Structural & Construction Deficiencies Of Nepalese Buildings
Common failure patterns were observed in the buildings of Nepal due to 7.8 Recter earthquake. Several field visits in the affected districts were conducted and associated failure/damage patterns have been identified and analyzed. Several types of damage patterns were observed for reinforced concrete buildings, as well as for unreinforced masonry and adobe houses during the reconnaissance survey performed immediately after the earthquake of 25 April 2015. This article covers damage patterns in non-engineered buildings, middle and high-rise buildings, commercial complexes, administrative buildings, schools and other critical facilities from Kathmandu valley as well as other affected districts. The construction and structural deficiencies are identified as the major causes of failure, however local soil amplification, foundation problems, liquefaction associated damages and local settlement related damages are also significantly observed during this earthquake.
Construction & Structural Deficiencies & Associated Damages In RC Buildings
About 10 % of buildings in Nepal are RC buildings. The construction of RC buildings only started after 1980; however the mushrooming number of RC construction was started only after 1990. Even though the RC construction was started in early 1980s, engineered construction was only felt after enforcement of building codes in 2006 and almost 70 % of existing RC buildings are either owner built constructions constructed with the help of contractors following by-laws or constructed as per the mandatory rules of thumbs as suggested by Nepal Building Code. Smaller fraction of buildings are structurally analyzed, designed and constructed. After more localized concentration of RC building damage during 2015 Gorkha earthquake, several field visits and case studies have reflected many of the deficiencies associated with construction or structural aspects have been highlighted in recent dates. Common structural and construction deficiencies and associated damages as observed during field reconnaissance performed after the earthquake are summarized in following sections.
During the field study performed immediately after the 2015 Gorkha earthquake, it is observed that soft storey failure in RC buildings is one of the most common causes of collapse, among with other type of structural deficiencies. The common type of building framing is moment resisting with monolithic slab cast in beams and columns. The foundation type usually adheres as isolated footing in residential level and combined mat and pile foundation for most of the high rise apartment buildings. The ground floor in almost 90 % RC buildings is used for commercial purpose and provided with shutters, however the upper stories of such buildings are provided with infill brick masonry walls. Similarly, in case of high rise constructions, the ground floor are left open for parking or sometimes basement parking is provided without infill walls. Such practices have led the soft storey failure during 2015 earthquake in most of the damaged RC buildings. In addition to this, it was observed that most of the buildings in residential level were found to be practicing “weak column-strong beam” as depicted by the massive beams and smaller columns during field reconnaissance. Due to lack of infill wall on ground floor, the increased flexibility has significantly triggered the increased displacement in ground floor thus majority of the cracks or minor to severe damages were found to be concentrated therein during reconnaissance.
Longitudinal Reinforcement Detailing
The longitudinal rebar’s are mostly limited to 12–16 mm diameter and usually in four to eight in numbers. In most of the collapsed buildings, the longitudinal rebar’s were usually four, however majority of the building columns were found to be constructed with six rebar in vertical direction. The intense damage was significantly governed by the horizontal re-bars (stirrups). The stirrups were found to be 6 mm diameter bars usually gaped in regular interval of 0.15 m or more and in some cases, such gap was measured up to 0.40 m. Reinforcement in infill walls were not found to be provided in residential constructions with exception to some apartments. Due to lack of reinforced walls in most of the brick kiln chimneys, almost all chimneys suffered from partial to complete collapse in Kathmandu valley. More significantly, the connection between the structural components was also found to be poor. In some cases, it was observed that people preferred welding for connection of vertical reinforcements rather than anchoring thus failures were observed in those columns. The larger gap of stirrups was noticed to be significantly contributing in the failure of columns. Moreover, the structural damage and building collapse was found to be more governed by too less, smaller diameter and tying the vertical re-bars outwardly. Similarly, the beam column connection and layout of the reinforcement was found to be random leading to minor to serious damages. The workmanship defect along with the connection deficiencies were widely observed.
In order to cover up maximum area, upper stories are constructed more in balconies. In such practices, floating columns from first storey were commonly observed in residential construction within as well as outside Kathmandu valley. The floating columns are frequently used for increasing the built up area from upper stories. Due to lack of continuous load path during earthquakes, the lateral forces are not effectively transferred to the foundation. The overturning forces developed lead to buckle the columns of ground floor and subsequent damage was noticed in some buildings. Due to poor mechanism of construction monitoring, many of the urban constructions were noticed to be haphazardly increasing the housing size beyond structural system. Figure 3d is a residential construction in Lalitpur sub-metropolitan city which is taken as the successful example of building code implementation framework in Nepal, however the enforcement of building code was not found to be effectively monitored.
Concrete Mixing & Placement
In residential construction, the MRT has provisioned the minimum crushing strength to be 15 MPa as per NBC 205, though due to insufficient mixing, placement and workmanship the strength is not assured. Segregation and bleeding were commonly observed in the damaged buildings. Quality of fine and coarse aggregates, water cement ratio significantly affect the strength of concrete, however in Nepal water cement ratio is usually exceeded for achieving workability compromising strength of concrete. In most of the damaged houses, corrosion of re-bars was recurrently visible. Use of rounded aggregates and poor binding was also exposed in some buildings outside Kathmandu valley. Rounded aggregates have led in poor stiffness and improper binding with concrete causing the out of plane failure.
Majority of the RC buildings in Nepal are constructed in phase, in this regard the variation in quality of construction materials and technology. This has led in non-homogenous and non-monolithic construction showing wider variation in terms of structural performance. It has been observed in around 80 % of buildings with minor cracks to major damage was constructed in phase.
Building Asymmetry & Other Deficiencies
Building asymmetry in terms of plan as well as elevation is more common in Nepal. Due to unavailability of spaces for construction, slender structures are also frequent in urban neighborhoods. Nepal building codes restrict the height to breadth and length to breadth ratio less than 3, though these regulations are not properly enforced and monitored even in the only one metropolitan city of Nepal. Due to lack of construction monitoring system, owners add stories themselves hiring local contractors. In addition to slender structures, asymmetrical structures constructed following the shape of the land plot are more common in Nepal and such structures sustained heavy damage during 2015 Gorkha earthquake. In the outskirts of Kathmandu valley, the localized damages are more intense. Previously those areas used to be the villages or suburbs so that only mandatory rules of thumbs (NBC, 205) or experience of the local level contractors were effective as construction guideline. However due to lack of monitoring, owners themselves constructed some more stories without design approval, at the meantime such buildings were observed to be facing severe damages or even collapse. Before implementation of MRT, 0.23 × 0.23 m was commonly adopted as the column size, however it provided as guideline for up to three stories in Nepal. Later, with the commercialization of areas, houses with the same column size were made up to seven stories. This was the significant cause for damage in most of buildings in Nepal during 2015 Gorkha earthquake. In addition to this, several houses were found to be constructed with combination of load bearing walls and columns, in doing so, it was observed that columns were too less and heavy RC slabs were provided for load bearing wall system. In some sites of Kathmandu valley, some of the construction flaws like joining masonry as well as RC buildings were observed.
Load Accumulation In Upper Stories
In some locations of Kathmandu valley, traditional masonry houses were found to be added with reinforced concrete construction in third or four storey. Many of the buildings in Kathmandu are constructed with higher load concentration in upper stories and reducing the column size while constructing upper stories. Water tanks were found to be installed in every RC building in Nepal for gravity distribution in taps, though the weight of water tank was found to be nevertheless accounted during structural analysis and design before construction even if it was obvious to install. At least one to five water tanks of varying capacity were found to be installed during field reconnaissance. Due to continuation of single column for water tank and construction of short column, some of the buildings sustained damages in those structures. Also in some houses, the load of telecommunication tower was found to be established without any structural analysis and design.
Due to close proximity of buildings in Kathmandu valley and other affected parts of Nepal, pounding solely or with several combinations with pounding has led moderate to severe damages in building. Due to wider variation in dynamic properties of adjoined buildings and unequal storey height, the pounding is observed intense in some areas where building proximity is common and adjoining buildings were constructed at different time with wider variation in construction materials, technology and workmanship. The major urban and sub-urban centers in Kathmandu valley have high built up area to total land plot, so almost all portion of land is covered by the structural system. Due to such close proximity led pounding, some structures were found to be separated some 3 m.
In columns, due to large spacing between the stirrups, weak column strong beam, insufficient development length of rebar, among others were found to be governing the damage concentration in columns. Plastic hinge formation was also observed in some of the structures along with concrete spall caused due to buckled re-bars on ground floor.
In Nepalese RC construction practice, the infill walls are not accounted during design and analysis phase rather such walls are provided haphazardly through the knowledge of contractors or sometimes masons. Regarding the infill walls behavior in-plane and out-of plane damage was observed in several cases. The most common failure of the non-structural element is related with the in-plane damage (diagonal and horizontal cracks), however it was also observed several case of out of plane failure of walls, most probably due to the combined effect of in-plane and out-of-plane behavior. Due to lack of anchorage between building diaphragm and infill walls, such failure was commonly observed during field reconnaissance. Infill walls have effectively contributed in damage with formation of captive column in many damaged RC buildings. The gross area of openings in infill walls is restricted to less than 10 % in NBC though for commercial purpose, most of the infill wall area is found to be covered by shutters or plywood separations.
Damages in staircase without proper anchoring with structural members was also noticed. Staircases could effectively perform as diagonal bracing members but due to lack of sliding joints in design most of the damage concentration was found to be in staircase. Also staircases are never isolated from structural elements as well, so the performance of buildings during earthquakes is also affected by the heavy staircases.
Failure of water storage tanks was noticed during the reconnaissance. Water tanks are not accounted during building design and also it is found that owners themselves hire masons to construct water tanks. Moreover, there is no proper regulation to monitor how many water tanks a single building can install is not governed by any by-law or design code. Due to this fact, many houses in Kathmandu were found to be installing water tanks, heavy purification system and water heating systems.
Damage in parapet walls was observed in some RC buildings which were not reinforced and excessively high. The anchorage with roof diaphragm was found to be often lacking in case of damaged parapet walls.
The boundary walls were among the severely collapsed non-structural members during 2015 Gorkha earthquake. In order to separate property line or for security purpose, boundary walls were found to be constructed in almost every house within Kathmandu valley in newer settlements. However, in case of older settlements and outside Kathmandu valley boundary walls were not frequent. Due to poor workmanship and inferior quality of plain cement concrete construction proper binding was not assured so around 90 % of boundary walls with the thickness of 0.115 m fell in urban neighborhoods. In contrast, majority of the boundary walls with 0.23 m thickness survived significantly.
Majority of constructions in Nepal follow the natural terrain relief for starting foundations. Thus for the inclined or terraced land, stepped foundation is more common. Isolated footing is the dominant foundation type and also it is observed that such footings are many on occasions placed in terraced construction site. Due to construction of footings in terraced land plot, numbers of footings were found to be varied in lower stories of structures. This ultimately led enhanced torsion in structural elements and damage is found to be concentrated in those buildings which are constructed in stepped foundation in Chautara, the most affected area in Sindhupalchowk district in central Nepal. As there is no trend of geotechnical observation and associated design variation in local scale in Nepal and many settlements are established on river banks, reclaimed sites and rice fields as well. During field reconnaissance, it was observed that the buildings constructed in river banks were facing rocking and sliding of foundation problem even if the structural system was undamaged. Moreover, both foundations as well as structural deficiencies led building collapse were also noticed in some locations within Kathmandu valley.
Soil Investigation & Foundation Site Selection
Soil investigation before construction is not performed for residential buildings in Nepal. Even metropolitan cities, sub-metros and municipalities are not able to enforce mandatory geotechnical investigation before construction. The severely damaged areas of Sitapaila and Gongabu within Kathmandu metropolitan city have soil bearing capacities of 52 and 106 KN/m2 respectively. As per Nepal building code these sites are classified as weak to soft foundation types though effective remedies weren’t incorporated for building constructions and common construction system as of other areas in Kathmandu valley were found to be incorporated during building construction. Instead of foundation improvement, additional stories were constructed in weak columns of 0.23 × 0.23 m.
Due to lack of site specific design spectrum and localized design guidelines, construction practices are similar for all over Nepal. Though MRT is mandatory in many urban areas, most of the recently declared municipalities lack the basic earthquake resistant features in residential level. This has also triggered the intensity of damage in many newer urban centers and suburbs. The effect of topographic amplification, ridge effect and local site effects are clearly identified due to the more localized nature of damage during 2015 Gorkha earthquake.
Construction & Structural Deficiencies & Associated Damages In URM Buildings
Most of the urban nuclei in Kathmandu valley and older settlements outside consists majority fraction of unreinforced masonry building stocks constructed from sun-burnt or fired clay bricks. The common failure characteristics of masonry buildings interlinked with the analysis of structural and construction technology based deficiencies are disseminated in following sections.
Almost all, except some URM buildings were found to be not consisting any bands at various levels like; sill, lintel or gable. Due to lack of proper bonding in masonry load bearing wall, out of plane collapse was more commonly observed in Kathmandu valley and other settlements with abundance of URM buildings. In most of the URM building, the orthogonal walls were found to be behaving differently due to lack of proper connection between such walls showing poor integrity. Also due to lack of integration of several members within the structural components, out of plane failures were more intense than any other type of failure. In case of masonry structures, due to poor quality of binding materials, delamination of wythes was also common in stone masonry houses outside Kathmandu valley, however such delamination was also noticed in some thick walled constructions in Kathmandu valley monuments. Use of mud-mortar or clay with high water quantity was noticed on some reconstruction efforts during field reconnaissance this might significantly contribute in generating larger voids and poor binding of masonry units so as to contribute in delamination of wythes.
The structural integrity depends also on monolithic and homogenous behavior of masonry structures; however the mud mortar used for binding the brick/stone units were found to detached already and brick/stone units were found to be behaving separately. This binding problem led the severe devastation during 2015 Gorkha earthquake in the historic settlements of Kathmandu, Lalitpur, Bhaktapur, Sankhu, Harisiddi, Barpak, among others. Even after the earthquake damage people were found to be using the same binding material for reconstruction. This could be detrimental in future events.
Load Path Discontinuity
Due to mixed up system of construction and added reinforced concrete portions in upper stories along with cantilevered constructions, many of the buildings in Kathmandu valley were observed with load path discontinuity. The reentrant corners and diaphragm discontinuities were also noticed in masonry construction practices in Nepal. Masonry buildings up to six stories were found inside Kathmandu valley and in case of outside Kathmandu valley the number of stories for masonry buildings is limited to four. Struts in some masonry buildings performed the function of transferring the cantilevered load into structural wall, but majority of masonry structures were found to be without having struts even if the roofing was too heavy constructed with roof tiles.
The connection between walls, walls and floor and wall and roof was found to be poor in masonry buildings. In many buildings, it is found that due to weak connection and lack of corner post or stone, two orthogonal walls were found to be isolated leading to out of plane failure, evidently this type of failure was most prevalent in majority of damaged masonry houses. In case of timber elements used as flooring and load transfer components, those were found to be surviving even after the out of plane collapse of external walls. Several traditional practices to tie up the building elements like wooden pegs and wooden bands exist in Kathmandu valley as well as throughout Nepal. However, those collapsed or severely damaged buildings seldom constituted such elements.
Age Of Buildings
Masonry buildings in Nepal during the 2015 Gorkha earthquake were up to of 100 years age used by at least three generations. Many of the building at Sankhu and Bhaktapur were of around 100 years of age and it is also noticed that majority of collapsed buildings were of 70–80 years old. Due to deterioration of strength, buildings were in vulnerable stage even before the earthquake. Aside from very old age of masonry buildings, it was noticed that none of the houses were strengthened or retrofitted after construction. All the damaged houses at least sustained some damages during the 1988 Uadypur earthquake, however houses weren’t strengthened. Due to lack of repair and maintenance, most of the old houses were heavily damaged.
Shared Walls & Wall Thickness
In case of traditional masonry constructions most of the urban fabrics with row housing were observed to be sharing walls in one or both side of walls and found to be damaged during 2015 Gorkha earthquake. Such practices were seen in the row housing system of traditional settlement where single house is divided into two and phased construction or repair was done in either unit. The thickness of brick masonry wall was found up to 0.60 m but poorly connected with mud mortar. Such mud mortar binding was found to be detached from brick units many years back and delamintaion of wythes was commonly observed in those buildings with segregated mud mortar.
Shallow and spread foundation is commonly adopted in masonry constructions in Nepal. Beside this, natural terrain was found to be adopted directly for foundations so that inclined foundations were commonly observed in majority of masonry buildings constructed across and outside Kathmandu valley. It is observed that the buildings resting on inclined foundations were more damaged than those buildings resting on leveled foundation.
In majority of the masonry construction in Nepal, roof tiles are commonly used as roofing materials with thick layer of mud mortar. Also in some masonry buildings in Kathmandu valley, RC slab was found to be used as roof and ultimate destruction was caused by such heavy load concentration in the third or fourth storey. The roofing materials are not also tied or anchored properly with other structural members and found to be severely damaged during reconnaissance.
The gable portion of most of the masonry buildings was found to be constructed with thick brick wall; this ultimately led to most of the gable portion failure. The performance of thick gable wall was weak throughout Nepal in stone masonry and brick masonry structures. Although, some areas used to have small to big gable openings, due to heavy concentration of stacked and unsupported masonry units, failure was commonly observed in central Nepal, eastern Nepal as well as inside Kathmandu valley.
Due to lack of diagonal bracing in masonry constructions, most of the out of plane failure were common. Diagonal bracings are not practiced in Nepal in any form, thus structural integrity is seldom assured in owner built constructions as well historical and monumental constructions. The widespread damage in epicentral district of Barpak (Gorkha), Melamchi and Chautara (Sindhupalchowk), Mankhu (Dhading), Beshisahar (Lamjung), Dhunche (Rasuwa) and other areas were suffered from brittle collapse due to lack of diagonal or any type of bracing on walls.
Pounding and progressive failure
Most of the urban fabrics within Kathmandu valley consist row housing. In such housing framework, due to variation in dynamic properties of masonry units, pounding in terms of wall damage or bulging out of the wall was commonly observed within Kathmandu valley. The variations in storey height and construction materials and building components usually control the intensity of damage significantly. Meanwhile, the masonry buildings on the edge of row housing setup were found to be more damaged than the buildings in between. The intensity of damage in case of row houses was observed to be higher in the buildings on edge for all settlements in central Nepal, however those masonry structures with wooden frames were not found to be completely collapsed like the stone or brick masonry load bearing system. Many of the masonry buildings in epicentral districts and areas with row houses were observed to be suffered from progressive failure mechanism.
Construction & Structural Deficiencies & Associated Damages In Random Rubble Masonry Buildings
Majority of the fraction of buildings in Nepal is comprised of dry stone masonry construction practice . These are non-engineered constructions without following any earthquake resistant construction guideline and mud mortar is either used or sometimes constructed without mortar as well. In case of absence of mud mortar, the voids in between masonry units were found to be filled with stone chips or aggregates. Beside the irregular stones, in majority of houses, there were no cornerstones and even the shape of stone units was too irregular leading to heavy damage. Timber bands were seldom noticed in most of the damaged houses in all reconnaissance sites, however, in case of presence of timber framing those frames were found to be undamaged or sustaining very little damage. Mud mortar was found to be used in stone masonry buildings. Due to irregular shape of the stones the binding material was used variably, this may have contributed to the performance of buildings during earthquake. Smaller chips to heavy stone pieces were found to be used to construct walls, sometimes such arrangement itself was found to be triggering the pancake destruction houses due to lack of structural integrity. Those houses with timber elements were found to be less damaged than the homogenously constructed stone houses. Though rural constructions are isolated type building units with lower height, usually from one to three storied; yet deficiencies in terms of structural composition and construction technology were widely noticed during field reconnaissance. Heavy wall of rubble stones with irregular shape and size up to 0.53 m thick was observed. The stone masonry houses constructed with piling of stones were severely affected like in the epicentral village of Barpak. Beside this, very heavy roofs constructed from stone slices were found to be dominant than any other roofing system in Barpak leading to severe collapse of houses.
Construction & Structural Deficiencies & Associated Damages In Adobe Buildings
Adobe buildings are also non-engineered constructions prevalent in many urban fabrics as well as in suburbs and villages in Nepal. Most of the adobe constructions constitute either handmade brick walls, walls mixed with clay and some bamboo elements or even sometimes clay units of irregular shapes. The roofing may be of tiles, thatched or sometimes stones as well. The structural integrity is not justified in adobe houses due to poor binding and non-homogenous construction. Somehow, the components of construction behave separately rather than combined action against the earthquake motion. The roof if provided with tiles is also heavy thus damages were frequent in such houses during 2015 Gorkha earthquake. Adobe constructions are also found to be one to three storied and houses with more heights were found to be affected more than the single storied adobe houses. The primary damage mechanism was observed as complete collapse, gable collapse and detachment of orthogonal walls. As recent construction technology seldom follows adobe construction system in Nepal, almost all structures were of old to very old age and also those structures sustained minor damage during 1988 Udaypur earthquake.