geotechnical behaviour of shell foundations
TRANSCRIPT
PRESENTED BY SABNA THILAKAN
Semester III
ME (Found. Engg)
Guided by
Prof. Nisha Naik
4-02-2015
GEOTECHNICAL BEHAVIOUR OF
SHELL FOUNDATIONS
Objective of study
Provide and insight into the geotechnical
behaviour of shell foundations
Showcase its advantages over conventional
foundation structures
Popularize the concept of shell foundations
as an economical replacement to
conventional flat footings
Shells
Structures that derive its strength from its
geometry rather than mass
Three dimensional manifestation of the arch-
action
SHELLS
Commonly found shells in nature
Shells as structures
Used widely as roof structures both in the
past as well as present
Gives maximum strength with minimum
material consumption
Very economical as they can cover large
space
Aesthetics
Shell roofs in Germany showed
remarkable resistance to bombing during
World War II
Some famous shell structures
Shells as foundations
Many buildings in India have been supported on
inverted brick arches
IS 9456-1980 Code of Practice for Design and
Construction of Conical and Hyperbolic Parabolic
Type of Shell Foundations
Shells as foundations
Classification
Shells
Curved
Singly curved
Doubly curved
Folded Plates
Upright Inverted
Doubly curved
Synclastic(Curved in same direction)
Eg. Spherical dome
Anticlastic(Curved in opposite direction)
Eg.Hyperbolicparaboloid
Synclastic and Anticlastic
Shells Used in Foundations
Hypar (Hyperbolic paraboloid)
Made up of two parabolas one over the other
Anticlastic
Also formed as a warped surface using straight line
generatrix
Conical
Simplest form of shell
Pyramidal shell foundation
Square or rectangular in
plan
Combine to form raft
Spherical shell
Complex geometry
Does not possess
straight line property
Difficult to construct
Costly
Elliptical paraboloid shell foundation
Synclastic shape
Used a single unit of
foundation
e.g towers, chimneys
Built with edge beams
iver which columns can
be supported
Inverted Dome Shell
Foundation
Paraboloid and Hyperboloid of
revolution shells
Suited for tower shaped
structures
Literature Survey
Hanna and Abdel- Rahman (1998)
studied behaviour of three types of shells namely,
triangular strip, conical and pyramidal
Compare results with conventional flat counterparts.
Loose, medium, dense sands
Experimental, numerical (PLAXIS) and theoretical
analysis
Special tests on coloured sands
On surface and embedded
Plain
strain
Axisymm
etrical
Three
dimensio
nal
Flat strip Flat
circular
Flat
square
Rise to
half
width =
½
Triangula
r 1
Conical 1 Pyramidal
1
Rise to
half
width =
1
Triangula
r 2
Conical 2 Pyramidal
2
Square flat and Pyramidal model
at ultimate stage
Typical Load-Settlement Curves
for Plain Strain conditions
Comparison
Shell efficiency factor (η )
Settlement factor (Fδ)
Qus is the ultimate load of shell
footing
Quf is the ultimate load of flat
footing
Fδ= Non dimensional settlement factor
δu= Settlement at ultimate load
γ = Unit weight of soil
Ah= area of footing as projected
horizontally
Qu=Ultimate load
Ultimate load higher for shells as compared to flat
foundations
Bearing capacity increases with shell angle θ
Shell gain factor decreases with ϕ
Settlement factor was less for shells as compared to
flat footings
Rupture area was shallower as compared to that of flat
foundations
Calculation of bearing capacity (Abdel,1996)
qu = c Nc + γ Df Nq + γ b Nγ
Nc, Nq and Nγ are bearing
capacity factors as
functions of ϕ and θ
Kurian and Devaki (2005)
Conical,Spherical ,Hypars(a/b=1)
Experimental and FEM analysis
Bearing Capacity
Settlement characteristics
Comparison with flat counterparts (circular and square)
Parameters chosen:
• Interface roughness (µ =0, tan(2/3 ϕ) , 100)
• Type of soil ( c, ϕ and c - ϕ soils)
• Type of loading (vertical loading , horizontal loading, moments)
Huat and Mohammed (2006) FEM analysis
(PLAXIS)
Hypar , conical and
spherical
Load carrying capacity
Effect of edge beams
Effect of depth of
embeddment
Effect of
Embeddment
Effect of edge
beams
Huat et al (2007)
Triangular shell
Upright and inverted
Effect of shell thickness t and shell angle θ
Lab tests
Field tests
Numerical tests ( 2D and 3D using LUSAS)
Field test set up
Endalkachew (2009)
Conical shells and flat circular footing
Numerically modeled in PLAXIS
Soil used was red Clay
Mohr coulomb model was used in FEM
Azziz et al (2011)
Hypar shell footings
Best shell configuration for foundation
Easy construction because of straight line
property
Economical, Savings in construction costs
Rinaldi (2012)
Studied upright
and inverted shells
Modeled in PLAXIS
deflection
soil stresses
contact pressures
Triangular
modelsCylindrical
modelsPlaxis modeling
SummaryAdvantages of shell foundations over conventional flat
foundations
shells are simple to construct (especially hypar, conical)
Greater load capacity
Greater stability
Minimum material consumption.
Lesser construction costs.
Inverted shells have higher load capacity than upright
ones
Advantageous in developing countries with high
material-labour costs
Conical foundation suitable for chimney and tower like
structures.
Easier to construct than roof
Disadvantages
Limited shapes as foundations
Difficult to prepare formworks (esp. spherical )
Skilled labour required
Soil subsidence can pose problems
Uncertainties when dealing with soils with large
volume changes
References
Abdel-Rahman, M.M, and A.M.Hanna (1990) Experimental Investigation of Shell Foundations
on Dry Sand. Canadian Geotechnical Journal, J.35,pp.847-857.
Abdel-Rahman, M.M, and A.M.Hanna (1990) “Ultimate Bearing Capacity of Triangular Shell
Strip Footing on Sand”.Journal of Geotechnical Engineering ASCE, 116(2):1851-1863
Abdel-Rahman, M., (1996).” Geotechnical behavior of shell foundations”. Ph.D Thesis,
Department of Civil Engineering, Concordia University, Montréal, Canada.
Ángel E. C., William C., Yoermes G, José Á.(2011).”A look at half a century of shells
foundations, methods of calculation and associate research in Cuba” Revista Ingeniería de
Construcción ,Vol. 26 No3, pp 245-268
Fernando N., Sendanayake E., Sendanayake D., De Silva N., (2011) “The Experimental
Investigation of Failure Mechanism and Bearing Capacity of Different Types of Shallow
Foundations” Department of Civil Engineering, University of Moratuwa.
Hanna, A.M. and M. Abdel-Rahman, (1998). “Experimental investigation on shell foundations on
dry sand”. Can. Geotech. Journal., 35: 828-846.
Huat, B.B.K. and Mohammed, T.A. (2006). “Finite Element Study Using FE Code (PLAXIS) on
the Geotechnical Behaviour of Shell Footings”.Journal of Computer Science. USA: NY. 2(1):
104-108.
.
References
Huat B. B. K.,Mohammed T. A., Abang Ali A. A. A. and Abdullah A.A,(2007), “Numerical and Field
Study on Triangular Shell Footing for Low Rise Building”, International Journal of Engineering and
Technology,l. 4- 194-204
IS 9456 – 1980, Code of Practice for Design and Construction of Conical and Hyperbolic
Paraboloidal types of Shell Foundations
Kurian, N. P. & Jeyachandran, S. R. (1972). “Model studies on the behaviour of sand under two and
three dimensional shell foundations”. Indian Geotechnical Journal, Vol. 2, No. 1, pp. 79-90.
Minami K.,’Foundation Construction Using Hollow Shells to Minimize Settlement,” Part 1, General
Concepts, Trans., Arch. Institute of Japan, No. 37, pp 52-57, Tokyo
Nainan P. Kurian and V.M. Jayakrishna Devaki, (2005), “Analytical studies on the geotechnical
performance of shell foundations,” published on the NRC Research press web site at http://cgi.nrc.ca
on 19 May 2005
Rinaldi R.(2012).”Inverted Shell Foundation Performance In Soil”.A Thesis in the Department of
Building, Civil & Environmental Engineering. Concordia University, Montreal, Quebec, Canada
Varghese P.C.,(2009) .”Design of Reinforced Concrete Foundations”.Published by Ashok K Ghosh,
PHI Learning Private .pp 27