Citation: Delgado, C., T. Azuhata (2020), Seismic retrofitting of an existing RC building in Mexico City
using hysteretic steel damper “ADAS”, Synopsis of IISEE-individual study report.

SEISMIC RETROFITTING OF AN EXISTING RC
BUILDING IN MEXICO CITY USING HYSTERETICSTEEL DAMPER “ADAS”
Carlos Hugo Delgado Rodriguez1

Supervisor:

Tatsuya Azuhata2

ABSTRACTOne of the most devastating earthquakes in the history of Mexico was the earthquake of 1985. The
seismic design code of Mexico City was urgently revised just after this earthquake. Furthermore, it wasmodified in 1987 and 2004, incorporating the earthquake damage analysis results and the latest
knowledge on earthquake engineering.
In 2017, the Puebla Earthquake occurred and struck Mexico City. About twenty buildings
collapsed, and more than 200 peoples died due to this earthquake. The survey results have found outthat most of the buildings that collapsed in this earthquake were designed based on the old seismic code
before 1985. Therefore, seismic performance evaluation and retrofit of these old buildings is still one of
the significant issues.
In this study, the author investigated the ADAS's potential as dampers to retrofit existing
reinforced concrete buildings. The ADAS, which means the "Added Damping and Stiffness," is one
type of steel hysteretic dampers. This ADAS damper yields in bending during an earthquake anddissipates seismic energy by its plastic deformation. The original developer devised its shape so thatplastic deformation occurs in a broader area without stress concentrating on a specific part.
The author took up one existing public office building and attempted to retrofit it using this ADAS. Hecarried out earthquake response history analyses and compared the building's dynamic behaviors before
and after retrofitting. The results cleared that we could reduce inter-story drift angles by inserting theADAS in the building.
Finally, the author considered a possibility of the damper's fatigue failure due to long-term
earthquake ground motions.
Keywords: Seismic retrofit, Hysteretic steel damper, Inter-story drift angle, floor response acceleration,
fatigue failure.

1. INTRODUCTION
On September 17, 2017, the Puebla Earthquake occurred in Puebla state of Central Mexico and caused
heavy seismic damages even in Mexico City, about 120 km away from the epicenter. About 20 buildings
collapsed, resulting in the death of more than 200 people.
After the well-known 1985 Mexico earthquake, they urgently revised the seismic code.
Moreover, they expanded their seismic observation network and worked hard to research the earthquake
ground motion characteristics, surface ground amplification, non-linear behavior of structures, etc.

12National Polytechnic Institute / InnovaSismo (IPN), Mexico, CDMX.
International Institute of Seismology and Earthquake Engineering, Building Research Institute.

1Based on these advanced research results, they furthermore revised the seismic design code in 1987 and2004.
As a result of these revisions, the seismic design force has been dramatically increased
compared to that before 1985. The zoning to evaluate surface ground application have become moredetailed. Also, design standards have been established to ensure sufficient ductility for structuralmembers. There can be a massive difference in seismic performance between buildings designed
according to these revised standards and old standards. They reported that most of the buildings
collapsing in the Puebla earthquake were such old ones. We can say that seismic performance evaluationand seismic retrofit of old existing buildings are remaining issues.
Based on this situational awareness, in this study, the author decided to examine a seismic
reinforcement method for existing reinforced concrete structures by inserting dampers. He used theADAS, one kind of the steel hysteretic dampers, for dissipating earthquake energy.

2. METHODOLOGY
2.1. Added Damping and Stiffness (ADAS)
The ADAS (Added Damping and Stiffness) is made up of a series of metal plates (generally ASTM
grade A-36 steel) in the shape of an “X”, connected in a way that has restriction at their ends. During anearthquake, its relative deformation must be close to that of the upper floor. As the upper part deforms
laterally by a force perpendicular to its plane, these plates are subjecting to shear forces; the sheer force
induces bending moments at the height of the plates. This bending occurs in the weak axis, giving adouble curvature of equal radius. Figure 1 shows the shape of the ADAS, the deformation, the moment’sdistribution, the plate’s width, and the curvature.
Making the plate’s width coincide with the moment’s distribution, as shown in this figure, canmake the steel plate yield in bending simultaneously along the height. At that time, we can suppose thecurvature as shown in the figure. We can derive the yield deformation and the strength capacity using
this curvature with material and sectional properties: Young's modulus, the yield point of steel, theplate’s height, width, and thickness.
This ADAS has the hysterical stability withstanding many cycles of the load without degrading.
The X-shaped section leads to a uniform deformation throughout their its height, providing good
resistance and deformation capacity for cyclic loads. By adjusting the number of plates and thedimensions of each plate, we can get required capacities of dampers relatively freely (Whittaker, 1989,Aguiar, Rodríguez, & Mora, 2016).

Figure 1. Moment and bending deformation of ADAS Steel Damper due to lateral force.

22.2. Target Building and Retrofitting Strategy
Figure 2 shows the plan of the building to be targeted in this study. Also, the author shows the locationsof the ADAS dampers for seismic retrofit in this figure. The original building was designed based on
the seismic code before 1985. Thus, we need to retrofit it. Figure 3 presents the conceptual strategy forthe retrofit. The structure is composed of one central part and two side parts. Each side has nine stories:
the central region has eleven stories. The author investigates the ADAS's potential for seismic retrofit
by comparing the building's dynamic behaviors before and after the retrofit.

Figure 2. Plan of target building and arrangement of ADAS steel dampers.

Figure 3. Retrofit Strategy using steel damper, ADAS.

3. RESULTS
3.1. Natural fundamental periods and static force-displacement relationships
In this study, the author inputs the ground motion record of the 1985 Mexico Earthquake to the building
model for response history analyses. Figure 4 shows the response acceleration spectrum of the input
ground motion and the fundamental natural periods of the model before and after the retrofit.
Figure 5 shows the relation between story drift angle and shear capacity for the original building modelcalculated by a push-over analysis. The author sets the retrofit design criterion for drift angle of eachstory as 0.015 to ensure the performance of “life-safety.” The yellow broken line shows this retrofit
design criterion in the Figure 5.
Figure 6 shows the capacity curves, which are the relations between the base shar and the roof(PH) displacement of the target structure before and after retrofit. The base shear is normalized by the3

toral weight and revealed as the spectral acceleration, Sa. We can see the retrofit’s effects in this figure.
The ADAS inserted in each story makes the Sa increase from 0.7 G to 0.8G in x-direction: from 0.4 G
to 0.5 G in the y-direction. These are the results against the monotonic loading calculated by static
analyses. Furthermore, we can expect the damping effects of the ADAS in the case of dynamic repetitiveloads.
Spectrum
accelerations
NS NS
Spectrum
accelerations
Response acceleration (cm/s/s)

700000700000Retrofitting (x)Original (x)Natural period (s)TxTy
Original
1.51
1.24
Retrofiting
0.81
0.745

60000060000050000050000040000040000030000030000020000020000010000010000000001213243

Natural period (s)545Capacity curve

Figure 4. Response acceleration0.9spectrum, and natural periods of target model
before and after retrofit.
1400

1200

1000

0.7
0.6

0.2

800

Qi/w

0.1
00.0000

600

4002F

PH_x3FPH_y

0.9

0.40.3

Capacity curve

1F

0.5

0.84F

Spectral Acceleration (Sa)Spectral Acceleration (Sa)0.8

PH_ADASX

0.75F

PH_ADASY

6F
0.67F

0.5

PH_x8F

0.49F
10.0000
10F
0.3

20.0000

30.000040.0000

PH_y50.0000

PH_ADASX

Spectral Displacement (Sd)11F

PH_ADASY

0.2

12F

0.1
fully operational

200

operational
00.0000life
safety000.005

0.01

0.015

0.02

0.025

pre collapse

0.03

10.0000

20.0000

30.0000

40.0000

50.0000

Spectral Displacement (Sd)Drift Ratio

Figure 5. Shear force - story drift angle(Before retrofit).

Figure 6. Capacity curves of target model before
and after retrofit.

3.2. Results of response history analyses
Figure 7 presents each story’s maximum story drift of the target model before and after the retrofit.
Before retrofitting, the story drift from the second to the ninth story exceeds the limit drift angle for“pre-collapse.” On the contrary, after retrofitting, all stories’ drift angles fall below this limit drift angle.
Some of them exceed the limit angle for “fully-operational”, but all of them fall below that for “lifesafety”, which is 0.015, in both x and y directions. Figure 8 and 9 respectively show the time history of
the shear deformation and the hysteresis loop of the ADAS arranged in the second story. The ADASrepeated oscillation a lot. We can say that the dampers effectively dissipated the earthquake energy due
to these cyclic behaviors and successfully reduced the earthquake responses of the target model.

4Deformation (cm)4Def. of ADAS

20-2-402040
Time (s)6080

Figure 8. Time history of ADAS’s
deformation.

Figure 7. The maximum story drift of target model
before and after retrofit.

Figure 9. Hysteresis loop of ADAS.

3.3. Discussion on a risk of fatigue failure
The latest seismic code of Mexico City was published in 2017. This year, the Puebla Earthquake
occurred. However, regardless of this earthquake, they had previously planned to revise the seismic code
to introduce the newest knowledge about seismology and earthquake engineering. This latest code
stipulates that they should consider two types of earthquakes to design building structures in Mexico
City. One is the subduction earthquake, and the other one is the intermediate-depth earthquake. We
generally think that the duration of a subduction earthquake is longer than the other one. The seismiccode of Mexico City supposes that the duration of the former one is 80 s: that of the latter one is 40 s.
When designing structural steel members against such a long-duration earthquake, we need to check if
there is a risk of fatigue failure.
We can predict the risk of fatigue failure for cyclic loading with constant amplitude by using
the S-N curve. For example, the following curve is used for the structural design of beam ends of highrise steel buildings in Japan.
  C  N f 1 3(1)where, : ductility factor, C: Coefficient derived from cyclic loading tests and Nf: the number of load’scycles when the structure fractures.
The coefficient C ranges from 2.0 to 10.0 for the beam ends of steel structures. To apply Eq,(1) to a random response due to earthquake ground motions, we generally use the well-known Miner’srule as shown below.

Dni
N fi

(2)

where, D: Failure index, ni: the number of load’s cycles in the specified deformation range and Nfi: Nffor the specified deformation range.
If the index D surpass 1.0, we should judge the structure will fracture. The author counts ni inEq. (2) from the results shown in Figure 8 by using the rain-flow method. Figure 10 shows the result ofcounting.
As long as the author knows, there is not sufficient test data to determine the coefficient C. If
we assume C = 43, the failure index D becomes 1.01, which is almost equal to 1.0. And this case almost

5meets the design criterion. This C value isrelatively high, comparing those of the beam
ends. However, we should consider thecharacteristics of ADAS again. It has an Xshape section, which makes the steel plate
yield through the height uniformly. Thisfeature brings stable performance against
cyclic loadings. We can reasonably assume
that the ADAS has a higher coefficient C. To
improve the ADAS dampers' reliability, theauthor considers that we need to acquire
their test data against cyclic loading more
and analyze them to reduce fatigue failure
risk.

Figure 10. Cycles counted by rein-flow method.

4. CONCLUSIONS
The author investigated the ADAS's potential as an energy dissipation device used to retrofit existingbuildings through the case study. The results of earthquake response analyses for the model retrofittedusing the ADAS presented their excellent performance to reduce displacement responses. We can also
expect stable performance even against cyclic loading with a long duration because of its mechanism
due to the X-shaped section. The author thinks that we need to acquire more test data and analyze themto demonstrate such a good performance for the cyclic loading by more quantitative evaluation scales
like the S-N curve.
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. I
want to express my sincere gratitude to Dr. Tatsuya Azuhata for supervising me and giving me beneficial
advice and suggestions during my study and also all the other staff members at IISEE/BRI for theirsupport and encouragement during this training program. Thanks, are also due to Dr. Matsutaro Seki forgiving me suggestions during my study. Further, I am thankful to Mr. Mauro Rangel Orihuela (MIDASLatin-American) for lending me a license from the MIDAS GEN program for the generation of mystructural model. I want to express my gratitude to JICA for giving me a chance to follow this course byfinancial supporting.
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