Neural network modeling of time-dependent creep deformations in masonry structures

Stresses and deformations in concrete and masonry structures can be significantly altered by creep. Thus, neglecting creep could result in un-conservative design of new structures and/or underestimation of the level of its effect on stress redistribution in existing structures. Brickwork has substantial creep strain that is difficult to predict because of its dependence on many uncontrolled variables. Reliable and accurate prediction models for the long-term, time-dependent creep deformation of brickwork structures are needed. Artificial intelligence techniques are suitable for such applications. A model based on radial basis function neural networks (RBFNN) is proposed for predicting creep and is compared to a multi-layer perceptron neural network (MLPNN) model recently developed for the same purpose. Accurate prediction of creep was achieved due to the simple architecture and fast training procedure of RBFNN model especially when compared to MLPNN model. The RBFNN model shows good agreement with experimental creep data from brickwork assemblages collected over the last 15 years.     

Numerical simulation of shrinkage and creep in patch-repaired axially loaded reinforced concrete short columns

This paper presents numerical results obtained from finite element simulations of the time-dependent behaviour of moderately loaded patch-repaired reinforced concrete short columns. Patch repair is a structural concrete repair method in which damaged concrete is replaced with one of a wide range of materials. Relative to the concrete substrate, the patch repair materials used in this study had different properties, such as the elastic modulus, shrinkage and creep. A priori, it would appear that simple theory such as the engineer's theory of bending cannot be used to quantify the behaviour of the patch-repaired member. Experimental evidence needed to clarify such issues remains scarce. The finite element simulations performed in this study indicate that shrinkage and creep cause the progressive shedding of the load carried by the patch repair to the concrete substrate. Finite element results compare favourably with the predictions of the engineer's bending theory. Relative to test results some qualitative agreement is observed though there is significant quantitative deviation.     

Application of ADINA and ADINAT in the creep analysis of heated concrete structures

ADINA and ADINAT have been used in the analysis of time-dependent changes of stress and strain occurring in heated concrete structures. The problems tackled include a simple concrete flat plate and a fast breeder reactor containment vessel. A direct method of solution, based on a variational principle, which does not require intermediate solutions at many small intervals of time, has been developed. This paper describes how ADINA can be used in conjunction with this method to predict time-varying stresses.     

Time-dependent creep and shrinkage analysis of composite beams curved in-plan

This paper develops a numerical formulation for the time-dependent creep and shrinkage analysis of steel–concrete composite beams that are curved in-plan under conditions of service load. The creep behaviour of the concrete is considered by using the viscoelastic Wiechert model, in which the aging effect of the concrete is taken into account. The curved composite beam model that is developed also accounts for the partial shear interaction at the deck-girder interface in the tangential (or longitudinal) direction, as well as in the radial (or horizontal) direction, due to the flexibility of the shear connectors. Models based on the developed formulation are validated by comparisons with sophisticated and computationally intensive ABAQUS shell element models, and with available results reported in the literature. The effects of initial curvature and partial interaction on the time-dependent behaviour of curved composite beams are also illustrated in the examples.     

Some applications of resultants to problems in computational geometry

Resultants were originally developed in the 18th and 19th centuries to solve certain simple algebraic problems. Here we shall present some applications of resultants to several important problems in computational geometry, including the implicitization, inversion, and intersection of parametric rational polynomial curves and surfaces.This paper was first delivered orally at the International Conference on Engineering and Computer Graphics in Beijing, China, held in August 1984     

Some applications of resultants to problems in computational geometry

Resultants were originally developed in the 18th and 19th centuries to solve certain simple algebraic problems. Here we shall present some applications of resultants to several important problems in computational geometry, including the implicitization, inversion, and intersection of parametric rational polynomial curves and surfaces.     

Tendon layout optimisation through configurational forces equilibration in plane stress analysis of prestressed concrete structures

The focus of this work is a novel application of configurational forces to tendon layout optimisation problems in prestressed concrete beams. To this end, the application of the method of the configurational forces is extended to two-dimensional plane stress analysis. The optimum tendon position is obtained by iteratively arriving at vanishing configurational forces. The performance of the optimisation algorithm is illustrated by means of numerical examples.     

FEM analysis of concrete structures subjected to mode-I and mixed-mode loading conditions

In this study, the finite element method (FEM) for a body containing displacement discontinuity is used for the investigation of tensile fracture behavior under mode-I and mixed-mode loading conditions in concrete structures. A mechanical model for the tensile fracture behavior is reduced to a mathematical problem, and the analysis method is proposed. With the aid of this method, several factors which govern tensile fracture are examined, such as the unloading path in the tension-softening behavior and the transmission of shear stresses across crack surfaces. A plain concrete beam without a notch is analyzed by first neglecting and then taking into account the unloading path in the tension-softening behavior to demonstrate the phenomenon of cracking localization in mode-I crack growth. Pullout test specimens of practical significance are analyzed in order to study the crack growth phenomenon under mixed-mode loading conditions. Cases with and without lateral confinement are considered and the results obtained from the present analysis are compared with those obtained from available experimental data. A simple model for shear transfer across crack surfaces is established. By incorporating this model in the program, a pullout test specimen with lateral confinement is analyzed to examine the influence of shear transfer across crack surfaces on cracking localization.     

Hydrodynamic loadings due to the motion of large offshore structures

A very effective non-reflecting boundary condition is proposed for the three-dimensional finite element analysis of hydrodynamic loads on offshore structures under motion due to earthquake, waves or machinery forces. The effect of surface waves is considered and the analysis is conducted in the frequency domain. The structure is assumed to be sufficiently large such that the non-linear effect of drag force is negligible. The unbounded extent of water surrounding the structure is assumed to be incompressible. The boundary condition, which is derived for the general analysis of structure-water interaction, is found to depend on the frequency of excitation, the location of the truncation boundary and the depth of water in the far field. Through some simplified numerical examples, it is shown that the proposed technique is very efficient for a wide range of the frequency of excitation. Incorporation of the boundary condition into a finite element program requires practically no extra effort.     

Thermo-elastic interactions in an infinite elastic solid due to distributed time-dependent heat sources in generalized thermo-elasticity III

The generalized theory of thermo-elasticity of Type III recently developed by Green and Naghdi is employed to study thermo-elastic interactions in a homogeneous isotropic unbounded solid having distributed instantaneous and continuous heat sources. The solutions are derived by using Laplace transform on time and then Fourier transform on space. It is found that the interactions consist of a wave travelling with the speed of dilatational wave and a diffusive part. The temperature and the deformation field are both continuous at the dilatational wave front while the stress field exhibits finite discontinuity at this location in case of instantaneous distributed heat sources. For continuous distributed heat sources, the thermal, deformation, and stress fields are however all continuous at the dilatational wave front. All the fields suffer exponential attenuation at the dilatational wave front and the attenuation is influenced by the thermo-elastic coupling and the thermal diffusivity of the medium. The results of the present analysis are compared to those derived by using other generalized thermo-elasticity theories such as L-S theory and G-L theory. The analysis reveals that G-N theory III eliminates some of the finite discontinuities and δ-function singularity in the deformation, temperature and stress fields derived by using other generalized thermo-elasticity theories in earlier investigations. Finally, numerical results applicable to a copper-like material are presented in order to illustrate the analytical result.     

Simulation of inelastic deformation in road structures due to cyclic mechanical and thermal loads

The paper presents modeling and simulation of inelastic deformation in road structures leading to rutting. Two material models, one for asphalt-concrete and another for unbound materials, have been calibrated to laboratory test data. The models account for the time and temperature dependent deformation of the asphalt-concrete as well as the friction and cyclic compaction of the unbound layers. The models have been used in simulations of a complete road structure exposed to cyclic mechanical and thermal loads. The simulation results are qualitatively similar to the rutting observed in reality. The simulations can either trace the complete history of the individual load cycles, or can be performed to directly obtain the accumulated effect after a certain number of load cycles. This makes it possible to get quickly the result due to a large number of loadings as desired in engineering design practice.     

Approaches to AI methodology in reinforced concrete structures: a case study

A system developed earlier for reinforced concrete halls layout is studied and justified from different aspects. Also, updates of both capabilities and limitations, which were recently incorporated in the system, are emphasized. Some of the original aspects of the system were reviewed and modified, to overcome some of the drawbacks in the earlier presentation. The system's organization remains almost the same. Some suggestions for further improvements to the system are presented as well. Finally the function of the system is illustrated by a sample run of the system.     

Large natural strains and some special difficulties due to non-linearity and incompressibility in finite elements

The finite element method is now a well established tool for the routine treatment of large linear problems, but the treatment of non-linear problems by the method is yet at the beginning.Section 1 of the present work extends the idea of natural strains and stresses to large strains in simplex finite elements.Section 2 applies some algorithms developed for structural dynamics to the problem of non-linear wave propagation including a case of shock development.Section 3 discusses with numerical examples special difficulties when displacement finite elements are used to solve problems with incompressible or nearly incompressible material.     

Semi-implicit representations of surfaces in P3, resultants and applications

In this paper we introduce an intermediate representation of surfaces that we call semi-implicit. We give a general definition in the language of projective complex algebraic geometry, and we begin its systematic study with an effective view-point. Our last section will apply this representation to investigate the intersection of two bi-cubic surfaces; these surfaces are widely used in Computer Aided Geometric Design.     

Cable stretching force optimization of concrete cable-stayed bridges including construction stages and time-dependent effects

This paper presents an optimization algorithm to compute the prestressing forces on concrete cable-stayed bridges to achieve the desired final geometry. The structural analysis includes the load history and geometry changes due to the construction sequence and the time-dependent effects due to creep, shrinkage and aging of the concrete. An entropy-based approach was used for structural optimization and discrete direct sensitivity analysis was used to evaluate the structural response to changes in the design variables. Numerical examples are presented and the results exhibit the importance of considering both the construction stages and the time-dependent effects for adequately predict the bridge behaviour and compute the cable prestressing forces.     

Unified concepts of constitutive modelling and numerical solution methods for concrete creep problems

In the first part of the paper an internal variable model is developed for concrete creep with the following features: creep strain is a linear functional of stress history; application or removal of stress produces instantaneous elastic response; upon removal of stress, creep deformations partially recover, approaching in the limit an irrecoverable part. The mechanism of each creep component is affected be temperature, humidity and loading age.In the second part the internal state variables are derived from concrete creep data with loading ages ranging from 7 to 4560 days and temperatures from 20° to 95°C. Several aging models are examined for which the parameters are determined via least squares optimization. A root mean square criterion is used to assess the approximation.In the third part a step by step algorithm is developed for incremental solution of the time-dependent creep process, noting that the governing equation of evolution lends itself to an economic computer solution without storing previous results. The one-dimensional constitutive statements are extended to multiaxial conditions and subsequently projected from the local level onto the structural level via finite elements (incremental initial load method).In the fourth part the creep behaviour of a prestressed concrete reactor vessel is examined for a loading history which includes hygrothermal effects due to drying and heating as well as prestress and cycling pressure.     

Optical behavior of some nanosized structures formed due to laser irradiation

With the help of laser method the metal nanoparticle suspensions (Ag, Zn, V) and diamond carbon films were formed. The effective size of metal nanoparticles in liquid media was defined due to the method of laser probing. The optical properties (transmission, index of refraction) of diamond carbon films and metal nanoparticle suspensions (absorption) were investigated. The dependence of carbon films thicknesses on deposition conditions were obtained.     

Modeling and simulation of damage in elastomer structures at high strains

A new model is proposed in order to simulate correctly the behaviour of elastomers for very large deformations. The hyperelastic Hart–Smith’s model is revisited in order to take into account the damage of elastomers and the opening and the closing of micro-defects. Applications concern structures with elastomer used in space industry. Shear loadings are typical solicitations for these structures. Whenever the material is able to undergo very large deformations (600–700%), the simulation of this type of experiment up to so high strains is not easy. The failure of industrial finite element code leads us to develop a prototype software using a non-incremental method (large time increment method) and a material formulation with “rotated” quantities. Some applications are shown for elastic or hyperelastic materials, especially for nearly incompressible materials.