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

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

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

Tests on composite slabs and evaluation of relevant Eurocode 4 provisions

The paper addresses some key issues related to the design of composite slabs with cold-formed profiled steel sheets. An experimental programme is first presented, involving six composite slab specimens tested with a view to evaluating Eurocode 4 (EC4) provisions on testing of composite slabs. In four specimens, the EC4-prescribed 5000 load cycles were applied using different load ranges resulting from alternative interpretations of the reference load Wt. Although the rationale of the application of cyclic loading is to induce loss of chemical bond between the concrete plate and the steel sheet, no such loss was noted in the tests for either interpretation of the range of load cycles. Using the recorded response of the specimens the values of factors m and k (related to interface shear transfer in the composite slab) were determined for the specific steel sheet used in the tests, on the basis of three alternative interpretations of the related EC4 provisions. The test results confirmed the need for a more unambiguous description of the m-k test and its interpretation in a future edition of the Code, as well as for an increase in the load amplitude range to be used in the cyclic loading tests, to make sure that the intended loss of bond between the concrete slab and the steel sheet is actually reached. The study also included the development of a special-purpose software that facilitates design of composite slabs; a parametric investigation of the importance of m-k values in slab design is presented in the last part of the paper

Efficiency of alternative intensity measures for the seismic assessment of monolithic free-standing columns

This paper deals with the dynamic response of a free-standing ancient column in the Roman Agora of Thessaloniki, Greece as a means to shed more light on the complex behaviour of rocking bodies under seismic excitation. Numerical analyses utilizing discrete element method were carried out with the use of multiple seismic records selected based on the disaggregation of the seismic hazard for the region of interest. To identify their impact on structural performance, earthquake Intensity Measures, such as Peak Ground Acceleration and Peak Ground Velocity are examined for the case of a column that sustained no visible permanent deformations during the Ms = 6.5 Thessaloniki earthquake of 1978. The analysis revealed a weak correlation of PGA and PGV with the response results and a significant influence of the mean frequency (fm) of the seismic motion. No coupling was found between the maximum displacement of the top during the oscillation and the permanent post-seismic deformations. The complementarity of both earthquake Intensity Measures in the structural vulnerability assessment is also depicted

Seismic Risk of Inter-urban Transportation Networks

AbstractThe paper presents a holistic approach for assessing and managing the seismic risk and potential loss in inter-urban highway networks in earthquake-prone areas. The vulnerability of all elements of the intercity transportation system (i.e., roads, bridges, abutments, retaining walls, and tunnels) is assessed considering the interdependency among the structural, transportational and geotechnical components of the network under different seismic scenarios. Both the direct earthquake-induced damage, as well as the indirect socio-economic loss attributed to reduced network functionality are taken into account in an explicit and transparent formulation that is then displayed in space through an ad-hoc developed GIS-based software. The methodology and the decision-making tools developed are adequately modular, for them to be utilized after appropriate adaptation by local authorities in identifying, prior to a major earthquake event, those vulnerable components of their network whose failure may have a disproportional socio-economic impact. In this way, a rational and effective emergency plan can be deployed to minimize potential human, social and financial loss after a future earthquake. The outline of a foreseen application is also presented for the case of the road network of the Region of Western Macedonia in Greece. Through this pilot application, the methodology is to be optimized in real conditions before being cast in the form of a fully parameterised seismic risk tool, to be used in other earthquake prone areas as well