Effectiveness of simple approaches in mitigating residual deformations in buildings

Developments in performance‐based seismic design and assessment approaches have emphasized the importance of considering residual deformations. Recent investigations have also led to a proposed direct displacement‐based design (DDBD) approach which includes an explicit consideration of the expected residual deformations as an integral part of the design process. Having estimated the expected residual deformations in a structure, engineers are faced with the problem of reducing them to meet the targeted performance levels under pre‐defined seismic hazard levels. Previous studies have identified the post‐yield stiffness as a primary factor influencing the magnitude of residual deformations in single degree of freedom and multiple degree of freedom structures. In this paper, a series of simple approaches to increase the post‐yield stiffness of traditional framed and braced systems for the purpose of reducing residual deformations are investigated. These methods do not utilize recentring post‐tensioned technology. This contribution addresses the feasibility of altering the lateral post‐yield stiffness of structural systems by: (i) using different reinforcement materials with beneficial features in their stress–strain behaviour; (ii) re‐designing the section geometry and properties of primary seismic‐resisting elements; and (iii) introducing a secondary elastic frame to act in parallel with the primary system. The efficiency of each of these techniques is investigated through monotonic and cyclic moment‐curvature and non‐linear time‐history analyses. Of these approaches the design and introduction of an elastic secondary system was found to be most effective and consistent in reducing residual deformations. A simplified design approach for achieving the desired increase of a system's post‐yield stiffness is also presented. Copyright © 2007 John Wiley & Sons, Ltd.     

An experiment proving that dilatancy is a pervasive volumetric property of brittle rock loaded to failure

Summary An Experiment Proving that Dilatancy is a Pervasive Volumetric Property of Brittle Rock Loaded to Failure. Dilatancy, that is, volumetric expansion relative to the compression resulting from elastic deformation is often observed in the compression of rock specimens to failure. This phenomenon is of considerable importance to the understanding of the processes of rock failure and in their application to geological and engineering problems. Most of the observations to date have been made on small cylindrical specimens of rock, with a high surface area to volume ratio; frequently by means of resistance strain gauges cemented to the surface of the specimen. The possibility of the measured dilatancy being a superficial phenomenon due to anomalous surface deformations cannot be excluded. Measurements of dilatant deformation on the inside and outside of hollow cylindrical specimens during compression under homogeneous stresses provide a means of resolving this uncertainty. Superficial dilatancy would result in a relative diminution of the cross-section of the hole as that of the outside of the specimen increases with deformation; whereas pervasive volumetric dilatancy would result in similar relative increases in the cross-section of both the hole and the specimen. Experiments on hollow cyclindrical specimens of quartzite show that dilatancy is a pervasive volumetric phenomenon.

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Zusammenfassung Experimenteller Nachweis zur Dilatanz als volumenbezogene Eigenschaft spröder, bis zum Bruch belasteter Körper. Dilatanz, d. h. relative Volumenvergrößerung im Vergleich zur elastischen Zusammendrückung, ist ein Phänomen, welches häufig bei der Druckbeanspruchung von Gesteinen beobachtet werden kann. Diese Erscheinung ist für das Verständnis der Bruchvorgänge von Gesteinen und deren Anwendung auf geologische und technische Probleme von beträchtlicher Bedeutung. Ein Großteil der bisherigen Untersuchungen zu dieser Frage wurde an kleinen zylindrischen Probekörpern mit großem Verhältnis von Oberfläche : Volumen durchgeführt. Bei diesen Versuchen wurden meist Dehnmeßstreifen an die Oberfläche der Probekörper geklebt. Demnach kann die Möglichkeit nicht ausgeschlossen werden, daß die beobachtete Dilatation infolge abnormaler Oberflächenverformungen ein Oberflächenphänomen ist. Messungen der Dilatation an der Innen- und Außenseite eines hohlen zylindrischen Probekörpers während der Druckbeanspruchung in einem homogenen Spannungsfeld bilden eine Möglichkeit, diese Ungewißheit zu beseitigen. Im Falle einer oberflächengebundenen Dilatation würde es zu einer relativen Verkleinerung des inneren Querschnittes des hohlen Probekörpers kommen, während die Dilatation der Außenseite mit der Verformung anwachsen würde. Demgegenüber würde es im Falle der Dilatation als einer volumetrischen Eigenschaft zu einer ähnlichen relativen Vergrößerung des inneren Querschnittes des hohlen Probekörpers und des Probekörpers selbst kommen. Die Experimente an hohlen zylindrischen Probekörpern aus Quarzit zeigen, daß die Dilatanz ein volumetrisches Phänomen ist.

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Résumé Expérience prouvant que la dilatance est répartie dans tout le volume des roches fragiles chargées jusqu'à la rupture. La dilatance, c'est-à-dire l'augmentation de volume par rapport à la déformation élastique en compression, se manifeste souvent quand on comprime des échantillons de roche jusqu'à la rupture. Ce phénomène a une importance considérable pour la compréhension des processus de rupture des roches, et pour les applications aux problèmes géologiques et géotechniques. La plupart des essais ont été faits à ce jour sur de petits échantillons cylindriques, dont la surface est grande par rapport au volume, le plus souvent au moyen de jauges à résistance collées sur la surface de l'échantillon. Il reste la possibilité que la dilatance mesurée ne soit qu'un phénomène superficiel, dû à des déformations anormales de la surface.Pour lever cette incertitude, on peut mesurer la dilatance à la fois sur la surface externe et sur la surface interne de cylindres creux soumis à des contraintes homogènes: une dilatance superficielle produirait une diminution de la section du trou et une augmentation de la section totale, tandis qu'une dilatance répartie dans tout le volume produirait des augmentations de section similaires. Les essais sur cylindres creux de quartzite montrent que la dilatance est un phénomène réparti dans tout le volume.

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With 4 Figures     

Climate Change and Winter Wheat Management: A Modelling Scenario for South-Eastern England

Crop models are useful tools for assessing the impact of climate change on crop production. The dynamic crop-growth model, CERES-Wheat is used to examine crop management responses, including yield, under six climate change scenarios for the years 2025 and 2050 on the Estate of Imperial College at Wye, Kent, U.K. Sensitivity analysis shows a dry matter yield decrease in response to increases in temperature alone. CERES-Wheat was then constrained to assess the crop performance under water-limited production scenarios with different soils, and the results show that crop grain yield actually increases, largely due to CO₂ fertilisation leading to increased rates of photosynthesis. Different management practices (planting dates and nitrogen application) were applied to find the best adaptation strategies. In general, `early' sowing (10th September) had the highest simulated yield, and `late' sowing (10th November) the lowest. For the soils tested, the highest and sustained crop production was obtained from Hamble soils (silt loam) compared with either the Fyfield (sandy) or Denchworth (clay). Adding nitrogen and other fertilisers would likely be necessary to take full advantage of the CO₂ fertilisationeffect and to compensate, in some cases, for yield losses caused by climate change where water shortage becomes serious.     

Spatial and temporal variability of ground water recharge in central Australia: a tracer approach

Two environmental tracer methods are applied to the Ti-Tree Basin in central Australia to shed light on the importance of recharge from floodouts of ephemeral rivers in this arid environment. Ground water carbon-14 concentrations from boreholes are used to estimate the average recharge rate over the interval between where the ground water sample first entered the saturated zone and the bore. Environmental chloride concentrations in ground water samples provide estimates of the recharge rate at the exact point in the landscape where the sample entered the saturated zone. The results of the two tracer approaches indicate that recharge rates around one of the rivers and an extensive floodplain are generally higher than rates of diffuse recharge that occurs in areas of lower topographic relief. Ground water 2H/1H and 18O/16O compositions are all depleted in the heavier isotopes (delta2H = -67 per thousand to -50 per thousand; delta18O = -9.2 per thousand to -5.7%o) compared with the long-term, amount-weighted mean isotopic composition of rainfall in the area (delta2H = -33.8 per thousand; delta18O = -6.3 per thousand). This indicates that recharge throughout the basin occurs only after intense rainfall events of at least 150 to 200 mm/month. Finally, a recharge map is developed to highlight the spatial extent of the two recharge mechanisms. Floodout recharge to the freshest ground water (TDS <1,000 mg/L) is approximately 1.9 mm/year compared with a mean recharge rate of approximately 0.2 mm/year to the remainder of the basin. These findings have important implications for management of the ground water resource.     

Adapting to climate change: Public water supply in England and Wales

This paper describes an assessment of the ways in which water supply companies in England and Wales are adapting to climate change, evaluated in the context of a model of the adaptation process. The four components of the model are (i) awareness of and concern about the potential impacts of climate change, (ii) adaptation strategy, (iii) the concept of an adaptation space from which options are selected, and (iv) the notion that three groups of factors influence awareness, strategy and option selection: susceptibility to change, internal characteristics of the organisation, and regulatory and market context.Public water supply in England and Wales is provided by private sector companies, subject to environmental and economic regulation. Hydrological simulations suggest that climate change has the potential to reduce the reliability of supply sources over the next few decades. The industry in December 2004 completed a review of investment requirements over the next five years.Awareness of climate change is high in the water industry, but by developing assessment procedures and incorporating them into the investment review the regulators forced companies to consider explicitly the potential impacts of climate change in a consistent and rigorous manner. These analyses combined climate change with other pressures on water resources, and in practice companies did not attribute specific investment decisions or proposals to climate change or indeed any other individual drivers. The broad strategy adopted by all water supply companies – to maintain standards of service – is determined by regulatory controls and market considerations, but the degree of concern about the impacts of climate change and precise adaptation options necessary to address supply-demand imbalances varied between water supply companies, reflecting local geographic conditions. The water supply companies and regulators have different perspectives on the relative merits of supply-side and demand-side measures, reflecting different organisational priorities.The 2004 investment review determined that no specific actions were necessary to deal with future climate change, but that measures set in place – in terms of methodologies and investment in investigations into specific resource developments – provided a sound foundation for more specific actions in the next investment review in five years time. The paper concludes by summarising the factors assisting and constraining adaptation over the next few decades.     

A Pb-Ra-Th-U study of Klyuchevskoy and Bezymianny volcanoes, Kamchatka

Lavas from Klyuchevskoy and Bezymianny volcanoes, Kamchatka, appear to show a link between the extent of partial melting in their mantle source region and the subsequent degree of fractionation suffered by the magmas during passage through the crust. This fractionation may have occurred on timescales significantly less than 1000 years if observed ²²⁶Ra excesses largely reflect variable residual porosity in the source melting region. Unlike most arc lavas, those with the highest MgO contents and Ba/Th ratios have the lowest ²²⁶Ra excess. Forward models suggest that those portions of the source which had undergone the greatest addition of U by fluids from the subducting plate also underwent the greatest extents of partial melting at the highest residual porosity. At Kluchevskoy, a change from eruption of high-MgO to high-Al₂O₃ basaltic andesites around 1945 is reflected in an increase in size of ²²⁶Ra excess which seems to require a simultaneous decrease in residual porosity and suggests a rapid changes in the melting regime. The eruption of andesites at Bezyminanny, simultaneous with the eruption of basaltic andesites at Klyuchevskoy, further suggests that different degree melts produced at differing residual porosity can be formed and extracted from the melt region at the same time. Thus, the melting processes beneath Klyuchevskoy and Bezyminanny are demonstrably complex. They have clearly been influenced by both fluid addition from the subducting plate and extension and decompression beneath the Central Kamchatka Depression. Finally, the ²¹⁰Pb data are, with one or two exceptions, in equilibrium with ²²⁶Ra, suggesting that there was restricted relative magma-gas movement in this highly productive magmatic system.     

Thermobarometry of the Bleikvassli Zn-Pb-(Cu) deposit, Nordland, Norway

The Bleikvassli massive sulfide ore deposit is hosted by Proterozoic pelitic, quartzofeldspathic, and amphibolitic rocks of the Uppermost Allochthon of the Scandinavian Caledonides. Staurolite-garnet-biotite and kyanite-staurolite-biotite assemblages indicate that metamorphism reached the kyanite zone of the amphibolite facies. Geothermobarometry was conducted on rocks in and around the deposit using a variety of silicate and sulfide calibrations. Temperature determinations are most reliant on the garnet-biotite exchange reaction, with analyses obtained from 259 garnet rims and adjacent biotite. Results from nine calibrations of the garnet-biotite geothermometer are considered, but compositional limitations of many calibrations involving high Ca and Mn contents in garnet and Al^VI and Ti in biotite make many of the coexisting mineral pairs unsuitable. Average temperatures calculated from the two calibrations that most closely address the garnet-biotite compositions observed at Bleikvassli are 584 °C ± 49 °C and 570 °C ± 40 °C. The application of two calibrations of the garnet-staurolite geothermometer on a limited number of samples yields 581 °C ± 2 °C and 589 °C ± 12 °C, assuming a _H2O=0.84, based upon calculations of the modal proportions of gaseous species. Pressure determinations are less constrained. Phengite and plagioclase-biotite-garnet-muscovite geobarometers give average pressures of approximately 5.0 kbar and 8.5 ± 1.2 kbar, respectively. Pressures obtained from the sphalerite-hexagonal pyrrhotite-pyrite barometer average 7.7 ± 1.0 kbar. In consideration of these results, the peak metamorphic conditions at the Bleikvassli deposit are estimated to be 580 °C and 8 kbar. Received: 18 June 1997 / Accepted: 14 May 1998