Journal of Structural Engineering

The experimental and analytical investigations carried out on lightly reinforced concrete beam-column joints subjected to seismic loading are presented in this paper. Five 3/4-scale reinforced concrete beam-column joints were tested to investigate the seismic behavior of the joints. The variables in the tested specimens include column orientations and the presence of slabs on the top of beams. The specimens were subjected to quasi-static load reversals to simulate earthquake loadings. Experimental results obtained include joint shear stresses, joint shear strains, observed cracking and initial stiffness. The test results provided some information about the seismic behavior of the specimens. However, due to the unique nature of the specimens, several key parameters could not be varied. Therefore, to elucidate further information, a numerical study consisting of three-dimensional nonlinear finite-element (FE) models was carried out. The FE models were then validated with the experimental results. This was followed by parametric studies, carried out to understand the effects of several critical factors, including column axial load, ratio of column depth to beam reinforcement bar diameter, and effective slab width, on the complex behavior of the joints.

Flat plate systems are commonly designed as a gravity-force-resisting system (GFRS) along with stiffer lateral-force-resisting systems (LFRS) such as shear walls, and moment resisting frames; however, deformation compatibility between the GFRS and LFRS results in positive moments (producing tension at the slab bottom) at the slab-column connections in flat plate systems. Continuous slab-bottom reinforcement (integrity reinforcement) passing through the column is required to prevent progressive collapse; however, for posttensioned flat plate (PT-flat plate) systems, there have been arguments among engineers on applying continuous slab-bottom reinforcement since ACI 318 and 352 provisions do not clearly specify the requirement on the slab-bottom reinforcement for PT flat plates. The purpose of this study is to investigate the effects of continuous slab-bottom reinforcement passing through the column core on the hysteretic behavior of the PT slab-column connections. Test results for six, 3/5 scaled specimens for interior PT flat plate connections subjected to constant gravity loads and quasistatic reversed cyclic lateral loads are presented and indicate that the slab-bottom reinforcement conforming to ACI-ASCE 352R.1 is not required to prevent collapse, but does significantly improves the hysteretic energy absorption capacity.

This paper proposes a new equation for the effective length factor ( $k$ -factor) for reinforced concrete columns in braced frames. The new formula is valid both for normal and high-strength concrete. The equation was obtained from a sensitivity analysis performed on a two-dimensional nonlinear finite-element numerical model that takes into account the inelastic behavior of the concrete columns (cracking, yielding, and second order effects). The numerical model was calibrated with 44 experimental tests performed by the writers’ research group. A comparative study was carried out between the numerical model and different national design codes, displaying important differences with respect to all of them: the ACI code (from 37 to −3%), the Spanish code EHE (from 26 to −9.26%), and the Eurocode 2 (from 14 to −14%). It was decided to propose two additional simplified equations: one for checking and the second for design.

The macroscopic Poisson ratio and elastic moduli of the cast-in-place hollow-core slab are researched in this paper. At first, the formulas for computing the Poisson’s ratio of cast-in-place hollowed slabs with bilateral ribs are deduced by theoretical means. Then, based on the characteristics of stress distribution, an effective sectional area method is proposed for computing both the bending and axial compressive elastic moduli of cylindrical hollow-core slabs at the transverse section, in which the boundary of the effective area is suggested to be at a radial distance with its vertical slope angle ${\alpha }_0=22.5°$ . Relevant formulas for computational purposes are also proposed. At last, the shear modulus of cylindrical hollow-core slab at the transverse section is proposed to be computed considering the bending-shear deformation of the varying rib and face sheets at a periodic segment, and relevant formulas for curved sides beam are also deduced. In the course of the study, the so-called transverse-rib-effect phenomenon in which the transverse rib of hollow-core slab may lead to macroscopic stiffness increase has been revealed, which is modeled by an appropriate mathematical relationship. The proposed methods and formulas are carefully evaluated analytically and numerically and may be the foundations for structural analysis of cast-in-place hollow-core slabs.

Eleven steel-concrete composite beams, four under pure torsion, and seven under combined bending and torsion were tested to study their torsional behaviors. Torsion-dominated and bending-dominated failure modes were observed, depending on the ratio between the applied bending and torsional moments. Testing results also showed that the reinforced concrete slab contributes mainly to the torsional resistance of composite beams and the contribution of steel joists to torsion is negligible. However, the steel joist plays a vital role in restraining the concrete slab from deforming longitudinally, which enhances the torsional strength of the concrete slab. Based on the experimental observations, a three-dimensional behavioral truss model capable of analyzing composite beam sections subjected to the combined bending and torsion was presented. In this model, the section is subjected to one- and two-dimensional stresses separately. The former resists the longitudinal stresses due to bending and torsion while the latter resists the shear stresses due to torsion. These two systems are related based on the compatibility of strains and the equilibrium of stresses in the longitudinal direction. The results predicted with this method are in good agreement with those obtained from the tests. Additionally, the interaction between bending and torsional strengths was discussed.

The flexural capacity of a new cold-formed hollow flange channel section known as LiteSteel beam (LSB) is limited by lateral distortional buckling for intermediate spans, which is characterized by simultaneous lateral deflection, twist, and web distortion. Recent research has developed suitable design rules for the member capacity of LSBs. However, they are limited to a uniform moment distribution that rarely exists in practice. Many steel design codes have adopted equivalent uniform moment distribution factors to accommodate the effect of nonuniform moment distributions in design. But they were derived mostly based on the data for conventional hot-rolled doubly symmetric I-beams subject to lateral torsional buckling. The effect of moment distribution for LSBs and the suitability of the current steel design code rules to include this effect for LSBs are not yet known. This paper presents the details of a research study based on finite-element analyses (FEAs) of the lateral buckling strength of simply supported LSBs subject to moment gradient effects. It also presents the details of a number of LSB lateral buckling experiments undertaken to validate the results of FEAs. Finally, it discusses the suitability of the current design methods and provides design recommendations for simply supported LSBs subject to moment gradient effects.

This paper presents an analytical model for the failure of lightly reinforced concrete slabs under elevated temperature, considering simply supported boundary conditions with planar edge restraints. This model is typically applicable to the failure assessment of composite floor slabs under fire, where the steel deck is assumed to lose strength relatively quickly, leaving a lightly reinforced concrete slab, as supported by experimental evidence. The proposed model accounts for membrane action, which arises at large slab deformations, and important it presents a rational failure criterion based on the rupture of the steel reinforcement. In this respect, this is the first analytical slab model to consider the influence of bond between steel and concrete on reinforcement rupture, while also dealing with elevated temperatures, including the potentially negative effects of thermal curvature. Based on principles of mechanics, detailed analytical forms of the model are first presented and verified for each of the ambient and elevated temperatures cases. In each case, a simplified form of the analytical model, which is more suitable for practical design-oriented application, is also proposed and verified. The successful verification of the proposed analytical models, demonstrated against the results of appropriate nonlinear finite-element analysis, paves the way for their application in the design of composite floor slabs with planar edge restraints under fire.

In many tunnel designs, lining cross section is selected based on construction requirements rather than design loads. A constant cross section is typically used along a tunnel especially for tunnels constructed by tunnel boring machines (TBMs). Factor of safety against failure is typically high at shallow depth regions of alignment. Minor repairable damage is targeted at rare events such as earthquakes, train derailments, explosions, and long-duration fires, and a reduction to service load factor of safety is applied in these types of events. The focus of this paper is to analytically investigate structural fire safety of circular tunnel linings in terms of reduction in service load safety due to time- and temperature-dependent material degradation and increase in load demand in a tunnel fire, and to develop recommendations for preliminary assessment of structural fire endurance of circular tunnel linings. Analytical methods accounting for thermal nonlinearity, material degradation, tunnel lining-ground interaction, and fire time stages are available to assess the structural fire safety of the concrete tunnel linings. Results of hydrocarbon fire tests of tunnel segments can be estimated by available analytical methods. TBM tunnels at soft soil can have a better fire performance compared to the ones located at stiff conditions with similar initial loading.

A new static-based method for damage detection of cable-stayed bridges using the changes in cable forces is presented in this paper. In recognizing the fact that damage incurring in bridge girders will cause a redistribution of forces in stay cables, the proposed method uses the measured changes in cable forces to detect the damage in bridge girders. In this method, the damage identification is formulated as an optimization problem in which the cable force error between measurement results and analytical model predictions is minimized. The sensitivities of cable forces with respect to the structural parameters of bridge girders for the linear and geometrically nonlinear bridge structures are obtained by using the direct differentiation method and the finite difference method, respectively, and are used to solve the optimization problem. Regularization technique is used to alleviate ill-conditioning in the solution course of the optimization problem. The validity of the method is illustrated by numerical studies of damage detection of the cable-stayed Sutong Bridge with a main span of 1,088 m. The effect of measurement noises and the number of measured cable forces on the accuracy of damage quantification is investigated. The proposed method correctly identifies the damage locations and damage magnitudes in bridge girders when using noise-free cable forces. It is also shown that structural damage can be quantified with acceptable accuracy in the case of low-level measurement noises and the damaged members being well spaced, and the identification accuracy is greatly improved when the forces in the cables connected to the damaged members are available for damage quantification.

In this paper, a simplified system identification procedure is developed to investigate the dynamic characteristics of base-isolated buildings equipped with lead-rubber bearings (LRBs). The multistory superstructure is assumed to be linear on the account of substantial reduction in seismic forces due to the installation of LRBs for which a bilinear hysteretic model is considered. The hysteretic model is in turn characterized by a backbone curve by which the multivalued restoring force is transformed into a single-valued function. With the introduction of backbone curves, the system identification analysis of inelastic structures is simplified to a large extent. The proposed algorithm extracts individually the physical parameters of each floor and the bearing system that are considered useful information in the structural health monitoring. A numerical example is conducted to demonstrate the feasibility of using the proposed technique for physical-parameter identification of partially inelastic based-isolated buildings.

Modeling the energy dissipation capacity of shear-yielding members is important in the evaluation of the seismic response of earthquake resistant structural systems. This paper presents the model of a frame element for the hysteretic behavior of these members. The model is based on a three-field variational formulation with independent displacement, stress, and strain fields. The displacement field is based on the Timoshenko beam theory. The nonlinear response of the element is derived from the section integration of the multiaxial material stress-strain relation at several control points along the element, thus accounting for the interaction between normal and shear stress and the spread of inelastic deformations in the member. With the derivation of the axial force-shear-flexure interaction of short members from the material response the proposed model is general, in contrast to existing concentrated plasticity models that require parameter calibration for different loading and support conditions. Furthermore, the model does not suffer from shear locking and does not require mesh refinement for the accurate representation of inelastic member deformations. Correlation studies of analytical results with available experimental data of the hysteretic behavior of shear-yielding members confirm the capabilities of the proposed model. Its computational efficiency makes it suitable for large scale simulations of the earthquake response of structures with shear-yielding members.

Real-time pseudodynamic (PSD) and hybrid PSD methods are a means to evaluate structural performance under dynamic loading when a component of a structural system cannot be accurately modeled and is load rate dependent. The real-time PSD and hybrid PSD testing of structures with full-scale elastomeric dampers is presented. A Bode diagram of the complete real-time testing system is used to design a velocity feed forward component that improves the tracking performance of the command displacements by the hydraulic actuator. To evaluate the accuracy of the tests, an on-line error tracking indicator is developed to monitor errors due to actuator lag or lead together with an amplitude indicator to reveal the amplitude errors in actuator measured displacements. The results from the real-time PSD and hybrid PSD tests are presented and evaluated using the error tracking indicators. The stability limits associated with a time delay for the tests are discussed and compared with delays that occurred in the tests.

This paper addresses a parameter identification problem of a linear dynamic system resulting from forced acceleration responses when input forces are unknown. The problem is attributed to an input force imposed on its impulse acceleration response function. An identification procedure is provided as a solution to reduce the error caused by the identification problem. The procedure applies the mode indicator function, the complex mode indication function, and the rational fraction polynomial method to identify the modal parameters of the dynamic system from the frequency response functions of the random decrement signatures. Applicability of this procedure is investigated through numerical simulations of a three degree-of-freedom dynamic system loaded by Gaussian white and color noise forces. For the purpose of comparison, the autoregressive model is also used to analyze the same example. Results show that the proposed procedure can improve the identification accuracy of the modal parameters from the acceleration-based random decrement signatures, when comparing with the use of the autoregressive model.