Showing papers by "Zdenek P. Bazant published in 1982"

Strain-Rate Effect in Rapid Triaxial Loading of Concrete

Abstract: Presented is a generalization of time-independent nonlinear triaxial constitutive relations for concrete to model short-time viscoelastic effects. The effect of strain-rate magnitude upon the initial elastic modulus, the peak stress, and the sharpness of the stress peak is taken into account. To obtain a model that also exhibits short-time rapid creep and stress relaxation, the incrementally linear constitutive law fully characterized by tangential moduli is replaced by a nonlinear first-order differential equation in stress, strain, stress rate and strain rate. This requires introduction of the (apparent) instantaneous modulus, the relaxation time and retardation time. The irreversibility at unloading, caused by the short-time viscoelastic mffects, is also analyzed. The model allows more realistic dynamic finite element analysis of concrete structures.

Input of creep and shrinkage characteristics for a structural analysis program

Abstract: Presented is a computer program for the input of creep and shrinkage properties for a structural analysis program. The program either accepts numerical data on the compliance function and the shrinkage function at discrete load durations and ages at loading, or uses the recent BP Model for creep involving the double-power law. The most useful characteristic is the availability of thirteen different options for specifying the creep and shrinkage parameters; sealed conditions or drying conditions, creep values specified and interpolation to be used, parameter of a creep formula specified, or creep formula automatically fitted to given creep data, short time data specified and extrapolation done by a formula, etc. In addition to returning the compliance function, the program also calculates the relaxation function and the age-dependent elastic moduli of a Maxwell chain model that is equivalent to the given creep properties. The program is particularly suitable for the input of creep characteristics for a large-scale finite element analysis.

Random Shrinkage Stresses in Aging Viscoelastic Vessel

Abstract: Shrinkage stresses caused by random variation of environmental humidity in a long cylindrical concrete wall sealed at the internal surface are analyzed, taking creep and aging of concrete into account. The random time-variation of humidity and stress is analyzed by the spectral method using the elastic-viscoelastic analogy. Spatial variations are described by Bessel and Kelvin functions. Numerical examples typical of a 1 m thick reactor containment and a thin shell of 10 cm thickness are given. The environmental humidity varies with a dominant period of one year. The results confirm that, due to low moisture diffusivity, as well as aging, the problem must be analyzed as nonstationary. The mean and the standard deviation are obtained as functions of time as well as location. Random stress fluctuations are found to be significant in all cases and the influence of aging to be strong. In the thicker containment only about 30 percent of the wall thickness ever feels the random variation, while in the thin shell the entire thickness is affected.

Seismic analysis of an earth dam based on endochronic theory.

Abstract: A two-dimensional finite element program using triangular plane strain elements has been developed to determine the stress and displacement fields that result in an earth dam subjpcted to earthquake loadinq, The governing ',"luations are derived fro!:l the principle of vlrtual work, and they account for the exi stence of two phases (solid and fluid) with coupling and daJll['inq due to relatlve accelerations and velocities. Endochronic (nonlinear inelastic) and linear elastic constitutive relationships are used to describe the soil behavior, and a step-by-step integration in ti",e is l,erformed by an implicit methoJ. Two Jifferent earthquake acceleration records with ,lrI,ruximately the same maximum acceleration were employed in the endochronic analysis. Cc11culated results demonstrate the importan~" of utilizing appropriate constitutive relationships and the effect caused by tho nature of the input earthquake record, Endochronic theory and the concept of soil as a two-phase medium are combined to conduct a dynamic finite element analysis of an earth dam. Calculations are made (a) with both endochronic and linear elastic constitutive relationships using a modified version of the N-S component of the E1 Centro accelerogram from the 1940 earthquake and (b) with an endochronic constitutive relationship using a dec onvoluted and enriched version of the Pacoima accelerogram from the 1971 San Fernando 559 earthquake. Treatment of the soils as twophase media accounts for the coupling between phases and allows the stress distribution to be determined in both the solid and fluid phases; as such, it is a more accurate and realistic representation of the actual behavior of soils. Virtually all of the available models that have been applied to this problem are based on the concept of a one-phase medium, and the pore pressure response is determined explicitly. ENDOCHRON Ie CONSTITUTIVE LAW FOR SOIL.S Meaningful advances in our analytical ability to solve field problems in soil dynamics requires, among other things, the formulation of more realistic and generally applicable constitutive relationships for soi Is. Toward this end endochronic theory, which was first applied to describe the mechanical behavior of metals (24), has been extended to characterize the response of concrete (3) and soils (2, 4,11,15). In endochronic theory the notion of an intrinsic time scale is formulated to account for the independent dissipative effects of accumulated strain and external time. The intrinsic time parameter, z, depends on external time (strain rate) only for rate-dependent materials, such as cohesive soils, and it should be a monotonically increasing function of strain and external time. Assuming that the development of inelastic strains is gradual, the intrinsic time increment, dz, can be expreased as a crement, dC ij , function of the strain inand the external time incre-