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Vol.16 No.2
structural engineers and architects. Mostly relevant for more
developed countries, there were a significant number of papers
on seismic isolation, damage avoidance design and new design
approaches like displacement-based design. These technologies
are being implemented slowly until damaging earthquakes strike,
such as has occurred in Christchurch, New Zealand, when
interest in them and their uptake suddenly increases.
Editorial: October 2012
Virtual Site Visit No. 30
The Sikkim-Nepal Border Earthquake of
September 2011
As far as materials useful for those practicing in developing
countries, several sessions were a reminder of the resources
available from the World Housing Encyclopedia. While various
tutorials including those on reinforced concrete and adobe have
been published, new information on stone masonry and
confined masonry construction was introduced. Confined
masonry, in contrast to unreinforced masonry and infill masonry
in reinforced concrete frame construction, has shown its
superiority in several recent quakes such as in Haiti and Chile.
Information on confined masonry construction is continuing to
be developed, both for design engineers and for builders and
professionals who are working in the field. Keep a watch on the
World Housing Encyclopedia (WHE) website. Regarding adobe
construction, a tutorial on this construction material can be
downloaded and this has recently been supplemented by the
publishing of a construction manual describing how to apply
straps cut from the treads of used car tyres to prevent the
collapse of adobe houses during earthquakes. An article about
this technology is included in this newsletter.
A summary of Tyre Strap Seismic
Reinforcement for Adobe Houses
Editorial: October 2012
I've recently returned from Lisbon where I attended the 15th
World Conference on Earthquake Engineering. Some 3000
delegates from many countries ensured a very full if not
overwhelming programme. As well as key note speakers several
thousand papers were presented either as e-posters or in parallel
sessions, making it very difficult to choose what sessions to
attend. A huge amount of research had been undertaken since
the previous conference four years ago and here many findings
were disseminated. As well as theoretical academic and more
practical research summaries, several sessions focused upon
previous recent earthquakes. They are always a rich source of
lessons to be learnt as we go into the future trying to improve
our buildings' resilience.
Since the conference has ended, for both those of who attended,
and those who did not, earthquake engineering is essentially
business as usual. For the most part, the way forward is to keep
attending to the basics – there are no shortcuts when it comes to
earthquake engineering. As far as buildings are concerned, good
seismic performance begins with a sound structural
configuration or layout of structural systems and elements. This
needs to be followed by careful and well-checked design and
detailing, and then completed by accurate and quality-controlled
construction. None of these steps is easy to get right, especially
in the context of a developing country. If we can work on
improving each step, and if we have some success, then this will
be worth sharing at the next World Conference on Earthquake
Engineering, to be held in Santiago, Chile, in 2016.
An announcement as to when the proceedings of this
conference will be publically available has yet to be made, but
papers from all past World Conferences can be obtained from
the International Association of Earthquake Engineering
(IAEE) website free of charge. To have these proceedings so
readily available is a tremendous resource for the world
earthquake engineering community. It's possible to undertake a
keyword search for each set of world conference papers
There did not appear to be any hugely significant breakthroughs
presented at the conference. Certainly there were many
developments reported upon of interest and relevance to
Earthquake Hazard Centre Newsletter, Vol. 16 No. 2, October 2012
Virtual Site Visit No. 30. Reinforced concrete shear walls and steel bracing
In this Issue we visit a car parking building at Auckland
Airport, New Zealand. Although there is nothing
particularly notable about the building from an
architectural perspective, its interior structure is highly
This is a four storey building of reinforced and precast
concrete construction with a light-weight roof (Figure 1).
From the façade the lateral load-resisting system is hidden
behind horizontal and vertical panels, suggesting a
moment frame in the longitudinal direction. However,
once inside, the actual gravity and seismic load resisting
structure is revealed.
Gravity floor loads are resisted by precast double-tee units
spanning transversely and supported by reinforced
concrete beams that in turn load quite short transverse
walls. These walls therefore resist all gravity loads as well as
transverse horizontal loads from wind and earthquake.
Fig. 3 Concentric steel bracing.
Although the walls are quite slender, with a relatively high
aspect ratio (height/length), and because the seismicity of
Auckland is low, they are adequately strong and stiff for all
load conditions. We can expect them to have been
designed using the Capacity Design approach. So, in a
seismic overload situation, ductile flexural yielding occurs
in the potential plastic hinge regions at the base regions in
the first storey before shear failure or any other less ductile
failure mechanism occurs.
The structural system resisting longitudinal forces could
not be more different in terms of material or structural
form since it consists of concentrically braced steel frames
(Figure 3). Several braced frames are placed along each line
of gravity-only beams. A frame consists of two diagonally
orientated steel tubes. Under the impact of horizontal
loads, one tube will experience tension stress, while the
other will be loaded in compression. The vertical
components of the axial actions in the tubes will cancel out
at where they meet at the mid-span of the beams.
Fig. 1 A four storey car parking building.
As there are no eccentric connections or structural fuses
provided, this system is not expected to possess significant
ductility, so it will be designed for far higher loads than
those to be resisted by the ductile reinforced concrete
transverse walls. Nevertheless, this system is very suitable.
From a functional perspective it allows each floor to be
very open and also it would have been quick and simple to
Fig. 2 Reinforced concrete shear walls in the transverse direction.
Earthquake Hazard Centre Newsletter, Vol. 16 No. 2, October 2012
A summary of ‘The Mw 6.9 SikkimNepal Border Earthquake of
September 18, 2011', an EERI
S p e c i a l E a r t h q u a ke Re p o r t ,
February 2012 based on observations of the
contributors listed in the original article.
An Mw 6.9 earthquake struck near the Nepal-Sikkim
border on September 18, 2011, at 18:10 local time. The
earthquake triggered a large number landslides and caused
significant damage to buildings and infrastructure. Sikkim
was the most affected state of India, followed by West
Bengal and Bihar. Neighbouring countries of Nepal,
Bhutan, Tibet (China) and Bangladesh sustained damage
and losses to varying extent. The maximum shaking
intensity is estimated to be around VI+ on the MSK scale.
The earthquake was followed by a series of aftershocks,
two of which were M4.5 and M5.0 and hit within 75
minutes of the main shock.
The landslides, rock falls, and mud slides were responsible
for most loss of life and damage to infrastructure, as well
as associated economic losses. There was also extensive
loss of Buddhist monasteries and temples; these heritage
structures are built in random rubble masonry with mud
mortar. Most multi-story reinforced concrete (RC)
buildings were non-engineered and sustained considerable damage due to earthquake shaking; a small number
of these collapsed or suffered irreparable structural
damage. Poor performance and widespread damage are
of concern in important government buildings, such as
the secretariat, police headquarters and legislative
assembly, perhaps some of the few engineered buildings
in Gangtok. The total loss of life in India is reported to be
78, 60 in Sikkim, and the rest in West Bengal and Bihar.
The total loss has been estimated at around US $500 million.
Fig. 4 Damage to houses due to rolling boulder at Lingzya and
Chungthang (photos: Arun Menon and Rupen Goswami).
Damage and losses were sustained by houses in the
severely shaken areas owing to three main reasons:
slides on weak mountain slopes, rolling boulder impacts,
and ground shaking-induced damage (Figure 4). In some
Earthquake Hazard Centre Newsletter, Vol. 16 No. 2, October 2012
Fig. 5 above, Typical ekra house at Gangtok (photo: Alpa Sheth)
below, typical house with wooden planks (Shee Khim) with damaged
rubble masonry plinth (photo: AR Vijayanarayanan).
cases, the latter type of damage may have been exacerbated by the 2006 event (if the structure was not
retrofitted). Scientifically based land use zoning should be
undertaken to demarcate obviously unsafe sites in the
state of Sikkim. Many instances were observed of soil
movement under buildings on hill slopes largely made up
of metamorphic and sedimentary rocks covered with soft
Housing Types
The state of Sikkim has been adopting three dominant
construction typologies: (1) traditional wood frame
construction with ekra/bamboo-matting walling, or
wooden plank construction (Shee Khim); (2) reinforced
concrete (RC) construction with moment frame
construction type; and (3) unreinforced masonry (URM)
construction with masonry units of stone, burnt clay
brick or cement blocks, mud or cement mortar, with NO
earthquake-resistant features. Of the above three
typologies, the third (URM) is less prevalent in recent
construction, and the second (RC) most prevalent.
However, most of the RC construction in the last two
decades is largely non-engineered. Most buildings built in
recent times in Gangtok, Chungthang, Pelling, Jorethang,
Naya Bazaar and other larger towns in Sikkim are of RC.
Fig. 6 - Top image, Pancaked nine-story Dzongpo House building
in Gangtok; three intermediate stories collapsed owing to sudden
change in lateral stiffness (photo: Alpa Sheth);
Left image, damage to storage facility building in Chungthang
(photo: Ajay Chourasia);
Middle and right image, collapsed three-story RC frame building at
Singtam (photo: Alpa Sheth);
Bottom image, collapsed four-story house at Dikchu (photo: Hemant
B. Kaushik).
Earthquake Hazard Centre Newsletter, Vol. 16 No. 2, October 2012
Ekra construction consists of a wood frame with crosswoven wood matting infill panels, and a light roof. The
matting is currently plastered. Another variation to this is
the use of wood planks in construction by the rich. When
made on flat ground, it rests on a relatively shallow and
uniform masonry plinth (made of stone and mud mortar),
and when made along hill slopes, it rests on a tapered stone
plinth. Four varieties of stone masonry plinths are
observed in Sikkim and the hills of Darjeeling: random
rubble masonry (RRM) with and without mud mortar;
dry dressed stone masonry; dressed stone masonry; and
dressed stone masonry with pointing. In recent times, the
plinth has been constructed with RC. During the earthquake, this type of housing was shaken to varying degrees,
but with the exception of distress in some plinths
(especially those made of RRM without any mortar), the
houses performed exceptionally well. In instances where
the cross-woven wood matting was replaced by clay-brick
masonry in cement mortar, damage was sustained by out
of plane collapse (Figure 5). The highly satisfactory
performance of this housing validates its appropriateness
in the Himalayan region and makes a compelling case for
its continuation. This traditional style (including hybrid
varieties with structurally designed basements) should be
encouraged in the state of Sikkim and neighbouring
stories. Despite a large number of deficiencies in the RC
buildings in Gangtok, one feature that may have saved
them from more damage is the use of a uniform grid in
most buildings. The spans vary from 3m to 4.5m,
depending on the site. As these structures are on sloping
ground, heights of columns vary, with some of them
short columns and some slender columns. Further,
because buildings are on hill sides, the width of the
building is smallest at the base due to the topography. The
roof of the building is partially or wholly in steel roof
trusses or joists and metal sheets. The concrete for
construction is hand-mixed, with neither control of the
water-cement or aggregate-cement ratio, nor any systems
for cube testing of concrete or testing of reinforcement.
No vibrators were being used for compaction in
construction observed during the reconnaissance. The
performance of RC frame buildings with unreinforced
masonry (URM) infills would have been better if one
brick-thick URM infill walls were used instead of halfbrick-thick URM walls that either collapsed out-of-plane
or were severely damaged in-plane. In cases where infills
were absent or poorly built (with too many openings), the
damage was significant. In RC frame buildings where the
URM infill walls were made of large-sized thin cement
concrete block units (350mm x 200mm x 75mm) masonry
walls, the performance of the building was poor.
Reinforced concrete
Buildings that had a sudden change in the stiffness pattern
of the infill panel walls suffered significant damage or
collapse (Figure 6). Buildings experienced torsion and
collapsed when distribution of stiffness was poor in plan
(Figure 6).
There are about 13,000 RC buildings in Gangtok alone,
almost 65-70% of which were built after 1995. Almost all
of these buildings are non-engineered (there are only two
qualified structural engineers resident in Sikkim). RC
residential or hotel buildings are built on steep sloping
sites on tight plots abutting each other. Only a few
institutional buildings are engineered, such as government
buildings, educational institutions and large hotels. RC
construction performed poorly during the earthquake,
even though the maximum intensity of ground shaking
was only around VI+ scale on the MSK scale in most of
the affected areas.
Masonry construction is found especially in the British
colonial government buildings in Sikkim and Darjeeling.
This type has very thick walls made in random rubble
stone masonry; dressed stone masonry was used in some
government structures. No earthquake-resistant features
are found in these structures — no bands, no throughstones, and no vertical reinforcement at corners and
around openings. Unreinforced masonry (URM) was used
to construct a large number of government schools and
primary health centres. This stock sustained dilation of
masonry starting from the upper stories.
Non-engineered RC buildings across Sikkim typically
have a grid of beams and columns in both plan directions.
The buildings are 3-8 stories high, except in villages like
Lachen and Lachung, where they are dominantly of two
Earthquake Hazard Centre Newsletter, Vol. 16 No. 2, October 2012
“declared”unsafe and ordered to be demolished without
professional inputs of engineers conversant with safety
assessment protocols. This caused severe disruption to
medical service, relief operations, and governance
Lessons Learned
This earthquake has brought into relief issues of disaster
mitigation and management in the inhospitable region of
the Himalayas which is one of the most seismically active
regions of the country.
1. The extent and type of damage in newly built RC
structures are not commensurate with the in tensity of
ground shaking. Most damage can be attributed to
irregular structural configuration, improper design and
detailing, poor construction materials and practice, or
complete absence of regulatory framework from the
government side to ensure earthquake-resistance in the
built environment.
Fig. 7 Collapse of masonry building of the primary school at Theng
(photo: Arvind Jaiswal).
Nonstructural damage
This moderate earthquake also highlighted the need to
address nonstructural elements (building contents,
systems, facades) formally while designing buildings.
Many losses were incurred in hospitals, offices, buildings,
monasteries, and residential buildings. Should the next
earthquake be of severe intensity, it may result in much
higher loss due to nonstructural elements.
2. No new URM structures should be permitted in Sikkim,
and existing ones should be retrofitted, especially the
critical, life-line and government structures.
3. Develop a comprehensive plan to Sikkim and West
Bengal. When even basic earthquake- resistant
construction is not known by local architects and
engineers, special assistance may be required from outside
these states to support this culturally and historically
critical work.
Older schools are made of traditional Ekra construction,
while the recent ones (including extensions and
replacements structures) are URM and RC. Ekra construction performed relatively better than RC buildings,
and damage to RC schools deprived the government of
safe havens for post-earthquake relief camps and
emergency services. Over 600 school buildings are said to
have suffered extensive damage or collapse. One
complete collapse of a school building was observed
(Figure 7).
4. There should be an aggressive promotion of traditional
Ekra housing by development of a manual of good
construction practices and inclusion of this as a formal
housing construction typology eligible for bank loans.
5. Post-earthquake damage assessment teams need to be
mobilized from out of state that have sound judgment on
usability of damaged structures and no stake in the new
construction. As well, technical information needs to be
disseminated to professional architects and engineers on
accepted methods for assessment and retrofit of damaged
Medical facilities
In most structures, pounding damage was noticed at
construction joints; frame-infill separation, cracking of
plaster and diagonal cracking on infill walls were observed
RC frame buildings, as was damage due to incorrectly
detailed seismic joints. Some government buildings and
hospitals in the state sustained nominal damage and could
have been used after the earthquake, but were
Earthquake Hazard Centre Newsletter, Vol. 16 No. 2, October 2012
6. Document all losses incurred by nonstructural
elements, and disseminate technical know-how to
architects and engineers on methods of protecting these
A summary of “Tyre Strap Seismic
Reinforcement for Adobe Houses"
by A.W. Charleson, School of Architecture, Victoria University
of Wellington, New Zealand, presented to the 15th World
Conference on Earthquake Engineering, Lisbon,
September 2012.
Fig. 8 Steps in the process of reinforcing an adobe house
with tyre straps. Step (a) is performed in a workshop or
factory and (b) to (d) on site. (Courtesy Matthew French)
Of all housing construction types worldwide, earthen
construction is among the most fragile with respect to
horizontal loads experienced during earthquakes.
Although there are many different types of earthen and
related construction, including random rubble and
dressed stone construction either laid dry or in mud
mortar, they all share two serious structural deficiencies:
(1) of having little if any tension strength, and (2)
brittleness. As tragically witnessed after every damaging
earthquake in developing countries, due to their high mass
and lack of tensile resistance that has the potential to tie
the elements of buildings, like walls together, the seismic
performance of these forms of construction is very poor.
Tyre strap reinforcement is one response to this
unfortunate situation. The basic steps of the application
of tyre strap technology is summarized visually in Fig 8.
Fig. 9 The main features of a strap-reinforced house.
centres pass underneath or through the foundations,
then rise up both sides of the walls, wrap over them and
are connected and finally nailed to roof timbers. This
type of reinforcing pattern is designed so as at least one
pair of straps, either vertical or horizontal, cross every
large potential crack that will open during an
earthquake. The reinforcement provides structural
strength and tying-action after the earthen wall material
has failed. Figure 9 illustrates the main features of the
The concept at the heart of this reinforcement system is
for tyre straps to be cut from discarded used tyres in
developed countries (where used tyres are generally not
too badly damaged and worn and may be costly to dispose
of) and then donated and transported to developing
countries, where at minimal cost homeowners
incorporate them in new or existing houses. Two very
desirable outcomes eventuate: both existing and new
adobe buildings are strengthened at minimal cost with a
material that is simple to install and plentiful in supply, and
a significant portion of used car tyres are recycled in an
environmentally acceptable manner.
This system is suitable for new and existing earthen
houses in areas of moderate to high seismicity. It could
also be employed shortly after a damaging earthquake
to enable seismically resilient reconstruction to proceed
using materials salvaged from badly damaged and
collapsed houses. Until further research is undertaken it
is proposed that strap reinforcement be applied to
earthen houses whose designs broadly comply with the
most recent unreinforced adobe construction
In this system, the circumferentially cut straps from the
treads of used car tyres that function as tension
reinforcement must be cut from steel-belted radial car
tyres. Although the steel wires in the two belts are not
continuous they give the straps sufficient strength and
stiffness to be used as reinforcement.
Technical Development of the Tyre Strap
After approximately six metre-long and 40 mm wide
continuous straps have been cut from tyre treads, they are
connected on site using a special yet simple nailed joint.
Once the walls of a house are constructed and holes
drilled or formed during construction to allow straps to
pass through, straps are then wrapped horizontally around
walls at 600 mm centres maximum vertically. Vertical
straps spaced horizontally at approximately 1.2 m
Earthquake Hazard Centre Newsletter, Vol. 16 No. 2, October 2012
Tensile tests have been conducted on tyre straps cut
from the treads of radial steel-belted car tyres. Test
results confirm that given the necessity for desirable
strength and stiffness, and the need to avoid short strap
lengths with large numbers of connections, 40 mm
wide straps are the most suitable.
They possess tensile strengths between 10 - 15 kN. Straps
are butted together and connected via two short lengths
of overlapping straps to form a butt joint (Figure 10). The
nails are bent carefully to prevent a premature nail pullthough failure mechanism of the joints.
Initial load tests using the tyre straps for in-plane and outof-plane test specimens on dry-stacked brick walls were
successful. They indicated the potential of the system to
provide large amounts of ductility and so the system was
further developed for use in adobe construction. A small
single room full-scale adobe house was built and
reinforced and subjected to four phases of earthquake
shaking on a uni-directional shaking table (Figure 11).
Assuming there is no cost for the tyre straps, the cost of
materials to provide straps to a small 52 m2 four roomed
house has been estimated as US$378. Almost 65% of this
cost is for the wall finishing paint.
Fig. 11 The adobe test module on the shaking table.
Costs of Materials and Labour Requirements
An estimate of time to complete each construction
activity was based on the times taken to reinforce the one
room module that was dynamically tested. In that case one
experienced mason worked with a person with little
building experience. It is feasible for most of the work to
be undertaken by an unskilled worker or the house owner,
with occasional supervision by a mason. The total time
required to install the straps and complete the work will
vary due to many factors, including the quality of the
equipment and the speed and efficiency of the workers.
For the single-roomed test module with a gross area of
10m2 and average wall height of 2m the estimated times
are shown in Table 1.
Construction activities
Mark the position of horizontal straps and vertical straps using chalk.
Cut rebates into adobe walls to accommodate straps.
Form holes under/through foundations for vertical straps.
Drill 50 mm by 10 mm holes at wall corners for horizontal straps to pass through, and
paint exterior rebates for straps.
Remove areas of roof and ceiling to pass vertical straps over rafters.
Place, cut, tighten, connect vertical and horizontal straps (requires two workers).
Apply corrosion protection to vertical straps where they pass under the foundations.
Drill 5 – 10 mm dia holes through walls, tie straps together and provide ties to top
course of adobe blocks.
Plaster over straps with mud mortar. After mortar is dry, paint over the straps with
water-resistant paint.
Table 1. List of construction activities and worker days.
A more detailed and comprehensive paper is available in the
article ‘Seismic reinforcement for adobe houses with straps
from used car tyres’, Earthquake Spectra 28:2, 2012.
The module that was reinforced and tested had 250 mm
thick walls with an average height of 2m and a gross area
of approximately 10m . The time taken to reinforce a
larger house can be approximately determined on a pro
rata basis. Extra time needs to be allowed for if walls are
thicker and higher, and if there are gable ends and/or
parapets. No allowance is made for painting or repainting
all the interior and exterior surfaces.
Earthquake Hazard Centre
Promoting Earthquake-Resistant Construction
in Developing Countries
The Centre is a non-profit organisation based at the School of
Architecture, Victoria University of Wellington, New Zealand.
Director (honorary) and Editor: Andrew Charleson,
ME.(Civil)(Dist), MIPENZ
Research Assistant: Kate Bevin (BAS)
Mail: Earthquake Hazard Centre, School of Architecture,
PO Box 600, Wellington, New Zealand.
Location: 139 Vivian Street, Wellington.
Phone +64-4-463 6200
Fax +64-4-463 6204
The Earthquake Hazard Centre Webpage is at :
Fig. 10 A nailed strap joint. Both ends of the nails are bent
during nailing.
Earthquake Hazard Centre Newsletter, Vol. 16 No. 2, October 2012

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