Building design based on energy dissipation: a critical assessment

The basic objective of this study is the assessment of the European seismic design codes and in particular of EC2 and EC8 with respect to the recommended behaviour factor q. The assessment is performed on two reinforced concrete multi-storey buildings, having symmetrical and non-symmetrical plan view respectively, which were optimally designed under four different values of the behaviour factor. In the mathematical formulation of the optimization problem the initial construction cost is considered as the objective function to be minimized while the cross sections and steel reinforcement of the beams and the columns constitute the design variables. The provisions of Eurocodes 2 and 8 are imposed as constraints to the optimization problem. Life-cycle cost analysis, in conjunction with structural optimization, is believed to be a reliable procedure for assessing the performance of structures during their life time. The two most important findings that can be deduced are summarized as follows: (1) The proposed Eurocode behaviour factor does not lead to a more economical design with respect to the total life-cycle cost compared to other values of q (q = 1, 2). (2) The differences of the total life-cycle cost values may be substantially greater than those observed for the initial construction cost for four different q (q = 1, 2, 3, 4).     

The influence of anisotropy on compaction bands: The case of coaxiality between stress and fabric anisotropy tensors

Compaction bands are localized failure patterns that appear in highly porous rock material under the effect of relatively high confining pressure. Being affected mainly by volumetric compression, these bands appear to be almost perpendicular to the most compressive principal stress of a stress state at the so-called “cap” of the yield surface (YS). In this study, we focus on the mechanism that leads to the onset of compaction bands by using a viscoplasticity model able to describe the post-localization response of these materials. The proposed constitutive framework is based on the overstress theory of Perzyna (1966) and the anisotropic clay plasticity model of Dafalias (1986), which provides not only the necessary “cap” of the YS, but introduces a rotational hardening (RH) mechanism, thus, accounting for the effect of fabric anisotropy. Following the analysis of Veveakis and Regenauer-Lieb (2015), we identify the compaction bands as “static” cnoidal wave formations in the medium that occur at a post-yield regime, and we study the effect of rotational and isotropic hardening on their onset. Moreover, we determine a theoretical range of confining pressures in triaxial compression tests for the compaction bands to develop. Under the assumption of coaxiality between stress and anisotropy tensors, the results show that the isotropic hardening promotes compaction localization, whereas the RH has a slightly negative effect on the onset of compaction localization.