_0/h$, CFRP reinforcement ratio ${\rho }_f$, and modulus $E_f$. It was shown that significant gains in axial strength, ranging from 17 to 90%, occur as the magnitudes of $\lambda$, ${\rho }_f$, $E_f$ and $e_0/h$ increase.

Performance of steel reinforced concrete (SRC) columns under fire is investigated in this paper. A three-dimensional finite-element analysis (FEA) modeling is developed for sequentially coupled heat transfer and structural analysis, in which the classical creep strain and the transient strain of concrete together with thermal strain are included explicitly by subroutines. Four SRC columns with H-shaped steel and cross-shaped steel are experimentally investigated, and the test results are adopted to verify the FEA modeling. The FEA modeling is then used to construct the model of a typical full-scale SRC column and perform analysis to the behavior of the column in fire. Extensive parametric studies are performed to investigate the fire resistance of the SRC column, and the key influencing parameters are identified. Finally, a simplified calculation method is proposed to predict the fire resistance of the SRC column.

A reliability-based design methodology is urgently needed in the fire resistant design of structures to ensure adequate reliability in the face of uncertainty. This paper presents the application of the analytical reliability methods, namely the first-order reliability method (FORM) and second-order reliability method (SORM), to the design of structures subjected to fire. In particular, the FORM and SORM algorithms are implemented in a sequentially coupled thermal-structural finite-element model. A protected steel column exposed to natural fire is presented as an example. In comparison to Latin hypercube sampling (LHS), the analytical reliability method yields sufficient accuracy and provides a significant saving in computational expense. Discrepancies between the analytical methods and LHS are analyzed in depth, and it is shown that the parametric fire model produces a response surface that is highly nonlinear. Despite differences between the FORM and LHS solutions, the utilization of FORM is recommended for the rapid estimation of the reliability of structures threatened by fire.

The peak deformation, acceleration, and the base shear experienced by the structures can be reduced by simulating yielding in an elastic system—also referred as apparent weakening. The negative stiffness device (NSD), used in this study, exhibits nonlinear-elastic negative stiffness behavior; by adding NSD to the elastic structure (primary structure), the resulting structure-device assembly behaves like a bilinear-elastic structure. In an elastic structure, the acceleration and base shear experienced by the structure can be reduced by adding the negative stiffness device, and the additional deformations caused from the reduced stiffness can be contained by adding viscous dampers. Previously, the authors have carried out experimental studies to demonstrate the effectiveness of apparent weakening in elastic structures, but little is known about the behavior of these systems when the primary structure itself yields. This paper focuses on the issues that may emanate with the addition of NSD to the systems prone to yielding while highlighting the response reduction achieved by proper design of the NSD. Shake-table studies of a three-story fixed-base structure (3SFS) braced in top two stories, that is acting as a single degree of freedom system (SDOF) are presented in this paper. Two NSDs and a viscous damper are installed in the first story of the SDOF-3SFS. The structure is subjected to strong ground motions, with and without the NSDs, so that the columns of the SDOF-3SFS in the first story yield. Analytical models for all the components, that can capture the observed experimental behavior, are also presented in this paper. To demonstrate the advantages of apparent-weakening in yielding SDOF structures, experimental and analytical responses of four different systems, (1) 3SFS, (2) 3SFS with damper, (3) 3SFS with NSD, and (4) 3SFS with NSD and damper, are compared for a suite of ground motions. Shake-table tests on SDOF-3SFS have confirmed that by adding the NSD and damper, the acceleration and base shear and deformation of the bilinear inelastic structure and NSD assembly will be reduced by more than 20% for moderate ground motions, and the collapse of structure can be prevented for severe ground motions.

This paper presents a general frequency domain framework for predicting multimode coupled buffeting response of long-span bridges to nonstationary winds. Within this framework, the wind speed field, aerodynamic forces, and bridge response are separated into deterministic time-varying mean and evolutionary random fluctuating components. Under the assumption of low variation rate of mean wind speed, the time-varying mean and self-excited and buffeting forces are modeled in static force coefficients, flutter derivatives, and admittance functions, determined in a wind tunnel under stationary wind, but with consideration of nonstationary wind characteristics in a quasi-steady manner. The time-varying mean bridge response is calculated through static analysis at each time instant. The random buffeting response is expressed in generalized modal displacements governed by the equations of a frequency-dependent linear time-variant (LTV) system. An equivalent frequency-independent LTV system is introduced, which permits calculation of response impulse functions directly from system modal properties at each frozen time. Formulations are presented for calculating time-varying RMS value and evolutionary spectrum of bridge response and its extreme value distribution. The traditional buffeting analysis framework for stationary wind excitations is the special case of this general framework. Based on this analysis framework, the buffeting responses of a long-span suspension bridge with a center span of approximately 2,000 m under various nonstationary winds are investigated, which shed insights on the general characteristics of nonstationary wind load effects on long-span bridges. The effectiveness and accuracy of the framework are also confirmed through time domain Monte Carlo simulations.