2735
Probabilist analysis of the foundation of a shopping center in brazil
Analyse probabiliste des fondations d'un centre commercial au Brésil
Gusmão A.
University of Pernambuco and IFPE, Brazil
Oliveira P.
Gusmão Engineers, Brazil
Ferreira S.
Federal University of Pernambuco and Unicap, Brazil
Maia G., Amorim M.
Gusmão Engineers, Brazil
ABSTRACT:
The Brazilian foundation code that was reviewed in 2010 defined the criteria for the use of probabilistic analysis in the
design of foundation, and there is a perspective that such analyzes are increasingly used in practice. The greatest difficulty of the
analysis is the small number of load tests available in the works, and particularly in the design phase. This article presents the main
aspects of design, execution and control of the construction of a shopping center with 280,000 m
2
area in Recife, Brazil, where they
were executed 4,000 continuous flight auger piles type (CFA piles). 40 static load tests were simultaneously performed with the
execution of the piles (1% of the total number of piles as recommended by the Brazilian code). With the favorable outcome of the first
15 load tests, the design was reviewed and there was a significant reduction in the number of piles and the total cost of the foundation.
It is presented a statistical analysis of results of static load tests, where it was verified the influence of the number of load tests
considered on the shape of the curve of resistance, global factor of security, characteristic safety factor and probability of failure. The
results showed that when it attained a number of load tests equivalent to 0.4% of the total number of piles, the probability of failure
are not showed more substantial changes
.
RÉSUMÉ :
La norme brésilienne sur les fondations qui a été révisée en 2010 a défini les critères pour l'utilisation de l'analyse
probabiliste dans la conception des fondations. Une
telle
analyse est susceptible d’être de plus en plus utilisée en pratique. La plus
grande difficulté de l'analyse est le petit nombre d’essais de
chargement
disponibles et en particulier en phase de conception. Cet
article présente les principaux aspects de la conception, l'exécution et le contrôle de la construction d'un centre commercial de
280 000 m
2
de surface à Recife, au Brésil, où ont été exécutés 4 000
pieux
forés en continu (pieux CFA). 40 essais de chargement
statique ont été réalisés simultanément à l'exécution des pieux (soit 1% du nombre total de pieux,
tel
que recommandé par la norme
brésilienne). Avec les résultats satisfaisants des 15 premiers essais de chargement, la conception été révisée, entraînant une réduction
significative du nombre de pieux et du coût total des fondations. Une analyse statistique des résultats des essais de chargement
statique est présentée, permettant de vérifier l'influence du nombre d
’essais
de chargement considéré sur la forme de la courbe de
résistance, le facteur global de la sécurité, le facteur de sécurité caractéristique et la probabilité de ruine. Les résultats ont montré que,
quand on atteint un nombre d
’essais
de chargement équivalent à 0,4% du nombre total de pieux, la probabilité de ruine ne se trouve
pas significativement modifiée.
KEYWORDS: Foundation, Statistical Analysis, Probability of Failure.
1 DESCRIPTION OF THE STRUCTURE AND
SUBSOIL
The building analyzed is a precast reinforced concrete structure
with openings, with approximately 280,000 m2 destined to
shopping areas and garages. There are portions of the structure
with up to six levels of concrete slabs. A total of 1,283 pillars
support permanent vertical loads ranging from 200 to 9,000 kN.
The arrangement of the loads also takes into account vertical,
horizontal, and moments loads resulting from wind action. The
project is designed with a ground floor level, deployed at an
elevation of +3.00. From the standpoint of topography, the
native terrain does not feature natural unevenness, with a
median average of 2.00. From the geological standpoint, the site
is located in the fluvial-marine plain, within an marine terrace
(Gusmão et al., 1998).
The geotechnical characterization carried out with an
arrangement of 61 percussion soundings permits, in a simplified
form (Fig.1) characterization of the foundation ground, which is
initially composed of a landfill layer of fine silty sand, light
brown, soft to moderately compact, to a level of 1.00; followed
by layers of sand with organic materials, or silty organic clays,
dark gray, very soft, to a level of -3.00. Below this level are
found subsequent layers of fine to medium sand, gray, lightly to
moderately compacted, interspersed with layers of silty clays,
medium, to levels of -21.00 -28.00. After this layer, a layer of
silty clays, or silt with very fine sand, gray, compact, extends to
the limit of the soundings that were performed (-35.00). The
water level was found to vary in levels from 0.00 to +1.50.
2 FOUNDATION DESIGN AND VERIFICATION
OF PERFORMANCE
CFA piles (400 and 500 mm diameter) were employed, with
allowable loads of 700 and 1,150 kN, respectively. The client
indicated that load tests be conducted over the course of the
execution of the foundation, due to the time frame for
completion and delivery of the project. At first, a total of 15
static load tests were performed, with satisfactory results. At
this point, during this first step, 2,563 piles had been placed.
Then, from the results of these pilot load tests, the project was
revised, and loads for the 400 and 500 mm piles were altered to
800 kN and 1,300 kN (an increase of 14% and 13%
respectively).
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