Journal of Structural Engineering

A new connection system for a concrete filled steel tube composite column and reinforced concrete beams is proposed. In this connection, the steel tube is interrupted while the reinforced concrete beams are continuous in the joint zone. Multiple lateral hoops that constitute the stiffening ring are used to confine the core concrete in the connection zone. The transfer of moment at the beam ends can be ensured by continuous rebars; the weakening of the axial load bearing capacity due to the interruption of the steel tube can be compensated by the confinement of the stiffening ring. Using these configurations, concrete casting and tube lifting can be made more convenient since welding and hole drilling in situ can be avoided. Axial compression experiments on six specimens and reversed cyclic loading tests on three interior column specimens and three corner column specimens were conducted to evaluate this new beam-column system; load-deflection performance, typical failure modes, stress and strain distributions, and the energy dissipation capacity were obtained. The experimental results showed that the effective confinement can be achieved by the stiffening ring, and an excellent axial bearing capacity can be obtained, as well as a superior ductility and energy dissipation capacity. As a new connection system for the concrete filled steel tube composite column with reinforced concrete beams, it can also be applied to other types of confined concrete columns.

The concept of confined concrete has been widely accepted throughout the structural engineering field, such as the spiral, steel tube, or carbon fiber composite shell confined concrete, etc. In the present study, this concept has not only been used in the structural columns, but also in joints. In this connection system, the steel tube is interrupted at the floor, the concrete in the connection zone is confined by a stiffening ring with multiple lateral hoops, and the reinforced concrete beams are continuously arranged through the joint. The structural behavior of this new connection system in axial compression tests and reversed cyclic loading tests is presented in a companion paper. In the present study, the bearing strength of composite columns and the joint in axial compression is obtained based on the stress and strain analyses. The critical volume fraction of transverse stirrups of the composite column and the effective confining radius in the stiffening ring are proposed based on the thick cylinder model with the assumption of plane stress state. By using these solutions, it is favorable to obtain the stress distribution in confining concrete and the bearing strength of the confined concrete. For the reversed cyclic loading tests, the Clough hysteresis model is used to simulate the hysteresis loops of the specimens without considering the stiffness degradation in the unloading process. The results of the theoretical modeling are generally in good agreement with the experimental observations.

Full-scale laboratory tests on $\mathrm{1,270}\mathrm{mm}$ $\left(50\mathrm{in.}\right)$ long, $305\mathrm{mm}$ $\left(12\mathrm{in.}\right)$ diameter steel pipes were carried out to investigate their postwrinkling behavior when subjected to monotonic and cyclic bending loads. Under monotonically increasing bending loads and deformations, the test specimens exhibited the ability to undergo significant plastic deformation and did not fail in fracture (leak or rupture in the pipe wall). However, when subjected to cyclic elastic-plastic strain reversals due to cyclic bending deformations, fracture developed at the wrinkle. This study indicated that the current pipeline design method using wrinkle limit strain is conservative. The paper presents test procedures, postwrinkling behavior, and fracture limit strain values obtained from four full-scale tests.

A comprehensive study has been carried out to investigate the correlation between maximum interstory drift demand of frame structures and a number of widely used ground motion intensity parameters. Nonlinear time–history analyses of 16 reinforced concrete frames were performed under a set of 80 ground motion records. The frames were selected to represent features of typical low- to mid-rise reinforced concrete structures. The records contained in the ground motion database were compiled from recorded ground motions with the intention to represent a wide range of seismic forces that impose various degrees of elastic as well as inelastic response of the frames. The maximum interstory drift ratios computed were compared with the ground motion intensity parameters employed and correlations between them were investigated through coefficients of correlation and determination. The results revealed that spectrum intensity parameters, having the strongest correlation, are superior to other parameters such as peak ground velocity, peak ground acceleration, and spectral acceleration.

Improved numerical integration procedures are essential for the extension of hybrid numerical and experimental simulation to large and complex structural systems. While implicit integration algorithms are widely used in pure numerical simulations for their superior stability and accuracy, their direct application to hybrid simulation has been partially limited by difficulties in estimating the tangent stiffness matrix of multi-degree-of-freedom experimental substructures. Current applications of hybrid simulation using integrators with improved stability have mostly resorted to methods that are noniterative or utilize the initial stiffness matrix for iterative corrections. To improve the accuracy of integration procedures for hybrid simulation, a new method for online estimation of experimental tangent stiffness is proposed. The stiffness estimation procedure is tailored for fast online applications by transforming the measurements into a coordinate system, which reduces the number of unknown stiffness coefficients that need to be updated during the simulation. The updated experimental stiffness matrix is used in a modified operator-splitting integration scheme to improve the accuracy of hybrid simulations with highly nonlinear experimental substructures. The application and effectiveness of the proposed approach is demonstrated through hybrid simulations with multi-degree-of-freedom experimental substructures.

The work described in the present article is concerned with the numerical investigation of the dynamic response of reinforced concrete (RC) beams subjected to high rates of transverse loading. Of interest herein is the case of localized impact loading, such as that encountered in the case of contact, impact, and ballistic problems, rather than the case of loads distributed over the entire span of RC structural elements arising particularly from far field explosions. The investigation is based on the use of a commercially available general purpose finite-element (FE) package for nonlinear static and dynamic analysis of three-dimensional FE models. A key feature of our study is the hypothesis that the material properties of concrete and steel reinforcement are independent of the loading rate. Based on this assumption, the effects of the applied loading rate on the exhibited structural response are primarily attributed to the inertia forces that develop within the beam and not to the loading rate sensitivity of the mechanical characteristics of the materials involved. This hypothesis constitutes a major departure from currently accepted design and numerical modeling practices, which adopt exactly the opposite view, thus, providing an alternative explanation as to the causes that affect the complex inelastic response of RC structural elements under high loading rates, as well as the cracking patterns, observed during testing. The results obtained correlate closely with the experimental observation that structural performance, in the form of stiffness, load carrying capacity, and deformability, depart significantly from those recorded under quasi-static loading as certain thresholds of applied loading rates are surpassed, with these changes becoming more pronounced as the rate of loading increases. From the analysis of numerical and experimental predictions, the causes that lead to the above change in behavior are established and a relatively simple design model is proposed, which is able to quantify the observed increase in load-carrying capacity exhibited by RC beams with increasing rates of applied loading as well as the concentration of damage.

This paper presents a statistical model of the dynamic loads produced by humans jumping on structures, which realistically captures the randomness in the jumping process. The model is based on a large database of experimentally measured individual jumping loads. Key parameters are introduced to characterize the timing and shape of the jumping impulses, taking account of the variability between jumps and between individuals. Probability distribution functions are fitted to these parameters and the correlation relationships between the various parameters are investigated. Through repeated sampling, a realistic ensemble of individual jumping loads can be generated. This can be used to assess the dynamic response of assembly structures, such as grandstands when excited by rhythmic motion of large crowds moving in time to a musical beat.

A finite-element model for nonlinear analysis of reinforced concrete flat plate structures is presented. A flexible layering scheme incorporating the transverse shear deformation is formulated in shell element environment. Each node of the layered shell element can be specified as either a normal node or a node with shear correction. A three-dimensional hypoelastic material model is implemented to model reinforced concrete. The cracking effects of tension softening, aggregate interlock, tension stiffening, and compression softening in multidirectionally cracked reinforced concrete are incorporated explicitly and efficiently. A flat plate, a flat slab with drop panel, and a large size flat plate with irregular column layout have been analyzed. The influence of the distribution of transverse shear strain on the punching shear failure mode has been identified in the numerical studies. The proposed finite-element model has been proved to be capable of simulating the localized punching shear behavior of slab–column connections and to be suitable for global analysis of structural performance of flat plate structures.

This paper presents both the results of an experimental program and numerical simulations, which have been conducted to investigate the ductility of composite beams in the hogging moment regions under fire conditions. The main objective is to study the ductility issue related to the inelastic behavior of composite beams at an elevated temperature. A second objective is to investigate the feasibility of using a finite-element model for parametric studies. To fullfil the first objective, a total of four composite beams with decking slabs were tested to failure. They were designed to represent the internal joint of a continuous beam. The segment between the plastic hinge over support and adjacent point of inflection was represented by one-half of a simply supported beam subjected to a midspan point load. The specimens were heated to a certain temperature before they were subjected to a static point load up to failure. These four test results were then used to validate finite-element models constructed from MSC. Marc Mentat. It is demonstrated that the finite-element analysis gives reasonable accuracy compared to test results. Hence, it provides an efficient, economical, and yet accurate tool to study the inelastic behavior of composite beams in fire. Finally, a parametric study was conducted to investigate the influence of several parameters on the moment-rotation response and the rotational capacity at elevated temperatures.

The consistency of measurements in various wind tunnels is of concern to designers and code writers. This study attempts to quantify the variability of wind effects estimates based on tests conducted at six wind tunnel laboratories. Pressure tap measurements were made on wind tunnel models of four buildings. Comparisons were made between estimated $50\text{th}$ percenttiles of (1) peak positive moments in a frame section near the knee joint and (2) peak pressure coefficients of a roof tap nearest a building corner. Modeling of suburban terrain contributes significantly to the variability. Other factors are eave height, wind direction, and frame location within the building. Coefficients of variation were about 10–40%. A subsequent phase of this research entails a detailed analysis of the reasons for the variabilities. The results could help future improvement of wind load factors that account for all relevant uncertainties in the estimation of wind effects and efforts to improve and standardize wind tunnel simulations.