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Seismic Performance of Masonry Buildings during Recent Gorkha
Earthquake in Nepal
Bharat Chalise
1
Dr. Rajan Suwal
2
ABSTRACT
Recent 7.8 Richter scale magnitude earthquake hit Nepal followed by 366 aftershocks having
magnitude more than 4 till July 31, 2015. During the earthquake 501,201 numbers of buildings
were collapsed and 272,177 numbers of building were damaged partially or fully. Most of the
building damaged by the earthquake were masonry buildings. This paper presents a
comprehensive overview of the performance of masonry buildings in the recent Gorkha
Earthquake affected areas. Photograph of damaged buildings taken after destructive earthquake
is presented succinctly. Compliance with existing code is discussed together with damaged
building. Several structural deficiencies by the recent earthquake are assessed including: poor
quality of materials and workmanship, pounding effect, weak storey, unconfined gable walls,
missing confining elements, vertical and horizontal irregularities, lack of corner treatment, heavy
balcony and poor anchorage for horizontal projections.
Key Words: Masonry Buildings, Gorkha Earthquake, Seismic Performance, Nepal
1
Department of Civil Engineering, Indian Institute of Technology Delhi, India. Email:naturead@gmail.com
2
Dr. Rajan Suwal, Department of Civil Engineering, Institute of Engineering, Central Campus, Tribhuvan University,
Nepal. Email:rajan_suwal@yahoo.com
INTRODUCTION
Recent 7.8 Richter scale magnitude earthquake hit Nepal followed by 366 aftershocks having
magnitude more than 4 till July 31, 2015 and a very powerful one is 6.8 magnitude on May, 12
and 6.7 magnitude on April 26, 2015.The entire territory of Nepal lies in high seismic hazard
zone. The country's high seismicity is related to the movement of tectonic plates along the
Himalayas that has caused several active faults. A total of 92 active faults have been mapped
throughout the country by the Seismic Hazard Mapping and Risk Assessment for Nepal carried
out as part of the Building Code Development Project 1992-1994 (MHPP, 2994). Earthquakes of
various magnitudes occur almost every year and have caused heavy losses of lives. Whole
country falls on the high intensity scale of MMI IX to X for generally accepted occurrence
period. The country has a long history of destructive earthquakes. In this century alone, over
11,000 people have lost their lives in four major earthquakes. A 1934 AD earthquake produced
strong shaking in Kathmandu Valley, and destroyed 20 percent and damaged 40 percent of the
valley’s building stock ( NSET 2012).
Countries where experienced in earthquake-engineering, most of the researches are focused on
the study of complex structures only such as high-rise buildings while little attention is given to
masonry buildings. In addition, observed damages on masonry buildings were very few in
comparison with the observations on reinforced concrete buildings. These structures have been
severely damaged during the destructive earthquakes and have caused a great number of human
death and a large amount of economic loss in some countries such as Nepal. Furthermore,
historical and monumental structures have been assumed to be cultural inheritance for a
community. Many of them were built as masonry structure on seismic zones in many countries.
To transfer this cultural inheritance to the next generations, it is very important that these
buildings will be survived during next earthquakes with only slightly damaged or even
undamaged. Therefore, further theoretical studies and damaged observations should be made to
put forward the realistic behaviors of masonry structures during the earthquakes. As known, site
conditions, properties of structural systems and earthquakes affect the damage level of structures
during earthquakes.
Figure 1: Intensity of Gorkha Earthquake Nepal 2015 and After Shocks. Left Bottom Caption:
Earthquake Timeline of Nepal (Source: USAID, 2015 & Aljazeera, 2015)
During the earthquake 501,201 nos. of building were collapsed and 272,177 numbers of building
were damaged (Ministry of Home Affairs, Nepal 2015). Most of the building damaged by the
earthquake were masonry building. In Nepal, generally three types of building structure have
been constructed: Reinforcement concrete cement frame structures, brick masonry structures and
stone masonry structures. Cement sand mortar and mud mortar have been used to construct these
masonry structures. Masonry buildings are commonly practiced in village are while RCC frame
May 2015
7.8
structures are constructed as modern infrastructure in cities. The damage information of masonry
building was collected from the site visit, local authorities and organizations. This paper presents
a comprehensive overview of the performance of masonry buildings in the recent Gorkha
Earthquake affected areas.
MASONRY BUILDING RESPONSE DURING EARTHQUAKES
During an earthquake when a structure is shaking, it gets damaged if it is not sufficiently strong
and/or flexible. The extent and degree of damage depend on how weak and/or rigid it is. The
type of damage depends on a number of factors including the type, direction and duration of the
earthquake forces, the frequency of the ground motion, the natural frequency of the building, the
nature of the underground geology and soil deposits, the shape of the structure, the type of
building technology, etc ( UNESCO 2003).
Earthquake is an event which vibrates the surface of earth where we made structures. During an
earthquake, the ground surface moves in all directions. The most damaging effects on buildings
are caused by lateral movements which disturb the stability of the structure, causing it to topple
or to collapse sideways. Since buildings are normally constructed to resist gravity, many
traditional systems of construction are not inherently resistant to horizontal forces. Thus design
for earthquakes consists largely of solving the problem of bracing a building against sideways
movement.
Damage to a structure is not an instantaneous event. It occurs over a span of several seconds as a
continuous process during the period of earthquake shaking. The degree of damage can range
from a low level to a high one that may include partial or total collapse. Since wall are the major
structural elements to bear the load of entire masonry structure damage pattern of wall is very
important (Doğangün, Ural and Livaoğlu 2008).
MASONRY BUILDINGS IN NEPAL
Buildings that are constructed with either brick or stone masonry with either cement sand mortar
or mud mortar is categorized as masonry structures. Generally most of the cases presented here
are non-engineered masonry buildings unless specified. Many buildings under this category were
also constructed with adobe brick. Masonry buildings can further divide into two categories
irrespective of mortar used in the structure.
1. Brick Masonry Buildings
The traditional brick masonry buildings in Nepal are 1 to 5 story high with a sloped roof at
the top as shown in figure 2 (a). The walls of the buildings are constructed using fired
bricks or fired and sun dried bricks in mud mortar. The thickness of the brick wall in typical
traditional residential buildings varies from 360 mm to 600 mm. The floors are constructed
using wooden beams, above which wooden planks are placed in the direction perpendicular
to the beams and a thick mud layer is used to provide the smooth surface for the floor (Figure
2 (b)). The sloped roofs are also constructed similar to the floors but with a layer of roof
tiles provided on top of the mud layer or above wooden beams (Figure 2 (c)). Such type of
buildings are very common in the old cities of Kathmandu Valley. Not only residential building
but several monuments, historic buildings, temples are also constructed in similar fashion. Most
famous historic 9 storied Bhimsen Tower, Dharahara which has been collapsed in this
earthquake was constructed with 600mm thick round brick masonry wall. The traditional brick
masonry buildings are characterized by its heavy mass. Since the bricks are laid on the mud
mortar, these structures possess very low strength and exhibit a brittle nature of failure resulting
in sudden collapse giving occupants inadequate time to escape during seismic events. The
performance of these buildings is usually better than those of stone masonry buildings, however,
these buildings cannot be considered as seismic resistant buildings unless proper retrofitting
techniques are implemented.
Modern brick masonry buildings are constructed by using cement sand mortar and standing up to
six storey. RCC rigid slab is used on the floor level and roof structure. Many of the buildings are
non-engineered but comparatively less vulnerable to the traditional one. Bricks are laid on
cement sand mortar thus bonding between the brick element is high. Many of this type of
building failed in recent earthquakes but not much as traditional brick masonry and stone
masonry buildings.
Figure 2: Detail Photographs of Brick Masonry Building (Photos Source: Wikipedia)
2. Stone Masonry Buildings
Traditional stone masonry buildings are mainly constructed of random rubble stone
masonry, in which rough stones are piled up without any mortar or with mud mortar. Such
buildings are constructed based on traditional techniques using locally available material and
Roofing Technique
Wooden Planks
a) Brick Masonry Building
b) Flooring Technique
c) Roofing Technique
labor. Stone masonry buildings are common in most of the villages in Nepal, due to easy
availability of construction material and associated low cost. Seismic forces are ignored
while constructing these buildings. Either slate or corrugated sheet is used as roofing material
(Figure 3 (a)). The stone masonry buildings are characterized by heavy mass, very low strength
compared to the mass density and brittle nature of failure. The absence of reinforcements and
poor quality of mud mortar or no mortar, leads to buildings that fail in brittle manner, without
allowing for energy dissipation and providing no warning to the occupants before collapse
during seismic events (K.Shakya, et al. 2013).
At the moment people are using cement sand mortar to make strong bonding strength. Thickness
of these type of wall varies from 375-500mm having two layer of wall. This type of buildings
possess better seismic performance because of strong bonding between wall elements. The floors
are constructed using wooden beams, above which wooden planks are placed in the direction
perpendicular to the beams and a thick mud layer is used to provide the smooth surface for the
floor.
Figure 3: Detail Photographs of Stone Masonry Buildings (Photos Source: Wikipedia)
Floor Level
Roofing
a) Typical Stone Masonry House in Nepal
b) Slate Roofing and Floor Construction
Method
DAMAGE ASSESSMENT OF MASONRY BUILDINGS
The frequent occurrence of huge earthquake results catastrophic losses to the people’s life and
property, which is mainly caused by the devastation of many buildings. Severe damage on
masonry buildings caused by the 7.8 magnitude earthquake is an opportunity for engineering
community to improve the seismic performance of these types building. The study focused on
the masonry buildings because most of the losses occurred on these structures.
No official data revealed for the types of building collapsed but analysis of census data shows
more than 80% of the building might be damaged by recent earthquake. According to the 2011
National Population and Housing Census, the total number of individual households in Nepal i
5,423,297, while the population was 26,494,504. The census data indicate that mud-bonded
brick/stone masonry buildings are the most common in all geographical regions of Nepal
(44.2%), followed by wooden buildings (24.9%). In urban areas (e.g., Kathmandu Valley),
buildings with cement-bounded brick/stone (17.6%) and cement concrete (9.9%) are popular.
Figure 4 (a) shows the damage level of earthquake on the village nearby epicenter while 4 (b)
shows the total human loss and building damage by the Gorkha earthquake. There are no steel
structures build in Nepal and modern RCC frame structures are confined only on the city area.
The seismic damage of masonry buildings investigated in this area is mainly stated in the
following section.
Figure 4: Human and Property Loss by Gorkha Earthquake, 2015 (Source: USAID, 2015)
1. Poor Mortar, Masonry and Weak Wall
Mortar to be used in brick masonry building for load bearing walls shall be Cement-sand mixes
of 1:6 and 1:4 shall be adopted for one-brick and half brick thick walls, respectively. The
addition of small quantities of freshly hydrated lime to the mortar in a lime-cement ratio of ¼:1
to ½:1 will increase its plasticity greatly without reducing its strength (NBC 202:1994 1994).
Although code came very late but it is not generally observed that quality was the same as
defined in the NBC 202: 1994.
Figure 5: Quality of Mortar and Bricks Used in Construction of Buildings
a) Damage extent of Barpak VDC Gorkha, Nepal
(Epicenter of Gorkha Earthquake 2015)
b) Human & Building Loss during
Gorkha Earthquake 2015
The photo shown in figure 5 indicates the poor quality mortar and bricks used in the building
which cannot bear the horizontal deformation during the earthquake. This cause brittle failure
with high possibility of casualties. This photo was taken at Bhaktapur, where most of the
building cross the age of 50 yrs. No code compliance can be seen there which caused massive
damage in the city with hundreds of loss of lives.
Quality of mortar and its use is not observed as per the standard practices. Poor quality mud or
cement mortar caused in the disintegration of masonry units and loss of support to floors as
shown in Fig. 5. Quality of bricks and stone also matters in the bonding process. Here link
elements were of inconsistent size and not having strength as mentioned in code. Code talks
about the standard brick “The bricks shall be of a standard rectangular shape, burnt red,
handmade or machine-made, and with a crushing strength not less than 3.5 N/mm2. The higher
the density and the strength, the better they will be. The standard brick size of 240 × 115 x 57
mm with 10 mm thick horizontal and vertical mortar joints is preferable”. All the things hardly
compliance with code. Code should follow strictly and can improve the bonding capacity by
keeping wooden keys and structural elements to prevent the complete and brittle collapse of
building. Generally outer wall of brick masonry buildings are made of 9” thick and partition wall
are made off 5” only. Load bearing masonry structures masonry wall acts as structural element
and only of 5” means high probability of failure of the building.
2. Pounding effect
Seismic pounding between adjacent buildings can cause severe damage to the structures
under earthquakes, when owing to their different dynamic characteristics. Commonly, pounding
damage only receives a passing mention, or is described in case studies of specific buildings.
Figure 6: Pounding of Building during Gorkha Earthquake, Nepal
Several buildings as shown in figure 6, damaged on urban area especially in Bhaktapur city were
constructed by adjoining side by side with adjacent buildings. Nepal National Building Code is
also silent about pounding mechanism. Street survey of Kathmandu shows that pounding effect
is more serious when dynamic properties of adjacent building is different.
3. Irregularities in Plane and Vertical Directions
These are common problem of masonry buildings in Nepal. Since most of the masonry buildings
are non-engineered and constructed haphazardly performance of these structures were extremely
measurable during earthquakes.
Figure 7: Damaged Irregular Structures
4. Lack of vertical confining elements
As per NBC 202:1994 (1994) steel bars shall be installed at the critical sections (the corners of
walls, junctions of walls, and jambs of doors) right from the foundation concrete. They shall be
covered with cement concrete in cavities made around them during the masonry construction
This concrete mix should be kept to 1:2:4 by volume, or richer.
Figure 8: View of Damaged Buildings in Kathmandu Valley without Vertical Confining
Elements
Code further elaborates; the vertical steel at openings may be stopped by embedding it into the
lintel band, but the vertical steel at the corners and junctions of walls must be taken into either
the floor and roof slabs or the roof band. Despite this provision in code almost all masonry
buildings were without vertical confining elements. Following pictures show the exact situation
of the structures which are prone to structures. As seen in figure 8 it was very hard to find the
building with such elements. Reasonably these buildings suffered more damage.
5. Weak Out of Plane Response
Unreinforced load bearing structures are more vulnerable in flexural failure. Insufficient
anchorage or bonding between walls and beams/slabs cause the toppling of the walls in out of
plane. Rural structures in Nepal with flexible slab are more vulnerable. Following figure shows
some plane failure of the buildings during Gorkha Earthquake in Nepal.
Figure 9: Out of Plane Failure of Walls in Bhaktapur City during Gorkha Earthquake
6. Unconfined Wall Corners
Corner is a most critical section of masonry building. Performance of corner during earthquake
determines the stability of building. NBC 202:1994 (1994) mentioned about the vertical joints
between the orthogonal walls. For convenience of construction, builders prefer to make a toothed
joint which is later often left hollow and weak. To obtain full bond, it is necessary to make a
sloped or stepped joint. It should be constructed so as to obtain full bond by making the corners
first to a height of 600 mm, and then building the wall in between them. Alternatively, the
toothed joint shall be made in both the walls in lifts of about 450 mm. similarly to confine its
corner with wall horizontal band is proposed in the same code. Steel dowel bars shall be used at
corners and T-junctions to integrate the box action of walls. When assessment of the buildings all
the rules were non-compliance with the codal provisions. Most of the non-collapsed buildings
are suffered with gable wall failure because of inadequate anchorage.
Figure 10 : Damage on Corner due to Confinement Missing
7. Weak Storey
There is a strong provision on NBC 202:1994 (1994) for the limit of opening in the building.
Total horizontal length of the opening should not exceed one third of its wall width. Opening
should start minimum of 600 mm from both the corner of the wall. However, many building in
urban area are not following these thumb rules. They are even making shutters and series of
multiple wooden door for the purpose of business. During the earthquake these first floor
performed as a weak story to increase the extent of the damage.
Figure 11: Views of Failure due to Weak Story was a Serious Problem of Non Engineered
Structures during Gorkha Earthquake
CONCLUSION
The heavy weight of the building without considering any engineering practice led to greater
scale of damage. Many buildings in Kathmandu valley found constructed by two or more layers
of adobe and modern clay bricks and mortar. These houses are heavily damaged by delamination
effect. Most of the damaged masonry structures were poorly constructed without following the
design guidelines while minor or no structural damages were observed in well-constructed
masonry buildings. Infilled brick walls are largely damaged during the earthquake. Several
structural deficiencies by the recent earthquake are assessed including: poor quality of materials
and workmanship, pounding effect, weak storey, unconfined gable walls, missing confining
elements, vertical and horizontal irregularities, lack of corner treatment, heavy balcony and poor
anchorage for horizontal projections. Cultural heritage sites that suffered heavy damage were
very old monuments where there was deterioration of the construction material. Lack of cyclic
maintenance and unpreparedness for strong ground shaking. The major deficiencies indicating
non-compliance with codal provisions should be noted and strengthening of existing building
may increase the performance of the building on coming days.
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