De La Salle University – Manila
Gokongwei College of Engineering
Civil Engineering Department
Graduate Studies Program
CASE STUDY
In Partial Fulfillment
Of the Course Requirements in
CIV575M – Earthquake Disaster Mitigation
Term 2 – Academic Year 2020 – 2021
PREPARED AND SUBMITTED BY:
LEGASPI, LAURICE JEAN D.
Contribution to the project: Accomplished Hazard Vulnerability Asset Framework, Time-History Analysis
of the original building, and Retrofitting Strategies.
LIM, CARL HADRIAN C.
Contribution to the project: Accomplished the Abstract, Introduction, Structural Model and Methodology,
and Pushover analysis of the original building.
NAVAL, JELEAN NADINE G.
Contribution to the project: Accomplished analysis of the Retrofitted Models, Comparison of Seismic
Performances, and Conclusion.
SEISMIC PERFORMANCE OF A BUILDING WITH WEAK FIRST
STORY IRREGULARITY
Laurice Jean D. Legaspi1, Carl Hadrian C. Lim1, Jelean Nadine G. Naval1
1
Civil Engineering Department, De La Salle University, Manila, Philippines
ABSTRACT:
The Philippines is one of the most populous and disaster-prone countries in the world. Numerous
structures have been built due to the rapidly growing population. Most of these structures are prone to
seismic hazards since the Philippines rests on the Pacific Ring of Fire, where the tectonic activities result
in almost 90% of earthquakes in the world. In Metro Manila, the looming threat of the West Valley Fault
challenges the seismic design of the structures. Building irregularities such as vertical and plan irregularities
make these structures vulnerable to hazards, especially earthquakes. Seismic retrofitting methods must be
adapted to eliminate the vulnerabilities. In the study, a five-story building with a weak first story was
utilized and two retrofitting methods were applied in order to determine its seismic response. Pushover and
time-history analyses were employed to determine the building’s performance under the vertical
irregularity. The results of the study may be used to determine the critical members of the building, which
may fail under seismic loads, and the effective retrofitting methods that can be applied to mitigate the
building vulnerabilities. The study has found that weak links and heavily stressed regions are found where
the weak story is located. In addition, the results show that the retrofitting through addition of chevron
bracing and cross-bracing may significantly improve the seismic capacity by increasing the lateral
resistance of the building.
KEYWORDS: Earthquake, Hazards, Retrofitting, Vulnerability, Weak Story
INTRODUCTION
Several earthquakes in the Philippines have already demonstrated critical damages to many
structures. One type of extremely vulnerable structure to seismic activity is a building with vertical
irregularity, specifically, a weak-story building. These weak-story buildings exhibit lower lateral resistance
in one floor, typically the ground floor, compared to other floors (Guevarra-Perez, 2012). In the event of
seismic activity, the inadequate lateral resistance in the floor level may cause major damage to the building
due to its inability to resist ground motion loads such as lateral, vertical, and moments. These types of
vertical irregularities are commonly seen in buildings where: the ground floor or intermediate floors have
fewer columns or shear walls than other floors, one floor has a greater height than other floors, and one
floor has less seismic-resistant elements compared to other floors. According to the National Structural
Code of the Philippines (NSCP 2015), a weak-story irregularity occurs when the story strength is less than
80% of that in the story above. Weak-story is often confused with soft-story. Their main difference
is that weak-story exists when there is weakness in lateral resistance, while soft-story exists when
the lateral stiffness of a story is lower than the other stories. These two irregularities can exist
together in a building and cause damages (Guevarra-Perez, 2012).
Several examples of weak-story buildings in the Philippines that failed during an earthquake can
be seen below. In Figures 1 and 2, during the 1990 Luzon Earthquake in Baguio City, the first two stories
of the FRB Hotel and the ground floor of the Hotel Nevada collapsed. Sharpe (1994) stated that the weak
first-story buildings in Baguio City failed due to the first story being taller than the others and having limited
seismic resistance. Sharpe also noted that the upper stories of the buildings had minimal damage.
Fig. 1 & 2. The collapse of FRB Hotel and Nevada Hotel
In addition, weak-story buildings are also common in other countries. During the 2011 Van
Earthquake in Turkey, several apartment buildings collapsed due to weak-story irregularity. According to
Erken et al. (2013), the ground floor story height of the building was more than 50% taller than the upper
stories. In Figure 4, the Imperial County Services Building suffered ground floor column failures during
the 1979 California Earthquake. The columns failed due to weak-story irregularity caused by discontinuous
shear walls (Bertero, 1997).
Fig. 3. Column failures of Imperial County Services Building
The objective of this case study is to present and discuss the seismic response of the building when
subjected to earthquake loads through Push-Over and Time History analysis. Two seismic retrofitting
methods will be applied to the building. This study will then compare the base shear, story displacement,
and story drift of the buildings. The seismic response and performance of the original and retrofitted
buildings will be discussed and analyzed. From this study, the failure of the structural members of the
building due the vertical irregularity will be identified and the various effective retrofitting methods will be
determined.
HAZARD-VULNERABILITY-ASSET FRAMEWORK
Risks happen when vulnerable buildings are subjected to hazards such as being in high seismicity
areas. This is critical for multi-story buildings that have irregularity in their structure. The weak-story
irregularity being a type of vertical irregularity is a building vulnerability that could cause extensive
destruction and disaster during a hazard, specifically earthquake. Weak-story irregularities could lead to
the collapse of the building, injuries, and loss of lives. Although disasters cannot be prevented, they can be
mitigated and reduced. By identifying the potential vulnerabilities of the building and hazards on its
location, disaster risks could be lessened. Disaster Risk Reduction can be applied to buildings with weakstory irregularities. One way of mitigating the disaster risk is reducing the vulnerability of the building.
This can be done by retrofitting and strengthening its structural members or adding lateral resistant
elements.
STRUCTURAL MODEL
The structural model used in this study is a 5-story Reinforced Concrete Building with weak first
story irregularity. It is intended to be used as a typical commercial building in Metro Manila. Using STERA
3D, a three-dimensional (3D) representation of the building has been created. The building is rectangular
with a total length of 32m and a total width of 16m. The bottom story has a height of 5m, while the upper
story heights are 3m. The total height of the building is 17m. STERA 3D-Model, floor plans, and elevation
of the building are shown in Figures 5-8. Furthermore, the bottom story has taller and fewer columns
compared to the upper stories to simulate a weak-story irregularity.
Fig. 4. 3D Model of the Building
Fig. 5 & 6. Ground floor and typical upper floor plan of the building
Fig. 7 & 8. Front and side elevation of the building
Material properties of structural members of the building are recommended properties used in the
design and construction of low-rise to mid-rise buildings in Metro Manila. The compressive strength of
concrete is 3000 psi (20.7 MPa), the tensile strength of the main reinforcements bars is 60 ksi (414 MPa),
while the ties and stirrups are 40 ksi (275 MPa). The Basic Seismic-Force Resisting System of the building
is a Special Moment Resisting Frame composed of reinforced beam and column system. Section dimensions
and reinforcements are shown in Figures 10 and 11.
Fig. 9 & 10. Beam and column dimensions
Design loadings are in line with the National Structural Code of the Philippines, NSCP 2015 7 th
edition. Loads used in the structural model can be seen in the table below. The total weight per floor was
determined by multiplying the design loads and floor areas.
Table 1. Design loads
METHODOLOGY
Push-Over analysis and Time-History analysis were performed on the building. The earthquake
data used for the analyses is El Centro Earthquake (1940). This earthquake has similar seismic
characteristics to past earthquakes in Luzon and Metro Manila. Hence, it was chosen as data for the
analyses. STERA 3D was used in the analysis of the building. It is software used for seismic analysis of
various structures such as reinforced concrete, steel, steel-reinforced concrete, seismic isolation, and
response control.
The study was performed in two stages, the analysis of the original model and a retrofitted model.
In the first stage, the seismic response was calculated using the existing structural elements of the building.
Moreover, potential weaknesses of the irregular building were identified. Meanwhile, a retrofitted and
strengthened building through the addition of bracings was used for the second stage. The results and
outputs of the two stages were compared and discussed.
SEISMIC PERFORMANCE
Push-Over Analysis
As mentioned earlier, push-over analysis was performed on the structural model. Since the
push-over analysis represents a static nonlinear analysis through the incremental increase of the
magnitude of the lateral force, the weak links and model failures can be identified. The structural
model will be pushed to its limits until a collapse or failure mechanism is developed (Naphade &
Patil, 2015). A maximum drift ratio of 1/50 was used in X and Y directions for the pushover
analysis. The lateral force distribution used was UBC as per NSCP 2015. The results of the analysis
show that the heavily stressed region and possible collapse are found at the bottom story of the
building. The strength of the ground floor to resist the story drift is drastically lower compared to
the upper floors as seen in the graph below. The drift-shear relation curve shows the relationship
between the story drift and story shear coefficient.
Fig. 11 & 12. Drift-Shear relation curve in X and Y-direction
Members with ductility factors greater than five are exclusively found on the ground floor. A
ductility factor of one to five indicates light to moderate failure, while members with a ductility of greater
than five indicate severe and critical failure. Ductility of the structural elements is the basis of the damages
on the structural elements in STERA 3D, therefore, the results of the push-over analysis on the building
revealed that due to the weak first story irregularity, critical failures on members may occur on the ground
floor. The ductility of critical members can be seen in the figure below.
Fig. 13 & 14. Member response in X and Y-direction
The push-over curve can be generated by plotting the Base Shear and Top Displacement of the
building. The curve represents the capacity of the structure to resist lateral and seismic forces. According
to Dya and Oreta (2015), the building encounters different performance levels during the analysis. The
damage incurred by the building becomes severe as the displacement increases. The push-over curve is
shown in Figure 15, while performance levels on the push-over curve can be seen in Figure 16. For the xdirection, the maximum value of top displacement is 34 cm while the max base shear is 4667 kN. For the
y-direction, the maximum top displacement is 34 cm while the max base shear is 3945 kN.
Fig. 15 & 16. Push-Over curve of the building and performance levels
TIME-HISTORY ANALYSIS
Using El Centro Earthquake, the time-history analysis of the irregular building was performed.
Figure 18 shows the input ground acceleration of the El Centro Earthquake and Figure 19 shows its input
ground displacement. The ductility factor starts at a lower value but increases as the period progress.
Medium damages under ductility factor between 1 to 5 can be observed to be larger in lower and lower
levels on upper levels.
Fig. 17. Input ground acceleration
Fig. 18. Input ground displacement
The base shear and top displacement can be plotted as shown in Figure 19. For the xdirection, the maximum value of top displacement is 11.53 cm while the max base shear is 3333
kN. For the y-direction, the maximum top displacement is 10.59 cm while the max base shear is
2971 kN.
Fig 19. Base Shear vs Top Displacement of Original Building
SEISMIC RETROFITTING STRATEGIES
Seismic retrofitting strategies reduce the risks caused by hazards. It is required for vulnerable
buildings that are susceptible to damage or destruction by seismic forces. Retrofitting increases the
structural strength and ductility of the building as well as its resistance to any seismic or geotechnical
hazards due to earthquakes. Seismic retrofitting strategies can be classified as global and local. Global
methods focus on the whole structure while local methods focus on the individual elements of the structure.
Global methods such as the addition of shear walls, infill walls, and bracing help weak story irregularity.
The Local Methods include Concrete Encasement, Steel Jacketing, and Fiber Overlays.
Fig. 20. Sample of Global Methods of Retrofitting
Fig. 21. Sample of Local Methods of Retrofitting
It has previously discussed the different effects of the various retrofitting methods listed in Figure
20. In previous studies, Cross-brace and Chevron have a comparative performance. Hence this will be
investigated in this case study using the STERA 3D. Only the most common types will be applied: crossbracing and chevron. This is Type 3 and Type 4 under the steel brace choices of the STERA 3D Software.
The layout of the retrofit is the same for the two types to simplify the comparison. The floor plan is shown
in figure 22.
Fig. 22. Floor Plan showing the location of braces
The brace is made of W200X15 with a material grade of A992. A small cross-section is applied to
ensure that its self-weight is negligible, and the overall capacity is highly dependent on the large capacity
of the bracing. The bracings are placed on the inner spans of the structure to reinforce the members with
the least resistance. It has also been applied on the succeeding floors to reduce the chance of having a weak
story throughout the structure. Masonry Walls and Diaphragms are not included in the model to see the
overall effect of the retrofitting applied.
COMPARISON OF SEISMIC PERFORMANCE OF RETROFITTED/ORIGINAL BUILDINGS
Fig. 23. Shear-Drift Relationship of the ground floor
The Shear-drift relationship of the ground floor shows that both retrofitting strategies are effective
in eliminating the weak story of the structure. Not only did they increase the floor stiffness and reduce the
floor deflection. Based on the values presented on the drift-shear relation graph, the chevron utilizes a
higher story shear over the cross-brace to meet the target drift of 1/50.
As for the overall performance of the structure, the retrofitting strategies change the location and
number of plasticized elements/joints. It did not eliminate the structural member having a ductility factor
of more than 5 but it reduces the elements with the same failure clustered at the ground floor of the original
model. Another observation is the number of elements/joints with ductility factors within the range of 1 to
5 was increased and are found on the upper floors. This is expected as the retrofitting equally distributes
the load on the members and allows most of the joints to plasticize before failure.
Fig. 24. Ductility of the Retrofitted structures with shear acting on X and Y-axis
In terms of story drift, both strategies improved the ground floor’s drift value compared to its upper
floors. However, the story drift of the ground floor retrofitted with chevron did not meet the 1/50 drift limit.
This can easily be solved by increasing the cross-section of the brace.
Another that can be found in the story drift is evenly distributed on each story eliminating the weak
story irregularity at the ground floor. However, since the weight of the bracing has not been added to the
model, the overall drift of the building remains at 34cm. Therefore, the increase of stiffness does not
correlate with the reduction of story drift. In terms of the effectiveness of distributing the story drift, CrossBrace is better than Chevron.
Fig. 25. Story Drift of the Ground Floor
As for the base shear in the Cross-Bracing model, the maximum value of base shear in the xdirection is 13740 kN, while for the y-direction, the maximum base shear is 20250 kN. For the base shear
in the Chevron model, the maximum value of base shear in the x-direction is 12380 kN. For the y-direction,
the maximum base shear is 16010 kN. The push-over curves for the retrofitted models are shown in Figures
27 and 28
Fig. 26: Comparison of the Base Shear and Top displacement for the Pushover Analysis
Fig. 27 & 28. Pushover curves for the retrofitted building with Chevron Bracing (left) and
Cross Bracing (right)
Time-History was also conducted to the two retrofit methods. Same accelerograms from the El
Centro earthquake of 1940 were used.
Fig. 29 & 30. Input ground acceleration for Chevron (right) and Cross Bracing (left)
For the Chevron method, the base shear and top displacement are plotted as shown in Figure 31.
For the x-direction, the maximum value of top displacement is 7.59 cm while the max base shear is 6435
kN. For the y-direction, the maximum top displacement is 9.64 cm while the max base shear is 10050 kN.
Fig. 31. Base Shear vs Top Displacement of Retrofitted Building using Chevron method
On the other hand, for the cross-brace method, the base shear and top displacement are
plotted as shown in Figure 32. For the x-direction, the maximum value of top displacement is 6.95
cm while the max base shear is 6258 kN. For the y-direction, the maximum top displacement is
8.63 cm while the max base shear is 8834 kN.
Fig. 32. Base Shear vs Top Displacement of Retrofitted Building using Cross-Brace method
Fig. 33. Comparison of the Base Shear and Top displacement of the three (3) models
Values for top displacements were lower in the retrofitted buildings. It can also be observed that
the base shear values from the retrofitted buildings are 1.8-3 times higher than the original building. It
shows that the buildings under retrofitting are much stiffer due to the addition of bracing and therefore give
lesser displacements. It is observed that under earthquake response, the cross-brace method gives lesser
displacements compared to the chevron method.
CONCLUSION
Both retrofitting has its own merits and setbacks. The chevron governs in increasing the stiffness
of the ground floor while cross-bracing evenly distributes the drift on each floor. Although the increase in
stiffness often results in lesser drift, Chevron is attached to the beam and not to the column. This allows
deformation to happen on the beam-column joint which cross-brace directly resists.
There are some improvements that can be done in this study. One of which is the configuration of
the bracing applied. The brace applied is a result of the limitation of the software to provide separate and
pre-defined bracing per beam span. Cross brace could have been a brace for two spans. Another is to explore
the effect of the placement of the brace in different locations since this study is only limited to two
retrofitting models.
It is recommended to take further studies on the following: Explore the effects of different steel
bracing configuration on other irregularities, comparison of steel bracing and concrete bracing with the
same configuration, and explore the effects of a steel column at the unsupported side of the bracing.
REFERENCES
Bertero, V. (1997) Distribution of Mass, Stiffness & Strength, Structural Engineering Slide Library, W.
G. Godden Editor. Set J: Earthquake Engineering. NISEE On-line, U.C. Berkeley.
Dya, A. & Oreta, A. (2015). Seismic Vulnerability Assessment of Soft Story Irregular Buildings
using Pushover Analysis. EACEF-5: Surabaya East, Java, Indonesia.
Erken, A., Taskin, B., Tugsal, U., & Sezen, A. (2013). The Aftermath of 2011 Van Earthquakes:
Evaluation of Strong Motion, and Geotechnical and Structural Issues. Bulletin of Earthquake
Engineering, Vol. 11, 285-312.
Guevarra-Perez, L. (2012). “Soft Story” and “Weak Story” in Earthquake Resistant Design: A
Multidisciplinary Approach. 15 WCEE: Lisboa, Portugal.
Naphade, A. & Patil, G. (2015). Pushover Analysis of RCC Buildings with Soft Storey at Different
Levels. IOSR Journal of Mechanical and Civil Engineering, 100-108.
(2015) National Structural Code of the Philippines, Volume 1. Association of Structural Engineers of
the Philipines (ASEP), Manila.
Sharpe, R. (1994). July 16, 1990 Luzon (Philippines) Earthquake. Earthquake Engineering, 10th World
Conference: Balkema, Roterdam, Netherlands.
ABOUT THE AUTHORS
Laurice Jean D. Legaspi is currently working as a Structural Engineer in TCGI Engineers. She can be
contacted through his email address: lauricejeanlegaspi@gmail.com
Carl Hadrian C. Lim is currently working as a Structural Engineer in D.A. Domingo Engineering
Consultant. He can be contacted through his email address: limhadrian@gmail.com
Jelean Nadine G. Naval is currently working as a Structural Engineer in Kin Long Industrial Philippines.
She can be contacted through her email address: jelean.nadine@gmail.com
ACKNOWLEDGEMENT
The authors would like to express their gratitude to Dr. Andres Winston Oreta for his lectures and
guidance in the course. Additional thanks to Engr. Revin Berces for his tutorial on the modeling and analysis
using STERA 3D software.
Appendix A : Original Method -STERA 3D Results
At Ground Floor
At Upper Floors
Floor Plan
Appendix A : Original Method -STERA 3D Results
At X-Axis
At Y-Axis
Pushover Analysis Results
Appendix A : Original Method -STERA 3D Results
Input Ground Acceleration
Input Ground Displacement
Time History Analysis Results
Appendix A : Original Method -STERA 3D Results
Top Building Displacement
Base Shear Top Drift
Time History Analysis Results
Appendix A : Original Method -STERA 3D Results
At X-Axis
At Y-Axis
max_structure.txt
Appendix B : Cross-Brace - STERA 3D Results
At Ground Floor
At Upper Floors
Floor Plan
Appendix B : Cross-Brace - STERA 3D Results
At X-Axis
At Y-Axis
Pushover Analysis Results
Appendix B : Cross-Brace - STERA 3D Results
Input Ground Acceleration
Input Ground Displacement
Time History Analysis Results
Appendix B : Cross-Brace - STERA 3D Results
Top Building Displacement
Base Shear Top Drift
Time History Analysis Results
Appendix B : Cross-Brace - STERA 3D Results
W1 Brace Properties
Brace Property
Brace Properties
W2 Brace Properties
Appendix B : Cross-Brace - STERA 3D Results
At X-Axis
At Y-Axis
max_structure.txt
Appendix C : Chevron - STERA 3D Results
At Ground Floor
At Upper Floors
Floor Plan
Appendix C : Chevron - STERA 3D Results
At X-Axis
At Y-Axis
Pushover Analysis Results
Appendix C : Chevron - STERA 3D Results
Input Ground Acceleration
Input Ground Displacement
Time History Analysis Results
Appendix C : Chevron - STERA 3D Results
Top Building Displacement
Base Shear Top Drift
Time History Analysis Results
Appendix C : Chevron - STERA 3D Results
W1 Brace Properties
Brace Property
Brace Properties
W2 Brace Properties
Appendix C : Chevron - STERA 3D Results
At X-Axis
At Y-Axis
max_structure.txt