Citation: Phadera, D., Haruhiko SUWADA (2019), A study on seismic performance and retrofit
approach for current RC buildings with soft first story in Nepal, Synopsis of IISEE-GRIPS master report.

A STUDY ON SEISMIC PERFORMANCE AND
RETROFIT APPROACH FOR CURRENT RC
BUILDINGS WITH SOFT FIRST STORY IN NEPAL
Dhira Phadera1,2

Supervisor:Haruhiko SUWADA3ABSTRACT
The soft first story RC buildings proved to be vulnerable in Nepal during the 2015 Gorkha earthquake.
The RC buildings in Nepal are constructed with a weak frame, lack of ductile detailing, and brickmasonry infill. In the city area, due to urbanization and lack of land area, the upper story is used as brickmasonry, and the first story is used for shopping and parking purpose. This research intended to assessand compare the seismic vulnerabilities of RC buildings with soft story in Nepal namely, (i) building
constructed by following NBC 205:1994 i.e. Mandatory Rule of Thumb (MRT) with poor detailing andmodified later by adding story (NBC) and (ii) building constructed based on modified MRT of 2010
recommended by DUDBC, with poor detailing and modified later by adding story (NBC+). Seismic
performance is evaluated by JBDPA guidelines of seismic evaluation, FEMA-356, NBC 1994, nonlinear
static pushover analysis and dynamic analysis by Gorkha Earthquake motion. The results are discussed
in terms of story shear, capacity curve, maximum roof displacement, inter story drift and damage pattern.
Retrofit by wing wall and shear wall is proposed. The research found that seismic behavior, ductility
demand, and inter-story drift pattern of RC building with the soft first story are different from those withanalysis of bare frame. The soft first story suffered extreme inter story drift change causing severe
damage. Seismic vulnerability of both buildings before and after retrofit showed NBC+ building
performed better than NBC building. Both model buildings sustain Gorkha earthquake motion which
reveals the building collapsed during earthquake are of low concrete strength, poor detailing, and lackof reinforcement constructed before the application of NBC. A combination of RC shear wall and wing
wall proved to be effective in eliminating stiffness differences and controls excessive inelastic lateraldrift to keep the usability of open space in the first story.
Keywords: Soft-first story, Seismic Performance, Retrofit, Wing wall, Shear wall.

1. INTRODUCTION
Reinforced Concrete (RC) building construction has begun from the late 1970s and increased rapidly inthe last 3-4 decades in Urban areas of Nepal. The increasing population and scarcity of land havecompelled people to construct non-engineered multistory buildings with open space on the first floor for
shopping and parking purposes. As infill material is commonly used for the upper story of brick masonry1

Department of Urban Development and Building Construction (DUDBC), Kathmandu, Nepal.
IISEE-GRIPS master course student
3
International Institute of Seismology and Earthquake Engineering (IISEE), Building Research Institute (BRI).
2

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structure, lateral stiffness differences occur between the upper and the first story. Soft story building
proved to be very vulnerable and performed poor during 2015 Nepal Earthquake.
Most of the buildings are of owner-driven poorly constructed with inferior masonry quality
in light reinforced frames lacking ductile detailing. However, to improve the quality of construction andenhance the seismic behaviour of buildings, Nepal Building code (NBC) was implemented from 2003.
NBC contains mandatory Rules of Thumb (MRT) that provides the facility for ready-to-use in terms ofdimensions and details for structural and non-structural elements for up to three-story RC, framed,ordinary residential buildings commonly built by owner-builders in Nepal (NBC). Later in 2010, theDepartment of Urban Development and Building Construction (DUDBC) published an additional
recommendation for the construction of Earthquake Safer Buildings in Nepal with the assistance ofUNDP (NBC 1994). It is an improved version of MRT in NBC 2003 which specifies the increased
minimum size of columns, including the rebar detailing for the three-story building. Hence, people
constructed buildings following both MRT of 2003 and 2010 additional recommendation to some extent.
However, with urbanisation and increase in the land price, owners tended to add the other story to their
existing buildings, which were not initially designed for this condition. This tendency makes the building
more vulnerable.
Since Nepal lies in the subduction region of the Eurasian plate and Indian plate, it poses ahigh risk of an earthquake. During earthquake force, inadequate lateral strength in the first floor of thesoft-story building to withstand massive displacement of the building may cause extensive damage oreven collapse. Moreover, it is estimated that more than 90% of the buildings, designed and constructedaccording to old standards, are non-engineered RC frame and owner-built, posing a high risk to life andproperty. Those structures constructed and modified without considering soft-story effects are under
threat of severe damage or collapse during a future earthquake. In this context, understanding the seismic
behaviour of the prevalent soft-first story buildings constructed in Nepal and the employment of thestrengthening mechanism has become a subject of paramount importance.

2. THEORY AND METHODOLOGY
The prototype 5-story RC building with the soft first story is intended to represent a typical residencewith commercial RC building in the urban area of Nepal. The global dimensions of the sample building,story height, number of story's and bays, including span length, concrete quality and steel type typically
represent the existing scenario. In this study, the two variations of the typical soft first story moment
resistant frame (MRF) with infill masonry are considered. The first type corresponds to a typical 5-story
residential cum commercial building with soft first story constructed somehow following MRT, i.e.
NBC-205:1994 (called as NBC building) with later adding another story, which makes buildings morevulnerable. Similarly, the second type is also a 5-story building with a soft first story based on a modified
version of MRT after recommendation by DUDBC at 2010 (called as NBC+) and modified later byadding stories.
JBDPA guidelines of seismic evaluation is done to clarify the possessed performance oftypical soft first-story buildings in Nepal. Nonlinear static pushover analysis is done to understand thestructure's capacity by the capacity spectrum method. After evaluating the seismic vulnerability of 5story buildings with a soft first-story, anti-seismic measures with RC wing wall and the shear wall isproposed and reevaluated to conduct the structure's safety. Nonlinear Time history analysis (NTHA) isperformed using the ground motion of Gorkha Earthquake 2015, Nepal to understand the vulnerabilityof building before and after the earthquake.
Seismic demand index (Iso) is the level of seismic capacity needed for a structure to remain
safe against a ground motion or code defined ground motion. For this study, Iso value is taken as 0.6,
which is calculated from the design horizontal seismic force coefficient of NBC 105:1994 for thebuilding with importance factor 1.5 to be used even after the earthquake. The building is supposed to belocated in zone 1 and medium soil and performance factor of 4 without following ductile detailing.
Moreover, for load combination considering earthquake, 25% is increased. Hence the total value
becomes the product of basic seismic coefficient in code, i.e. 0.08, importance factor, zone factor and2structural performance factor. Demand spectrum is prepared to find the performance point based on thecapacity spectrum method.
JBDPA standard for seismic evaluation considers only RC wall. So, for brick masonry infill
wall, shear strength of infill panel is taken as 0.2 Mpa from the proposed value from Seki et al. Similarly,
for ductility index the proposed value of 1.5 from the Al-Washali et al. is taken.
STERA 3D, the finite element modelling software is used for both nonlinear static anddynamic analysis. Equivalent strut method is used to represent brick infill masonry in modelling. For
equivalent strut compressive strength (f’m) is taken from formula proposed by Paulay and Priestley in1992. The modulus of elasticity of masonry prism is taken from the equation proposed by FEMA.

3. SEISMIC PERFORMACNE OF SOFT FIRST STORY
3.1. Outline of target building
The prototype 5-story RC building with the soft first story is intended to represent a typical residential
cum commercial RC building in the urban area of Nepal. It has three-span in both transverse andlongitudinal direction. The story height is 2.84 m. The global dimensions of the sample building, story
height, number of story's and bays, including span length, concrete quality and steel type typically
represent the existing scenario. The buildings have open first floor used for shops and parking areas andbrick infill masonry in the upper floor having a thickness of 115 mm in partition and 230 mm in outerside of building. The foundation is isolated footing.
In this study, the two variations of the typical soft first story moment resistant frame (MRF)
with infill masonry are considered. The first type corresponds to a conventional soft first story residential
cum commercial building construction somehow following the MRT, i.e. NBC-205:1994 (called asNBC building) and later adding story making building more vulnerable. The second type is a 5-story
soft first story building based on a modified version of MRT after recommendation by DUDBC at 2010(called as NBC+).

Figure 1. First floor plan.

Figure 2. Front elevation.

Figure 3. Side elevation.

3.2. Story displacement and inter story drift
Homogenous lateral displacement in every story in bare frame case is observed. Unlike this, in soft firststory case, large inelastic deformation is concentrated with subtle inter-story displacement in upper story.
The drift at soft first story exceeds the allowable drift limit (1%) as mentioned in NBC 105:1994 andimmediate occupancy for FEMA. Thus, soft-story columns are very vulnerable to earthquakes if they
do not have adequate ductility and strength to meet the demand. Sudden change of story drift in the soft
first story enhances the possibility of forming non-uniform plastic hinge in soft first columns and severe
damage or even collapse during earthquakes. However, in the case of bare frames, uniform change of3the inter-story drift is observed. However, in the case of bare frames, uniform change of the inter-story
drift is observed as in Figures 4 and 5.

Figure 4. Inter story drift (NBC building).

Figure 5. Inter story drift (NBC+ building).

3.3. Damage distribution
Inelastic analysis is vital to know the modes of failureand collapse sequence when any structure's elastic
capacity exceeds during earthquake. From the nonlinear pushover analysis, the failure mechanism for thestructural members in the longitudinal direction of theNBC building at grid D-D for bare frame and at theperformance point of the capacity spectrum method forsoft first story is shown in Figure 6. In transverse direction also similar damage pattern was observed atperformance point. NBC+ building also shows similar
Figure 6. Bare frame case and soft first casedamage pattern.
for NBC building.
3.4. Seismic evaluation by JBDPA standard
Seismic evaluation by the JBDPA standard 1st levelscreening for bare frame of both NBC and NBC+building shows the lack of seismic capacity in all story.
In 2nd level screening considering infill wall, NBCbuilding model has a deficiency in earthquake resisting
capacity in first, second and third story at the longitudinal
direction as in Figure 7. Whereas, NBC + building has
only soft first story has inadequate earthquake resisting
capacity.

Figure 7. Second level screening (NBC+).

4. RETROFIT
4.1. The retrofit strategy for the soft first story
Many retrofitting mechanisms have been practiced to upgrade the strength and ductility of the vulnerable
buildings. Among them, column jacketing is widely used in Nepal in RC building. Whereas bracing,insertion of RC shear walls, steel plate jacketing, structural slit, FRP wrapping and base isolation are
popular in other countries. Retrofitting strategies for soft first story is different from the conventional
retrofitting of RC buildings. The main goal of soft first story retrofitting is to eliminate the extreme
stiffness difference, control excessive story drifts beyond the elastic limit, and increase seismic
performance behaviour of structure up to the desired level.

4The columns of the study
buildings are very slender, which
makes
retrofitting
even
more
challenging. Using the column
jacketing, which is a popular one in
Nepal, would not eliminate the extreme
stiffness difference. Other retrofitting
like FRP jacketing, steel platejacketing, bracing, base isolation needs
high-level technicians and is expensive
too. Under such circumstances, thewing wall retrofitting method is timeand cost-effective and does not hamper Figure 8. Plan and elevation of retrofitted NBC+ building.
the usage of the soft first story (Nakamura et al.). Moreover, it helps to eliminate stiffness difference,
control drift and helps to achieve the desired seismic performance of the building. Retrofitting with only
wing wall was not adequate to meet the strength demand, so the addition of shear wall in the proper
location was done.

Figure 9. 2nd level screening NBC building and NBC+ building.

Figure 10. Inter story drift (NBC+).

Figure 11. Performance of retrofitted buildings.

In retrofitting of NBC+ building only first story need retrofit whereas NBC building firstsecond and third story in the longitudinal direction and first and second story in the transverse directionneed a retrofit. The retrofit by wing wall and shear is easy to apply, provides more inner spaces. The
building gives better performance after conducting nonlinear static and dynamic analysis andrecommended by this study. In Figure 8 the retrofitting plan for first story of NBC+ building is shown.
In Figure 9, seismic index before (original Is) and after retrofit (Final Is) are presented. Is after retrofit
are within allowable limit. Improvement in capacity of spectral acceleration of both NBC and NBC+model building after retrofit is shown in Figure 11 which ensures the structural safety. The inter story

5drift after retrofit of NBC+ building is presented in Figure 10 which ensures the within the range ofimmediate occupancy of FEMA and drift limit of NBC 105.
4.2. Dynamic response history analysis by Gorkha earthquake 2015
Dynamic response time history analysis (DRTHA) by the input of 2015
Gorkha Earthquake strong ground motion recorded by DMG isconducted using STERA 3D software, a finite element method-based
software to check inter-story drift and damage pattern. The story drift ofNBC and NBC+ building is reduced significantly after retrofitting, which
are within the immediate occupancy of FEMA and drift criteria of NBC.
Though both NBC and NBC+ building does not have an adequate seismic
capacity from JBDPA evaluation, in DRTHA of Gorkha the building aresafe within life safety.
Both NBC and NBC+ building show moderate masonrycrack and some moderate hinges in beam and columns before retrofit Figure 12. Gorkha
whereas after retrofit no hinge formation occurred.
earthquake motion on NBC.
5. CONCLUSIONS AND RECOMMENDATIONS
The bare frame model of NBC and NBC+ building shows uniform story displacement. The soft firststory model of both NBC and NBC+ building after nonlinear static and dynamic evaluation, a sudden
increase in displacement in the soft first story is observed. This abrupt change of inter-story drift due tostiffness difference is the leading causes of soft-story damage. NBC+ model building show better
performance than NBC model building in terms of roof displacement, inter-story drift and maximum
shear capacity. By retrofitting with a combination of shear wall and wing wall eliminates the stiffness
difference and control excessive without altering the occupancy use of the first floor. Both NBC andNBC+ model building in this study survives with the moderate hinge in soft first story after time history
analysis of Gorkha earthquake 2015. The soft first story RC building collapsed during the 2015 Gorkha
earthquake were weak building constructed haphazardly adding multiple stories with low-strength
concrete in the structural members with inadequate longitudinal and shear reinforcement and poor
detailing.
Soil structure interaction is not incorporated in this research. It could be incorporated infuture research. Investigation of masonry unit can be done to find exact shear and ductility index ofbrick masonry.
ACKNOWLEDGEMENTS
This research was conducted during the individual study period of the training course “Seismology,Earthquake Engineering and Tsunami Disaster Mitigation” by the Building Research Institute, JICA,and GRIPS.I would like to express my sincere gratitude to the supervisors Haruhiko SUWADA, andcourse coordinator Tatsuya AZUHATA for their continuous support, valuable suggestion andinstruction during my study.
REFERENCES
FEMA-356, 2000, Federal Emergency Management Agency, Washington, USA.
Japan Building Disaster Prevention Association (JBDPA), 2001.
Nakamura A., Teshigawara M., Inuoue Y., Ohta T., 2012, 15th WCEE, Lisboa.

NBC (Nepal Building Code), 1994, Ministry of Housing and Physical Planning, Government of Nepal.
Saito T., 2019, STERA 3D, Version 10.2., http://www.rc.ace.tut.ac.jp/saito/software-e.html.
Seki M. 2017, 16th WCEE, Santiago, Chile, Paper No. 566.

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