Multi-objective optimum selection of ground motion records with genetic algorithms

Existing ground motion selection methods for the seismic assessment of structural systems consider only spectral compatibility as selection objective. Other important earthquake parameters such as those related to regional seismicity, local soil conditions, strong ground motion intensity and duration are considered indirectly by setting them as selection constraints. This study presents a new framework for the optimum selection of earthquake ground motions, where more than one objectives are considered explicitly in the selection procedure including objectives that are not directly related to spectral matching. To address the multi-objective nature of the optimization problem examined herein, the weighted sum method is used that supports decision making both in the pre-processing and post-processing phase of the selection procedure. The optimum selections are conducted by the use of a mixed-integer genetic algorithm that is able to track near-global optimal solutions of constrained problems with both discrete and continuous design variables. It is found that proposed methodology is able to select ground motion sets that are both spectrum compatible and representative of the seismic conditions of the structural system under investigation

Selection of earthquake ground motions for multiple objectives using genetic algorithms

Existing earthquake ground motion (GM) selection methods for the seismic assessment of structural systems focus on spectral compatibility in terms of either only central values or both central values and variability. In this way, important selection criteria related to the seismology of the region, local soil conditions, strong GM intensity and duration as well as the magnitude of scale factors are considered only indirectly by setting them as constraints in the pre-processing phase in the form of permissible ranges. In this study, a novel framework for the optimum selection of earthquake GMs is presented, where the aforementioned criteria are treated explicitly as selection objectives. The framework is based on the principles of multi-objective optimization that is addressed with the aid of the Weighted Sum Method, which supports decision making both in the pre-processing and post-processing phase of the GM selection procedure. The solution of the derived equivalent single-objective optimization problem is performed by the application of a mixed-integer Genetic Algorithm and the effects of its parameters on the efficiency of the selection procedure are investigated. Application of the proposed framework shows that it is able to track GM sets that not only provide excellent spectral matching but they are also able to simultaneously consider more explicitly a set of additional criteria

Probabilistic seismic risk assessment of adjacent colliding r/c inelastic structures accounting for record-to-record variability

Seismically excited adjacent buildings with equal floor heights and with inadequate clearance may interact/collide due to out of phase response developing slab-to-slab pounding forces which, although do not induce local damage to structural members, they do influence the overall seismic structural response. Herein, a numerical study is undertaken to quantify, statistically, the above influence in terms of fragility curves conditioned on different limit states widely used in performance-base seismic risk assessment of yielding structures. This is accomplished through incremental dynamic analysis (IDA) for 72 far-field GMs and facilitated by introducing a novel intensity measure, the geometric mean of spectral acceleration at the fundamental periods of the colliding structures, shown to improve efficiency in accounting for GM record-to-record variability. Probabilistic models (fragility curves) are derived for a case-study scenario of two colliding reinforced concrete (r/c) structures with unequal number of floors and ductility capacities (i.e., 12-storey ductility class high and 8-storey ductility class low according to Eurocode 8) modelled as equivalent inelastic SDOF systems. It is found that seismic pounding has significant impact to the median and standard deviation (shape) of fragility curves for a clearance of 10% the minimum required distance specified by Eurocode 8, especially for the higher (12-storey) structure for which pounding is detrimental

Influence of bi-directional seismic pounding on the inelastic demand distribution of three adjacent multi-storey R/C buildings

Interaction between closely-spaced buildings subject to earthquake induced strong ground motions, termed in the literature as “seismic pounding”, occurs commonly during major seismic events in contemporary congested urban environments. This influence is not taken into account by current codes of practice and is rarely considered in practice at the design stage of new buildings constructed “in contact” with existing ones. Thus far, limited research work has been devoted to quantify the influence of slab-to-slab pounding on the inelastic seismic demands at critical locations of structural members in adjacent structures that are not aligned in series. In this respect, this paper considers a typical case study of a “new” reinforced concrete (R/C) EC8-compliant, torsionally sensitive, 7-story corner building constructed within a block, in bi-lateral contact with two existing R/C 5-story structures with same height floors. A non-linear local plasticity numerical model is developed and a series of non-linear time-history analyses is undertaken considering the corner building “in isolation” from the existing ones (no-pounding case), and in combination with the existing ones (pounding case). Numerical results are reported in terms of averages of ratios of peak inelastic rotation demands at all structural elements (beams, columns, shear walls) at each storey. It is shown that seismic pounding reduces on average the inelastic demands of the structural members at the lower floors of the 7-story building. However, the discrepancy in structural response of the entire block due to torsion-induced, bi-directionally seismic pounding is substantial as a result of the complex nonlinear dynamics of the coupled building block system