eqrisk-report 13-012 building typologies in guwahati 130916 dhl
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EQTRANSCRIPT
Building Classification Scheme
for the City of Guwahati, Assam
Jayanta Pathak
Assam Engineering College
Guwahati, India
Dominik H. Lang
NORSAR
Kjeller, Norway
Report no.13–012
Kjeller (Norway) – Guwahati (India)
September 2013
A Collaboration Project funded by the Royal Norwegian Embassy to India, New Delhi
This report was prepared under the EQRisk (Earthquake Risk Reduction on the Indian Subcontinent) project
under collaboration of NORSAR and Assam Engineering College, Guwahati. It can be openly accessed on the
EQRisk webpage at www.eqrisk.info.
Please cite this report as:
Pathak, J., and Lang, D.H. (2013). Building Classification Scheme for the city of Guwahati, Assam, EQRisk project
report, Report no. 13–012, Kjeller – Guwahati, September 2013, 20 pp.
EQRisk is a collaboration project funded by the Royal Norwegian Embassy to India (New Delhi) and
administered by the Research Council of Norway (Oslo).
© 2013, Stiftelsen NORSAR
Table of Contents
Preamble......................................................................................................................................... 5
General observations ...................................................................................................................... 6
Non‐engineered construction typologies ............................................................................................ 6
Urban area building type scenario ...................................................................................................... 9
Building classification procedure ................................................................................................... 13
Wall materials .................................................................................................................................... 14
Roof types .......................................................................................................................................... 15
Building typologies ............................................................................................................................ 16
References .................................................................................................................................... 20
Preamble
The city of Guwahati (state of Assam, Northeast India) is one of the test beds considered for detailed
earthquake damage and loss assessment studies within the EQRisk project. The totally selected study
area represents the 2025 Guwahati Master Plan Area (Figure 1, area demarcated by green line). This
development area of 262 km2 was demarcated by the Guwahati Metropolitan Development Authority
(GMDA). It includes the Guwahati Municipal Corporation (GMC) area including North Guwahati Town
Committee area, Amingaon Census Town and 21 revenue villages, i.e., Abhaypur, Rudreswar, Namati,
Jalah, Gouripur, Silamohekaiti, Tilingaon, Shila, Ghorajan, Mikirpara, Kahikuchi, Kahikuchi, Mirjapur,
Jugipara, Borjhar, GaralGaon, Dharapur, Janisimalu & Jansimalu (NC), Kalitakuchi (NC), Kharghuli,
Bonda, Bondagaon and Bonda Grant (I&II), and Birkuchi. In addition to these areas come the Narengi
Cantonment areas, Guwahati Refinery (IOCL) area, and the NF railway Colony which show planned
clusters of buildings.
The Guwahati Municipal Corporation (GMC) area is subdivided into 60 municipal wards as delineated
by the Guwahati Municipal Corporation (Figure 1, red‐shaded subareas demarcated by red lines).
None of these wards shows any typical formation of building clusters which makes it imperative to
collect data of a large number of sample buildings by thorough building stock inventory surveys.
Some of the demarcated areas, i.e. Narengei cantonment (a geounits that is bordering ward no. 46)
show planned and clustered development. This also applies to the Guwahati refinery area (included
in ward no. 46) and the railway colony area (partly included in wards no. 8 and 9), which have well‐
defined cluster formations with a homogeneously arranged building typologies.
The footprints of individual buildings were studied from satellite imagery such as GoogleTMEarth. The
survey of representative samples from these areas had been conducted during numerous field
studies and the building data are generated based on sample cluster survey and assigning a
corresponding typology to the building clusters.
Figure 1. Overview map indicating the considered study area, i.e. the 2025 Guwahati Master Plan Area (green line) as well as the 60 municipal wards of Guwahati (red‐shaded and outlined areas).
General observations
The revenue villages around Guwahati are mostly dominated by houses for which bamboo‐
reinforced biomass was traditionally used as cladding material while thatch or corrugated iron sheets
were used as roofing materials. The majority of houses in these villages around the city of Guwahati
are of the traditional Assam‐housing typology (WHE report #154: Kaushik and Babu, 2009) where the
roofing system consists of timber/bamboo trusses supporting thatch or CGI sheets. The wall system
is made of Ikra1 and/or bamboo‐reinforced biomass cladding. However, a gradual shift towards
confined masonry houses (i.e. burnt brick masonry walls, 6 6” RC posts, RC bands at lintel level, roof construction made of timber or tubular steel trusses supporting CGI sheet roofing) can be
observed.
The various traditional construction practices and housing types that are prevailing today in the
different regions have evolved based on technology transfer from one generation to the next by
word of mouth or by some kind of documentation done by practicing masons and end users. What is
important to note is that these various construction schemes often have characteristics that address
the prevalent local conditions of weather and other environmental and natural hazards, i.e.
earthquakes, floods or cyclones.
Non‐engineered construction typologies
The variety of non‐engineered construction typologies is quite limited in the city of Guwahati as well
as surrounding sub‐urban and rural areas. In general, these building typologies are constricted to the
traditional Assam‐type house as well as both unreinforced and confined clay brick masonry houses.
The Assam‐type (Ikra) house is a vernacular construction typology made of timber. Traditionally, it
was the most common building type throughout Northeast India which is located in the country’s
most severe seismic zone, i.e. Zone V – corresponding to a MSK‐intensity IX (9.0). The majority of
these houses were and still are used for residential purposes while the construction technology has
been transferred traditionally from one generation of local masons to the next. However, during the
colonial era, the British made this type of construction popular by preparing specifications and
improving the construction technology. The advantage of this both lightweight and eco‐friendly
construction was well understood by the successive governments before and after India’s
independence. During this period of time, the practice was largely adopted for construction of
government offices and residences across the Northeast Indian states. The Assam Public Works
Department (APWD) included specifications of this type of housing in their building schedule. In rural
areas, these houses are typically built with lightweight locally available material like bamboo,
wooden planks, thatch etc. and are generally termed as ‘thatch house’. These housing types have
traditional systems of bamboo posts in rural areas and wooden posts in more urban areas. Bamboo
posts are inserted into the ground to act as compression members and are tied with horizontal
bamboo/wooden girders with the help of coconut fiber ropes or bamboo ropes to give a proper
shape and framing action. Bamboo or seasoned wood has natural resistance to decay, which makes
these houses sustainable and low on maintenance. It was noted that the majority of houses survived
the recent big earthquakes in 1897 (Assam earthquake) and 1950 (Assam Tibet earthquake). 1 in local Assamese language, ‘Ikra’ is pronounced as ‘Ikara’
Ikra houses are generally single‐storied structures consisting of brick or stone masonry walls up to
about 1 m above the plinth. This masonry supports the walls consisting of bamboo woven together
with a wooden frame, and plastered with cement or mud plaster. Even though this building typology
is more prevalent in rural areas, a significant percentage of this type of housing can be also found in
the cities of the region, also in the city of Guwahati. However, in the last two decades a decrease of
these traditional houses especially in urban areas has been observed.
Figure 2 illustrates a traditional Assam‐type building in an urban residential area. This traditional
practice has been replaced by confined or unreinforced masonry buildings in the last two decades.
There has been a legal ban on felling of trees, which has reduced the supply of timber as building
material. This situation has forced or encouraged masonry constructions for smaller houses and RC
framed constructions for larger or multistoried houses.
Figure 4 shows the timber truss used in the roofing of typical Assam‐type building built in the 1950–
60 by Assam PWD on the Assam Engineering College campus. Figure 5 illustrates the two‐way slope
of the roofing system, which provides efficient bracing action at the post plate level against out‐of‐
plane failure. Figure 6 shows the typical half‐wall in bricks above plinth level, which interface with
the timber frame panels to contain the upper part of the wall made of reed with cement mortar
plaster work. The timber posts are bolted to the plinth. These kind of lighter and well‐integrated wall
systems are now replaced with heavier brick work, which are sometimes not well integrated and
thereby increasing the falling hazards of walls during earthquake.
Nowadays, traditional Assamese houses coexist along with RC constructions that have become more
popular (Figure 7). The new urban and semi‐urban landscape is gradually dispensing with this time‐
tested, earthquake‐safe and potentially sustainable traditional housing. The main strength of these
Assam‐type houses, which sustained several major earthquakes in northeastern India, lies in their
structural configuration. They are further characterized by a number of advantageous design
features such as regular plan shapes, the sensible use of locally available building materials, the
integration of wooden beams over the total height of the building as well as small openings.
Over the years, this particular traditional construction form has gone through various
transformations. The current Assam PWD specification for Assam‐type houses includes RC
foundations according to design with a depth of foundation of 1.0 m from the original ground level
with plinth beam as per design, connecting the foundations. The plinth beams are laid over 75 mm
thick cement concrete of proportion 1:3:6. The outer plinth wall shall be 225 mm thick and of first
class brick work in proportion 1:5 with 15 mm thick plaster (proportion 1:6) on exposed faces with
neat cement slurry finish. Inner plinth walls shall be 112 mm thick. The posts/columns are shall be
made of RC and 130 mm 130 mm in size with reinforcement. The floor shall be of 65 mm thick
cement concrete flooring in panels with 50 mm under layer of cement concrete in proportion 1:3:6
and 15 mm topping in proportion 1:1:2 over one flat brick soling or 150 mm thick stone soling. The
wall system of these type of building are now specified as 112 mm thick first class brick interlocked
wall in cement mortar in proportion 1:5, from plinth up to post plate with 15 mm cement plaster on
the outside face and 10 mm cement plaster on the inside. The plinth height is kept at 600–750 mm
and the height of the building is between 3050–3600 mm. The roofing is generally of 0.63 mm CI
sheeting over undressed wooden or timber trusses.
Figure 2. Assam‐type house around the city of Guwahati with typical wooden beams and columns.
Figure 3. Traditional Assam‐type house (left) and masonry house (right).
Figure 4. Assam‐type building with timber roof trusses.
Figure 5. Assam‐type house with two‐way sloped roof.
Figure 6. The timber posts are bolted to the plinth and confine half of the brick wall.
Figure 7. Traditional Assam‐type houses replaced by masonry houses in the city.
Figure 8. An Assam‐type office building. Figure 9. Unconfined masonry house with lightweight roofing.
Figure 10. Confined masonry building with lightweight roofing. Gable end is not braced against out‐of‐plane failure.
Figure 11. Mixed construction with timber and brick masonry in business area.
Figure 12. Mixed construction with timber and brick masonry in business area.
Urban area building type scenario
Figures 13 to 18 illustrate some randomly chosen buildings that are located in various wards of the
Guwahati Municipal Area. The municipal area of Guwahati city is largely characterized by
multistoried RC frame structures. The dwelling units are mainly 2 to 3 stories high with an average
story height of 3.0 to 3.3 m. Most of the residential dwelling units are designed and constructed by
laymen without engineering consultancy or quality control, but in principle have some earthquake‐
resistant features such as the presence of tie beams or minimum column sizes of 250250mm. In
most of these buildings the column reinforcements are 4–16 TOR2. Only few of these buildings exist
that were built around 1965–70, where old ductile steel was used as reinforcement.
Most of the residential buildings in Guwahati are constructed following socio‐economic rather than
engineering considerations. The general construction of RC framed buildings are of nominal concrete
of M15 grade (mix proportion 1:2:4) ranging from 3 to 4 story (story height: 3.0–3.3 m, RC slabs:
120–140 mm thickness, infill walls of brick masonry in cement mortar (1:6): 200–250 mm thickness).
Figures 13 and 14 represent examples of residential buildings in Guwahati city which are largely
ductile or non‐ductile RC frame structures with lightweight roofing systems. The roofs are generally
pitched due to high rainfall in the rainy season.
2 TOR, also called Cold Twisted Deformed (CTD) steel reinforcement bars, HYSD – high yield strength
deformed bars
Since the year 1995/96 there has been a phenomenal increase in construction of multistoried (mostly
G+53 to G+8) apartment and commercial buildings. These nearly exclusively RC frame constructions
are engineered in a sense that they are designed by engineers as per Indian earthquake code IS 1893
(2002), which has been made mandatory by the Guwahati Municipal Corporation (GMC) and
Guwahati Metropolitan Development Authority (GMDA) some 20 years ago.
The promotion for housing finance under the national housing policy fuelled the growth of
apartment housing in Guwahati since the year 1990. Both GMDA and GMC introduced registration of
practicing technical personnel (architects and engineers) in Guwahati during 1994‐95 in order to
bring in accountability to the construction business. Since 1998, the Urban Local Bodies (ULB) in
Guwahati (i.e. GMDA and GMC) introduced mandatory certifications of buildings by structural
engineers while applying for building permits. However, the Urban Local Bodies (ULB) that were
engaged in order to take care of construction development control were not yet ready for handling
the phenomenal rise in multistoried construction during the decade 1990–2000. As a result, several
hundreds of such multistoried buildings were added to Guwahati’s building stock without reliable
lateral load‐resisting systems such as the provision of shear walls, appropriate bracings systems etc.
Most of these multistoried buildings were constructed with an open ground story for parking
purpose. The area covered by the open parking story was not included in the FAR4 calculation in
order to encourage the provision of more parking spaces in apartment buildings. The introduction of
any walls in the otherwise open ground story would have caused that the parking space area had to
be included in the FAR calculations, hence reducing the alienable building floor area. Understandably,
builders and real‐estate developers were therefore reluctant to provide any type of wall to the open
ground story (stories) as it would entail the inclusion of some area into the FAR calculation. However,
all of these multistoried buildings generally dispose of RC cores (shafts) for elevators, though not
placed from structural design viewpoints but rather due to functional requirements given by the
architects. Though not placed strategically, these RC cores provide at least some lateral resistance to
the building depending upon their arrangement in plan.
In 2006, this situation was partly corrected when GMDA and GMC adopted the revised building
byelaws (GMDA 2006) in line with the Model Building Byelaws proposed by the Committee
constituted in 2004 by the Ministry of Home Affairs, Govt. Of India, on the recommendation of the
National Core Group on Earthquake Mitigation set up by the Ministry. The revised building byelaws of
2006, introduced the detailed proof‐checking of multistoried buildings in Guwahati by senior
structural engineers considering earthquake safety. The revised building byelaws of 2006 also
introduced mandatory features like shear walls and bracings, especially in case where the building is
designed with open ground floor for parking.
3 G+5, i.e. ground floor plus five upper stories 4 FAR – Floor Area Ratio (in India: also FSI – Floor Space Index is used); FAR = (total covered area on all floors
of all buildings on a certain plot‐ unenclosed parking area) / (area of the plot)
Figure 13. RC frame building for (left) commercial and residential use, (right) residential use.
Figure 14. RC frame building of residential use (with lightweight roof construction).
Figure 15. Older (non‐ductile) RC frame building for (left) residential use, (right) commercial use.
Figure 16. Newer (ductile) RC frame building. Figure 17. (Ductile) RC frame building under construction (residential use).
Building classification procedure
The building classification scheme that is described herein resulted from various inventory surveys in
Guwahati conducted by Assam Engineering College (AEC) in recent years as well as more recent
investigations on the prevalent building stock. It reflects the building typologies and materials
identified in the Guwahati urban area as well as in several revenue villages around Guwahati city. The
building classification may thus be only representative to the municipal and suburban area around
the city of Guwahati and may require adjustment or amendment if applied to surrounding regions or
other Indian cities. However, the identified typologies can be considered as representative for the
entire building stock in the urban and rural areas of Assam.
The classification is provided on different steps:
1. Available wall materials (i.e., clay brick, stone, concrete etc.)
2. Available roof and floor types (i.e. rigid, flexible, heavy etc.)
3. Available (structural) building typologies (certain building typologies may summarize
different variations of a main building typology)
Wall materials
A variety of wall materials can be found in the investigated region (Table 1). Many of these may be
used for both the main load‐bearing system and as infill material (e.g. of RC or timber frames).
Table 1. General characteristics of different wall materials.
Wall material (units) Illustration
Burnt clay bricks (rectangular)
Dimensions 10 5 3 ‘ (23 11 7,6 cm) Locally produced in Guwahati Unit price Rs. 7.5 p.p. Used for URM buildings as well as for infill walls of RC frame and steel frame structures (bricks of first class are used for load‐bearing buildings)
Autoclaved Aerated Concrete (AAC) blocks
Dimensions 100–300 62,5 12,5–20 cm Produced in Kolkata Mixture of fly ash, lime, gypsum and cement as binding agent, water and aluminum powder acting as foaming agent Advantage: 1/3 of brick weight Price Rs. 9.0 (w.r.t. brick unit price)
Cellular Light Concrete (CLC) blocks
Dimensions 100–300 62,5 12,5–20 cm Locally produced Mixture of fly ash, lime, gypsum and cement as binding agent, water and aluminum powder acting as foaming agent Advantage: 1/3 of brick weight Price Rs. 8.0 (w.r.t. brick unit price)
Ikra
Composite wall material of Ikra‐canes and cement plaster (in older days: mud plaster) Mostly used for one‐story Assam‐type (Ikra) houses Construction technology of walls: (1) Timber frame is erected, (2) Frames are filled in with ikra panels (ikaras are cut in size of the panel and laid vertically), (3) Horizontal members are usually bamboo canes; after mounting the ikara it is left to dry for few days, (4) Walls are plastered on both sides with cement or mud mortar; 3 layers of plaster are done alternatively after drying of each layer, (5) Finished coating is done with a liquid mix of cement/mud and cow dung.
Roof types
A small variety of roof types and materials are applied in the investigated region (Table 2).
Table 2. Roof types and roofing materials.
Roof type and material Illustration
Light sloping roofs made of timber frame construction
with corrugated metal sheet
with corrugated asbestos sheets (discontinued now)
RC solid slab – Rigid diaphragm
(left): with RCC solid slabs supported on RC frames at all levels working as rigid diaphragm (right): with RCC solid slabs supported on RC frames at lower stories and a light‐sloping roof over the top story
Building typologies
As already indicated, Guwahati’s building stock is dominated by three groups of building typologies:
‐ traditional Assam‐type houses,
‐ confined clay brick masonry houses, as well as
‐ ductile and non‐ductile RC frame structures.
In the semi‐urban and rural areas around the city, the traditional Assam‐type houses had been and
are still being replaced by confined masonry houses while a wide range of variations (especially with
respect to used building materials) can be observed. However, these houses can also be found in
large numbers in urban areas. The urban and sub‐urban housing stock is dominated by reinforced‐
concrete frame buildings (with clay brick masonry infill walls) up to 8 stories high. There are a few
commercial buildings with greater story numbers, i.e. ranging from ground floor plus 8 (G+8) to 10
stories (G+10). The observed construction technology and workmanship are considered to be fairly
good.
Table 3. Building typologies.
Building typologies Illustration
IK – Ikra
i.e. W5 – Wattle and daub (bamboo mesh with mud)
sloping roofs with light CI sheets supported by timber trusses mainly single story
UFB4L – Unreinforced burnt clay brick masonry in cement mortar
sloping roofs with light CI sheets supported by timber trusses bands at tie and lintel levels temporary construction for retail shops etc. mainly single story
RM3L – Confined brick masonry in cement mortar using burnt clay bricks
sloping roofs with light CI sheets supported by timber trusses confined burnt clay brick masonry with bands at tie levels, lintel levels and post plate levels
Building typologies Illustration
C1 – Ductile reinforced‐concrete frames with infill walls
C1L – low‐rise
1–3 stories
C1M – mid‐rise
4–6 stories
C1H – high‐rise
7+ stories
C1s – Ductile reinforced‐concrete frames with infill walls and soft ground story
C1sM – mid‐rise
4–6 stories
C1sH – high‐rise
7+ stories
Building typologies Illustration
C3 – No ductile reinforced‐concrete frames with infill walls
C3L – low‐rise
1–3 stories
C3M – mid‐rise
4–6 stories
C3H – high‐rise
7+ stories
S3L – Steel light frames – low‐rise
Buildings of this type are located in the Pandu Port near Guwahati. They are long‐span structures with CGI sheet cladding side walls, which are used as handling facilities in the river port.
S5L – Steel frame with unreinforced masonry infill walls – low‐rise
Table 4. Building classification scheme.
No. Index Building typology Wall material (brick/mortar) Roofing/flooring type Story range, N
PAGER 1) HAZUS 2) Risk‐UE 3) PSI 4) EMS 5)
1 W5, IK Ikra (Assam‐type, wattle and daub)
Large timber frames with wattle‐and‐daub infills, cement plaster
Timber or steel truss, CI sheet
1 (2) W5 – – AE2 E
2 UFB4L Unreinforced fired clay brick masonry with cement mortar
Rectangular fired clay bricks, cement mortar
Timber or steel truss, CI sheet
1 UFB4 – M3.3 BB1 C
3 RM3L Confined masonry Rectangular fired clay bricks in cement mortar with reinforced concrete confinements
Timber or steel truss, CI sheet,
1‐2 (3) RM3 – M4 BB2 D
4 C1L Ductile RC moment frame with masonry infills
Ductile RC frames, rectangular fired brick masonry infills
RC slabs, (for low‐rise: timber/steel trusses, CI sheets)
1‐3 C1L C1L RC31L DC1 D – E
5 C1M 4‐6 C1M C1M RC31M DC1 D – E
6 C1H 7+ C1H C1H RC31H DC1 D – E
7 C1sM Ductile RC moment frame with masonry infills, soft ground story
Ductile RC frames, rectangular fired brick masonry infills
RC slabs 4‐6 C1M C1M RC32M DC1 D
8 C1sH 7+ C1H C1H RC32H DC1 D
9 C3L Nonductile RC moment frame with masonry infills
Ductile RC frames, rectangular fired brick masonry infills
RC slabs, (for low‐rise: timber/steel trusses, CI sheets)
1‐3 C3L C3L DC2 C
10 C3M 4‐6 C3M C3M DC2 C
11 C3H 7+ C3H C3H DC2 C
12 C6L RC dual system RC moment frames with RC shearwalls, masonry infill walls
RC slabs, (for low‐rise: timber/steel trusses, CI sheets)
1‐3 C6L – RC4L DC3 C – E
13 C6M 4‐6 C6M – RC4M DC3 C – E
14 C6H 7+ C6H – RC4H DC3 C – E
15 S3L Steel light frames (Industrial use) Steel trusses with CI sheets
1 (2) S3 S3 S1L DS1 E
16 S5L Steel frame with URM infill walls
(Industrial use) Steel trusses with CI sheets
1 (2) S5L S5L S3L DS3 E
1) Jaiswal and Wald (2008) 2) FEMA (2003) 3) Lungu et al. (2001), Milutinovic and Trendafiloski (2003) 4) Spence et al. (1991) 5) Grünthal, ed. (1998)
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