Fiber Element for Cyclic Bending and Shear of RC Structures.?II: Verification

The fiber beam element with shear modeling developed in the companion paper is calibrated and verified by comparison with test data. The verification is carried out for the material constitutive behavior and for single beam and column elements using available test results from literature. A structural analysis of a shear sensitive viaduct pier subjected to ground input motion is presented. Details of the algebraic expressions used for the concrete and steel constitutive behaviors are provided.     

Probabilistic Nonlocal Theory for Quasibrittle Fracture Initiation and Size Effect.?I: Theory

The nonlocal generalization of Weibull theory previously developed for structures that are either notched or fail only after the formation of a large crack is extended to predict the probability of failure of unnotched structures that reach the maximum load before a large crack forms, as is typical of the test of modulus of rupture (flexural strength). The probability of material failure at a material point is assumed to be a power function (characterized by the Weibull modulus and scaling parameter) of the average stress in the neighborhood of that point, the size of which is the material characteristic length. This indirectly imposes a spatial correlation. The model describes the deterministic size effect, which is caused by stress redistribution due to strain softening in the boundary layer of cracking with the associated energy release. As a basic check of soundness, it is proposed that for quasibrittle structures much larger than the fracture process zone or the characteristic length of material, the probabilistic model of failure must asymptotically reduce to Weibull theory with the weakest link model. The present theory satisfies this condition, but the classical stochastic finite-element models do not, which renders the use of these models for calculating loads of very small failure probabilities dubious. Numerical applications and comparisons to test results are left for Part II.     

Hydrodynamics of Turbid Underflows.?I: Formulation and Numerical Analysis

A mathematical model is developed for unsteady, two-dimensional, single-layer, depth-averaged turbid underflows driven by nonuniform, noncohesive sediment. The numerical solution is obtained by a high-resolution, total variation diminishing, finite-volume numerical model, which is known to capture sharp fronts accurately. The monotone upstream scheme for conservation laws is used in conjunction with predictor-corrector time-stepping to provide a second-order accurate solution. Flux-limiting is implemented to prevent the development of spurious oscillations near discontinuities. The model also possesses the capability to track the evolution and development of an erodible bed, due to sediment entrainment and deposition. This is accomplished by solving a bed-sediment conservation equation at each time step, independent of the hydrodynamic equations, with a predictor-corrector method. The model is verified by comparison to experimental data for currents driven by uniform and nonuniform sediment.     

Probabilistic Nonlocal Theory for Quasibrittle Fracture Initiation and Size Effect.?II: Application

The nonlocal probabilistic theory developed in Part I is applied in numerical studies of plain concrete beams and is compared to the existing test data on the modulus of rupture. For normal size test beams, the deterministic theory is found to dominate and give adequate predictions for the mean. But the present probabilistic theory can further provide the standard deviation and the entire probability distribution (calculated via Latin hypercube sampling). For very large beam sizes, the statistical size effect dominates and the mean prediction approaches asymptotically the classical Weibull size effect. This is contrary to structures failing only after the formation of a large crack, for which the classical Weibull size effect is asymptotically approached for very small structure sizes. Comparison to the existing test data on the modulus of rupture demonstrates good agreement with both the measured means and the scatter breadth.     

Adaptive FE Analysis of RC Shells.?II: Applications

This paper deals with the application of an adaptive calculation scheme to ultimate load analyses of reinforced concrete (RC) plates and shells. The influence of the user-prescribed accuracy on the numerical results, especially on the ultimate load, is investigated. Three examples are considered: (1) a shear wall panel; (2) a circular plate; and (3) an RC cooling tower subjected to dead load and wind load. In addition to adaptive finite element (FE) analyses, single-mesh calculations on the basis of uniformly refined meshes are performed.     

Flutter and Buffeting Analysis.?I: Finite-Element and RPE Solution

A finite-element formulation is developed for analyzing flutter instability and buffeting response of long-span bridges and their interaction. The flutter derivatives, instead of the indicial functions used by previous researchers, are applied in the random parametric excitation (RPE) analysis. This application makes finite-element formulation possible and results in much less computational effort in RPE analysis than those of previous analyses. With the finite-element program developed in the present study, as many modes as desired can be easily included in the flutter and buffeting analyses. Users have the choice of RPE or eigenvalue method for flutter analysis and RPE or spectral method for buffeting analysis.     

Numerical Modeling of Silo Filling.?I: Continuum Analyses

Two main computational methods have been used in recent years to model the behavior of particulate solids in silos. These are the finite element method and the discrete element method. To assess the current state of the art in the two methods applied to silo problems, and to evaluate their capabilities without bias, an international collaborative project was set up to compare predictions of several silo phenomena. The first of these computational challenges was deemed the simplest: that of filling a silo or container with particulate solid. This paper presents an overview of the findings of this first problem, based on a total of 38 independent calculations. Here the collaborative project is briefly outlined and some deduced outcomes from calculations by continuum analysts are compared. The results of discrete element calculations are described in a companion paper, which also compares the two methods and comments on their strengths and weaknesses.     

Optimal Furrow Design.?I: Time of Advance Equation

A particularly simple equation is developed to explicitly calculate the time of advance in surface irrigation. The proposed equation can be used for routine furrow irrigation design. The equation is derived as an extension of the small-time, large-time explicit solutions. Systematic comparisons of dimensionless solutions indicated that the prediction of the equation is in agreement with the accurate zero-inertia numerical solutions. The accuracy of the simple U.S. Soil Conservation Service explicit time of advance empirical equation was also evaluated. Comparisons have shown that the U.S. Soil Conservation Service equation generally performed poorly. The suggested equation is also compared with observed furrow data and with the accurate zero-inertia model. The provided predictions are in agreement with the numerical model and observed data.     

Finite Strain, Anisotropic Modified Cam Clay Model with Plastic Spin.?I: Theory

There are several approaches that can take into account the micromechanical and anisotropic behavior of soils. One is Dafalias's plastic spin approach. It is a convenient approach that utilizes an internal variable called “the plastic spin tensor.” The plastic spin is a function of the embedded stress (back stress) that causes the induced anisotropy and, consequently, the isotropic soil models cannot account for it. This paper presents a formulation of the plastic spin and related constitutive equations based on Dafalias's “anisotropic modified Cam clay model” with an updated Lagrangian reference frame. The required parameters are the typical consolidation and stress-strain parameters. The only additional parameters used in this work are the back-stress parameters c and x. These features are relatively simple and easy to use compared to other models.     

Speciation and Chemical Interactions in Nitrifying Biofilms.?I: Model Development

Steady-state models of nitrifying biofilms are developed taking into account the mass transfer of neutral and ionic species, electroneutrality, pH-dependent Monod kinetics, chemical equilibrium, and the presence of a boundary layer. Due to the assumption of bulk conditions, the rigorous biofilm model often predicts the washout of one of the biofilm species involved in a series of sequential biofilm transformations such as nitrification. Coupling the rigorous biofilm model with reactor mass balances yields the rigorous reactor model, which calculates the bulk conditions where both of the biofilm species remain and is thus more widely applicable. Substantial changes in the pH across the boundary layer and the biofilm are predicted; therefore, rates of nitrification may be improved by increasing bulk pH to high (alkaline) values, which results in optimum (neutral) pH within the biofilm. Varying influent conditions such as pH and buffer capacity can result in significant changes in reactor performance with respect to complete nitrification. The validity of the nitrification models are tested by comparison of profile predictions with that of experimental data, with encouraging results.     

Seismic Repair and Strengthening of Lap Splices in RC Columns: Carbon Fiber–Reinforced Polymer versus Steel Confinement

A design approach, developed specifically for seismic bond strengthening of the critical splice region of reinforced concrete columns or bridge piers, is presented and discussed. The approach is based on providing adequate concrete confinement within the splice zone for allowing the spliced bars to theoretically develop enough postelastic tension strains demanded by large earthquakes before experiencing splitting bond failure. The accuracy of the approach was validated experimentally by evaluating the seismic behavior of full-scale gravity load-designed (as-built) rectangular columns that were strengthened or repaired in accordance with the proposed approach. Three types of confinement were used and compared, namely, internal steel ties, external fiber polymer reinforced jackets, and a combination of both. The repaired/strengthened columns developed sizable postyield strains of the spliced bars, considerable increases in the lateral load and drift capacities, and much less concrete damage within the splice zone when compared with the as-built columns. As a further support of the adequacy of the design strengthening approach, the backbone lateral load-drift response of the strengthened columns showed a good agreement with the envelope response generated using nonlinear flexural analysis assuming perfect bond between the column reinforcement and concrete.     

Analysis of Surface Longitudinal Crack of Slab and Crack of Cold Bending for Cu-P-RE Weathering Steels

The surface morphology of longitudinal crack at corner of slab and the crack morphology of cold bending for Cu-P-RE weathering sleels were analyzed by metalographic examination and SEM.The feature of erack and the composition of inclusions on the surface of erack were detected by EMPA,     

Developing Dislocation Subgrain Structures and Cyclic Softening During High-Temperature Creep–Fatigue of a Nickel Alloy

The complex cyclic deformation response of Alloy 617 under creep–fatigue conditions is of practical interest both in terms of the observed detriment in failure life and the considerable cyclic softening that occurs. At the low strain ranges investigated, the inelastic strain is the sole predictor of the failure life without taking into consideration a potentially significant environmental influence. The tensile-hold creep–fatigue peak stress response can be directly correlated to the evolving dislocation substructure, which consists of a relatively homogenous distribution of subgrains. Progressive high-temperature cycling with a static hold allows for the rearrangement of loose tangles of dislocations into well-ordered hexagonal dislocation networks. The cyclic softening during tensile-hold creep–fatigue deformation is attributable to two factors: the rearrangement of dislocation substructures into lower-energy configurations, which includes a decreasing dislocation density in subgrain interiors through integration into the subgrain boundaries, and the formation of surface grain boundary cracks and cavity formation or separation at interior grain boundaries, which occurs perpendicular to the stress axis. Effects attributable to the tensile character of the hold cycle are further analyzed through variations in the creep–fatigue waveform and illuminate the effects of the hold-time character on the overall creep–fatigue behavior and evolution of the dislocation substructure.     

Are Available Models Reliable for Predicting the FRP Contribution to the Shear Resistance of RC Beams?

In this paper the trustworthiness of the existing theory for predicting the fiber-reinforced plastic contribution to the shear resistance of reinforced concrete beams is discussed. The most well-known shear models for external bonded reinforcement are presented, commented on, and compared with an extensive experimental database. The database contains the results from more than 200 tests performed in different research institutions across the world. The results of the comparison are not very promising and the use of the additional principle in the actual shear design equations should be questioned. The large scatter between the predicted values of different models and experimental results is of real concern bearing in mind that some of the models are used in present design codes.     

Cyclic deformation of ferritic steel—I. Stress-strain response and structure evolution

The complete CSS-curve has been established for a low alloyed structural steel tested under plastic strain control. The curve which can be divided into three separate regimes has a plateau region between cyclic strain levels of 10⁻⁴ and 8·10⁻⁴. PSBs are formed on the specimen surface when the plastic strain range corresponds to the plateau regime. The PSBs are sites for crack nucleation. The substructure evolution as seen going along the CSS-curve and with accumulating numbers of cycles is documented in detail and includes: dislocation loops, veins, walls including the ladder-like walls usually associated with PSBs, labyrinths, cells, subgrains, banded cells and subgrains. At low and intermediate plastic strain ranges the surface grains contain a more fine-scaled substructure and develop features which appear in the interior at higher plastic strain ranges. At larger cyclic strain levels microbands and noncrystallographic deformation bands become dominating features. Heavily displaced and serrated grain boundaries are observed at intermediate and high plastic strains both in interior- and surface grains containing microbands, banded cells or banded subgrains.