Journal of Structural Engineering

Although the first progressive collapse event occurred in precast concrete (PC) buildings due to gas explosions, the behavior of PC buildings resisting progressive collapse is still unclear because related tests are few, and the types of PC connections vary. To study the progressive collapse resistance of monolithic PC beam-column assemblies under a middle column removal scenario, an experimental program including four PC assemblies and one reinforced concrete (RC) assembly is conducted. These PC assemblies contain different types of beam-column connections, including connections of lap-splice, 90° hook anchorage, U-shaped bar, and combined U-shaped bar and 30° hook anchorage. The test results demonstrate that flexural action, compressive arch action (CAA), and catenary action were mobilized in sequence in the beams of RC and PC assemblies. Thus, in general, the development of load resisting mechanisms of PC assemblies is similar to that of RC assemblies, although different types of connections may affect the magnitude of each mechanism. The PC assemblies with 90° hook anchorage and U-shaped bar connections attained a CAA capacity similar to that of the RC assembly, whereas the PC assembly with the combined connections had a greater CAA capacity because of plastic hinge relocation at the beam ends near the middle column. The PC assembly with lap-splice connections achieved the lowest CAA capacity due to the anchorage failure of the beam bottom rebar. However, this PC assembly attained the greatest catenary action capacity, because the beam bottom rebar did not fracture and could still develop tension. Subsequently, an analytical model is proposed to reveal the nature of CAA and to better understand the influence of critical parameters.

Fire hazard has been one of the most critical factors to threaten the safety of steel structures. Steel structures may not collapse when suffering fire and cooling down, but the serviceability of building after the fire needs to be evaluated before reusing. Hence, the postfire mechanical properties of steel are necessary. With the high- and ultrahigh-strength steel applied in the construction industry, superhigh-tension bolts (SHTB) are thus encouraged to apply in engineering. However, the current studies on the postfire mechanical properties mainly focus on the Grade 8.8s and 10.9s high-strength bolts. There is limited research work conducted to study the postfire mechanical properties of SHTB. In this study, coupon tests of Grade 14.9 SHTB were conducted, and the postfire performance was evaluated. Failure mode, stress-strain curve, elastic modulus, yield stress, tensile strength, tensile strain, percentage elongation after fracture, and percentage reduction of area were tested and analyzed. The postfire mechanical properties of different types of bolts were compared. The test results indicated that temperatures beyond 750°C cause decrease in the ductility of Grade 14.9 SHTB, regardless of the cooling condition. Accordingly, predictive formulas with good precision are proposed for the postfire mechanical properties of Grade 14.9 SHTB cooled down by air and water, respectively.

This paper presents the results of 10 laboratory tests performed on closely spaced built-up members fabricated from hot-rolled angle sections. Such members are of special interest in the construction and reinforcement of steel lattice towers used for telecommunication or electrical transmission lines. The angle sections used in this study were star-battened with pinned support conditions. In particular, two configurations were studied: star-battened angles with equal sections (SBE) (composed of two chords with the same section), and star-battened angles with unequal sections (SBU) (composed of two chords with different sections). The influence of connection spacing along the members and different types of connections (fully preloaded bolts, partially preloaded bolts) were also studied. In addition, the important influence of geometric imperfections on failure load and failure mode were analyzed. In this paper, a numerical model is introduced after the laboratory tests are presented. The model was built in ANSYS version 18.2 and was validated based on the results obtained during the experimental campaign. The numerical results highlighted the sensitivity of failure modes to initial imperfections (out-of-straightness, load eccentricity). Last, the validated numerical model is thought to be used in a wider parametric study with the aim of developing new design recommendations.

The deformation compatibility behavior of glue-laminated (glulam) beam-to-column connections is not yet well understood. To investigate this behavior, 10 specimens representing five different glulam beam-to-column connections were tested under lateral cyclic loading. Three of the connections were custom-designed connections and two were pre-engineered connections. Specimens were large-scale glulam beam-to-column connections, and testing included combined gravity and lateral loading as well as the presence of a cross-laminated timber (CLT) deck. Connection characteristics such as yield force, initial stiffness, secant stiffness, and energy dissipation were calculated and compared across connections as well as observations made during testing. Results from the experimental testing indicated that pre-engineered connections demonstrated overall better performance than the custom connections. Pre-engineered connections had the largest remaining secant stiffness by the end of the loading protocol, greater energy dissipation, and larger average yield forces than the custom connections. Although all connections tested exceeded their lateral design capacities at drift levels less than 2% drift, all connections maintained load-carrying capacity throughout testing.

Abstract

The galloping problem of a three-degree-of-freedom (3-DOF) system with strongly coupled vertical-horizontal-torsional motions has drawn wide attention due to the high risk of galloping and subsequent serious damages caused, but the galloping mechanism remains unclear because of the complex interaction of structural coupled motion and aerodynamic forces. In the present work, a 3-DOF system with close frequencies in three directions was analyzed to reveal the galloping mechanism under a strongly coupled motion. Based on quasi-steady theory, theoretical models of a 3-DOF system with a single section and multiple subsections were studied by using a perturbation method based on order analysis, and a galloping stability criterion considering aerodynamic stiffness and small-frequency detuning was established. This criterion reveals the promoting mechanism of aerodynamic stiffness on galloping initiation. Galloping tests on a segmental 3-DOF model of an eight-bundled conductor with D-shaped ice accretion were conducted to examine the validity of the theoretical model and proposed galloping stability criterion under various wind conditions. For the strongly coupled system in the tests, aerodynamic stiffness was able to trigger galloping even under positive aerodynamic damping in all directions, which could be explained by the proposed criterion. Numerical examples were also employed to validate the proposed galloping stability criterion under attack angles of 0°–180°. The numerical study showed that aerodynamic stiffness can promote galloping regardless of attack angles, especially under high wind speeds. The results reveal the galloping mechanism of a strongly coupled 3-DOF system initiated by aerodynamic stiffness and provide a new insight into the prediction of galloping.

Practical Applications

Galloping can cause great damages to slender structures, especially transmission lines. Understanding the galloping mechanism is essential for researchers, engineers, and designers because the galloping mechanism is the foundation for antigalloping design. It has long been suspected that the structures with closely tuned frequencies are prone to galloping because the coupling effect between different directions is very strong theoretically. For example, bundled conductors are more likely to gallop compared with single conductors, and the close frequencies of bundled conductors have been considered as an important reason. However, whether the strong coupling effect can promote or suppress galloping remains unclear, because there is a lack of an analytical solution to clarify this galloping mechanism. This study answers the question of why structures with close frequencies are prone to galloping. With the proposed criterion, the prediction of galloping initiation is greatly simplified, and this can provide important information for antigalloping design. Engineers can know more easily the wind attack angles where galloping is likely to occur, and appropriate galloping-suppression measures can be adopted, such as separating frequencies and antigalloping devices for conductors with close frequencies.

The paper presents an adaptive section discretization scheme for the inelastic response analysis of structural members with cross sections that can be decomposed into rectangular and circular subdomains. Each subdomain can consist of a different material. As long as the largest strain in a subdomain does not exceed the specified trigger strain values, the subdomain contribution to the section response is determined by the numerically exact cubature rule for the subdomain. Once the largest strain reaches the trigger value for a subdomain, it is discretized with a fiber mesh and the numerical evaluation of its contribution to the section response is determined with the midpoint integration rule. The fiber mesh with the midpoint integration rule remains in effect for the activated subdomain until the end of the response history. The paper applies the adaptive discretization scheme to the thin-walled sections common in metallic structures and investigates the effect of different trigger strain values on the accuracy and computational efficiency of the inelastic response analysis of wide-flange steel sections and multistory steel frames under static and dynamic excitations.

A self-centering square concrete-filled steel tubular column-to-steel beam connection with bolted angles was designed and its behavior analyzed theoretically. Three ½-scale specimens differing only in the loading procedure of the vertical force were tested under cyclic loading conditions. A finite element (FE) model of the self-centering connection was established in the ABAQUS software and its validity verified against experimental results. Compared with experiments, an extended parameter analysis of the variable axial force with a wider variation range of axial force and variable axial force in imitation ground motion was conducted using the proposed FE model. The experimental results showed that specimens with larger axial forces and variable axial forces consumed more energy and had clearer residual drifts compared with those of specimens with smaller axial forces and constant axial forces, respectively. The simulation results showed that a larger axial force considerably decreased the resistance capacity of the lateral load, resulting in a descending skeleton curve. A clear residual drift and asymmetry in the maximum relative rotation were observed under positive and negative displacement upon increasing the axial forces. In addition, a larger range of change in the axial force resulted in a more prominent residual drift compared with a smaller range of change in the axial force under a relatively small lateral displacement, decreasing the reliability of the self-centering capacity of the connection during an earthquake.

Information on natural frequencies and damping ratios is essential for structural dynamic analysis. To develop predictive models of modal parameters applicable for modern high-rise buildings with increasing heights, an advanced interferometric radar system was employed to measure the displacement responses of 38 supertall buildings with heights ranging from 200 to 600 m under ambient excitations. Then, a combined approach of the stochastic subspace method and an uncertainty bound estimation method are adopted for modal identification, and its accuracy is validated by a numerical simulation study. By applying the modal identification method to the field measurements, modal parameters of the 38 supertall buildings are identified. Based on these results, predictive models for fundamental natural frequency and damping ratio of supertall buildings are accordingly proposed, which are more applicable for the wind-resistant serviceability assessment or design of high-rise buildings with heights over 200 m than existing empirical models.

Both integral and implicit gradient consistent nonlocal formulations are developed for a force-based beam element with material and geometric nonlinearities. The element is based on the Timoshenko beam theory, which accounts for shear deformations. Material nonlinearity is considered by using inelastic constitutive relationships, and geometric nonlinearity is considered by using the corotational formulation in the global system and a curvature-shear-based displacement interpolation (CSBDI) in the local system. Integration point dependency for strain-softening responses is addressed by using the section deformation as the nonlocal variable. The weak form of the implicit gradient-type governing equation is derived, and an efficient strategy is proposed to solve it. Consistent element flexibilities for both the integral and implicit gradient formulations are derived. To implement the proposed elements, a new and simplified state determination algorithm is developed. Finally, four illustrative numerical examples are presented to demonstrate the utility of proposed element and validate it. The results indicate that the proposed element can accurately capture both material and geometric nonlinearities, and offers consistent response predictions for any number of integration points due to its nonlocal regularization.

Real-time hybrid simulation (RTHS) provides a cyber-physical technique for large- or full-scale experiments in size limited laboratories when parts of the structure are difficult for accurate modeling. Traditional practice of RTHS assumes deterministic structural properties therefore could not account for uncertainties in global response prediction in an efficient and effective way. Previous studies have shown that metamodeling enables efficient uncertainty quantification through limited number of expensive physical experiments or computational simulation. More recent studies indicate that multifidelity Co-Kriging can achieve better accuracy with fewer experiments or less simulation. This study presents an experimental study of the influence of low-fidelity model accuracy on Co-Kriging metamodeling for uncertainty quantification. Laboratory RTHS through are considered as high-fidelity (HF) simulation and conducted in parallel with low-fidelity (LF) computational simulation of the same structure. The Co-Kriging metamodeling is then applied to integrate multifidelity simulation to render accurate response prediction over the entire sample space of uncertainty input variables. Different parameter values are used for same computational model to emulate different LF simulation for Co-Kriging metamodeling. RTHS tests are conducted for a single-degree-of-freedom (SDOF) structure with self-centering viscous damper (SC-VD). The Co-Kriging metamodels established from experimental results are then evaluated through validation tests and further compared with corresponding Kriging metamodels. A multifidelity Co-Kriging with LF model updating is further proposed to improve the convergence and accuracy in response estimation for uncertainty quantification.

Following the 2010/2011 Canterbury and 2016 Kaikoura earthquakes, a number of reinforced concrete (RC) beams in high-rise structures developed a single primary crack at the beam-column interface without the formation of distributed secondary cracks along the beam length. Detailed assessments showed that these beams have conforming longitudinal steel ratios and the single-crack mechanism may be due to design and/or construction practices for beam-column joints in the 1980s. In order to investigate the seismic behavior of reinforced concrete beams with detailing that inhibited the spread of flexural yielding, an experimental program was carried out on RC beam specimens, having similar reinforcement detailing to that of beams that developed a single crack at their ends during the Kaikoura earthquake to understand their seismic behavior, postearthquake repairability, and residual low-cycle fatigue life. Experimental results showed that the beams were able to undergo significant inelastic drift demands without loss of lateral resistance and have sufficient residual drift capacity following moderate and large earthquake demands. The response of the beam specimens was dominated by hinge rotation via the bond-slip mechanism. Comparisons showed that the measured drift capacities of the beams exceeded the predicted drift capacities computed using state-of-the-practice procedures.

The nonlinear-inelastic response of reinforced concrete moment-resisting frame buildings due to an adjacent excavation in soft to medium clays is presented. An advanced numerical model is used to account for building-soil-excavation interactions, post-yielding behavior of the structural members, and solid-fluid coupled behavior of supporting soils. The effects of the excavation on the internal force redistributions of structural members, lateral and vertical distortions, ground surface settlements, and lateral wall movements are presented in terms of angular distortions and drifts, curvature ductility, axial forces, and flexural moments. Results showed how beams progressively developed plastic deformations as deep-seated movements occurred during the excavation of soft and medium clays, reaching maximum curvature ductility ratios larger than 2. As the excavation advanced, maximum curvature ductilities of up to 5 were computed in beams closer to the excavation, causing localized inelastic mechanisms in the building. This paper shows how even well-designed buildings to withstand large levels of lateral load demands using strong column–weak beam approaches can still be subjected to significant excavation-induced nonlinear-inelastic demands that can cause damage.

In the current standards/specifications for cold-formed steel structures, the direct strength method (DSM) has not been developed for the shear strengths of perforated members, which is currently based on an empirical design approach using a $q_s$ factor. To extend the DSM design to members in shear with various hole shapes and sizes, new design procedures have been developed to determine shear loads including the elastic shear buckling load ($V_{crh}$), shear yield load ($V_{yh}$), and ultimate shear strength ($V_n$). $V_{crh}$ can be calculated via a linear approximation for buckling coefficients ($k_v$) based on the hole and section dimensions. $V_{yh}$ can be computed based on a theoretical Vierendeel mechanism or a proposed alternative simplified model to permit an easier implementation in design checks. $V_n$ can be determined using proposed DSM shear curves for members with and without full-transversely stiffened webs. To validate the new DSM approach, a collection of reliable historical experiments in perforated sections under shear was used for strength comparison. The experimental shear strengths were also further used for the calibration of the DSM-based proposal to determine the resistance factor ($\varphi$) for shear design, which is traditionally based on the LRFD methodology.

Due to the excellent tensile properties and long-term durability, using ultrahigh-toughness cementitious composite (UHTCC) at the hogging moment regions of long-span composite bridge decks is a new solution to the cracking issue. Composite degrees and the longitudinal reinforcement ratio have been suspected of influencing the flexural performance of steel-UHTCC composite bridge decks under hogging moments, but the extent of the influence remains unclear. This study aims to investigate the influence of these two core design parameters. Six specimens with different stud spacings and longitudinal reinforcement ratios were designed to investigate the failure modes, flexural capacity, deformation behavior, and cracking characteristics. The results show that the cracking stresses ${\sigma }_{0.05}$ and ${\sigma }_{0.1}$ in the steel-UHTCC composite bridge decks were 58.6%–216.8% and 58.9%–213.5% higher than those in the steel-normal-strength concrete (NSC) composite bridge decks. As the longitudinal reinforcement spacing reduced from 125 to 75 mm, the growth of the crack width was effectively restrained, and the cracking stresses ${\sigma }_{0.05}$ and ${\sigma }_{0.1}$ increased by 36.6% and 71.2%, respectively. Finally, a modified theoretical model is established to evaluate the influence of composite degrees on the elastic flexural performance of steel-UHTCC composite bridge decks. The dimensionless composite degree $\eta$ is defined as a core parameter in this model. For practical elastic design, the threshold value of the dimensionless composite degree can be determined as 0.005.