Nikolaos Bakas

AUTOMATIC MINIMIZATION OF THE RIGIDITY ECCENTRICITY OF 3D REINFORCED CONCRETE BUILDINGS

The objective of this paper is to obtain the optimum design of 3D reinforced concrete buildings in terms of their performance under earthquake loading. This goal is achieved by considering the minimisation of the eccentricity between the mass centre and the rigidity centre of each storey layout as the optimisation objective in order to produce torsionally balanced structures. This problem is considered as a combined topology and sizing optimisation problem. The location and the size of the columns and the shear walls of the structure of each storey layout constitute the design variables. Apart from the constraints imposed by the seismic and reinforced concrete structure design codes, architectural restrictions are also taken into account. The test examples showed that a reduction in the structural cost of the building is achieved by minimising the eccentricity between the mass centre and the rigidity centre of each storey layout. Evolutionary optimisation algorithms and in particular a specially tailored algorithm based on Evolution Strategies is implemented for the solution of this type of structural optimisation problems.

Optimum Design Approaches for Improving the Seismic Performance of 3D RC Buildings

In this article, a number of design approaches for 3D reinforced concrete (RC) buildings are formulated in the framework of structural optimization problems and are assessed in terms of their performance under earthquake loading. In particular, three design approaches for RC buildings are considered in this study. In the first, the initial construction cost is considered as the objective function to be minimized. The second one is formulated as a minimization problem of the torsional response, while a combined formulation is also examined as the third design approach. The third approach is considered with two distinctive formulations. According to the first approach, the torsional behavior is minimized by minimizing the eccentricity between the mass and rigidity centers, while the second one is achieved by minimizing the eccentricity between the mass and strength centers. It is shown that the optimized designs obtained according to the minimum eccentricity of the rigidity center behave better in frequent (50/50 hazard level) and occasional (10/50 hazard level) earthquakes, while the designs obtained according to the minimum eccentricity of the strength center formulation was found better in rare (2/50 hazard level) events. Designs obtained through a combined formulation seem to behave equally well in the three hazard levels examined.

Nature of coupling in non-conservative distributed parameter systems attached to external damping sources

Non-conservative distributed parameter systems connected to external damping sources and possessing non-normal modes are analyzed in this work. The mathematical model of such systems is presented and real valued modal analysis is used to obtain the coupled modal equations of motion. A decoupling technique is developed using Fourier expansion, fictitious damping ratios, modal coupling parameters and pseudo forces. The method is applicable to all types of excitation. A normal mode criterion in the form of non-proportionality indices is also provided. The theoretical predictions are verified through application to a non-conservative Euler–Bernoulli beam with non-proportional damping configuration and various types of boundary conditions. Numerical examples emphasize the response errors associated with the proportional damping assumption and reveal the advantages of the proposed approach over the exact method.

HIGH COMPUTATIONAL EFFICIENCY THROUGH GENERIC ANALYTICAL FORMULATION FOR LINEAR SOIL PRESSURE DISTRIBUTION OF RIGID SPREAD RECTANGULAR FOOTINGS

Abstract. The primary objective of this paper is to resolve and provide generic analytical formulas concerning the linear pressure distribution of rigid spread rectangular footings and, consequently, limit computational costs. All five distinct regions of the eccentricity diagram are related to five possible forms of footing deformations and five discrete shapes of compression zone. For each region, the linear soil pressure distribution in soil-footing interface, the neutral axis position, the maximum pressure and the pressure values at the four corners, are expressed in closed forms as functions of biaxial eccentricities, mean soil pressure and footing dimensions. Several special cases are also presented, verifying the correctness and the consistency of the developed analytical formulas and revealing the physical meaning of the eccentricity diagram. The explicit expressions for responses and resultants enable algorithmic implications without iterations, providing high computational efficiency with low computational cost when forming envelopes for shear forces and bending moment or optimizing the design of footing geometry and footing reinforcement, etc. Through developed computer software, a provided simple example of a rigid spread footing under variable eccentric loading, demonstrates how the theoretical content of this article is used to perform numerical calculations. The software itself comprises 3D visualization technology to facilitate visual examination and validation of the results.

ROT INDEX FOR ASSESSING THE TORSIONAL EFFECT ON THE SEISMIC PERFORMANCE OF RC BUILDINGS

Due to the eccentric setback nature of structures, torsional moments will be in- duced by a set of lateral loads on buildings. Although the effect on the response of reinforced concrete buildings was the subject of intensive research over the last decades still lacks a cri- terion valid in both elastic and plastic region. In this study a criterion capable of assessing torsional effect is proposed. In order to evaluate the proposed criterion with code provisions, two test examples are considered. A torsionally stiff, mass eccentric one-storey and a four- storey horizontally irregular building designed according to Eurocode provisions subjected to bidirectional excitation are considered. Nonlinear dynamic analyses are carried out imple- menting natural record selected for three hazard levels. The performance of the proposed cri- terion is evaluated and its correlation with other structural response quantities like interstorey drifts, displacements, base torque, shear forces and diaphragm rotation is pre- sented. Through this investigation it was found that the proposed criterion is able to provide a reliable prediction of the magnitude of torsional effects for all test cases considered.