Research on Shear Behavior of Sand–Structure Interface Based on Monotonic and Cyclic Tests

In order to study the shear behavior of the interface between sand and structure, a series of shear tests were carried out using an HJ-1 ring shear apparatus (Nanjing, China). First, through the monotonic shear tests, the loose sand and dense sand were sheared at the steel interface with different roughnesses. The results showed that when the interface was relatively smooth, the shear stress–shear displacement curves of loose sand and dense sand both exhibit strain hardening characteristics. When the interface was rough, the dense sand showed strain softening. The initial shear stiffness of the sand–steel interface increased with the increase in normal stress, interface roughness, or sand relative density. Then, considering the influence of initial shear stress, through the cyclic shear test, this work analyzed the shape of the loading and unloading curves and the development law of cumulative normal deformation, and discussed the change of loading and unloading shear stiffness under different stress level amplitudes and the residual deformation generated during the cycle. The research results showed that loose sand and dense sand generally shrunk in volume during the cycle. The initial loading process was similar to the case of static loading. In the later dynamic loading process, the shear shrinkage per cycle was relatively small and continued to develop. Additionally, it was found that the unloading stiffness of the sand–steel interface is always greater than the initial loading stiffness. As the number of cycles increases, the loading stiffness increases, and it may eventually approach the unloading stiffness.

Experimental and Numerical Investigations on High Performance SFRC: Cyclic Tensile Loading and Fatigue

In the present study, the capability of high-strength short steel fibers to control the degradation in high-performance concrete was experimentally examined and numerically simulated. To this end, notched prismatic high-performance concrete specimens with (HPSFRC) and without (HPC) short steel fibers were subjected to static and cyclic tensile tests up to 100,000 cycles. The cyclic tests showed that the rate of strain increase was lower for HPSFRC specimens and that the strain stagnated after around 10,000 cycles, which was not the case with HPC specimens. The microscopic examinations showed that in HPSFRC, a larger number of microcracks developed, but they had a smaller total surface area than the microcracks in the HPC. To further investigate the influence of fibers on the behavior of HPSFRC in the cracked state, displacement-controlled crack opening tests, as well as numerical simulations thereof, were carried out. Experiments have shown, and the numerical simulations have confirmed, that the inclusion of short steel fibers did not significantly affect the ultimate strength; however, it notably increased the post-cracking ductility of the material. Finally, the unloading/reloading behavior was examined, and it was observed that the unloading stiffness was stable even for significant crack openings; however, the hysteresis loops due to unloading/reloading were very small.

Evaluation of Linear Deformation and Unloading Stiffness Characteristics of Asphalt Mixtures Incorporating Various Aggregate Gradations

Optimum stiffness and linear deformation in the unloading phase are fundamental properties of asphalt mixtures required for the durability of flexible pavements. In this research, blends of six different aggregate gradations were used for two base course (BC) and four wearing course (WC) asphalt mixtures. Stability and indirect tensile strength of resulting asphalt mixtures were evaluated to relate to viscoelastic unloading deformation and resilient moduli (instantaneous (MRI) and total (MRT)) at 25 °C using a 40/50 binder for 0.1 and 0.3 s load durations. Results indicated that an increase in coarse aggregate proportion from 48 to 70% for BC has shown a 12% and 14% increase in MRT for 0.1 and 0.3 s load durations, respectively, and an increase in coarse aggregate proportion from 41 to 57.5% for WC has caused a 26% and 20% increase in MRI for 0.1 and 0.3 s load durations, respectively. The same coarse aggregate proportions showed an increase in linear viscoelastic deformation at 0.1 s load duration from 54.6 to 68.2% for WC and from 53.0 to 62.7% for BC, whereas for 0.3 s load duration linear viscoelastic deformation increased from 58.1 to 69.1% for WC and 64.3 to 69.2% for BC. The findings of this study will assist in the selection of aggregate gradations to be used in wearing and base course asphalt mixtures for pavement design, construction and maintenance.

EXPERIMENTAL RESPONSE OF A LOW-YIELDING, SELF CENTERING, ROCKING COLUMN BASE JOINT WITH FRICTION DAMPERS

The sliding hinge joint (SHJ) is a supplemental energy dissipation system for column bases or beam-to-column connections of steel Moment Resisting Frames (MRFs). It is based on the application of symmetric/asymmetric friction dampers to develop a dissipative mechanism alternative to the column/beam yielding. This typology was initially proposed in New Zealand and, more recently, is starting to be tested and applied also in Europe. While on the one hand this technology provides great benefits such as the damage avoidance, on the other hand, due to the high unloading stiffness of the dampers in tension or compression, its cyclic response is typically characterized by a limited self-centering capacity.To address this shortcoming, the objective of the work herein presented is to examine the possibility to add to these connections also a self-centering capacity proposing new layouts based on a combination of friction devices (providing energy dissipation capacity), pre-loaded threaded bars and disk springs (introducing in the joint restoring forces).In this paper, as a part of an ongoing wider experimental activity regarding the behavior of self-centering connections, the attention is focused on the problem of achieving the self-centering of the column bases of MRFs by studying a detail consisting in a column-splice equipped with friction dampers and threaded bars with Belleville disk springs, located above a traditional full-strength column base joint. The main benefits obtained with the proposed layout are that: (i) the self-centering capability is obtained with elements (threaded bars and Belleville springs) which have a size comparable to the overall size of the column-splice cover plates; (ii) all the re-centering elements are moved far from the concrete foundation avoiding any interaction with the footing. The work reports the main results of an experimental investigation and the analysis of a MRF equipped with the proposed column base joints.

Static and dynamic effective moduli of elastic-perfectly plastic granular aggregates under normal compression

Based on an existing simplified theoretical model for the normal contact interaction between two elastic-perfectly plastic spherical particles, we derived explicit expressions for the static and dynamic normal and dynamic tangential contact stiffnesses of elastic-perfectly plastic two-particle combination at pre-yield, yield, and post-yield conditions of normal loading. We used “static stiffness” or “loading stiffness” to refer to the slope of the force-displacement curve during monotonically increasing load. The “dynamic stiffness” or “unloading stiffness” refers to the stiffness that controls the speed of infinitesimal strain elastic waves propagating through the contacts. The static and dynamic contact stiffnesses are compared with numerical modeling of a two-sphere combination using the finite-element method. Furthermore, we used the explicit expressions for contact stiffnesses with the commonly used statistical averaging scheme to derive the static and dynamic effective bulk and shear moduli of a dry, random packing of identical elastic-perfectly plastic spherical particles. Elastic contact/mechanics-based effective medium models are unable to model the growth of contact area between inelastic (e.g., plastic) particles under normal force, which results in inaccurate predictions of contact stiffnesses and effective moduli. Once the particle reaches the limit of elasticity with onset of plastic deformation (yielding), further loading of two elastic-perfectly plastic spherical particles leads to a larger contact area than for two elastic particles under the same normal loading. As a result, after yielding, the dynamic effective moduli become stiffer than the corresponding moduli in the elastic case, whereas the static effective moduli remain constant, rather than increasing as in the elastic case.

HYSTERESIS CHARATERISTICS OF SQUARE CFT COLUMNS SUBJECTED TO AXIAL FORCE AND BENDING MOMENT

Hysteresis characteristics of concrete filled steel tubular columns have been proposed. However, it has not been clarified if the skeleton curve could be applied for slender columns, and there are few studies about the unloading stiffness of the hysteresis loop. The purpose of this study is to compare the skeleton curves of square CFT columns, which have been proposed by the moment-rotation angle relationships and the experimental data by the previous research. The relationship between the initial stiffness obtained by the theoretical calculation and the unloading stiffness obtained by the experimental study are shown. Also the effects of the axial force ratio and the width-thickness ratio on the above mentioned relationships are discussed. The loading condition of the experimental study is monotonic and cyclic loading. It is found that the skeleton curve evaluates conservatively the bending moment-rotation angle relationships obtained by the test in both loading conditions, except for the widththickness ratio 43 specimen with cyclic loading. The ratio of unloading stiffness exKr obtained from the experimental data and calculated theoretical stiffness K was shown. The effect of the axial force ratio on the exKr/K was observed when the rotation chord angle was more than 1% and the width-thickness ratio increased as exKr/K decreased.

Study on Hysteresis Model of Welding Material in Unstiffened Welded Joints of Steel Tubular Truss Structure

The weld form of intersecting joints in a steel tubular truss structure changes with the various intersecting curves. As the key role of joints in energy dissipation and seismic resistance, the weld is easy to damage, as a result the constitutive behavior of the weld is different from that of the base metal. In order to define the cumulative damage characteristic and study the constitutive behavior of welded metal with the influence of damage accumulation, low-cycle fatigue tests were carried out to evaluate overall response characteristics and to quantify variation of cyclic stress amplitude, unloading stiffness and energy dissipation capacity. The results show that the cyclic softening behavior of welding materials is apparent, however, the steel shows hardening behavior with the increase of cyclic cycles, while the cyclic stress amplitude, unloading stiffness, and energy dissipation capacity of the welding materials degenerate gradually. Based on the Ramberg–Osgood model and introducing the damage variable D, a hysteretic model of welding material with the effect of damage accumulation was established, including an initial loading curve, cyclic stress-strain curve, and hysteretic curve model. Further, the evolution equation of D was also built. The parameters reflecting the damage degradation were fitted by the test data, and the simulation results of the model were proved to be in good agreement with the test results.

Seismic Performance of Steel Frames with Semirigid Connections

The nonlinear stiffness matrix method was incorporated to investigate the structural performance of steel portal frames with semirigid connections. A portal frame with unstiffened extended end-plate connection was designed to demonstrate the adequacy of the proposed method. Besides, the seismic performance of steel portal frames with semirigid connections was investigated through time history analysis where kinematic hysteresis model was assigned to semirigid connections to account for energy dissipation and unloading stiffness. Based on the results of the study, it was found that generally semirigid connections influenced the force distribution which resulted in the decrease in base shear and lighter frame compared to the rigid one. The results also indicated that there was no direct relationship between maximum displacement at the top and connection stiffness in high-rise frames.

Damage Analysis and Evaluation of High Strength Concrete Frame Based on Deformation-Energy Damage Model

A new method of characterizing the damage of high strength concrete structures is presented, which is based on the deformation energy double parameters damage model and incorporates both of the main forms of damage by earthquakes: first time damage beyond destruction and energy consumption. Firstly, test data of high strength reinforced concrete (RC) columns were evaluated. Then, the relationship between stiffness degradation, strength degradation, and ductility performance was obtained. And an expression for damage in terms of model parameters was determined, as well as the critical input data for the restoring force model to be used in analytical damage evaluation. Experimentally, the unloading stiffness was found to be related to the cycle number. Then, a correction for this changing was applied to better describe the unloading phenomenon and compensate for the shortcomings of structure elastic-plastic time history analysis. The above algorithm was embedded into an IDARC program. Finally, a case study of high strength RC multistory frames was presented. Under various seismic wave inputs, the structural damages were predicted. The damage model and correction algorithm of stiffness unloading were proved to be suitable and applicable in engineering design and damage evaluation of a high strength concrete structure.

A Study on the Mechanical Properties of Disc Spring Vibration Isolator with Viscous Dampers

In present study, the mechanical properties of disc-spring vibration isolator with viscous dampers is investigated by finite element analysis (FEA) and experiments. The result of FEA and static test reveals that frictional damping has significant effect on the static stiffness of combination vibration isolator, the ability to strengthen loading stiffness and lower unloading stiffness. However viscous damper has no effect on the static mechanical properties. The dynamic experiment shows that the dynamic performance of combination vibration isolator can be greatly improved with viscous dampers. Viscous dampers have little effect on dynamic stiffness of vibration isolator, but significant effect on damping characteristics. These conclusions are of significant importance in the analysis and design of combination disc-spring vibration isolators.