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The behaviour of partially grouted (PG) masonry shear walls is complex due to masonry's anisotropic nature and the nonlinear interactions among its constituents. Currently available code- and research-based shear strength equations provide highly variable results when predicting the in-plane shear strength of PG masonry walls. It is crucial to develop a greater understanding in this area, as sudden shear failures of masonry walls can lead to catastrophic losses of human life and property. This study presents the development of several new in-plane shear strength models for PG masonry walls using stepwise regression and model trees with data compiled from 292 experimentally tested walls. The models are found to significantly outperform existing code- and research-based shear strength equations. It was found that, of the variables studied, the most significant ones are the axial load, wall geometry, compressive strength of mortar, and area of interior vertical reinforcement. Key words: partially grouted, masonry wall, prediction models, shear strength, stepwise regression, model trees, 1. Introduction The behaviour of masonry shear walls is complex due to the interactions between concrete blocks, mortar, grout, and horizontal and vertical reinforcing steel (Hung 2018). Partially grouted (PG) [...]

In many injection molded parts weld lines are often unavoidable. These cause optical defects and a reduction of the mechanical properties of the part. Therefore, the predictability of the weld line strength at an early stage of development would provide a significant advantage by avoiding costly iterations of the mold and increases the understanding of the correlation between process history of the melt and weld line strength. For this purpose, a calculation routine has been developed to predict the weld line strength based on injection molding simulation. Different models to calculate the healing of a weld line are compared and analyzed. By adding a factor to consider the shear rate in addition to the temperature and the pressure and after calibration to one design of experiment setting of the experimental data, the prediction of the weld line strength shows good agreement for all examined process setpoints of the experimental data for polystyrene. KEYWORDS amorphous thermoplastics, injection molding, modeling, polystyrene, simulations, 1 | INTRODUCTION Weld lines can reduce the static strength of the part by up to 50%. The extent of this reduction in strength is highly dependent on the material [...]

Results of tests performed on joints used in timber-frame construction allow characterizing the variability of their mechanical behavior, which differs substantially from one joint to the next. The parameters of a constitutive model of the joints and their variability are identified. Finite element (FE) models of a shear wall and a timber-frame house are used in nonlinear dynamic calculations to study the propagation of uncertainty through the structure. It demonstrates that for a single-story 6 m x 6 m house, the variations in mechanical strength of each connection do not significantly affect the structural behavior of the house. Both the numerical and experimental results (on a shaking table) are quite similar, proving the model accuracy, its ability to study the propagation of uncertainty and its relevance for future development (non-regular, multi-story buildings...). Moreover, a sensitivity analysis performed on a FE wall model under uncertain seismic loads reveals the importance of earthquake motion modeling. Key words: timber joint resistance, variability, wood shear walls, probabilistic propagation of uncertainty, seismic loading. Cet article synthetise de nombreux resultats d'etudes experimentales aux echelles d'assemblages, de murs, et de maison a ossature en bois. Le nombre important de resultats a l'echelle des assemblages permet de caracteriser la variabilite de leur resistance. Un calage probabiliste d'une loi de comportement des assemblages est propose. Une etude de propagation d'incertitudes a travers les modeles elements finis de murs de contreventement est ensuite menee. L'influence de la variabilite materielle est estimee a l'echelle structurale via des modeles 2D et 3D, qui sont confrontes a des resultats d'essais sur table vibrante de murs et d'une maison. L'influence de la variabilite materielle se revele faible. Ces resultats montrent que le modele numerique reproduit de maniere satisfaisante les resultats experimentaux, qu'il permet de travailler sur la propagation d'incertitudes materielles et qu'il propose une base solide pour le futur developpement de modeles de structures plus complexes (irregularite en plan, en elevation, batiments de plusieurs etages). Enfin, la variabilite du chargement sismique est evaluee en utilisant une methode de generation d'accelerogrammes, les resultats a l'echelle du mur montrent qu'elle est non negligeable. Mots-cles : resistance d'assemblages, variabilite, murs de contreventement, propagation d'incertitudes probabiliste, chargement sismique., 1. Introduction Timber structures featuring metal connectors are fairly resistant under cyclic and seismic loadings thanks to joint ductility. The high energy dissipation due to the semi-rigid behavior of the [...]

One important component in the design of tunnels in urban areas is a correct assessment of the interaction between the underground excavation with other structures in the vicinity. In this sense a correct stress-strain response by the model representing the rock mass behavior is essential. The shear and normal displacement of rock discontinuities and their shear and normal stiffness control the distribution of stress and displacement within a discontinuous rock mass. In conditions where an equivalent continuum based approach is not applicable, the joint material model should be able to describe important mechanisms such as asperity sliding and shearing, post-peak behavior, asperity deformation, and the effect of soft infilling. The distinct element code UDEC was used to simulate the direct shear tests on a natural joint profile, and the prediction of two existing models of discontinuity strength and deformation were then compared with a new soil-infilled joint model and with experimental data for clean and soil-infilled rock joints. A numerical modeling of a cavern excavated in a jointed medium is also presented to illustrate the response of different models. The proposed soil- infilled joint model described more comprehensively the occurrence of dilation and compression with lateral displacements and also better represented the double peak shearing in relation to the adopted squeezing mechanism that could not be captured by the two existing models. DOI: 10.1061/(ASCE)GT.1943-5606.0000284 CE Database subject headings: Rocks; Joints; Displacement; Deformation; Numerical models; Tunneling; Excavation. Author keywords: Rock joint models; Soil-infill; Shear-displacement behavior; Numerical modeling; Tunneling.

The main purpose of this paper is to propose an incorporating improved grammatical evolution (GE) into the genetic algorithm (GA), called GEGA, to estimate the compressive strength of high-performance concrete (HPC). The GE is a recently developed evolutionary programming type system. It is used to automatically discover complex relationships between significant factors and the strength of HPC. This method is transparent and can enhance our understanding of the mechanisms of HPC strength. A GA was used afterward with GE to optimize the appropriate function type and its associated coefficients. In addition, macroevolution algorithm was processed to improve search efficiency during the GA optimization procedure. The case study includes over 1,000 examples of HPC for which experimental data were available. This novel model, GEGA, can obtain a highly nonlinear mathematical equation for predicting the HPC's compressive strength. The results show that GEGA has lower estimating errors, which outperforms another evolutionary strategy called genetic programming and two popular types of traditional multiple regression analysis. DOI: 10.1061/(ASCE)CP.1943-5487.0000031 CE Database subject headings: Algorithm; Compressive strength; High-strength concrete; Computer programming; Regression analysis. Author keywords: Grammatical evolution; Genetic algorithm; Compressive strength; High-performance concrete; Genetic programming; Regression analysis.

A general contact stiffness model is proposed in this paper to study the contacts between rough surfaces of machined plane joints. The proposed model uses fractal geometry for surface topography description, elastic-plastic deformation of contacting asperities, and size-dependent contact stiffness of microcontacts, where the contact stiffness is derived from Hertz contact theory. Three cast iron specimens are produced from different machining methods (milling, grinding, and scraping), and their rough surface profiles are extracted. The structure function method was used to calculate each profile's fractal dimension and scale coefficient. Both theoretical analysis and experimental results of contact stiffness are obtained for these specimens under different contact loads. The comparison between the theoretical contact stiffness and the experimental results at the interface indicates that the present fractal model for the contact stiffness is appropriate and the theoretical contact stiffness is consistent with the experimental data. [DOI: 10.1115/1.4000305] Keywords: machined plane joint surfaces, normal contact stiffness, tangential contact stiffness, fractal theory

An improved stiffness model is proposed for bolted joints made of similar and dissimilar plates. A novel approach is used to obtain an expression for the effective area used for determining the joint stiffness. More accurate estimate of the joint stiffness provides a more reliable prediction of the joint behavior both during its initial assembly, as well as under subsequently applied tensile loads in service. The effect of the grip length-to-diameter ratio, joint sizes, underhead contact radii ratio, hole clearance, and plate material/thickness ratio are investigated. Experimental data are used for determining the envelope angle [alpha] in the proposed analytical model Finite element modeling is used for evaluating the accuracy of the proposed stiffness model. Keywords: bolted joint stiffness, effective joint area [DOI: 10.1115/1.4000212]

A recently developed nonlinear approach is adapted and used to predict crack initiation in stringer terminations of discretely assembled composite panels made from skin and stiffeners. A linear elastic fracture mechanics based submodel is used to simulate the crack initiation in the critical regions and the influence of ply drop-off is captured. The von Karman formulation for moderately large deflections in plates is used to capture the nonlinear structural behavior; three-dimensional assemblies are schematized and the effect of eccentricity is included in the simulation. An optimum design criterion is sought and basic guidelines for good design are provided. Furthermore, a method for preliminary assessment of structural strength is proposed and predictions are validated against detailed nonlinear finite element analysis.

The low tensile strength and limited ductility, the unavoidable deficiency, of concrete can be overcome by the addition of fibres. High strength concrete (HSC) of 60 MPa containing hybrid fibres, combination of steel and polyolefin fibres, at different volume fraction of 0.5, 1.0, 1.5 and 2.0% were compared in terms of compressive, splitting tensile strength and flexural properties with HSC containing no fibres. Test results showed that the fibres when used in hybrid form could result in enhanced flexural toughness compared to steel fibre reinforced concrete [HSFRC]. The compressive strength of the fibre-reinforced concrete reached maximum at 1.5% volume fractions and the splitting tensile strength and modulus of rupture improved with increasing volume fraction. Strength models were established to predict the compressive and splitting tensile strength and modulus of rupture of the fibre-reinforced concrete. The models give prediction matching the measurements. Key words: HSFRC, HYFRC, polyolefin fibre, steel fibre, INTRODUCTION With the development of high grade cement and availability of proper mineral admixtures and chemical admixtures, it has become quite common to adopt concrete with compressive strength of 60 [...]

One of the fundamental components of the moment-rotation behavior of a steel or composite partially restrained (PR) connection is the load-deformation behavior associated with high-strength bolts bearing on steel plates. For many PR connections, the writers believe that this bearing behavior can be approximated by the behavior of a single bolt lap plate connection. One of the primary components of the lap plate connection behavior is the local load-deformation behavior associated with a single bolt bearing on a single plate. This paper presents an experimental and analytical study of the single bolt single plate behavior. The results of the paper include specific methods for predicting the initial stiffness, strength, and overall load-deformation behavior of the single bolt single plate connection. CE Database subject headings: Bolts; Connections; Plates.

Many theoretical and experimental studies have been carried out on fiber-reinforced plastic (FRP)-confined circular concrete specimens, leading to a variety of models for predicting their axial compressive strengths. In this paper, a large test database assembled from an extensive survey of existing studies is presented and employed to assess available axial strength models for FRP-confined concrete. The test database is also deployed to examine the effect of various factors on the performance of FRP-confined concrete. This study shows that the confinement effectiveness of FRP based on reported test results depends little on unconfined concrete strength, size, and length-to-diameter ratio of test specimens and FRP type, but depends significantly on the accuracy of the reported tensile strength of the FRP. The inherent variation of unconfined concrete strength also causes some scatter of data at low confinement ratios. Using those test data with accurate FRP tensile strengths, only two of the nine existing models are found to give close predictions. A new simple model is finally proposed, based on the observation that a linear relationship exists between the confined concrete strength and the lateral confining pressure from the FRP. DOI: 10.1061/(ASCE)0733-9445(2002)128:5(612) CE Database keywords: Concrete columns; Confinement; Compressive strength; Fiber reinforced materials.

An efficient numerical model was devised to investigate the postbuckling behaviors of delaminated composites. The analysis was based on the one-dimensional beam-plate model with multiple delaminations. A global-local approach for postbuckling problems with multiple delaminations was developed. The numerical model and solution process considers the effect of initial imperfections and contact condition between delaminated surfaces. The global-local approach reduces the global degrees of freedom but still accurately models the delaminated zone.

In 1994, California's steel building frames withstood the 6.8 magnitude Northridge earthquake without collapse or casualties. Severe damage, however, was sustained by the welded flange-bolted web connections: at the column-weld interface brittle fractures were discovered. Using linear elastic fracture mechanics this unexpected failure mechanism is explored in this work. Post-Northridge connection test results and recent finite-element studies supply the springboard for this investigation of the fracture strength of welded flange-bolted web connections. A method of predicting the brittle fracture strength of these connections is presented and validated. The probability of brittle fracture is also investigated.

Local buckling behavior of symmetrically perforated lipped channels is investigated experimentally and analytically. A total of 63 stub column tests have been carded out to investigate the effects of the size, position, and aspect ratio of rectangular holes on the ultimate compressive strength of perforated columns. A formulation is proposed to predict the ultimate load-carrying capacity of such channels. In the proposed approach the web plate with two rectangular holes is idealized as two unstiffened elements and a central column. The effective width of an unstiffened element is estimated by the Cold-Formed Steel Design Manual, published by the American Iron and Steel Institute in 1996. The Perry-Robertson approach is modified to predict the strength of the central column. The proposed formulation is shown to predict the ultimate strength of perforated columns accurately. The size and positions of holes are the most influential factors determining the weakening effect. The aspect ratio of the rectangular holes has only marginal effects for lipped columns.

One of the goals of this research is to apply nonlinear fracture mechanics to design problems encountered by the engineering society. Nonlinear fracture mechanics makes it possible to study the size effect with respect to the pullout cone failure of headed anchors embedded in concrete under various boundary conditions. This investigation shows that the fictitious crack approach can be extended with two orthogonal rod elements to become a new technique that can successfully predict the size effect on the pullout tests. In this study the numerical investigation that describes the behavior of headed anchors under tension loading has been carried out by a computer simulation using our original program ANACS (Advanced Nonlinear Analysis of Concrete Structures). To judge the validity of the results, the numerical results were compared with Eligehausen and Sawade's empirical equation and test results of a report by the Reunion International des Laboratoire d'Essais et de Recherches sur les Materiaux et les Constructions.

A model for the fatigue analysis of prestressed concrete bridges is developed. The uncertainties in the geometrical and mechanical variables involved in the problem are included. Traffic actions, thermal effects, and differential settlement are considered. The model also includes the nonlinear behavior under the action of external loads causing cracking. The S-N curves and the Palgrem-Miner hypothesis were chosen for the definition of the fatigue strength of the studied materials (reinforcing and prestressing steels). The fretting fatigue effect is considered in the definition of the resistance of posttensioning steel strands. This effect causes a very significant drop in the fatigue resistance of these elements predicted after tests in air. Weibull distributions are adopted for describing the uncertainty in the definition of the fatigue strength of the steel elements. The limit state functions for the fatigue assessment of reinforcing and prestressing steel are derived. Finally, the statistical definition of the variables affecting the fatigue resistance of posttensioning steel strands under fretting conditions is given. The variability of the random variable damage of Miner at failure is derived from the variability in the S-N curves and a model for the calculation of the parameters of the cumulative distribution function is also proposed. The paper shows that the calculation of the reliability index of posttensioned concrete bridges under fatigue is a straightforward process.

In this paper, a novel neural network architecture, augment-neuron network, is proposed and examined for its efficiency and accuracy in modeling concrete strength with seven factors (water/cement ratio, water, cement, fine aggregate, coarse aggregate, maximum grain size, and age of testing). The architecture of the augment-neuron network is that of a standard back-propagation neural network, but augment neurons, i.e., logarithm neurons and exponent neurons, are added to the input layer and the output layer of the network. Two hundred examples were collected from actual experimental data from 15 sources. The system was trained based on 100 training examples chosen randomly from the example set, and then tested using the remaining 100 examples. The results showed that the logarithm neurons and exponent neurons in the network provide an enhanced network architecture to improve performance of these networks for modeling concrete strength significantly. A neural network-based concrete mix optimization methodology is proposed and is verified to be a promising tool for mix optimization.

An exact analysis of nonprismatic beams with general boundary conditions is presented. The analysis is based on the boundary integral method. The fundamental solutions for nonprismatic beams of linear and parabolic profiles are derived. The fundamental solution is used as a weighing function to transform the governing equation and the boundary condition equations into four algebraic equations. The resulting algebraic equations offer exact relations among the physical variables at the beam supports. The method is tested through two numerical examples to show its accuracy.

This paper investigates the cyclic-localization behavior of axisymmetric plastic buckling patterns of cylindrical shells in terms of its influence on strength deterioration. Since localization occurs due to a plastic bifurcation in the postpeak range, we first present a rigorous method to analyze the plastic bifurcation under the two-axial stress state, which is characteristic of the axisymmetric buckling. In this analysis, we also use an accurate cyclic plasticity constitutive model. The validity of the proposed theoretical approach to localization behavior is confirmed by a cyclic loading test on thick cylindrical shells. Then, using the proposed method, we precisely examine the effects of structural parameters as well as boundary conditions on localization behavior.

Available models are not suitable for calculating the load rating of bearings when an infinite life is required. This paper presents a new model that has been developed from a detailed analysis of the damage mechanism responsible for failure of bearings operating under EHD pure rolling conditions. It is based on the comparison between the local shear stress concentration built up around inhomogeneities present in steels and the microyield stress of the martensitic matrix. The predictions of this model are in good agreement with some literature data concerning the fatigue limit of bearing steels. Finally, the influence of material parameters and operating conditions on the endurance limit of bearings is investigated.

Traditional smeared-crack models for concrete fracture are known to suffer by stress locking (meaning here spurious stress transfer across widely opening cracks), mesh-induced directional bias, and possible instability at late stages of the loading process. The present paper suggests to overcome these deficiencies by combining the standard rotating crack model with the scalar damage concept. The combined model keeps the anisotropic character of the rotating crack but it does not transfer spurious stresses across widely open cracks. This is documented by examples including the three-point bending, wedge splitting, and four-point shear single-edge-notched specimens. The model is then extended to a nonlocal formulation, which not only acts as an efficient localization limiter but also alleviates mesh-induced directional bias. Transition to damage can prevent a special type of material instability arising due to negative shear stiffness of the rotating crack model.

Lateral stresses in shocked solids were determined in relation to piezoresistance gauge data using a simplified analysis. Dynamic, two-dimensional computations were used to analyze the equilibrium response of lateral manganin foil gauges in shocked ceramics. The results of several simplified methods of reverse analysis were compared with the ideal solution. It was demonstrated that a one-dimensional calculation combined with a simple plane strain analysis offers a simplified forward analysis that approximates the ideal solution.

A study is done to compare two models (that of Hobbs and Raoof from 1982, and that of Jolicoeur and Cardou from 1996) that use a semicontinuous approach to predict the behavior of multilayered wire strands under axial and bending loads. Theoretical differences and similarities between the two models are highlighted. Static stiffness results are obtained for a seven-wire steel strand and for a multilayered electric overhead conductor. For stiffness in tension and torsion, both models appear quite equivalent but, for bending stiffness, the Jolicoeur and Cardou model agrees better with more classical models and with published experimental results. Recommendations are given to improve predictions from both models.

A refined method for interpreting the Vickers composite microhardness measurement for multilayer materials having layers of arbitrary plating thickness is first presented. The position of an 'effective substrate' is found using the concept of the 'plastic boundary,' and the depth-wise deformation of each layer is considered in a double-iterative procedure which converges fast. This computational method is then extended from pyramidal indenters to conical and spherical indenters (e.g., Meyer's). For its confirmation, experimental investigations are carried out for two configurations of Cu/Ni/Au sandwiches, using different diameter spherical indenters and spherical tipped cones, through and well above the microhardness load range. The general rules for composite action are established.

Concrete is a highly heterogeneous multiphase material, and its constitutive and damage behavior has some very special characteristics. In this paper an elastoplastic constitutive relationship is proposed for damaged concrete materials based on a thermomechanically consistent mechanical model. The proposed relationship takes into account the interaction between volumetric and deviatoric inelastic deformation, hardening, anisotropic damage, unilateral effect of damage, and different failure modes. The validity of the proposed model is verified by comparing theoretical and experimental results of concrete subjected to triaxial stress histories with strain-controlled cyclic loading and unloading.

Two analytical models are presented for the identification of the actual static scheme and tensile force of metallic tie-rods included in masonry arches and vaults. The reference structural system consists of a moment-resistant tie-beam with elastic rotational springs at the extremities. Symmetrical restraint conditions are initially hypothesized, thus obtaining a two-parameter model in which the unknowns are the springs' rigidity along with the axial force. The preceding assumption is then removed by defining a more general, three-parameter model in which two different spring rigidities are considered. The mutual experimental data are the middle-section vertical displacement under a concentrated load and the fundamental vibration frequency; the second vibration frequency is adjunctively assumed for the three-parameter model. Closed-form equations are formulated in both cases. Laboratory tests show a notable agreement between analytical estimates and experimental measures. A statistical balance of the application of the proposed models to over 200 rods belonging to several Italian monumental as well as common ancient buildings is presented along with more-detailed information about a significant case study.

The theory of plasticity and the deformation compatibility equations are used to evaluate the development of lateral confinement of rectangular concrete columns and the resulting increases in strength and ductility. Concrete is modeled as an elastoplastic, strain hardening/softening material, with stress-path dependent strength and a nonassociative flow rule. The geometric modeling concentrates of the effects of the arrangement of transverse reinforcement on the type and magnitude of lateral confinement. The solution is based on the coupling of the elastoplastic relations for concrete and steel, and the compatibility of deformations of the concrete core and the transverse reinforcement. The ability of the method to predict the response of confined compression members is demonstrated based on several published experimental results.

The behavior of Voronoi-Delaunay parallel-brittle networks of truss elements as models for material behavior is studied. Means and variances of both the number of elements in a network and their length are given. The uniaxial stiffness of the networks is found equal to the element axial stiffness. The expected value of the networks' uniaxial-tensile strength is proportional to the square root of the expected number of elements and to the expected value of the strength of the elements. The toughness of the networks is proportional to the square of the expected value of element strength and to the square root of the expected number of elements. An application to pullout of an anchor is presented. Daniels systems and chain-of-bundles systems are also analyzed to provide perspective. Statistical size effects for parallel-brittle networks are quite different from those associated with weak-link systems.

A theoretical basis for the prediction of the properties of mixtures of virgin and reprocessed polymers is presented. The concept is developed using equations that describe the loss of polymer properties as a function of processing cycles. These equations correspond to properties that are important in characterization and quality control, such as melt flow index and tensile and impact strengths. Experimental data were determined and used to test the models derived. For glass-fiber-reinforced thermoplastics, the data on the degradation of fiber lengths and mechanical properties can help to understand the effect of reprocessing on the fiber-matrix interaction., INTRODUCTION Primary recycling of thermoplastics is done in industry using standard processing operations, and homogeneous, noncontaminated scrap, to fabricate parts with properties similar to those of the products from which [...]

A simple analytical expression, amenable to hand calculation, is developed for estimating the ultimate capacity of partially grout-filled damaged tubular members. The analysis is based on the simplifying assumptions that: (1) The bond between the grout and the tube wall could totally fail prior to the attainment of the ultimate capacity; and (2) grout provides sufficient support in the damaged region of the tube and makes the steel cross section fully effective. The load-carrying capacity is then obtained using a beam-column analysis similar in form to that used in the current design codes. The analysis uses the weighted average rigidity of the cross sections (taking into account the effect of the grouted segment) while using the dented section as the critical section. The first yield collapse criterion has been used. Experimental work has been conducted on 10 small-scale specimens. It is shown that the proposed expression provides acceptable lower-bound estimates of the experimental results. A parametric study is performed to establish the relationship between various parameters and their effects on the ultimate strength of partially grout-filled damaged tubes., A model is developed for determining the ultimate strength of partially grout-filled damaged tubular members. The model is based on current design codes for beam-column analysis using the first yield collapse criterion. Analytical and experimental results were compared and the model was found to provide accurate lower-bound estimates of the experimental values. A parametric study was also performed to study the relationships between different parameters and their effects on ultimate strength.

A study on the improvement of beam strength at design ductility factor demands was evaluated. Although the procedure for analyzing ultimate moment in the study was commended, additions for the concrete intergral functions are described. It was also recommended that ultimate moment be computed after obtaining theta(sub u) from the curvature ductility factor. An alternative method for computing the curvature ductility factor is also presented.

An analytical model is proposed to construct a stress-strain relationship for confined concrete. The model consists of a parabolic ascending branch, followed by a linear descending segment. It is based on calculation of lateral confinement pressure generated by circular and rectilinear reinforcement, and the resulting improvements in strength and ductility of confined concrete. A large volume of test data, including poorly confined and well-confined concrete was evaluated to establish the parameters of the analytical model. Confined concrete strength and corresponding strain are expressed in terms of equivalent uniform confinement pressure provided by the reinforcement cage. The equivalent uniform pressure is obtained from average lateral pressure computed from sectional and material properties. Confinement by a combination of different types of lateral reinforcement is evaluated through superposition of individual confinement effects. The descending branch is constructed by defining the strain corresponding to 85% of the peak stress. This strain level is expressed in terms of confinement parameters. A constant residual strength is assumed beyond the descending branch, at 20% strength level. The model is compared against a large number of column tests. Circular, square, and rectangular columns, with spiral and rectilinear reinforcements, as well as welded wire fabric, are used for comparison. Comparisons include concentric and eccentric loadings, as well as slow and fast strain rates. The results indicate good agreement., A model for the stress-strain relationship of confined concrete is developed. The model is based on the lateral confinement pressure produced by circular and rectilinear reinforcement with strength and ductility improvements. Comparison with testing results of circular, square and rectangular sections confined with spirals, rectilinear hoops, cross ties and welded wire fabric showed good agreement. Concrete behavior under cocentric and eccentric loadings are also described.

A life model for insulating materials under combined thermal and electrical stress that was proposed by A.C. Gjaerde is erroneous because of some simplifications he made to derive the model from a well-known model. These simple modifications resulted in the omission of the exponential dependence of aging on electrical stress and the actual trend of the threshold line in Gajaerde's model. These results suggest that phenomenological models simpler than the well-known model are difficult to find.

Several fracture mechanics models are evaluated. The two-parameter fracture model (TPFM), the size-effect model (SEM) and the linear fracture mechanics model (LEFM) are considered. It is shown that two material parameters defining load for different beam sizes are required for the application of the TPFM or SEM models. In addition, the LEFM defines concrete tensile strength.

More than 20 years have passed since a Terzaghi Lecture focused on the topic of deep foundations. However, considerable research has been performed, and experience gained, in this subject area in the intervening period. The objective of this paper is to update the earlier references on deep foundations by summarizing results of important recent research on a few aspects of the topic of side resistance, most notably (1) driven piles in saturated clay, (2) driven piles in siliceous sand loaded in compression and uplift, (3) drilled shafts in clay, and (4) drilled shafts in soft rock. It is concluded that, while simple design relations are available for topic (1), much is still to be learned. Under topic (2), the case is made that loading the pile in compression and uplift produces different values of unit side-shearing resistance. Regarding topics (3) and (4), the effects of details related to construction--such as stress relief, moisture migration from the concrete to the geomaterial, borehole roughness, and borehole smear--are shown to be significant. The final point made is that the design of deep foundations is a complex matter that should be addressed in a design context by engineers who are experienced in the observation of pile behavior, theoretical modeling, and the appropriate use of design methods.