Journal of Structural Engineering

During the last three decades, considerable research efforts have sought to improve the seismic design of California highway bridges. However, the in-span hinge (ISH) regions of concrete box girders have not been studied adequately. ISHs are classified as disturbed regions because of the concentrated bearing loads and the possible existence of utility and maintenance openings, which induce a three-dimensional (3D) behavior. Nevertheless, ISHs are commonly designed as two-dimensional short cantilevers, following standard procedures in “Building Code Requirements for Structural Concrete and Commentary” of the American Concrete Institute. These designs typically lead to congested reinforcement, causing constructability concerns from practical and economical aspects. In this study, the strength of current ISHs is assessed by using a combined experimental and computational approach. For the experimental part, two $1/3$-scale specimens were tested at the University of California, Berkeley. The computational part adopted nonlinear 3D finite elements considering embedded reinforcement and concrete cracking. Failure modes were identified, and realistic idealizations of the behavior of the ISHs were developed. The findings from this study revealed that ISHs fail with a combination of two failure modes: (1) beam shear and (2) punching shear.

Previous studies primarily focused on the seismic performance of high-strength concrete (HSC) members and the static behavior of high-performance concrete (HPC) members. The work presented here aims at studying seismic performance of HPC frames under low reversed cyclic loading to provide guidance to support the design of HPC structures in seismic regions. Experimental studies are conducted to investigate failure patterns, failure mechanism, strains of longitudinal bars, $P$-$\Delta$ curves, characteristic loads, displacement-restoring capacity, displacement ductility coefficient, stiffness degradation, and the energy-dissipation capacity of two-bay, two-story HPC frames under low reversed cyclic loading. Studies indicate that both nonprestressed and prestressed HPC frames show good seismic performance during testing. The two HPC frames both fail in a partial beam sidesway mechanism. The hysteresis area of the nonprestressed frame is larger than that of the prestressed frame, whereas both frames still show relatively high energy-dissipation capacity. The applied prestressing in frame beams could enhance the displacement-restoring capacity of frames but decreases the energy dissipation. The prestressed frame exhibits a better ductile manner than the nonprestressed frame.

This paper discusses the modeling of reinforced concrete structural elements for use in geotechnical centrifuge modeling of soil-structure interaction problems. Centrifuges are employed in geotechnical modeling so that the nonlinear constitutive behavior of soil in small-scale models can be correctly modeled at prototype scale. Such models typically necessitate large scale factors of between $1:20$ and $1:100$, which is significantly larger than most conventional small-scale structural modeling. A new model concrete has been developed consisting of plaster, water, and fine sand as a geometrically scaled coarse aggregate that can produce a range of model concretes with cube compressive strengths between 25–80 MPa. Reinforcement is modeled using roughened steel wire (beams) or wire mesh (slabs). To illustrate the validity of the modeling technique, a series of three- and four-point bending tests were conducted on model beams designed to represent a $0.5\times 0.5\mathrm{m}$ square section prototype beam at $1:40$ scale, and model slabs designed to represent a prototype slab with plan dimensions of $4.8\times 4.8\mathrm{m}$ and 0.4 m deep (also at $1:40$ scale). The amount of longitudinal reinforcement was varied and tests both with and without shear reinforcement were conducted. The models were able to accurately reproduce both shear and flexural (bending) failures when loaded transversely. The load capacity (strength), bending stiffness, and ductility were shown to be simultaneously and appropriately scaled over a range of scaling factors appropriate for geotechnical centrifuge testing, and the technique therefore provides a significant improvement in the ability to accurately model soil-structure interaction behavior in centrifuge models.

In the conventional seismic analysis of a system, the input motion is defined either by a probabilistic response spectrum or by ground-motion histories whose spectra “match” the probabilistic response spectrum. In both cases, it is implicitly assumed that the spectral values at different periods on a probabilistic response spectrum are fully correlated with one another. A nonlinear system changes its “effective” period during seismic shaking; hence, its response depends on spectral values at many natural periods. The assumption of complete correlation between spectral values at different periods results in an overestimation of the response of a nonlinear system. For the case analyzed in this paper, the conventional use of a 500-year mean return period (MRP) response spectrum in nonlinear analysis produces responses that have 20–30% longer MRPs. Also, the larger-component response acceleration was 11% higher than the geometric-mean response acceleration, and the larger component response deformation was 33% higher than the geometric-mean response deformation. This paper presents a more accurate way of using the response spectrum in the probabilistic analysis of nonlinear systems.

This experimental work focuses on enhancing the lateral cyclic behavior of RC columns by using an innovative active-confinement technique. The thermally triggered recovery stress of prestrained shape memory alloy (SMA) spirals is used to apply the active-confinement pressure. Four $1/3$-scale RC columns, three of which are retrofitted with different schemes, are tested under quasi-static lateral cyclic loading. The plastic hinge region of the first retrofitted column is actively confined using the new SMA spirals, whereas for the second column, traditional passive confinement is applied by using a glass fiber-reinforced polymer (GFRP)/epoxy jacket. The third column is retrofitted by using a hybrid confinement approach, which is applied by using SMA spirals and GFRP jacket simultaneously at the plastic hinge zone. The confinement pressure of the three retrofitted columns is designed to be the same. The results show a significant increase in the flexural ductility capacity and energy dissipation capability of the columns retrofitted with SMA spirals compared with those of the as-built column and the GFRP-retrofitted column. SMA spirals also show an outstanding ability to mitigate the damage sustained by the columns under extreme lateral drifts.

Reliable collapse assessment of structural systems under earthquake loading requires analytical models that are able to capture component deterioration in strength and stiffness. For calibration and validation of these models, a large set of experimental data is needed. This paper discusses the development of a database of experimental data of steel components and the use of this database for quantification of important parameters that affect the cyclic moment-rotation relationship at plastic hinge regions in beams. On the basis of information deduced from the steel component database, empirical relationships for modeling of precapping plastic rotation, postcapping rotation, and cyclic deterioration for beams with reduced beam section (RBS) and other-than-RBS beams are proposed. Quantitative information is also provided for modeling of the effective yield strength, postyield strength ratio, residual strength, and ductile tearing of steel components subjected to cyclic loading.

Cantilevered steel sign structures are widely used for on-premises freestanding signs. A welded sleeve connection detail is commonly used to splice pipes of different diameters. This type of structure can be flexible and the connections are vulnerable to fatigue cracking attributable to wind-induced vibration. Fatigue has not been considered in design, although the connection has an inherently low fatigue resistance. To investigate the cause of failure at the sleeve connection and to mitigate the problem, a study of 10 sample failure cases and a series of finite-element analyses were performed. The finite-element analyses showed that the actual load-transfer mechanism in the sleeve connection was not very consistent with that assumed in design. In addition, very high-stress concentration caused the weld-toe crack to propagate into the pipe section. The common practice of strengthening the connection by welded gusset plates did not alleviate the problem. A frequency analysis indicated that the second-mode resonance because of wind-induced vortex shedding may cause a large number of damage cycles. Two alternative connection details were proposed to mitigate weld cracking.

Tubular-section members are commonly used as seismic-resistant braces because they have a higher moment of inertia than open cross sections of equivalent area. However, the cumulative cyclic deformation capacity of tubular sections after buckling is smaller than that of open sections, such as H-shaped sections, because fracture is initiated by local buckling of circular tubular sections. To evaluate the seismic performance of such diagonal braces, it is essential to predict the cumulative cyclic deformation capacity of these braces before any fracturing. In this study, the cumulative cyclic deformation capacity of circular tube braces under local buckling was assessed by performing cyclic loading tests for a range of slenderness and diameter-to-thickness ratios. The mechanism of strain concentration in the tubular braces was studied in various types of analysis, and a method is proposed for assessing the cumulative deformation capacity before fracture based on the entire axial deformation of the braces.

Inspection and monitoring of deteriorating structures are generally performed to assess structural integrity and predict the remaining service life. For deteriorating structures, a reliable prediction of damage occurrence and the time-dependent damage evaluation depend on the accuracy of deterioration models under uncertainty. A probabilistic approach to establish optimum cost-based inspection and monitoring schedules of fatigue-sensitive structures is proposed in this paper. The inspection schedule is the solution of an optimization problem to minimize the expected total cost, including the costs of inspections or monitorings, and the expected failure cost. The failure criterion uses damage detection time and time for damage to reach the critical state. The uncertainties associated with damage occurrence/propagation and damage detection time are both considered. The solution of the optimization provides the inspection times and quality of inspections. The formulation of the optimization for inspection scheduling is extended to monitoring scheduling. Effects of the failure cost on inspection and monitoring scheduling are also studied. The proposed approach is applied to a fatigue-sensitive structure.

This paper describes and demonstrates modifications of the harmony search method to support multimodal structural optimization. Several researchers have recognized the potential of population-based optimization methods, such as genetic algorithms and particle swarm optimization, to support multimodal optimization, that is, generating a range of good alternative solutions, rather than a single best solution. Among these population-based methods is the harmony search method, which has been demonstrated to be efficient and effective in many unimodal structural optimization problems. Toward the goal of making the harmony search method more effective in multimodal optimization, this paper describes a new strategy for generating solutions called close-harmony improvisation, and a new strategy for replacing solutions called local replacement. Examples demonstrate the effect of the two strategies used individually and in tandem. The discussion compares results with conventional harmony search and finds that close-harmony improvisation consistently improves the fitness of the search results, although the effect is sometimes mild, whereas local replacement is quite effective in increasing the diversity of the search result.

Advances in wind pressure measurement and computer technology have made time-domain analyses of wind effects on high-rise structures possible in recent years. Time-domain solutions use aerodynamic and wind climatological databases and provide full phase information on wind-induced response that is lost in the frequency-domain approach; therefore, they can account rigorously for the superposed effects of any number of modes of vibration of any shape; for mode coupling; for wind directionality effects; and for the joint contributions of axial forces, bending moments, and shear forces in interaction equations used for structural design. Unlike the frequency-domain approach, in the time-domain approach, the process of determining wind effects and the structural design process, referred to jointly as database-assisted design (DAD), are integrated, transparent, and fully auditable. The objective of this study is to present the DAD approach as applied to high-rise reinforced concrete (RC) buildings. Given the time histories of pressures, measured in the wind tunnel at a sufficient number of taps on the exterior faces of the building envelope for a sufficient number of mean speed directions and a preliminary design of the building, the structural engineer can calculate, as functions of wind speed and direction: (1) demand-to-capacity indexes for any number of members and cross sections, (2) interstory drift, (3) and top floor accelerations. These responses are properties of the structure independent of the wind climate, and constitute response databases used in conjunction with a wind climatological database to obtain the requisite wind effects for any specified mean recurrence interval. The design, which accounts for both wind and gravity effects, is performed iteratively until the design specifications are satisfied.

A corotational element is developed directly from the governing second-order differential equations of beam theory. The corotational element includes not only the effect of axial force on the bending moment ($P$-delta effect) but also the additional axial strain caused by end rotations. Hinged and semirigid end conditions are also included so that plastic hinges could be considered. The resulting local element tangent stiffness matrix is compared to traditional local element elastic and geometric stiffness matrices. The method is implemented and executed on two example problems in which only one element per member is used. Results compare favorably to those from a nonlinear commercial program in which several elements per member are used. A third example problem includes both geometric and material nonlinearity to develop a pushover curve.

This paper presents a computational procedure to evaluate the shear strength contribution provided to a reinforced concrete (RC) beam by a system of near-surface mounted (NSM) fiber reinforced polymer (FRP) strips. This procedure is based on the evaluation of (1) the constitutive law of the average-available bond-length NSM FRP strip effectively crossing the shear crack, and (2) the maximum effective capacity it can attain during the loading process of the strengthened beam. Because of complex phenomena such as (1) interaction between forces transferred through bond to the surrounding concrete and the concrete fracture, and (2) interaction among adjacent strips, the NSM FRP strip constitutive law is largely different than the linear elastic one characterizing the FRP behavior in tension. Once the constitutive law of the average-available bond-length NSM strip is reliably known, its maximum effective capacity can be determined by imposing a coherent kinematic mechanism. The self-contained and ready-to-implement set of analytical equations and logical operations is presented along with the main underlying physical-mechanical principles and assumptions. The formulation proposed is appraised against some of the most recent experimental results, and its predictions are also compared with those obtained by a recently developed more sophisticated model.

This paper presents a finite-element formulation for the dynamic analysis of I-beams curved in-plan, in which geometric nonlinearities owing to large deflections and inelastic material behavior are taken into account. The developed finite element is based on the total Lagrangian formulation. Wagner and warping effects are considered in the analysis. The nonlinear dynamic equilibrium equations are solved by using Newmark’s step by step numerical integration procedure. The von Mises associative isotropic hardening model is used to formulate the metal plasticity. The elastic predictor-radial corrector algorithm is used to consider gradual yielding on the cross section and along the beam. Simpler finite-element formulations are also developed by excluding the effects of geometric and material nonlinearities. Numerical examples are provided to illustrate the significance of the effects of geometric and material nonlinearities on the dynamic behavior of I-beams that are curved in-plan.

Lumber has the advantages of being a lightweight construction material, easy to handle, and environmentally benign. However, large creep deflections and significant issues with sound transmission, particularly the footfall problem, generally limit lumber use to small spans and low-rise buildings. A concrete topping on lumber subfloors may mitigate some of these issues, but even with well-engineered wood systems, the spans are relatively short. In this study, a new structural system is analytically explored. Called a lumber-boxed concrete structural system, the dual system utilizes the positive attributes of both dimensional lumber and reinforced concrete; composite action is not a requirement of the system. A stress-block approach is developed to calculate strength and deformation behavior. An analytical stress-block-based moment-curvature analysis is performed on the lumber-boxed concrete structural elements. Results show that the structural lumber-boxed concrete members may have better strength and ductility capacities when compared to an equivalent size ordinary reinforced concrete member.

This paper discusses connections between Cross-Laminated-Timber (CLT) floor slabs and steel framework of hybrid tall buildings. Using the example of a 24-story building analysis, it is shown that such connections can be made using simple fasteners like large screws to enforce efficient diaphragm action in the critical case of CLT slabs subjected to lateral seismic or wind events. Results of laboratory tests on connections between CLT slabs and structural steel members are presented to prove this.