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Disaster Advances
Vol. 7 (11) November 2014
Evaluation of seismic stability for mud houses based on
various existing and proposed layouts
Mohamad Shahrukh S.*, Majumder Deepnath, Ramanathan Abhishek, Tangudu Sweeya and Bhattacharya S. P.*
Department of Architecture and Regional Planning, Indian Institute of Technology, Kharagpur, INDIA
*
[email protected], spb @ arp.iitkgp.ernet.in
A further revision was carried out after the occurrence of
the Killari earthquake in the year 1993. In this case, some
parts of Deccan and Peninsular India were included in
Seismic Zone III (a zone of moderate hazard).2 Current
research activities also claim that some more areas which
were initially considered as low hazard zones may be redesignated to higher levels of seismic hazard.
Abstract
Midnapore, a district located in southern West
Bengal, India, falls under a mild earthquake hazard
zone. But during the recent earthquakes at Andaman
and Nicobar Islands and Sikkim, this region
experienced not so ‘mild’ shocks. The repercussions
of such earthquakes in the urban areas are
indisputable. But we cannot overlook the fact that
rural areas still contain more than 60% of the
population in India. 3 High levels of poverty and lack
of resources and expertise to mitigate and respond to
disasters render these denizens defenseless against
such hazard events. Also the physical conditions of
their housing are poor and hence are presently
exposed to greater seismic risk.
Earthquakes have origins in several areas of West Bengal.
For example, there are several active faults in the vicinity
of Siliguri. Also, the Garhmayna-Khanda-Ghosh fault runs
in a north-south direction along the western part of the state
and terminates near Sahibganj in the north.1 Majority of the
earthquakes take place in the Himalayan ranges in North
Bengal while some are deep earthquakes within the Bengal
fan. The worst case recorded till date is the earthquake of
1897 which caused massive damage to the city of Kolkata.
Recent evidences include the instances at the Bengal basin
region, which falls under the seismic zone II of BIS: 2002
nomenclature. The first of those instances is a shallowdepth earthquake of magnitude 4.3 which took place on 6th
February, 2008 whereas the second is one of magnitude
4.0, recorded on 13th December, 2005.
This study presents a seismic evaluation of the rural
built forms, predominantly mud houses and prescribes
design provisions to mitigate the same. The rural
buildings are modeled and seismic forces are
evaluated based on a physical survey. Various planar
building layouts are examined and a comparative
analysis is made. Analysis has also been done to
investigate various options for expansion and a
comparison chart is prepared. In addition to this,
economically viable methods of retrofitting have been
suggested.
“A mild earthquake, which recorded 4 on the Richter scale,
took place in Burdwan, Bankura, West Midnapore and
Birbhum on Wednesday morning. The tremor, lasting for
about six seconds, started around 11.40 am. A number of
mud houses have collapsed and several concrete houses
cracked in the quake.”– The Times of India, Kolkata.8
Keywords: Rural mud houses, disaster mitigation,
earthquake resistant design, seismic stability, seismic
retrofit.
The caving in of unreinforced structures due to ground
motion is one of the major causes of human casualties
during earthquake disasters around the world. This
contributes to more than sixty percent of the structural
damage of masonry structures.7
Introduction
India consists of a large number of tectonic units such as
the Himalayan collision zone in the north, the IndoBurmese arc in the north-east, the Andaman Sumatra trench
in the south-east and failed rift zones in the interior areas of
the country. In spite of these hardcore facts, the subcontinental regions away from the Himalayas and other
inter-plate boundaries were always considered to be less
prone to massive earthquakes. However, in recent times,
even these areas have suffered from devastating
earthquakes; though the magnitudes are much lower
compared to the ones in the Himalayan regions. After the
Koyna earthquake in 1967, the seismic zoning map (based
on the probability of earthquake occurrence) was revised
leading to the deletion of the non-seismic zones from it.
The regions surrounding Koyna were also re-designated to
Seismic Zone IV (a highly hazardous zone).
Now, there are many sophisticated and expensive
techniques to make structures earthquake resistant but
taking into account the issues like economic viability,
cultural adaptability, material and technological
availability, this paper focuses on cost effective techniques
to improve mud houses. It also focuses on seismically
sound rural housing modules for the West Midnapore
district of West Bengal.
The research process involved an on-site study of rural mud
houses in the West Midnapore district, following which
some common layouts of houses were identified, evaluated
and modified in terms of layout of walls and alteration of
materials such that they do not fall apart due to differences
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Disaster Advances
Vol. 7 (11) November 2014
in shear forces in the adjacent walls.
At the end, a realistic comparative analysis among the
different building shapes is adopted through a point rating
system. The comparative analysis is established on the
basis of the values of σb and the maximum shear stress in
both X and Y directional walls. The shapes with higher
values of σb, σx and σy are assigned lower rating points. In
the final step, numerical scores are assigned for each grade
and the best shape is determined as the one having the
highest score among all.
Physical survey and Description of Building
Model
The buildings considered for the study are typical rural
constructions found in the villages of the West Midnapore
district. They are single-family houses mainly occupied by
the weaker economic sections. The construction is mainly
carried out by the inhabitants and occasionally by unskilled
local masons and hence, the workmanship is very poor.
This type of building is classified as grade-A (most
vulnerable to seismic forces) as per the IAEE building
classification and IS Code 1893:1984.
Some common features observed during the survey of the
mud houses are listed below:
The houses are mainly load-bearing structures
consisting of mud walls (generally 250 mm thick),
which carry the roof load and bamboo poles which act
as supporting members. However, these are not
structurally integrated.
The houses have a built up area varying between
35sq.m and 50sq.m.
They follow a wide variety of layouts, the common
ones being the square, L, Z and U (figures 1 to 4).
All the houses are single storied.
Each house consists of a single door (1mX2.5m) and
few small openings (0.3mX0.45m).
The roofs are pitched roofs having local terracotta tiles
supported over bamboo framework.
The height of the door is the same as that of the walls.
The roofing system has a typical span of 2.5 meters.
Figure 1: Square Layout
Based on the case studies and physical surveys, few cost
effective techniques have been identified in order to
increase the seismic resistance of these mud structures.
They are divided into three major categories: (i) layout of
building plan and future expansion possibilities, (ii) wall
stiffness improvement strategies and (iii) use of various
indigenous materials.
Seismic Analysis
Methodology
of
Building
Layouts:
A seismic response investigation has been attempted under
the prescribed methods suggested by IS-1893:2002.
Initially, the self-weight of the building, natural time period
and spectral acceleration are computed. In the next stage,
the direct and torsional shear forces are estimated based on
typical governing equations of statics. The computation
accounts for three different parametric variables: (i) layout
of the buildings (ii) configuration of internal walls (iii)
position of fenestrations. Finally, two observations are
estimated as base shear per unit plan area of the building
(σb ) and maximum total shear stress in the walls in both X
and Y directions (σx and σy respectively).
Figure 2: L shaped Layout
Result and Discussion
Initially, four basic layouts are examined to determine the
best seismic planar stability. The calculations yield the
result that σb is highest for the shape Z and lowest for the
square shape [Fig.5]. The outcome of the analysis is well
expected as the square is the most symmetrical shape
whereas Z is the most asymmetrical one. Fig. 6 illustrates
the values of maximum shear stress for the walls in X and
Y directions. σx is maximum for the square layout and
minimum for the U-shaped layout whereas in the case of
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Disaster Advances
Vol. 7 (11) November 2014
σy, the situation is just the reverse. Since the total number
of internal walls in a particular direction shares the total
shear, the results are justified.
arrangement of internal walls, distance of walls from the
centre of rotation etc. However, analyzing those problems
in detail requires higher levels of calculations which is
beyond the scope of this paper and hence has been avoided.
Base Shear per Plan Area (kN/sq.m)
1.5
1.30
1.40
1.28
1.11
1
0.5
0
Basic square
Figure 3: Z shaped Layout
L-Shape
Z-Shape
U-Shape
Max. Shear Stress at X & YDirection Walls
Figure 5: Comparison of σb for four basic layouts
125
100
108.89
82.45
75
50
45.84
35.13
28.18
41.76
30.01
24.84
L-Shape
Z-Shape
U-Shape
25
0
Basic
square
σx (kN/sq.m)
σy (kN/sq.m)
Figure 6: Comparison of σx and σy for four basic layouts
Expansion Scope of Building Layout: One of the major
problems faced by the people dwelling in the mud houses
of Midnapore is the dearth of space, combined with large
family sizes. Hence, expansion of the house becomes a
foreseen necessity. Some of the possible layouts that may
be obtained by the expansion of the basic layouts (i.e. L, U
and Z) are also studied and a comparison is carried out to
determine which layout performs best from the seismic
point of view. As shown in table 2, the two layouts possible
from L are T and Cross. T is obtained by the addition of
one extra space to the left side of L whereas the Cross is
obtained by further addition of one space to T on the upper
side. The space between the left and the right arms of U is
used to form the modified Rectangular layout. In another
case, a separate space is added to the top of U to get a
layout in the form of a Tuning Fork. Finally, in the case of
Z, two separate spaces are added, one on either side, to get
a rectangular plan similar to the one obtained by modifying
the U-shaped layout.
Figure 4: U shaped Layout
On the basis of numerical values, the different layouts are
graded depending on their stability and rating points are
assigned for each of them. The best stable layout to the
worst one is rated as 1.00, 0.66, 0.33 and 0.00. The best
shape of the building is determined by this comparative
rating analysis. The tabulated data for the four basic layouts
is given in table 1.
From the overall analysis, it may be concluded that the
square is the best layout from the seismic point of view
followed by U and L whereas Z is the worst. The main
purpose of the argument is to develop an idea as to which
type of layout may be suitable for a mud house from the
seismic point of view. In some cases, the results may be a
little biased due to certain problems such as location and
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Disaster Advances
Variable
σb (kN/sq.m)
σx (kN/sq.m)
σy (kN/sq.m)
Total
Vol. 7 (11) November 2014
Basic square
1
0
1
2
BEST
Table 1
Rating Analysis of Four Basic Layouts
L-Shape
Z-Shape
0.33
0
0.66
0.33
0.33
0.66
1.32
0.99
U-Shape
0.66
1
0
1.66
Table 2
Values of σb, σx and σy for Modified Shapes- T, Cross, Rectangle, Tuning Fork
Present Layout
Modified Layout
σb (kN/sq.m)
σx (kN/sq.m)
σy (kN/sq.m)
L
T-Shape
1.4056
34.4706
44.3161
Cross
1.4306
123.2235
63.0668
Rectangle
1.0817
39.2845
126.2828
Tuning Fork
1.3162
69.9137
34.704
Rectangle
1.0817
39.2845
126.2828
U
Z
The modified layouts are analyzed on the similar
parametric basis of σb, σx and σy. They are also individually
compared with the original layouts by using a similar type
of grading system. From tables 2 and 3, it can be observed
that the original basic layout „L‟ is the most seismically
stable one in comparison to its modifications. Out of the
two modifications, „T‟ is quite stable whereas the cross
type layout is highly unstable. Hence, it should not be
preferred.
practiced by the beneficiaries themselves for their own
house, to upgrade the houses of others and to avail a source
of income generation. In many rehabilitation projects in
Gujarat as well as in Uttarakhand, this approach has been
successfully implemented.
Stiffeners can be utilized for strengthening both new and
existing constructions.
The suggested provisions are not complex and can be
completed easily by local masons. A schematic diagram for
structural strengthening of mud houses using different
techniques is shown below.
The original U-shaped layout is better than its
modifications as per the calculations. Among the
corresponding modifications, the rectangular layout
performs much better than the Tuning Fork type during
earthquakes due to its planar symmetry. Hence, for future
extension in case of U-shaped layout, augmenting it into a
rectangular layout would be advisable. Regarding the
layout Z and its rectangular transformation, it can be
inferred that both are quite stable. It is distinct that the basic
layouts i.e. L, U and Z are more stable than the modified
ones in all the cases. Despite that, some layouts like the
Rectangle, T, etc. exhibit reasonably good performance in
case of earthquakes and hence, may be used in case of
future expansion whereas some like the Cross, Tuning Fork
etc. perform very poorly and should be avoided.
Seismic Performance Improvement Strategies
Stiffeners: Stiffeners may be plates, sections or members
which help in stiffening beams, walls etc. in order to
minimize deformations. Stiffeners can prolong the life of a
present temporary rural hut at a cost, 60 to 70 % less than
new construction. If a 30 - 35 sq. m house with economic
specifications is to cost Rs. 100,000/-, a retrofitted house
can be around Rs. 30,000/- Besides many of these
construction methods are relatively simple and can be
Figure 7: T shaped Layout
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Disaster Advances
Vol. 7 (11) November 2014
Figure 8: Cross shaped Layout
Figure 11: Schematic diagram for structural
strengthening of mud houses (Plan view)
Figure 9: Rectangle shaped Layout
Figure 12: Schematic diagram for structural
strengthening of mud houses (Sectional view)
Materials
Non-erodible mud plaster:4 Dry soil (clay – 20 to 25%,
sand – 40 to 45%, 6 to 20 plasticity index), free from
inorganic material, is taken and wheat straw is added to it
and thoroughly mixed. The mixture is kept wet and well
kneaded. Finally, a mixture of about 53kg of bitumen and
10.5kg of dry soil is added per cu.m of dry soil to get the
desired product.
Fire retardant thatch 5: Pressed thatch panels are prepared
by laying thatch between two bamboo mats held together
by binding wires. After the panels are laid on the roof, they
are treated with ordinary mud plaster in the first layer (mud
Figure 10: Tuning Fork shape
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Disaster Advances
Vol. 7 (11) November 2014
Bamboo 6: Bamboo, the fastest-growing, renewable natural
resource known to us can be used in various building
components such as foundations, framing, scaffolding,
flooring, walls, roof and trusses. Researches prove that it is
possible to use bamboo splints as reinforcements for
masonry structures. Though the tensile strength of bamboo
is about one-third of that of steel, it is sufficient for
masonry structures and provides a more economical and
environment-friendly alternative that is readily available to
the society.
and straw) and by non-erodible mud plaster in the second.
Cracks in the dried roof are filled with mud plaster and then
two coats of cow dung slurry with water repellent are
applied.
Adobe/mud stabilized with hay, jute fibers: This material
can be used for stabilizing the parts above the hip or lintel
level, which pose major damage to life and property with
high reserve of potential energy when it falls apart during
an earthquake.
Table 3
Rating Analysis of Modified Layouts
Variable
L-Shape
T-Shape
Cross
σb (kN/sq.m)
σx (kN/sq.m)
σy (kN/sq.m)
Total
1
1
0.5
2.5
BEST
0.5
0.5
1
2
GOOD
0
0
0
0
Not Preferred
Variable
U-Shape
Tuning Fork
σb (kN/sq.m)
σx (kN/sq.m)
σy (kN/sq.m)
Total
0.5
1
0.5
2
BEST
Modified
Rectangle
1
0.5
0
1.5
GOOD
Variable
Z-Shape
σb (kN/sq.m)
σx (kN/sq.m)
σy (kN/sq.m)
Total
0.5
1
1
2.5
BEST
0
0
1
1
Not Preferred
Modified
Rectangle
1
0.5
0.5
2
GOOD
Table 4
Strengthening of existing construction
A. Strengthening of existing construction
Seismic Deficiency
Description Of Seismic Strengthening Provisions Used
No Connection Between Adjacent
Walls
Poor Connection Between Roofing
Elements
Opening
- Provision of wooden bracings at regular intervals in the walls
Wall
- Filling of cracks with good fiber-reinforced mortar
- Bamboo support at corners of walls where cracks develop.
- Wood seismic bands at plinth, lintel and roof levels
Building
- Strengthening of roofing elements through bracings; securely tying rafters to roof
truss after removal of all tiles and purlins at the roof level
- Provide additional strength to openings by means of frames
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Disaster Advances
Vol. 7 (11) November 2014
Table 5
Strengthening new construction
B. Strengthening new construction
Seismic Deficiency
Description Of Seismic Strengthening Provisions Used
Wall Span
- Provide a wooden cross bracing if span exceeds 5 m.
Planning
- Place roof truss and rafters in a symmetric or regular arrangement.
Wall
- Reduce height of wall to ensure height/thickness ratio less than 8.
Need for strengthening of
Eaves level
- Continuous concrete band may be provided to strengthen the eaves not only
against seismic activity but also for anchorage against wind force
Table 6
Materials being worked with around the world to make rural houses seismically sound
Pakistan
Haiti
Peru
Indonesia
Most Destructive
Earthquake
October 8, 2005
January 12, 2010
May 31, 1970
December 26, 2004
Location
Port-au-Prince area
Chimbote
Sumatra
Magnitude
Northern Pakistan/
Kashmir
7.6
7.0
7.9
9.1
Fatalities
75,000
222,500
70,000
Earthquake
resistant measures
- Light walls and
gables.
- Light sheet metal
and wooden roofs
instead of heavy
concrete roofs.
- Eucalyptus or
bamboo reinforced
walls.
227,900
(Including global
tsunami deaths.)
- Corner column and
crown beam of
concrete.
- Tires fitted with
stone used as shock
absorbers between
floor and
foundation.
- Use of compressed
straw bales held by
nylon netting and
sandwiched by
plaster
- Few small
windows to reduce
weak spots.
- Adobe walls
retrofitted with
plastic mesh.
2. Parvez Imtiyaz A., Vaccari Franco and Panza Giuliano F., A
deterministic seismic hazard map of India and adjacent areas,
Geophysical Journal International, 155(2), 489–508 (2003)
Conclusion
Hence, according to the study, it is clear that among the
existing configurations of houses, the basic square shaped
layout is the most effective and for future expansion,
rectangular metamorphosis of layouts works the best
against the hazards of earthquake. So, it would be copacetic
to implement these layouts respectively, in conjunction
with the suggested strategies and materials to make the
village households seismically sound, thereby mitigating
the largely overlooked menace of earthquakes in rural
areas.
3. India facts, July 20, 2011, India Census 2011. Retrieved from
http://indiafacts.in/india-census-2011/urban-rural-population-oindia/ (2011)
4. CSIR-CBRI, Roorkee, Non-erodible mud plaster on mud wall
for rural houses, http://hdl.handle.net/123456789/222 (2011)
5. Central Building Research Institute, Roorkee, Improved
method of making durable and fire retardant thatch roof.
http://cbri.res.in/CSIR800/Web%20Technologies/WebTech/RBE
A1-Fire_Retardant_Thatch.pdf (2009)
References
1. Amateur Seismic Centre, http://www.asc-india.org/, Pune,
India (last accessed – February 19, 2014) (2014)
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Disaster Advances
Vol. 7 (11) November 2014
6. Meguro Kimiro, Soti Rajendra, Navaratnaraj Sathiparan and
Numada Muneyoshi, Dynamic Behavior of Masonry Houses
Retrofitted by Bamboo Band Meshes, 15 WCEE, Lisboa (2012)
8. TNN Feb 7, 2008, 02.10 am IST, Quake shock in four districts,
The
Times
of
India
http://articles.timesofindia.
indiatimes.com/2008-02-07/kolkata/27781606_1_quake-richterscale-tremors (2008).
7. Jagannadha Rao M., Jaya Raj Aaron A. and John Paul K., A
note on recent earthquakes in Bengal Basin, Current Science,
95(9), 1127-1129 (2008)
(Received 12th April 2014, accepted 20th June 2014)
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