Seismic Assessment of Asymmetric Single-storey R/C Buildings by Two New Methodologies: Enforced Displacement-Based and Forced-Based Pushover Procedures

Τwo new documented non-linear static (pushover) procedures on asymmetric single-storey R/C buildings are presented in detail herein, aiming directly at the Near Collapse state. Both procedures apply relative to the “Capable Near Collapse Principal reference system” of the single-storey building. The main objective of the two proposed procedures is to fully consider the coupling between torsional and translational vibrations of the floor-diaphragm under translational seismic excitation of the building’s base. The first pushover procedure, which is a Direct Displacement-Based one, uses floor enforced-displacements as action. In the second pushover procedure, which is a Force-Based one, the floor lateral static forces are applied eccentrically to centre of mass using suitable inelastic design eccentricities (dynamic plus accidental ones). The floor enforced-translations/rotation and the appropriate inelastic dynamic eccentricities used in the two proposed procedures derive from extensive parametric non-linear response history analysis and are given by figures or equations. In order to clarify in detail and evaluate the new pushover procedures, a torsionally-flexible, double-asymmetric, single-storey R/C building is seismically assessed. The validation of both procedures relative to the results of non-linear response history analysis shows that both predict with safety the in-plan displacements of the building.

Effects of Prior Stress Relaxation on the Prediction of Creep Life Using Time and Strain Based Methods

A critical requirement for both next generation conventional and nuclear plants is the development of simplified inelastic design and fitness for purposes procedures that give a reasonably accurate prediction of the complex multiaxial time dependent stress strain behavior. The accumulation of this inelastic strain in the form of coupled creep-fatigue damage over time is one of the principal damage mechanisms which will eventually lead to crack initiation in critical, high temperature equipment. Two main procedures that address creep-fatigue loading are generally used, either a time fraction or a ductility exhaustion approach. It is generally accepted that these methods enable conservative predictions within a factor of 2 to 3 and hence are reliable methods for code based design and fitness for purpose type assessments. However, for complex cycles, this may not be the case; for example, prior relaxation cycles are found to accelerate the creep rupture of the material with the result that a significant reduction in creep life can be observed. An investigation was undertaken into the influence of prior relaxation on resultant failure using a typical low alloy ferritic power station steel. Both time-based and strain based methods were used to predict the damage caused by the stress relaxation cycles followed by operation at steady state. The predictions found that while ductility exhaustion methodologies based on mean properties were adequate in predicting the failure life, time fraction methods were found to be extremely nonconservative for mean properties and only lower bound solutions provided an estimate of remaining creep life. The ASME time fraction approach, using isochronous curves was found to be extremely conservative for K = 0.67, but was able to predict similar damages to ductility exhaustion when K = 1 was used. The Monkman-Grant approach resulted in predictions that erred on the conservative side. The results have implications for both current and future conventional and nuclear power stations as it may be difficult for time based approaches to account accurately for complex cycling, shakedown conditions or stress relaxation at welds.

Numerical Analysis on Bending Strength and Ductility of Compact Section I-Girders with HSB800 Steels

New high performance steel HSB800 have been developed in South Korea, the yield strength of which is equal to 690MPa. High strength steels typically show a lower ductility and a larger yield ratio than conventional steels. Since the inelastic design procedure of current AASHTO specifications is not permitted to the flanges and webs of nominal yield stress exceeding 485 MPa, this study is intended to present an assessment result of the compactness requirements for I-section girders by performing 3D nonlinear finite element analyses. On the hypothetical specimens designed to be compact-section girders, the numerical analysis was performed using the finite element code ABAQUS. From the analysis results, it is found that the compact section limits speculated in the 2007 AASHTO LRFD could be applicable to the design of HSB800 steel I-girders to reach the plastic moment. However, the inelastic design procedure does not surely confirm a sufficient flexural ductility. More extensive studies of HSB800 I-girders are still in need.