Improved external device for a mass‐carrying sliding system for shaking table testing

Shaking tables are suitable facilities to assess and validate the behavior of structures and nonstructural components under actual seismic actions. Because of the size and weight limitations of the tables, some approaches, like testing reduced‐scale models or testing only the main structural components, are deemed necessary. In these cases, to comply with modeling requirements, large amount of extra‐mass should be added to the specimen. Therefore, to avoid the risk of lateral instability of models, to maintain the weight of test specimens within table payload, while maintaining the amount of mass needed, an external device for transmitting the inertia forces to the models using an improved sliding system is proposed. Although friction devices for similar purposes have been developed using sliding bearings (Teflon pads or rollers), the measured coefficient of dynamic friction and the energy dissipated by friction have been very high. In order to drastically diminish the damping added to the specimen response when a friction device is used, the improved device employs a linear motion guide system (LMGS) with very low friction. Shaking table tests to collapse of reinforced concrete walls were used to evaluate the effectiveness of the proposed device. Measured dynamic friction coefficients, spectral accelerations and hysteresis loops show that friction developed in the LMGS did not add any significant amount of damping into the specimen response. Thus, the proposed device is a reliable and suitable mass‐carrying sliding system (MCSS) for dynamic testing using medium‐size shaking tables. Copyright © 2010 John Wiley & Sons, Ltd.     

Acceptance limits for performance‐based seismic design of RC walls for low‐rise housing

Acceptance limits of the structural response of walls for low‐rise concrete housing were developed. Proposed values are applicable within a performance‐based seismic design framework. Acceptance limits are based on performance indicators of structural response–allowable story drift ratios, width of residual cracks and residual damage index, and expected damage of walls. Cracking limits were defined from parameters obtained at the unloading stage of walls (i.e., residual cracking stage). The residual cracking stage may be used for structural damage evaluation and cost estimation of structural rehabilitation after an earthquake has occurred. The performance indicators proposed herein were derived from test observations and measured response of 39 RC walls' specimens during shaking table and quasistatic testing, as well as from limiting values and results of previous studies. Copyright © 2012 John Wiley & Sons, Ltd.     

Damage index based on stiffness degradation of low‐rise RC walls

Widely used damage indices, such as ductility and drift ratios, do not account for the influences of the duration of strong shaking, the cumulative inelastic deformation or energy dissipation in structures. In addition, the formulation and application of most damage indices have until now been based primarily on flexural modes of failure. However, evidence from earthquakes suggests that shear failure or combined shear‐flexure behavior is responsible for a large proportion of failures. Empirical considerations have been made in this paper for evaluating structural damage of low‐rise RC walls under earthquake ground motions by means of a new energy‐based low‐cycle fatigue damage index. The proposed empirical damage index is based on the results of an experimental program that comprised six shake table tests of RC solid walls and walls with openings; results of six companion walls tested under QS‐cyclic loading were used for comparison purposes. Variables studied were the wall geometry, type of concrete, web shear steel ratio, type of web shear reinforcement, and testing method. The index correlates the stiffness degradation and the destructiveness of the earthquake in terms of the duration and intensity of the ground motions. The stiffness degradation model considers simultaneously the increment of damage associated to the low‐cycle fatigue, energy dissipation, and the cumulative cyclic parameters, such as displacement demand and hysteretic energy dissipated. Copyright © 2014 John Wiley & Sons, Ltd.     

Artificial macrophytes as fish habitat in a Mediterranean reservoir subjected to seasonal water level disturbances

Artificial macrophytes were experimentally deployed in Boadella Reservoir (NE Spain) and assessed for fish use throughout the first 3 months of the 2007 summer drawdown. In total, 1,832 individuals of seven fish species were recorded through visual censuses, with introduced perch Perca fluviatilis being the prevalent species. Fish richness and abundance were significantly higher in artificial macrophytes than in rocky shores and sandy beaches, displaying a unimodal variation through time. P. fluviatilis, Rutilus rutilus and Abramis brama were significantly more abundant in artificial macrophytes than in the other two naturally submersed habitats, where no individuals of these species were recorded. The abundances of Squalius laietanus, Lepomis gibbosus, Micropterus salmoides and Cyprinus carpio were overall greater in artificial macrophytes than in rocky shores but displayed significant habitat × time interactions. A decrease in predation risk was apparently the key-factor of the intensive use of artificial macrophytes by small fish (≤100 mm TL) in Boadella Reservoir, since most species reduced drastically or completely the use of submerged structures when body size increased. This study demonstrates that adding artificial structures mimicking aquatic macrophytes can be an interesting tool to mitigate the adverse effects of water level fluctuations on fish assemblages in structure-less and homogeneous ecosystems, and to understand the mechanisms affecting habitat use and species replacement.     

Experimental investigation on dynamic and quasi‐static behavior of low‐rise reinforced concrete walls

Experimental evidence supporting the fact that results from quasi‐static (QS) test of low‐rise reinforced concrete walls may be safely assumed as a lower limit of strength and displacement, and energy dissipation capacities are still scarce. The aim of this paper is to compare the seismic performance of 12 reinforced concrete walls for low‐rise housing: six prototype walls tested under QS‐cyclic loading and six models tested under shaking table excitations. Variables studied were wall geometry, type of concrete, web steel ratio, type of web reinforcement and testing method. Comparison of results from dynamic and QS‐cyclic tests indicated that stiffness and strength properties were dependent on the loading rate, the strength mechanisms associated with the failure mode, the low‐cycle fatigue, and the cumulative parameters, such as displacement demand and energy dissipated. Copyright © 2012 John Wiley & Sons, Ltd.     

The detrital record of late‐Miocene to Pliocene surface uplift and exhumation of the Venezuelan Andes in the Maracaibo and Barinas foreland basins

A multidisciplinary approach, combining sediment petrographic, palynological and thermochronological techniques, has been used to study the Miocene‐Pliocene sedimentary record of the evolution of the Venezuelan Andes. Samples from the Maracaibo (pro‐wedge) and Barinas (retro‐wedge) foreland basins, proximal to this doubly vergent mountain belt, indicate that fluvial and alluvial‐fan sediments of similar composition were shed to both sides of the Venezuelan Andes. Granitic and gneissic detritus was derived from the core of the mountain belt, whereas sedimentary cover rocks and uplifted foreland basin sediments were recycled from its flanks. Palynological evidence from the Maracaibo and Barinas basins constrains depositional ages of the studied sections from late Miocene to Pliocene. The pollen assemblages from the Maracaibo Basin are indicative of mountain vegetation, implying surface elevations of up to 3500–4000 m in the Venezuelan Andes at this time. Detrital apatite fission‐track (AFT) data were obtained from both stratigraphic sections. In samples from the Maracaibo basin, the youngest AFT grain‐age population has relatively static minimum ages of 5 ± 2 Ma, whereas for the Barinas basin samples AFT minimum ages are 7 ± 2 Ma. With exception of two samples collected from the Eocene Pagüey Formation and from the very base of the Miocene Parángula Formation, no evidence for resetting and track annealing in apatite due to burial heating in the basins was found. This is supported by rock‐eval analyses on organic matter and thermal modelling results. Therefore, for all other samples the detrital AFT ages reflect source area cooling and impose minimum age constraints on sediment deposition. The main phase of surface uplift, topography and relief generation, and erosional exhumation in the Venezuelan Andes occurred during the late Miocene to Pliocene. The Neogene evolution of the Venezuelan Andes bears certain similarities with the evolution of the Eastern Cordillera in Colombia, although they are not driven by exactly the same underlying geodynamic processes. The progressive development of the two mountain belts is seen in the context of collision of the Panama arc with northwestern South America and the closure of the Panama seaway in Miocene times, as well as contemporaneous movement of the Caribbean plate to the east and clock‐wise rotation of the Maracaibo block.     

Adsorption of arsenic by natural aquifer material in the San Antonio-El Triunfo mining area, Baja California, Mexico

 Several experiments of arsenic (As) adsorption by aquifer material of the San Antonio-El Triunfo (SA-ET) mining area were conducted to test the feasibility of this material acting as a natural control for As concentrations in groundwater. This aquifer material is mineralogically complex, composed of quartz, feldspar, calcite, chlorite, illite, and magnetite/hematite. The total iron content (Fe₂O₃) in the fine fraction is ∼12%, whereas Fe₂O₃ in the coarse fraction is <10 wt%. The experimental percent total As adsorbed vs. pH curves obtained match the topology of total As adsorbed onto iron oxi-hydroxides surface (arsenate + arsenite; high adsorption at low pH, low adsorption at high pH). A maximum of about 80% adsorbed in the experiments suggests the presence of arsenite in the experimental solutions. The experimental adsorption isotherm at pH 7 indicates saturation of surface sites at high solute concentrations. Surface titration of the aquifer material indicates a point of zero charge (PZC) for the adsorbent of about 8 to 8.5 (PZC for iron oxyhydroxides =7.9–8.2). Comparison between experimental and modeled results (using the MICROQL and MINTEQA2 geochemical modeling and speciation computer programs) suggests that As is being adsorbed mostly by oxyhydroxides surfaces in the natural environment. Based on an estimated retardation factor (R), the travel time of the As plume from the SA-ET area to La Paz and Los Planes is about 700 to 5000 years. Received: 17 March 1997 · Accepted: 8 September 1997     

Assessment of seismic damage of thin and lightly reinforced concrete walls using fractal dimension of cracking

Length, maximum width, and residual width of cracks are key indicators of structural damage. However, pattern and propagation of cracks on the affected structural component should be also considered. In addition, damage evaluation based on visual inspection is a subjective and capricious procedure because the damage assessment relies on the expertise and judgment of the inspector engineer. In order to assess a rapid and reliable evaluation approach of seismic damage, pattern and propagation of cracks observed in thin and lightly reinforced concrete walls for low‐rise housing subjected to seismic demands are evaluated in this study by means of fractal dimension of cracking pattern. The proposed parameters are based on the results of an experimental program that comprised 39 low‐rise RC wall specimens having typical variables of this type of housing, such as low compressive strengths of concrete, thin walls, low axial loads, low reinforcement ratios, and web shear reinforcement made of deformed bars and welded‐wire meshes. A statistical analysis is carried out for computing values of fractal dimension associated to cracking patterns at key damage conditions. Recommendations of this study can help the inspector in estimating the current limit state or performance level of the wall and the story‐drift ratio experienced by the wall during shaking. Copyright © 2016 John Wiley & Sons, Ltd.     

Seasonality and anomalies of sea surface temperature off the coast of Nayarit, Mexico

Sea surface temperature (SST) harmonic and empirical orthogonal function (EOF) analyses covering 18 years were performed for the area located from 114° to 105° W and from 18° to 25° N. The results indicate that the influence of the annual signal predominates over the semi-annual signal, and the closer to the coast, the stronger the annual harmonic. Several interannual anomalies arose that are connected with the main global indexes, especially the Oceanic Niño Index. Pearson correlations between the first temporal mode of the SST and regional rainfalls in Nayarit indicate that maximum correlations (r?>?0.7) are observed when there is a +1-month lag between the series. However, this result indicates that SST is delayed with 1 month after rainfall occurrence, which shows that the dominant influence in this relationship is not the SST forcing.