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Characterisation of model uncertainty for reliability-based design of pile foundations
Get PDFDissertation (PhD)--University of Stellenbosch, 2007.ENGLISH ABSTRACT: To keep pace with international trends, the introduction of geotechnical limit state design in
South Africa is inevitable. To pave the way for implementation of limit state pile design in
the country, the study quantifies model uncertainty in the classic static pile design formula
under the Southern African geologic environment. The generated model uncertainty
statistics are used to calibrate partial resistance factors in a reliability-based design
framework.
A series of pile performance predictions by the static formula are compared with measured
performances. To capture the distinct soil types for the geologic region of Southern Africa
as well as the local pile design and construction experience base, pile load tests and
associated geotechnical data from the Southern African geologic environment are used. The
methodology of collecting, compiling, and analyzing the pile load tests to derive the
measured ultimate pile capacities is described. To facilitate the computation of the
theoretical capacities, the site specific geotechnical data in the database are transformed to
the desired engineering soil properties through well established empirical correlations.
For a given pile test case, model uncertainty is presented in terms of a model factor
computed as the ratio of the measured to the theoretical capacity, leading to n realisations of
the model factor. To facilitate further interpretation and generalisation of the model factor
realisation data, statistical analysis is carried out. The statistical analysis comprises of
graphical representation by histograms, outliers detection and correction of erroneous
values, and using the corrected data to compute the sample moments (mean, standard
deviations, skewness and kurtosis) needed in reliability analysis. The analyses demonstrate
that driven piles depict higher variability compared to bored piles irrespective of materials
type. Furthermore, for a given pile installation method (driven or bored) the variability in
non-cohesive materials is higher than that in cohesive materials.
In addition to the above statistics, reliability analysis requires the theoretical probability
distribution for the random variable under consideration. Accordingly it is demonstrated that
the lognormal distribution is the most appropriate theoretical model for the model factor.
Another key basis for reliability theory is the notion of randomness of the basic variables.
To verify that the variation in the model factor is not explainable by deterministic variations
in the database, an investigation of correlation of the model factor with underlying pile
design parameters is carried out. It is shown that such correlation is generally weak. Correlation can have a significant impact on the calculated reliability index if not accounted
for. Accordingly, the effects of the exhibited correlation is investigated through an approach
based on regression theory in which systematic effects of design parameters are taken into
account (generalised model factor). The model factor statistics from the conventional
approach and those from the generalised model factor approach are used to determine
reliability indexes implied by the current design practice. It is demonstrated that no
significant improvement in values of the reliability indexes is gained by taking into account
the effects of the weak correlation.
The model factor statistics derived on the basis of the standard model factor approach are
used to calibrate resistance factors. Four first order reliability methods are employed for the
calibration of resistance factors. These include; the Mean Value First-Order Second
Moment approach, an Approximate Mean Value First-Order Second Moment approach, the
Advanced First-Order Second Moment approach using Excel spreadsheet, and the
Advanced First-Order Second Moment approach (design point method). The resistance
factors from the various calibration methods are presented for the target reliability index
values of 2.0, 2.5, and 3.0. The analyses of the results demonstrate that for a given target
reliability index, the resistance factors from the different methods are comparable.
Furthermore, it is shown that for a given material type, the resistance factors are quite close
irrespective of the pile installation method, suggesting differentiation of partial factors in
terms of materials types only. Finally, resistance factors for use in probabilistic limit state
pile design in South Africa are recommended.AFRIKAANSE OPSOMMING: Ten einde in pas te bly met internasionale neigings, is dit onafwendbaar dat geotegniese
limietstaat-ontwerp in Suid Afrika ingevoer word. Ter voorbereiding vir die plaaslike
toepassing van limietstaatontwerp op heipale, kwantifiseer hierdie ondersoek onsekerheid
rondom die model vir klassieke statiese heipaalontwerpformules in die Suid Afrikaanse
geologiese omgewing. Die statistiek van modelonsekerheid wat gegenereer is, word
gebruik om parsiële weerstandsfaktore in ’n betoubaarheid-gebasseerde ontwerpraamwerk
te kalibreer.
’n Reeks voorspellings van die gedrag van heipale volgens die statiese formules word
vergelyk met die gemete gedrag. Om die kenmerkende grond-tipes in die geologiese gebied
van Suidelike Afrika sowel as die plaaslike ondervinding met heipaalontwerp en -
konstruksie vas te lê, word heipaaltoetse en die gassosieerde geotegniese data vanuit hierdie
geologiese omgewing gebruik. Die metodiek vir die versameling, saamstelling en analise
van heipaaltoetse om uiterste kapasiteite daarvan te bepaal, word beskryf. Terreinspesifieke
geotegniese data in die databasis word getransformeer na die vereisde ingenieurseienskappe
volgens gevestigde empiriese korrelasies.
Vir ’n gegewe heipaaltoets word modelonsekerheid weergegee in terme van ’n modelfaktor
wat bereken word as die verhouding van die gemete tot die teoretiese kapasiteit waaruit n
uitkomstes van die modelfaktor dus gegenereer word. Om verdere interpretasie en
veralgemening van die modelfaktordata te vergemaklik, word ’n statistiese analise daarop
uitgevoer. Die statistiese analise bestaan uit grafiese voorstellings deur middel van
histogramme, uitkenning van uitskieters en verbetering van foutiewe waardes, waarna die
statistiese momente (gemiddeld, standaardafwyking, skeefheid en kurtose) vir gebruik in
betroubaarheidsanalise bereken word. Volgens die analises toon ingedrewe heipale ’n
groter veranderlikheid as geboorde pale, ongeag die grondtipe. Verder is die
veranderlikheid van heipale in kohesielose materiale hoër as in kohesiewe materiale, ongeag
die installasiemetode (ingedrewe of geboor).
Bykomend tot bogemelde statistiek, vereis betroubaarheidsanalise die teoretiese
waarskynlikheidsdistribusie van die ewekansige veranderlike onder beskouing.
Ooreenkomstig word illustreer dat die log-normale verspreiding die mees toepaslike
verspreiding vir die modelfaktor is. ’n Verdere sleutelvereiste vir betroubaarheidsteorie is die mate van ewekansigheid van die basiese veranderlikes. Om te bepaal of die variasie in
die modelfaktor nie deur deterministiese veranderlikes in die databasis verduidelik kan word
nie, word ’n ondersoek na die korrelasie van die modelfaktor met onderliggende heipaalontwerpfaktore
uitgevoer. Sodanige korrelasie is in die algemeen as laag bevind.
Korrelasie kan ’n belangrike invloed op die berekende betroubaarheidsindeks hê indien dit
nie in ag geneem word nie. Dienooreenkomstig word die effek van die getoonde korrelasie
ondersoek met behulp van die metode van regressie-analise waarin sistematiese effekte van
ontwerpparameters in berekening gebring word (veralgemeende modelfaktor). Die
modelfaktorstatistiek wat volg uit die konvensionele benadering en dié van die
veralgemeende benadering word gebruik om betroubaarheidsindekse te bepaal wat deur die
bestaande ontwerppraktyk geïmpliseer word. Die bevinding is dat daar nie ’n
noemenswaardige verbetering in die waardes van die betroubaarheidsindekse is wanneer die
effek van die swak korrelasie in berekening gebring word nie.
Die statistiek van die modelfaktor wat afgelei is volgens die standaardbenadering word
gebruik om die weerstandsfaktore te kalibreer. Vier eerste-orde betroubaarheidsmetodes
word gebruik om die weerstandsfaktore te kalibreer, naamlik die Gemiddelde Waarde
Eerste-Orde Tweede Moment benadering, die Benaderde Gemiddelde Waarde Eerste-Orde
Tweede Moment benadering, die Gevorderde Eerste-Orde Tweede Moment benadering
waarin ’n Excel sigblad gebruik word en die Gevorderde Eerste-Orde Tweede Moment
benadering (die ontwerppuntmetode). Die weerstandsfaktore vanaf die verskillende
kalibrasiemetodes word weergegee vir waardes van 2.0, 2.5 en 3.0 van die teikenbetroubaarheidsindeks.
’n Ontleding van die resultate toon dat vir ’n gegewe teiken
betroubaarheidsindeks die weerstandsfaktore vanaf die verskillende metodes vergelykbaar
is. Verder word getoon dat vir ’n gegewe grondsoort, die weerstandsfaktore vir verskillende
metodes van installasie van die heipaal nie veel verskil nie. Dit wil dus voorkom asof
parsiële faktore in terme van die grondsoort uitgedruk kan word. Ten slotte word
weerstandsfaktore vir gebruik in plastiese limietstaatontwerp van heipale in Suid Afrika
aanbeveel.Doctora
Resistance model uncertainty factors for ultimate limit state design of pile foundations
Get PDFCITATION: Dithinde, M. & Retief, J. V. 2014. Resistance model uncertainty factors for ultimate limit state design of pile foundations. Journal of the South African Institution of Civil Engineering, 56(2):37-45.The original publication is available at http://www.scielo.org.zaENGLISH ABSTRACT: The current limit state design procedure for pile foundations presented in geotechnical codes (e.g. SANS 10160-5, EN 1997-1) stipulates that, when the pile capacity is determined using an analytical approach, such as the static analysis using engineering properties of the soil as determined from laboratory or in-situ field testing, the prescribed partial resistance factors (γr) need to be corrected by a partial factor for the uncertainty in the resistance model (γR, d) or the resistance model uncertainty factor. The international position is to derive model uncertainty factors for both actions and resistances from available experimental data. Accordingly this paper makes use of a local pile load test database to derive the appropriate γR, d values for ultimate limit state design of pile foundations. The analysis indicates that γR, d values of 1.3 and 1.5 for piles in cohesive and non-cohesive materials respectively are appropriate. Alternatively, a single value of γR, d = 1.4 for all piles in all soils can be adopted with different γr values for the two distinctive pile classes of piles in cohesive and non-cohesive soils.AFRIKAANSE OPSOMMING: Geen opsomming beskikbaarhttp://www.scielo.org.za/scielo.php?script=sci_abstract&pid=S1021-20192014000200005&lng=en&nrm=iso&tlng=enPublisher's versio
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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