Experimental and FE Analysis of Funicular Shells

ISSN: 2277-3754
ISO 9001:2008 Certified
International Journal of Engineering and Innovative Technology (IJEIT)
Volume 4, Issue 9, March 2015
178
Abstract: Shells belong to the class of stressed skin
structures which, because of their geometry and small
flexural rigidity, tend to carry loads primarily by direct
stresses acting in their plane. Concrete funicular shells of
square ground plan, double curvature with 80 mm rise are
analysed for uniformly distributed load (One-way action).
Specimens of size 1080 mm x 1080 mm in plan with
rectangular edge beam of 50 mm x 40 mm are prepared
using cement concrete of grade M20 for which the mix
design is carried by Indian Standard method. The
precasting of the Concrete shallow funicular shell
specimens is carried by masonry mould method. The
specimens are prepared with thickness of 25 mm and 20
mm. The specimens are moist cured for 28 days before
testing. The uniformly distributed load over the shell
specimen is applied and the corresponding deflections,
strains are measured. The coordinates of funicular shells
are determined by developing a computer program. To
relate experimental results to theory, the finite element
technique (SAP 2000 Program) is utilized to analyse a
similar model in the elastic range. Finite element models
of funicular shells are developed by discretizing the shell
specimens into 20 elements along x direction and 20
elements along y direction. Behaviour of funicular shells
under uniformly distributed load is carried out.
Conclusions are made by comparing the experimental and
analytical results.
Keywords: Funicular shell, Edge beam, One-way action,
Finite element models, Mix-design, Discretization, SAP.
I. INTRODUCTION
Environmental degradation witnessed today is a result of an
irresponsible use of materials. Materials are being made to
perform contrary to their natural qualities. Most materials
behave best in compression but the over reliance on RCC has
resulted in tensile structures which are made to perform contrary
to the natural qualities of the materials. For example, a
conventional beam upon loading tends to bend at the centre. The
upper region of the beam is in compression while the lower part
is in tension. To counter the tensile stresses steel reinforcements
are required in the lower portion against its natural capabilities.
Upon inverting the structure, it is converted into a compression
structure, with a considerable reduction in the amount of steel
and cement. In this case, a nominal ring beam is capable of
taking the lateral thrust developed in the structure. Traditionally
compression structures in the form arches, vaults, domes,
catenaries and doubly curved structures also called funicular
shells have been used extensively in the temples / forts. The
problem of prefabricated shells of double curvature,
subjected to concentrated load and supported at two edges,
has previously been investigated. In practice there are cases
in which shells are supported on four edges, however, a
survey of the related literature reveals not much research has
done in this area.
II. LITERATUREREVIEW
The selection of available documents (both published and un
published) on the topic, which contain information, ideas,
data and evidence written from a particular standpoint to ful
fill certain aims or express certain views on the nature of
the topic and how it is to be investigated, and the effective
evaluation of these documents in relation to the research
being proposed. A literature review is a survey of already
existing writings (usually published) on a given topic or area
with a view to assessing their relevance to a proposed
project. The behavior of prefabricated shell units has been
investigated, both theoretically and experimentally, by many
researchers. Apart from construction schemes, the use of pre
fabricated shell units creates certain problems which have to
be investigated from the research point of view. One of the
most important is the structural behavior of the units when
subjected to the concentrated loads at various points along
their span. Another consideration of interest is the ultimate
strength and pattern of failure for these types of shells. The
shells of double curvature usually offer a higher ultimate
strength than the shells of single curvature, such as
cylindrical shells. The funicular shell roof is one such
compression structure, which ensures conservation of natural
resources by utilizing waste materials effectively and
optimizing the use of expensive steel and cement. Further, the
Experimental and FE Analysis of Funicular Shells
P. Sivakumar1
, K. Manjunatha2
, Harish B. A3
,
1
Chief Scientist, CSIR-Structural Engineering Research Centre, CSIR Road, Chennai-600113.
2
Professor, Department of Studies in Civil Engg, UBDT College of Engineering, Davangere-577004.
3
Assistant Professor, GM Institute of Technology, Davangere-577004.

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arch distributes the point load in all direction equally thus, is
able to withstand impact loading at any point. Diagonal grid of
funicular shell gives the illusion of a larger space.
III. REVIEW OF SELECTED TECHNICAL PAPERS
Albolhassan Vafai and Mehdi Farshad (1979) have done a
research work on theoretical and experimental study of
prefabricated funicular shell units. This paper presents the
results of a theoretical and experimental study of funicular
shell structures. A particular design is given for the
geometry, the form, and the type of reinforcement of the
units. Ten models are constructed based on this design. First,
six samples supported at two edges are loaded to a specified
load within the elastic region. Electrical resistance gauges
are mounted both inside and outside at several locations on
the surface of the shell on two different specimens. Also, dial
gauges are installed at several locations on the surface of four
shells. Following these non-destructive tests, all ten samples
are loaded to failure, subjected to a concentrated load at the
center. To relate experimental results to theory, the finite
technique is utilized to analyze a similar model. The
experimental values of membrane stresses along the central
section in the direction of the supports are calculated and
compared with the theory. The results are in close agreement
at some distance away from the supports, but the difference
becomes noticeable closer to the support. The same
phenomenon appears to be true when the experimental
values of vertical deflections along the longitudinal and
transverse sections of the shells are compared with the
theory. Also, the experimental failure loads are found to be
directly related to the amount of reinforcement, and the age
of the concrete shells.
Abolhassan Vafai, Massoud Mofid and Homayoon
E.Estekanchi (1997) have done a research work on
experimental study of prefabricated funicular shell units. The
experimental values of membrane stresses in the elastic range
along diagonal sections of funicular shell structures are
calculated and compared with the theory. The results are
generally found to be in close agreement with the theory.
Experimental values of vertical deflections along the
longitudinal and transverse sections of the shells also
compare favorably with the theory. The experimental failure
and crack loads are found and empirical equations,
expressing the relation between rise and failure crack loads,
are given. This paper contains the results of numerical and
experimental studies carried out in the area of funicular shell
units with square bases supported at four edges and subjected
to concentrated loading at the apex. A particular design is
given for the geometry, the form, the rise and the type of
reinforcement of the units. Forty-five models are constructed
based on this design. First, eight samples supported at four
edges are loaded to a specified load within the elastic region.
Electrical resistance gauges are mounted both inside and
outside at several locations along the diagonal on the surface
of the shell on two different specimens with a rise of 6 and
9cm, respectively. Also, dial gauges are installed at several
locations on the surface of the other six specimens with
varying rise and reinforcement types. Following these
non-destructive tests, all 45 samples are loaded to failure,
subjected to concentrated load at the centre. To relate
experimental results to theory, the finite element technique
(SAP90 Program) is utilized to analyze a similar model in the
elastic range.
John W. Weber, Kwong-chi Wu and Adholhassan Vafai
(1984) have done an evaluation of ultimate loads for shallow
funicular concrete shells. Shells belong to the class of
stressed skin structures which, because of their geometry and
small flexural rigidity of the skin, tend to carry loads
primarily by direct stresses acting in their plane. Ten shallow
funicular concrete shells were loaded to failure with a
concentrated central force. Five shells were randomly
reinforced with steel wires and the reminder with a wire mesh
through the middle surface. All of the shells were 90 cm x 90
cm in plan form. Strain gauges showed a linear relationship
between load and strain in the elastic range of the concrete,
whereas measured deflections were larger than those
determined analytically by small deflection theory, would be
more appropriate for theoretical investigations of shallow
funicular shells subjected to large concentrated loads.
Ultimate loads were not clearly related to type of
reinforcement, but were a function of the rise and thickness
of the shell in general the larger the rise parameter (square of
the ratio of rise to thickness), the larger the ultimate load.
Failure patterns for shells with both kinds of reinforcement
were the same. They observed that the mathematical
investigations of shallow funicular shells with large
concentrated loads should be based on large deflection
theory and the deflection characteristics of a shell vary
closely with its rise parameter.
S.Elangovan and A.R.Santhakumar (1988) have done a
research work on parametric study of funicular shells. The
funicular shell was analyzed by the finite element method
using isoperimetric elements with five degrees of freedom at
each node. A computer programme developed by the authors
was used in the analysis of funicular shells with clamped
boundaries. The behaviour of the shell under uniformly
distributed load, for various ratios of spans and rise/span was
studied analytically. Approximate expressions for the
calculation of bending moment at the edge of the shell,
in-plane force at crown, and deflection at crown had been
proposed. The funicular shell is a shallow shell of double

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curvature. The shape of such shells to suit any boundary
geometry can be found by analytical or experimental
methods. The behaviour of the shell under uniformly
distributed load, for various ratios of spans and rise/span was
studied analytically. The load applied was within the elastic
range and hence, the deflections, forces, and moments are
proportional to the load. So the intensity of the uniformly
distributed load was taken equal in all cases.
IV. METHODOLOGY
Funicular shells in square ground plan are considered for the
investigation. Numerical approach using advanced finite
element analysis based software called as SAP 2000 is
adopted. Based on reported literature of similar nature a FE
model is developed and an experimental study is conducted
to investigate the behaviour of funicular shells under
uniformly distributed. Concrete shallow funicular shells of
square plan, double curvature with various rises are analyzed
for concentrated central load.
V. GENERATING COORDINATES FOR
FUNICULAR SHELLS
The coordinates of funicular shells are determined by
developing a computer program. Here we are using Turbo C
as the compiler to compile the program. Steps followed in the
program are,
1. Declare integer values of x, y, s. c;
2. Initialize the integer values of f, a, b;
3. Declare Z as a float value
4. Take choice from user whether to continue or to stop
by pressing 1 or 0;
5. Take values of x and y by user;
6. Then calculate the value of Z, by using formula of
Where, Z = vertical ordinate at point x, y
f = maximum central rise
a = half length of the shell
b = half width of the shell
x, y = the co-ordinates of the grid point from the
origin, which is taken as the centre of the shell unit
7. The print the value of Z;
8. Take choice of user whether to continue or not;
VI. EXECUTION OF PROGRAM
To correlate experimental results with theory, a theoretical
study is carried out. The model consisted of a funicular shell
uniformly loaded at its top. The finite element technique and
a related computer program (SAP 2000) were utilized to
analyze structure. The finite element model used to analyse
the shell is based on the assumption that the material is
linearly elastic. Hence, the theoretical prediction is expected
to be valid only in the elastic region of the shell behavior.
The plan of the shell, its dimensions and other dimensions
are represented in the table.1.
Table.1. Shell Dimensions
Shell Plan in m Rise
in
mm
Thick
in
mm
Edge Beam
Reinforcement
FS I 1.08 X 1.08 80 25 6mm Dia rod all
round
FS II 1.08 X 1.08 80 20 6mm Dia rod all
round
The finite element model of funicular shell with square
ground plan is developed by using SAP 2000 finite element
package is shown in figure 1.
Fig.1 Funicular shell Model in SAP2000(3D View)
The plan of the shell, its dimensions, and mesh consisting of
400 elements, 54 X 54 mm each is shown in figure.2.
Fig.2. Finite Element Modelling and corresponding Node
Numbers.

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VII. EXPERIMENTAL INVESTIGATIONS
Following materials are used in construction of shell
specimens.
1) Cement: OPC 53 Grade.
2) Fly ash: 30% of cement is replaced by fly ash.
3) Aggregates: Fine and Coarse.
4) Super plasticizer: CONPLAST 430
CERAPLAST 400
Physical properties of cement are given in table.2.
Table.2. Physical properties of procured OPC
Particulars Test Results
Fineness of cement 2.96 %
Standard Consistency 29 %
Setting Time
Initial
Final
200 minutes
4 hr 10 minutes
Compressive Strength
3 Days
7 Days
16.22 N/mm2
19.86 N/mm2
VIII. CONSTRUCTION OF FUNICULAR
SHELL SPECIMENS
Precasting of the doubly curved shell units may be carried
out by any one of the methods given below.
1. Sagging Fabric Method.
2. Masonry Mould Method.
3. Mechanized Process.
In the present work masonary mould method has been
adopted.
To study the stress distribution, deflections of funicular
shells under uniformly distributed load specimens are
prepared and designated as
FS1: Funicular shell with rise 8 cm and thickness 25 mm.
FS2 : Funicular shell with rise 8 cm and thickness 20 mm.
The Mix Design for Funicular shell 1 has been arrived at
1:1.4:2.34. Cement is partially replaced with 30% of fly ash
and for better workability 0.2% Superplasticizer (Conplast
430) was added. Five concrete cubes and cylinders are cast
for compression and split tensile tests at 28 days.
The Mix Design for Funicular shell 2 has been arrived at
1:1.5:1.6. Cement is partially replaced with 30% of fly ash
and for better workability 0.2% Superplasticizer (Ceraplast
400) was added. Seven concrete cubes and cylinders are cast
for compression and split tensile tests at 28 days.
In masonary mould method, for any desired raise of the shell
of rectangular or square plan the ordinates on various points
can also be calculated using approximate formula. (Fig 3).
Fig.3. Fixing shell coordinates
The ordinates obtained shall be set off from a level platform
and the surface concreted , finished smooth. (Fig 4).
Fig.4. Finishing the surface with cement mortar
The finished surface shall be coated with oil, grease or any
other releasing agents. (Fig 5).
Fig.5. Oil coating

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Outer mould for the edge beams shall be set up. (Fig 6).
Fig.6. Fixing outer mould for the shell
The designed reinforcement for the edge beam shall be
placed between the outer edge beam mould and the masonry
platform built up. The minimum reinforcement in the edge
beams of the shell shall be one 6 mm diameter mild steel bar.
(Fig 7).
Fig.7. Placing the reinforcement in edge beam mould
Concrete of the specified mix shall be laid in the edge beam
and over the shell mould. (Fig 8).
Fig.8. Casting of edge beam
The thickness over the shell mould equal to the designed
thickness of the shell shall be controlled by thickness gauges.
(Fig 9).
Fig.9. Finished shell Surface
The edge beam mould should be released three hours after
casting. (Fig 10).
Fig.10. Removing of edge beam mould
The shell should be lifted off the mould using levers at the
four corners 24 to 48 hours after casting. Longer time up to
72 hours may be necessary in cold climates (below 250
c) as
also when pozzolana cement is used. In using levers ensure
that the levers are operated only on one side at a time and
never at the end at diagonals. (Fig 11).
Fig.11. Finished funicular shell

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The shell should be kept stacked and cured in the normal
way. The stacking of the shells may be done one above the
other supported at four corners only. Shells units up to 1.5 m
in size may be handled manually. Shells heavier than this will
need the help of hoisting equipment and appliances. For
small size shells where handling is done manually, provision
of lifting hooks are not necessary. The procedure is repeated
for casting the specimens FS1 and FS2 with rise 8 cm and
varying thickness. The shell specimens are moist cured for
28 days before testing.
IX. INSTRUMENTATION AND TESTING
To investigate the behaviour of the funicular shell subjected to a
uniformly distributed load at its top surface, the structure was
instrumented and loaded as follows.
Shells FS1 and FS2 are simply supported on a loading frame
and subjected to uniformly distributed load. To simulate
uniformly distributed load a number of 30 kg sand bags have
been prepared and placed on top of the shell in a uniform
manner. To check the elastic behavior of the shells, the
structures were loaded and unloaded several times. The two
shell specimens FS1 and FS2 were placed in a loading
apparatus designed for this purpose, as shown in figure 12.
Fig.12. Sand bags loading
Deflection measurements require more thought than might
be supposed. Since the shell is flexible, it is not very easy to
measure tangential displacements with a reasonable degree
of accuracy, except at its edges and it is best to confine
oneself, on the main part of the shell proper, to the
measurement of radial deflections. Two specimens with rise
of 8cm, thickness 25 mm and 20 mm were used for dial gauge
readings. Figure 7.2 shows the location of installed dial
gauges. Deflection measurements were also carried out for
shells FS1 and FS2 with rise of 8cm, thickness 25 mm and 20
mm respectively. (Fig 13).
Fig.13. Location of dial gauges
Two specimens with rise of 8cm, thickness 25 mm and 20
mm respectively, were chosen for strain measurement.
Electrical resistance strain gauge rosettes were mounted on
the top and bottom surface of the two shells (FS1 and FS2) at
different locations. Electrical resistance strain gauges have a
suitable sensitivity, they occupy little space, do not require
access once fixed and being read from a control panel so
electrical strain gauges were chosen for measuring strains.
Figure 14 shows the instrumented specimens and the
installed strain gauges respectively. Of these, the two
instrumented shells, FS1 and FS2 with 25 mm and 20 mm
thicknesses respectively, were loaded uniformly at the top
surface in the elastic range. Strain gauge readings were
recorded at different load increments. The results are
presented.
Fig.14. Instrumented shell and location of strain gauges
X. RESULTS AND DISCUSSIONS
Two shells of double curvature were constructed and tested.
From testing the specimens FS1 and FS2, the values of
strains and deflections for different loadings were recorded.
From the experimental strain values stresses are found out
and are compared with analytical stress values. Funicular
shell I and Funicular shell II are loaded up to 1145 Kg and
1140 Kg respectively. It is observed that the deflection of the
shell due to applied loads is small. From results obtained it is
found out that when the shell thickness is reduced the

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deflection of the shell also decreases for the same applied
load. Deflection is more in edge beam compared to shell
center.
Analysis of funicular shell 1(FS I)
The analysis of Funicular shell over square ground plan FS I
is made and from the results of the analysis, a graph is plotted
between the uniformly distributed load and membrane stress
distribution. From the visualization, the maximum deflection
under the load, the maximum stresses for the each analysis
are observed. Maximum tension stresses are developed at
shell corners. The comparisons of maximum stress with the
different applied load are shown in figure.15.
It should be noted that at most locations the theory yields
lower values than experiment. This would be because of
non-uniform applying of loads experimentally due to
practical difficulty of applying loads uniformly. The
discrepancies between the theory and experiment both in the
case of stress and deflection calculations could be attributed
to the possible errors involved in the computations in the
process of matrix inversion by single precision and also
non-consideration of material nonlinearity in stiffness
matrix. Also, the element sizes in the theoretical analysis are
not fine enough to simulate the physical structure. A stress
contour for maximum stress of Funicular shell I over square
ground plan is shown in figure 16. The maximum value of
tensile stress is observed on the shell edges.
Fig.15. Stress at strain gauge 1 for FS I
Fig.16. Stress contour for FS I
From the analysis results, the deflections of the concrete
funicular shell over square ground plan along vertical
direction are calculated and a plot is made between the
various points considered in the shell and the corresponding
deflections as shown in figure 17.
Fig.17. Deflection Curve for concrete funicular shell I
Analysis of funicular shell 1I (FS II)
The analysis of Funicular shell over square ground plan FS II is
made and from the results of the analysis, a graph is plotted
between the uniformly distributed load and membrane stress
distribution. From the visualization, the maximum deflection
under the load, the maximum stresses for the each analysis are
observed. Maximum tension stresses are developed at shell
corners. The comparisons of maximum stress with the different
applied load are shown in figure.18.
Fig.18. Stress at strain gauge 1 for FS II
It should be noted that at most locations the theory yields
lower values than experiment. This would be because of
non-uniform applying of loads experimentally due to
practical difficulty of applying loads uniformly. The
discrepancies between the theory and experiment both in the
case of stress and deflection calculations could be attributed
to the possible errors involved in the computations in the

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185
process of matrix inversion by single precision and also
non-consideration of material nonlinearity in stiffness
matrix. Also, the element sizes in the theoretical analysis are
not fine enough to simulate the physical structure. A stress
contour for maximum stress of Funicular shell I over square
ground plan is shown in figure 19. The maximum value of
tensile stress is observed on the shell edges.
Fig.19. Stress contour for FS II
From the analysis results, the deflections of the concrete
funicular shell over square ground plan along vertical
direction are calculated and a plot is made between the
various points considered in the shell and the corresponding
deflections as shown in figure 20.
Fig.20. Deflection Curve for concrete funicular shell II
XI. SUMMARY AND CONCLUSIONS
An experimental and analytical investigation has been
carried out to study the performance of funicular shells with
varying thickness using SAP 2000 software. Various
parameters like variation of deflection, stress distribution
have been considered in the study. The performance of
funicular shell under uniformly distributed load is presented
in this study.
The following conclusions are drawn from the test results:
1. The deflection of concrete funicular shell decreases
with increase in thickness.
2. Membrane stresses decreases with the increase in
thickness of concrete funicular shell.
3. Maximum tension stresses are developed at shell
corners.
4. Performance of funicular shell I is better when
compared to funicular shell II under uniformly
distributed load.
5. The deflection of shallow funicular concrete shell
decreases with increase in rise.
6. Membrane stresses decreases with the increase in rise
of shallow funicular concrete shell.
XII. SCOPE FOR FUTURE WORK
Precast doubly-curved shells have been in use for the past
few years as roofing and flooring elements. These shell units
are effectively used for replacing the solid RCC slabs. They
may be used in conjunction with precast joints or battens or
planks or as waffle shells by providing in situ ribs in two
directions. This type of construction has many advantages
over the conventional RCC slabs as it is lighter in weight and
saves reinforcing steel and concrete. It is also suited for
workshop floors and loading platforms carrying heavy
uniform loads. It eliminates the use of shuttering fully and
scaffolding partially. The sizes of the shells are chosen
depending upon handling facility available. The
experimental results presented in this work pertain to the size
of specimens chosen in the study, which has a potential for
application to industrial use. However, the exact nature of
behavioural response of large dimension shells is a topic for
further investigation. Analytical results can be improved by
considering non-linearity and also by using finer meshing.
ACKNOWLEDGEMENT
There are a number of people whose help and contribution I
would like to acknowledge. Without their support this work
would not have been accomplished. Primarily, I wish to
express my gratitude to, Dr. Nagesh R Iyer, Director,
CSIR-SERC, Chennai for providing me an opportunity to
carry out the project work at this esteemed organization.
I am thankful to Dr. P. Sivakumar, Chief Scientist, CAD
group CSIR-SERC, for allowing me to use facilities
available in CSIR-SERC.
I wish my grateful thanks to Dr. K. Manjunatha, project
guide for their invaluable support and guidance.
This project work has been done as part of CSIR-800
programme on “Sustainable Construction Technologies
for Social Development”.

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REFERENCES
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Estekanchi, “Experimental study of prefabricated funicular
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[3] Albolhassan Vafai and Mehdi Farshad “Theoretical and
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[4] G.S. Ramaswamy, N.V. Raman and Zacharia George “A
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[5] John W. Weber et al, “Ultimate Loads for Shallow Funicular
Concrete Shells”, Northwest, Volume. (58), No. 3, 1984,
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[6] P. Sachithanantham, S. Elavenil and S. Sankaran “Study on
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ultimate loads”, International Journal of Earth Sciences and
Engineering, ISSN 0974-5904, volume 04, No 06 SPL,
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[7] Ramaswamy G.S, “Design and construction of concrete shell
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[8] S. Rajasekaran and P. Sujatha, “Configuration of deep
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[9] IS: 2210-1988, “Criteria for design of reinforced concrete
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[10] IS: 6332-1984, “Code of practice for construction of floors
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Experimental and FE Analysis of Funicular Shells

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