SEISMIC EVALUATION AND RETROFITTING ...

SEISMIC EVALUATION AND RETROFITTING TECHNIQUE IMPLEMENTED TO A THREE STORY SCHOOL BUILDING OF RC FRAME STRUCTURE IN NICARAGUA A Master’s Thesis Submitted in Partial Fulfillment of the Requirement for the Master’s Degree in Disaster Management

By Rommel David ZAMBRANA AREAS (MEE15617) August 2016

Disaster Management Policy Program Seismology, Earthquake Engineering and Disaster-Recovery Management Policy & Tsunami Disaster Mitigation Course (2015-2016) Earthquake Engineering Course National Graduate Institute for Policy Studies (GRIPS), Tokyo, Japan International Institute of Seismology and Earthquake Engineering (IISEE), Building Research Institute (BRI), Tsukuba, Japan

ACKNOWLEDGEMENTS

I would like to express my sincere gratitude to M. Inukai, professor at the International Institute of Seismology and Earthquake Engineering (IISEE/BRI), for the time that he has shared his knowledge and given me the guidelines to follow, advice and suggestions, in order to improve the outcomes of my study and achieve my goals, being an important person during my individual study. Also, I’d like to express special gratitude to the Staff of teachers of the Earthquake Engineering course, as they provided us new knowledge and experiences, chiefly to Dr.T.Azuhata for his constructive comments and advice during the period of this work. I am extremely thankful to the staff members at IISEE/BRI for their support and encouragement during this training program. At the same time I am feeling so much grateful to the Japan International Cooperation Agency (JICA), for giving me the chance to participate in this academic program, providing me with the financial support and good conditions to perform this study. I would like to express my gratitude to my family for their love, patience and support that they gave me in this period outside my country.

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ABSTRACT

In Nicaragua the seismic evaluation and rehabilitation of reinforced concrete building are important, The Seismic Code RCN - 07 does not include detailed guideline to evaluate structures, for this reason the Japanese method of evaluation is used to determinate the seismic indices of the structure. As a case of study, the National Technological Institute is evaluated to understand the Japanese Seismic Evaluation Standard. The first and second level screening methods are applied to find the seismic capacity of the structure. The results demonstrate the structure in the longitudinal direction is weak against laterals loads and the columns do not have enough strength. The seismic rehabilitation of the structure is implemented in longitudinal direction of the building. A Steels braces system is selected as a retrofit technique and provide the necessary strength to the structure. Using the software Stera_3D the behavior of the structure before and after the rehabilitation are analyzed. This study helps to understand the seismic evaluation, which is a very useful tool to protect the schools buildings in the country. Future prospects are to incorporate a Seismic Evaluation Guideline to the seismic code and analyze more RC structures in Nicaragua.

Keywords:

Seismic Evaluation,

Rehabilitation, Retrofit

Technique,

Stera_3D, Seismic Code

The author works for the National Autonomous University of Nicaragua. UNAN-Managua, Campus Ruben Dario, Faculty of science and Engineering, Building Department.

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TABLE OF CONTENTS

ACKNOWLEDGEMENTS……………………………………………………………………………...i ABSTRACT……………………………………………………………………………….……………ii TABLE OF CONTENTS………………………………………………………………………………iii LIST OF FIGURES……………………………………………………………………….……………vi LIST OF TABLES……………………………………………………………………………..………vii LIST OF ABBREVIATIONS…………………………………………………………………………..ix 1.

INTRODUCTION .................................................................................................................... 10 1.1. Objective ............................................................................................................................... 11 1.1.1 General objective

11

1.1.2 Specific objective

11

1.2. Background ........................................................................................................................... 11 1.2.1. Geographical location of Nicaragua

11

1.2.2. Seismicity of Nicaragua

12

1.3. Earthquakes in Nicaragua ...................................................................................................... 13 1.3.1. Managua Earthquake 1931

13

1.3.2. Managua Earthquake 1972

15

1.4. Schools building in Nicaragua ............................................................................................... 17 1.5. National Building Regulation RCN- 2007, Nicaragua ............................................................ 18 1.5.1. Maximum distortion allowed 2.

20

METHODOLOGY ................................................................................................................... 21 2.1 Seismic evaluation process ..................................................................................................... 21 2.2 Methodological scheme .......................................................................................................... 21

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3.

SEISMIC EVALUATION OF EXISTING RC SCHOOL BUILDING. ..................................... 22 3.1 Target building: National Technological Institute.................................................................... 22 3.2 Preliminary calculation ........................................................................................................... 25 3.2.1 Structural weight and sustained force by columns

25

3.2.2 Story-shear modification factor for Eo.

26

3.3 First level screening method: Evaluation of longitudinal direction X ....................................... 26 3.3.1 Vertical elements categorization and shear stress at ultimate state

27

3.3.2 Strength index C

27

3.3.3 Basic seismic index of structure Eo .

28

3.3.4 Evaluation of seismic index of structure Is (longitudinal direction X)

29

3.4 First level screening method evaluation transversal direction .................................................. 30 3.4.1 Vertical element categorization and shear stress at ultimate state – columns and walls members.

30

3.4.2 Basic seismic index of structure Eo .

31

3.4.3 Evaluation of seismic index of structure Is (transversal direction y).

31

3.5 Final result of the first level screening in both directions. ........................................................ 32 3.6 Second level screening method ............................................................................................... 32

4.

3.6.1 Members’ strengths

33

3.6.2. The ultimate flexural strength

33

3.6.3. The ultimate shear strength

33

3.6.4 Ductility index F

34

3.6.5 Basic seismic index of structure Eo

35

3.6.6 Seismic index of structure Is

37

SEISMIC REHABILITATION DESIGN OF RC SCHOOL BUILDING .................................. 40 4.1 Strengthening first level screening. ......................................................................................... 40

4

4.1.1 Target performance and required strength

40

4.1.2 Strengthening using column jacketing

40

4.2 Strengthening second level screening. ..................................................................................... 41 4.2.1 Target performance and required strength

41

4.2.2 Strengthening using steel braces system

41

4.2.3 Basic index of the structure

43

5.

ANALYSIS RESPONSE STRUCTURE WITH STERA 3D SOFTWARE ............................... 46

6.

CONCLUSION ........................................................................................................................ 52

7.

RECOMMENDATION ............................................................................................................ 53

8.

ACTION PLAN ....................................................................................................................... 54

APPENDICES…………………………………………………………………...…………………….43 REFERENCES…………………………………………..…………………………………………….51

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LIST OF FIGURES Figure 1. Geographical location of Nicaragua. .................................................................................. 12 Figure 2. Tectonics plate and seismicity in Nicaragua, 2015. ............................................................ 13 Figure 3. National palace, Earthquake 1931, (http://www.skyscrapercity.com/). ............................... 14 Figure 4. Cracks and landslides on the roads, Managua, March 31, 1931. ......................................... 14 Figure 5. Damages to building in the capital, Earthquake 1972. ........................................................ 15 Figure 6. Hospital for The Retirement damaged by The Earthquake, 1972. ....................................... 16 Figure 7. Damage by strong seismic shaking to RC Buildings in the Earthquake 1972. ..................... 16 Figure 8. Prestige RC School built in Managua, Left Loyola Institute, and right Calasanz Institute. .. 17 Figure 9. National Regulation of Construction. ................................................................................. 18 Figure 10. Micro zone – mapping, RNC-07. ..................................................................................... 19 Figure 11. Methodological scheme for seismic evaluation. ............................................................... 21 Figure 12. Actual Conditions of the National Technological Institute................................................ 22 Figure 13. View from the second floor of the building. ..................................................................... 22 Figure 14. Studied frame area of the building. .................................................................................. 23 Figure 15. Result of seismic index of structure Is, longitudinal direction X. ...................................... 29 Figure 16. Result of seismic index of structure Is, transversal direction Y. ........................................ 31 Figure 17. Comparison of final results of seismic evaluation in both direction. ................................. 32 Figure 18. Result seismic evaluation second level screening, Is index. .............................................. 39 Figure 19. Retrofit techniques, Japan, summarized by S. Sugano. ..................................................... 42 Figure 20. Design and Strength of framed brace system. ................................................................... 42 Figure 21. Comparison of Is before and after retrofit, JBDPA & RNC-07 seismic index. .................. 45 Figure 22. Push over analysis in longitudinal direction before retrofit. .............................................. 46 Figure 23. Push over analysis in longitudinal direction after retrofit. ................................................. 47 Figure 24. Maximum displacement before and after retrofit. ............................................................. 47 Figure 25. Shear force ratio before and after retrofit. ........................................................................ 48 Figure 26. Capacity curve of the building before and after retrofit. ................................................... 48 Figure 27. Drift ratio angle before and after retrofit. ......................................................................... 49 Figure 28. Performance point of the Structure before and after retrofit. ............................................. 50 Figure 29. Push over analysis before retrofit, target drift 1/50. .......................................................... 51 Figure 30. Push over analysis after retrofit, target drift 1/50. ............................................................. 51

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LIST OF TABLES Table 1. Structure categorization Nicaragua RNC-07........................................................................ 20 Table 2. Maximum distortion allowed RNC-07. ............................................................................... 20 Table 3. General information of the building. ................................................................................... 23 Table 4. Members list – columns. ..................................................................................................... 24 Table 5. Members list – beams. ........................................................................................................ 24 Table 6. Members list – wall. ........................................................................................................... 24 Table 7. Verticals members. ............................................................................................................. 25 Table 8. Calculation of structural weights of the structure. ................................................................ 25 Table 9. Calculation of sustaining force of the structure. ................................................................... 26 Table 10. Story – modification factors. ............................................................................................. 26 Table 11. Classification of vertical members. ................................................................................... 27 Table 12. Vertical elements and shear stress at ultimate. ................................................................... 27 Table 13. Strength index C. .............................................................................................................. 28 Table 14. Basic index of structure E0 in longitudinal direction X. ..................................................... 29 Table 15. Classification verticals members – column. ....................................................................... 30 Table 16. Classification of vertical members – wall. ......................................................................... 30 Table 17. Basic index of the structure Eo transversal direction Y. ..................................................... 31 Table 18. Calculation strength members and failure mode. ............................................................... 34 Table 19. Ductility index F (Hoop spacing 200mm).......................................................................... 34 Table 20. Effective strength factor (hoop 200mm). ........................................................................... 35 Table 21. Result of basic seismic index E0, Eq. (11). ........................................................................ 36 Table 22. Result of basic index E0, Eq. (10). .................................................................................... 37 Table 23. CTU indices. .................................................................................................................... 38 Table 24. Is index of the structure..................................................................................................... 38 Table 25. Target performant and required strength............................................................................ 40 Table 26. Strengthening using column jacketing. .............................................................................. 40 Table 27. Result of new section of columns. ..................................................................................... 41 Table 28. Required strength using steel brace system........................................................................ 41 Table 29. Strength of framed brace system QBU. ............................................................................. 43 Table 30. Total numbers of brace for the building............................................................................. 43 Table 31. Strength of brace system. .................................................................................................. 44

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Table 32. Strength of existing columns. ............................................................................................ 44 Table 33. The seismic index of building after retrofit. ....................................................................... 44

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LIST OF ABBREVIATIONS UNAN

National Autonomous University of Nicaragua\

IISEE

International Institute of Seismology and Earthquake Engineering

BRI

Building Research Institute

JICA

Japan International Cooperation Agency

INETER

Nicaraguan Institute of Territorials Studies

MAT

Middle American Trench

RCN07

National Regulation of Construction Nicaragua 2007

JBDPA

Japan Building Disaster Prevention Association

STERA_3D Structural Earthquake Response Analysis 3D RC

Reinforced Concrete Building

Is

Seismic Index of Structure

Iso

Seismic Demand index of structure

IN

Seismic Index Non-Structural Elements

Eo

Basic Seismic Index of Structure

F

Ductility Index

C

Story-Shear Modification Factor

Ct

Cumulative Strength Index

ho/D

Column Clear Height

SD

Irregularity Index

D

Column Depth

Z

Zone Index

G

Ground Index

U

Usage Index

Ctu

Ultimate Cumulative Strength Index

B

Construction Index

f

Conformability Index

t

Deterioration Index

c

Risk Reduction Index

e

Location Index

W

Weight Index

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1. INTRODUCTION Nicaragua is a country with high level of seismicity due to the collision of Caribbean and Cocos plates, also by the active faults in the country, principally in all the pacific region and the capital Managua. The present research is regarding the seismic evaluation to a school building made by Reinforced Concrete structure, historically, in Nicaragua RC concrete and Concrete Block with RC Frame structures has been implemented by private companies for buildings of the capital, which has been common since 50 years ago. After the 1972 Managua Earthquake, however, those structures suffered several damages, especially school and hospital buildings. The main objective is elaborate a seismic evaluation and retrofit to an existing reinforced concrete building, the National Technological Institute, a school building with three stories located in Granada, Nicaragua. In the country all the school building are categorized as important structures, one of the characteristic those buildings have it is the structure has been suffered deterioration and the design was made by old seismic code implemented by internationals engineers, nowadays the actual regulations have been upgrading and new requirements are include in the national seismic code. Therefore the research in the field of earthquake engineering focuses on the seismic evaluation and rehabilitation to existing RC buildings, provide solutions for the development of new methods to perform seismic index of structures, the implementation of retrofit techniques help the building to be safer and more effective to resist the seismic force. Part of the theoretical framework this study will be realized following the Technical Manual for Seismic Evaluation and Seismic Retrofit of Existing Reinforced Concrete Buildings, 2001. Nicaragua does not have a Seismic guidelines to evaluate existing building, this tools will be very useful to elaborate this investigation and provide structural solutions to the target building. Earthquake Engineering investigates intense earthquakes possible to cause damages to manmade structures and studies the earthquakes´ effects, and the way to mitigate or reduce their destructiveness, with the aim of protecting lives and property.

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1.1. Objective 1.1.1 General objective To elaborate a seismic evaluation and retrofit a reinforced concrete building, the National Technological Institute, a school building with three stories located in Granada, Nicaragua. 1.1.2 Specific objective •

Analyze theoretical concepts of seismic evaluation, standards and criteria of retrofitting to improve the school building.

•

Calculate the actual Is of the RC building structure to verify if the school building is strong in different directions against earthquakes or lateral loads.

•

Determine the specific retrofit technique to upgrade the strength of the RC building structure, using The Guidelines of Seismic Retrofit of existing reinforced concrete buildings, 2001.

•

Elaborate the structural analysis of the RC building to know the behavior before and after the retrofit technique, and indicate the capacity of the school building Structure using the software Stera_3D developed by Dr. T. Saito

1.2. Background 1.2.1. Geographical location of Nicaragua Nicaragua is a country in Central America (Fig. 1), bordered by both the Caribbean Sea and the North Pacific Ocean, between Costa Rica and Honduras. It covers a total area of 130,370 square kilometers (119,990 square kilometers of which is land area). The population of Nicaragua is 6,100,000 and the most populous city in the country is the capital city, Managua, with a population of 1.2 million. Nicaragua occupies a landmass of 130,967 km2. Nicaragua has three distinct geographical regions: the Pacific lowlands - fertile valleys which the Spanish colonists settled, the Amerrisque Mountains (Northcentral highlands), and the Mosquito Coast (Atlantic lowlands/Caribbean lowlands). The low plains of the Atlantic Coast are 97 km (60 mi) wide in areas. They have long been exploited for their natural resources. On the Pacific side of Nicaragua are the two largest fresh water lakes in Central America— Lake Managua and Lake Nicaragua. Surrounding these lakes and extending to their northwest along the rift valley of the Gulf of Fonseca are fertile lowland plains, with soil highly enriched by ash from nearby volcanoes of the central highlands. Nicaragua's abundance of biologically significant and unique ecosystems contribute to Mesoamerica's designation as a biodiversity hotspot. Nearly one fifth

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of Nicaragua is designated as protected areas like national parks, nature reserves, and biological reserves.

Figure 1. Geographical location of Nicaragua. (https://www.google.com.ni/maps). 1.2.2. Seismicity of Nicaragua Nicaragua is located in a tectonically and seismicity active zone, on the western boundary of the Caribbean plate, where the Cocos plate subduction beneath the Cocos plate forms a trench, which is part of a large geological feature named the Middle American Trench (MAT). The main seismic activity is generated by the collision of the Cocos and Caribbean plates (DeMets et al. 1994). The earthquakes in Nicaragua always have presented events in shallow and intermediate depths. There are inland earthquakes because of high density faulting in many cities. Those inland events produced by the faults tend to cause heavy damages as the 1931 and 1972 earthquakes did (Hernandez et al. 1994). Also the seismic events in Nicaragua are caused by products of volcanic activities of the subduction zone. (Fig. 2) Due to its location on the plate boundary of the Coco and Caribbean tectonic plates, Nicaragua presents a high seismicity and active volcanism. Strong earthquakes can occur on the Pacific coast of Central America which can produce tsunamis. In 1992, (Talavera, Adan, 2014) a strong earthquake occurred off the coast of Nicaragua that generated a tsunami. This event was the largest in recent history. Monitoring of these phenomena is required for seismicity mapping, hazard assessment, early warning and planning of disaster prevention measures.

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Figure 2. Tectonics plate and seismicity in Nicaragua, 2015. (http://www.iris.washington.edu/).

1.3. Earthquakes in Nicaragua 1.3.1. Managua Earthquake 1931 The 1931 Managua Earthquake, which was an earthquake of magnitude 6.0 degrees on the Richter scale, destroyed the capital of Nicaragua on Holy Tuesday March 31, 1931. Its epicenter was located in The Failure of The Stadium. The death toll caused by this earthquake was about 1,200 to 1,500; as one of the main characteristics of the disaster was the bad design of the structures of the housings and buildings, when the seismic force is very strong, internal and fundamental structures of buildings are easily weakened. The seismic force was very strong making weaken of the internal and Foundations Structure of the Buildings. Approximately 833 buildings were totally destroyed. Among them were the Palace of Communications, the Central markets and San Miguel, the Variety Theatre, La Casa del Aquila, Candelaria church and the National Palace (Fig. 3). The Disaster sowed the seeds of the next Earthquake of December 23, 1972. As many houses and damaged buildings made of Taquezal and RC concrete were repaired improperly with plaster leaving cracks in its foundations and structures, they collapsed in the quake.

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Figure 3. National palace, Earthquake 1931, (http://www.skyscrapercity.com/). Managua suffered heavy losses of basic services and road infrastructure due to soil landslides and large cracks formed on the roads, and all the houses and buildings collapsed (Fig. 4).

Figure 4. Cracks and landslides on the roads, Managua, March 31, 1931. (INETER- Nicaraguan Institute of Territorials Studies).

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1.3.2. Managua Earthquake 1972 At 12:29 a.m. (local time), Sunday, December 23, 1972, an Earthquake of Magnitude 6.2 occurred at a depth of about 5 kilometers beneath the center of Managua, the capital of Nicaragua. Within an hour after the main shock, two aftershocks, one of magnitude 5.0 and the other 5.2, occurred. The Earthquake caused widespread damage to the houses and buildings (Fig.5), left a multitude of the dead and the injured. Approximately 5,000 of the 400,000 residents of Managua were killed while 20,000 were injured and some 250,000 were left homeless.

Figure 5. Damages to building in the capital, Earthquake 1972. (INETER- Nicaraguan Institute of Territorials Studies). The extent of damage indicated that The Earthquakes seriously affected an area of 27 square kilometers and destroyed 13 square kilometers in the heart of the city. At the time of the earthquake, Managua had one 17-story building, one 15-story building, approximately 5 buildings in the 7-to 9-story range, several dozen buildings in the 3-to 6-story range and a multitude of 1 and 2 story buildings. Most of these buildings suffered significant structural damage. Although a number of buildings were damaged by permanent ground displacement, most of the building damage was caused by strong seismic ground shaking (10-15 seconds during the main shock). Since all the fire-fighting equipment had been demolished, fires raged out of control in the downtown area for several days. Some 750 school rooms were affected by the earthquake, all four main hospitals (1650 beds) were rendered unserviceable (Fig. 6), and 53,000 units of family housing were lost or seriously damaged.

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Figure 6. Hospital for The Retirement damaged by The Earthquake, 1972. (INETER- Nicaraguan Institute of Territorials Studies).

The extensive damage to the RC Buildings Structures (Rojahn, Christopher 1973) was caused by the strong seismic shaking (depth of focus of the seismic event was shallow); permanent ground displacement along at least four major faults; and non-earthquake resistant characteristics of many of the structures (Fig. 7). Furthermore, the infrastructures were several damages, water and electrical power distribution systems were impaired. A large number of breaks occurred in the water mains and distribution pipes because of ground disturbance. Open fractures that trend north-south in the street pavement.

Figure 7. Damage by strong seismic shaking to RC Buildings in the Earthquake 1972. Left side: flexural failure mode in columns. Right side: shear failure mode. http://earthquake.usgs.gov

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1.4. Schools building in Nicaragua As the number of school buildings in Nicaragua has increased during the last few years, the scholar sector is divided in public and private area. Actually, a new model of schools building has been developed by nation’s private companies. The design incorporated a structural systems made with steels framework and light walls or concrete hallow blocks. This combination of materials has been accepted by the government to construct a new type of schools buildings in Nicaragua. However, in the capital of Managua, buildings older than 50 years have been conserved. As those structures are prestigious schools (Figure 8) and were built by private companies from Mexico, the United States and Germany during the ‘60s, international engineers support the structural design of those school buildings.

Figure 8. Prestige RC School built in Managua, Left Loyola Institute, and right Calasanz Institute.

Taking these two schools as examples of existing RC school buildings in the capital, this study will be an important tool to support and calculate future seismic indexes of existing RC schools buildings in Nicaragua using the Japanese Method (JBDPA) to evaluate the actual strength of the structures that have suffered from several earthquakes over the past years. The application of retrofitting techniques has to be implemented not only to those schools, but also to hospital and other structures made by reinforced concrete in order to improve the strength of the structures in longitudinal and transversal Direction. Furthermore seismic indexes of sides have to be determined to increase the strength of the building. However the support of government and private organization is needed as a foundation for protecting existing buildings to avoid the collapse of these structures from future earthquakes, and helping to avoid the loss of life and protect the infrastructures in Nicaragua.

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1.5. National Building Regulation RCN- 2007, Nicaragua

Figure 9. National Regulation of Construction. The first seismic design code in Nicaragua was issued in 1983, after the 1972 earthquake, but there are not specific seismic code for buildings, however, there are a National Regulation of Construction (RNC07), it was created in 2007 which provided in general the basic regulations and incorporated new technology for construction and basic topics of seismic design of buildings considering some basic dynamic properties of the soil. The regulatory standards of the National Regulation of Construction, 2007 establish requirements applicable to the design and construction of new buildings as well as repair and reinforcement of existing ones that need it, in order to: •

Prevent the loss of life and reduce the possibility of physical damage to persons.

•

Resisting minor earthquakes without damage.

•

Resisting moderate earthquakes with slight structural damage and moderate non-structural damage.

•

Prevent the collapse from the effects of high-intensity earthquake, reducing damage levels economically admissible.

•

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Resist, effects of winds and other accidental actions without damage.

The Nicaragua seismic code has been development by the effort of government, public and private institution, due to the lesson learned about past earthquakes. This code is the result of a compilation of information gathered of others codes of the region or recommendations made from other countries. The actual micro zone mapping of Nicaragua established by the RNC-07 (Figure 10), demonstrate that in the pacific region, zone C has a high seismic activities in the country, the capital Managua and important cities located in this zone are vulnerable to be affected for a future strong ground motion. This study will be an example to make a seismic evaluation for existing school building and stablish a future Seismic Index for the next upgrade of the Nicaraguan code.

High seismic level

Medium seismic level

Low seismic level

Figure 10. Micro zone – mapping, RNC-07. The schools building are considers important infrastructure in the country, categorized like essential structures (Table 1), those structures provide service to the population immediately after a disaster. An important fact to consider is upgrade the seismic code after occurs an earthquake, in order to improve the strength of the schools building. Also, evaluate the structures and provide basic knowledge about the actual condition of the infrastructure after occurs an earthquake and implement different retrofit techniques has it is mention in past sections in order to improve the behavior of the structure against future earthquakes and protect the human life.

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Table 1. Structure categorization Nicaragua RNC-07. Structures categorization Nicaragua RNC-07.

Group A

Essential Structures

Structures very important to provide service to the population immediately after a disaster. Is necessary to remain operational after an intense earthquake.

Group B

Structures Normal Importance

Those Structure which the degree of security required is intermediate.

Group C

Structures Less Importance

Isolated Structures are those whose total or partial failure not It threatens the lives of people.

Hospitals, Fire Stations, Police Stations, Government Buildings, Schools, call centers, Transport Terminals Homes, Office Buildings, Local Commercial, Industrial Buildings, Hotels, Warehouses Railings and Fences Height Less than 2.5m.

1.5.1. Maximum distortion allowed The maximum distortion allowed by the RNC-07 is demonstrate in table 2. The results of static and dynamic analysis to the structure have to comply different requirements stablish by the Nicaragua seismic code. Such as the maximum story drift angle should to be less than the stipulate according the classification of the structure. Table 2. Maximum distortion allowed RNC-07. Structural system Ductile reinforced concrete frames (Q=3 & 4)

Distortion 0.0300

Ductile steel frames (Q=3 & 4)

0.0300

Steel or concrete frames with limited ductility (Q=1 & 2)

0.0150

Flat slabs without walls

0.0150

Steel frames with eccentric outriggers

0.0200

Steel frames or concrete with eccentric outriggers

0.0150

Walls combined with ductile concrete frames (Q=3)

0.0150

Combined walls with concrete frames with limited ductility

0.0100

diaphragm walls

0.0060

Confined masonry bearing wall with horizontal reinforcement

0.0050

Confined masonry bearing walls solid piece

0.0040

Masonry bearing walls of hollow parts with internal reinforcement

0.0020

Masonry bearing walls that do not meet the specifications for confined masonry

0.0015

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2.

METHODOLOGY

The Methodology to carry out the seismic evaluation to the RC Building Structure has to be developed by gathering different information of the building such as drawings plans, general information about the structure, pictures, etc. Those tools provide a good result for the execution of the seismic evaluation. The process is based on a practical and descriptive investigation according to the established objectives using the Japanese Method (JBDPA, 2001), by making the collection of information to run the calculation of seismic index of structure and analyzing the conditions of the building. 2.1 Seismic evaluation process The seismic evaluation process starts with selection of a target building, calculation of the first level screening and second level screening method, and determination of the strength and seismic index of the structure in both directions. This is part of the quantitative methodology, where we apply different equations and conditions for the structure. Then we apply retrofit techniques and compare the behavior before and after retrofit using the software STERA _3D developed by Dr. T. Saito. 2.1.1. Methodological scheme The Following scheme shows the methodological process to follow during the study:

Figure 11. Methodological scheme for seismic evaluation.

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3. SEISMIC EVALUATION OF EXISTING RC SCHOOL BUILDING. 3.1 Target building: National Technological Institute. The Building, the former College Central America. Granada – Nicaragua, is located in Final Av. Spain # 1.This is a three story RC structure, categorized a school building, with four classrooms on each floor, 12 classrooms in total.

Figure 12. Actual Conditions of the National Technological Institute.

Figure 13. View from the second floor of the building.

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Table 3. General information of the building. General information of structure Floor area Total area of the building Total wall thickness Total height

308.99 m² 926.97 m² 0.15 m 10.55 m

Figure 14. Studied frame area of the building.

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Table 4. Members list – columns. Story

Direction

Member list C1

C2

1~3

bxD Main bar Hoop

450 x 550 10 - D22 2-ϕ9

450 x 450 8 - D 22 2-ϕ9

Table 5. Members list – beams. Story

Section

Beams list B-1

B-2

Section 1~3

bxD Main bar Hoop

300 x 600 5 - D22 (Top & Bottom) 2 - ϕ 9 @ 150

300 x 400 3 - D22 (Top & Bottom) 2 - ϕ 9 @ 150

Table 6. Members list – wall.

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Remark

Thickness

RC-1

150 mm

Wall list Wall reinforcement ϕ 9 @ 200 Single layered (Vertical and horizontal)

End reinforcement 1- ϕ 13

Table 7. Verticals members. Classification of vertical members

Y1 h0/D = 1500/550 = 2.72 h0/H0 = 0.53

Y2 h0/D = 1500/550 = 2.72 h0/H0 = 0.53

Y3 h0/D = 3000/450 = 6.66 h0/H0 = 1.0

3.2 Preliminary calculation

3.2.1 Structural weight and sustained force by columns The calculation of structural weight of the structure is assumed for specific unit area. In this case of study, it is W= 12.3 kN/m². The weight of each floor sustained by a column is calculated according to the area supported by the column and the W (Table 8). Table 8. Calculation of structural weights of the structure. Floor

Floor area(m2)

Structural weights Floor weights (kN)

3

36

389

389

2

36

389

778

1

36

389

1167

ƩW (kN)

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Table 9. Calculation of sustaining force of the structure. Column Sustaining Force Frame

Story

Supporting area A (m2) Width Length A

12.3*A(kN)

3 Y3

2

Y1

60.885 1.375

3.6

4.95

60.885

121.77

1

182.655

3

223.614

2

Y2

Sustaining force N(kN)

5.05

3.6

18.18

447.228

223.614

1

670.842

3

162.729

2

3.675

3.6

13.23

162.729

325.458

1

488.187

3.2.2 Story-shear modification factor for Eo. The Story- Shear modification factor is different in each floor of the building. This reduction factors will change. If the lateral force is applied to the structure of the building, the structural response of the building in the first story will increase, while the structural response at the top story reduces. For this reason the modification factors vary even in the same building. The calculated values are shown in Table 10. Table 10. Story – modification factors. Story - Shear modification factor for Eo Story

Modification factor

3

2/3= 0.666

2

4/5= 0.800

1

1

𝜼"𝟏 𝜼$𝟏

3.3 First level screening method: Evaluation of longitudinal direction X

The first level screening method is used to evaluate the seismic capacity of the structure using the sectional area of vertical elements, size of the column and concrete strength. In this case of study the concrete strength is 20.6 Mpa. The average shear stress of a column at ultimate state is defined according

26

to its shape. The stress multiplied by the modification factor βc based on concrete strength and the area of the column becomes ultimate strength of the column. 3.3.1 Vertical elements categorization and shear stress at ultimate state The vertical elements are categorized according to different parameter, and defined if each element is an extremely short column, a wall or a column. The current parameters are shown in Table 11. Table 11. Classification of vertical members. Vertical member Column

Definition Columns having ho/D larger than 2

Extremely short column Wall

Columns having ho/D equal to or less than 2 Walls including those without boundary columns

Table 12. Vertical elements and shear stress at ultimate. Story 3

2

1

Vertical element categorization and shear stress at ultimate stage Y1 Y2 Y3 Column 3C1 3C1 3C2 h0/D 2.73 2.73 6.67 Category Column Column Column τ (N/mm2) 1 1 0.7 Sectional Area A (mm2) 247500 247500 202500 Column 2C1 2C1 2C2 h0/D 2.73 2.73 6.67 Category Column Column Column τ (N/mm2) 1 1 0.7 Sectional Area A (mm2) 247500 247500 202500 Column 1C1 1C1 1C2 h0/D 2.73 2.73 6.67 Category Column Column Column τ (N/mm2) 1 1 0.7 Sectional Area A (mm2) 247500 247500 202500

3.3.2 Strength index C The calculation of Strength Index C is calculated using the Eq. (1) to (2) since the compression concrete strength of column is Fc= 20.6 Mpa > 20. And Eq. (3) is needed to define β.

𝐶𝑤 =

𝜏𝑤1𝐴𝑤1 + 𝜏𝑤2𝛢𝑤2 + 𝜏𝑤3𝛢𝑤3 ∑𝑤

(1)

27

𝐶𝑐 =

𝜏𝑐. 𝐴𝑐 𝛽𝑐 ∑𝑤

(2)

𝐹𝑐 𝛽𝑐 = 4 ; 𝐹𝑐 > 20 20

(3)

Where Cw is the strength index of the walls, Cc is the strength index of the columns, 𝛽𝑐 is the strength modification coefficient based on the compressive strength of concrete, in case the value is more than 20 N/mm². The values of Strength Index C for each story are obtained as in Table 13: Table 13. Strength index C. Strength index C β = Story 3 2 1

1.0149

Members category Column

C 1.29

Short column

0.37

Column

0.65

Short column

0.18

Column

0.43

Short column

0.12

3.3.3 Basic seismic index of structure Eo . The basic seismic index of structure is calculated using Eq. (4) and (5). After the result from both equation is obtained, it is necessary take the maximum value from both equations, which will be the seismic index of the structure Is. 𝜂+𝜄 (𝐶𝑤 + 𝛼1 ∗ 𝐶𝑐) ∗ 𝐹𝑤 𝜂+𝜄

(4)

𝜂+1 (𝐶𝑠𝑐 + 𝛼2 ∗ 𝐶𝑤 + 𝛼3 ∗ 𝐶𝑐) ∗ 𝐹𝑠𝑐 𝜂+1

(5)

𝐸𝑜 =

𝐸𝑜 =

Where, 𝜂 is the umbers of story of a building, 𝜄 is the number of story for evaluation, Csc is the strength index of the extremely short columns, Cw is the strength index of the walls, 𝛼1 is the effective factor of the columns at the ultimate deformation of the walls, which may be taken as 0.7. the value should be 1.0 in case of Cw=0, 𝛼2 is the effective strength factor of the walls at ultimate deformation 0.7, 𝛼3 effective strength factor of the columns at the ultimate deformation of the extremely short columns,

28

which may be taken 0.5, 𝐹𝑤 is the ductility index of the walls wich may be taken as 1.0, 𝐹𝑠𝑐 is the ductility index of the extremely short columns, which may be taken as 0.8. Table 14. Basic index of structure E0 in longitudinal direction X. Basic seismic index of structure Eo Modification Story factor 3

2/3

2

4/5

1

1

Member category Column

C

F

1.29

1

Short column

0.37

0.8

Column

0.65

1

Short column

0.18

0.8

Column

0.43

1

Short column

0.12

0.8

Seismic Index Is

Eo(Eq. 2)

Eo(Eq. 3)

0.8610

0.5416

0.86

0.5166

0.3250

0.52

0.4305

0.2708

0.43

3.3.4 Evaluation of seismic index of structure Is (longitudinal direction X) The result of seismic index of structure Is in longitudinal direction X is given in the Figure 15. The actual capacity of the building does not reach required strength Iso, though it is necessary to increase the strength of the building in longitudinal direction X using retrofitting technique. The results indicate that the rectangular geometry of the building in this direction is weak, due to the lack of structural walls in this direction. For this reason the calculated seismic index is low and it is necessary to be reinforced with different retrofit technique used in Japan. After the seismic indexes in both directions are calculated, it is necessary to make the retrofit design in order to improve the strength and capacity of the building.

RESULT OF SEISMIC INDEX LONGITUDINAL DIRECTION X 3 1

STORY NUMBER

0.86

LONGITUDINAL DIRECTION X 0.52 2

1 Iso REQUIRED STRENGHT

0.43

1

1 0

0.5

1

1.5

SEISMIC INDEX Figure 15. Result of seismic index of structure Is, longitudinal direction X.

29

3.4 First level screening method evaluation transversal direction 3.4.1 Vertical element categorization and shear stress at ultimate state – columns and walls members. The verticals elements categorization and shear stress at ultimate state in transversal direction Y are different from longitudinal direction X. In this case it is considered the existing structural walls, with a thickness equal to 0.15 cm, divide each classroom of the building. There are 4 classrooms on each floor, 12 classrooms in total in the building. To judge columns in the building it is necessary to define the category of columns if it is extremely short, or does not have problem to fail due to its shortness. In addition, it must be taken into account the measurement of the column Ho, from the bottom of the beam to the top of the floor. The current parameters are shown in Table 15. Table 15. Classification verticals members – column. Verticals elements categorization and shear stress at ultimate stage - column Story

3

2

1

Frame

D mm

b mm

Area

H0

h0/D

Category

Y1

550

450

247500

2.8

5.455

Column

Y2

550

450

247500

2.8

5.455

Column

Y3

450

450

202500

3.0

6.667

Column

Y1

550

450

247500

2.8

5.455

Column

Y2

550

450

247500

2.8

5.455

Column

Y3

450

450

202500

3.0

6.667

Column

Y1

550

450

247500

2.8

5.455

Column

Y2 Y3

550

450

247500

2.8

5.455

Column

450

450

202500

3.0

6.667

Column

τ (N/mm2)

F

1

1

1.015 1.820

1

1

1.015 0.910

1

1

1.015 0.607

βϲ

C

Table 16. Classification of vertical members – wall. Vertical element categorization and shear stress at ultimate state - walls

30

Aw m2

τ w(N/mm2)

F

βϲ

Cw

7.35 150

1.103

3

1

1.015

3.032

5

7.35 150

1.103

3

1

1.015

3.032

5

7.35 150

1.103

3

1

1.015

3.032

Story

Numbers

3

5

2 1

Iᴡ

Tᴡ

3.4.2 Basic seismic index of structure Eo . The basic seismic index of structure is calculated using Eq. (4) and (5). After the result from both equations are obtained, it is necessary to take the maximum value from both equations, which will be the seismic index of the structure Is. 𝜂+1 (𝐶𝑤 + 𝛼1 ∗ 𝐶𝑐) ∗ 𝐹𝑤 𝜂+1

(4)

𝜂+1 (𝐶𝑠𝑐 + 𝛼2 ∗ 𝐶𝑤 + 𝛼3 ∗ 𝐶𝑐) ∗ 𝐹𝑠𝑐 𝜂+1

(5)

𝐸𝑜 =

𝐸𝑜 =

Table 17. Basic index of the structure Eo transversal direction Y. Basic seismic index of structure Story 3 2 1

E0 (Eq. 2) 2.871 2.935 3.457

Seismic Index Is

E0 (Eq. 3) 2.0217 2.0620 2.4259

2.871 2.935 2.426

3.4.3 Evaluation of seismic index of structure Is (transversal direction y). The result of seismic index of structure Is in transversal direction Y is shown in Figure 16. The actual capacity of the building is more than the required strength Iso, and it is not necessary to think about the increase of the strength of the building in transversal direction Y. SEISMIC INDEX STRUCTURE TRANSVERSAL DIRECTION Y 3

STORY NUMBER

1

2.871 Iso REQUIRED STRENGHT

2.935

1 2

Is TRANSVERSAL

1

2.426

1 0

0.5

1

1.5

2

2.5

3

3.5

4

SEISMIC INDEX

Figure 16. Result of seismic index of structure Is, transversal direction Y.

31

3.5 Final result of the first level screening in both directions.

As shown in Figure 17, the final result of seismic evaluation in longitudinal X and transversal Y direction clearly demonstrates the structure does not comply with the Iso seismic strength required in longitudinal direction X. Accordingly, the building need retrofit technique to be implemented in order to increase the strength. On the other hand, in terms of the transversal direction Y, it is not necessary to apply retrofit because the seismic Index in this direction is upper than the required strength.

SEISMIC INDEX STRUCTURE IN BOTH DIRECTIONS 3 1

STORY NUMBER

0.86

2.871 Is LONGITUDINAL 2.935

0.52 2

Iso REQUIRED STRENGHT

1

Is TRANSVERSAL 2.426 0.43

1

1 0

0.5

1

1.5

2

2.5

3

3.5

4

SEISMIC INDEX

Figure 17. Comparison of final results of seismic evaluation in both direction. 3.6 Second level screening method

According to the second level seismic capacity evaluation, the seismic capacity of a structure is evaluated based on the performance of the vertical element on the assumption that girders are strong enough not to fail. The strength of members is calculated with available equations. The deflection angle at flexural yielding is derived from the column shape. Then the deflection angle at ultimate flexural strength and the deflection angle at ultimate shear strength are calculated considering the strength margin for shear failure. The hoop spacing of 200 mm are show in this section.

32

3.6.1 Members’ strengths The strength of the members in the second level screening are determinate with the ultimate flexural strength and the ultimate shear strength, using equation (6) and equation (8) obtained from the supplementary provision from the JBDPA. 3.6.2. The ultimate flexural strength The ultimate flexural strength is calculated using Eq. (6).

𝐼𝑓 𝑁 < 0.4 ∗ 𝑏 ∗ 𝐷 ∗ Fc

𝑀𝑢 = 0.8 ∗ 𝑎𝑡 ∗ 𝜎𝑦 ∗ 𝐷 + 0.5 ∗ 𝑁 ∗ 𝐷 ∗ S1 −

𝑁 U 𝑏 ∗ 𝐷 ∗ 𝐹𝑐

(6)

The shear force at the ultimate flexural strength Qmu can be calculated with Eq. (7), as follows on the assumption that the Mu at the top and the bottom of the column are the same. The flexural strength of each column can be calculated in the same procedure.

𝒬𝑚𝑢 = 2 ∗

𝑀𝑢 ℎ𝜊

(7)

3.6.3. The ultimate shear strength The ultimate shear strength is calculated with Eq. (8).

0.053 ∗ 𝑃𝑡0.23 ∗ (18 + 𝐹𝑐) 𝒬𝑠𝑢 = Z + 0.85^𝑃𝑤 ∗ 𝑠𝜎𝑤𝑦 + 0.1𝜎𝜊_ ∗ 𝑏 ∗ 𝑗 𝑀 + 0.12 (𝒬 ∗ 𝑑𝑒 )

(8)

Where 𝑎𝑡 length of reinforcement total, D column depth, b column width, 𝜎𝑦 is the material strength of the reinforced bars, Fc concrete strength, L’ total depth of the shear wall, N axial force in the shear wall, 𝑀𝑢 ultimate flexural strength at the bottom of the wall, ℎ𝜊 is the inflection height, 𝒬𝑠𝑢 ultimate shear strength, 𝑑a is the distance from the center of the tensile reinforcing bars to the extreme fiber of the wing wall in the compressive side mm, 𝑃𝑤 ∗ 𝑠𝜎𝑤𝑦 product of the shear reinforcement

ratio of the column and it is yields strength ( N/mm²). 𝜎𝜊 is the axial stress of the columns, j distance between the centers of stress in the section.

33

Table 18. Calculation strength members and failure mode.

Frame

Story 3 2 1 3 2 1 3 2 1

Y1

Y2

Y3

The strengths of members (hoop spacing of 200mm) h0 Mu Qmu h0/D N Qsu (kN) (m) (kN.m) (kN) 162.729 218.54029 291.3871 331.9351 1.5 2.73 325.458 259.00586 345.3411 344.9534 488.187 296.61482 395.4864 357.9718 223.614 234.01492 312.0199 336.8059 1.5 2.73 447.228 287.41764 383.2235 354.695 670.842 335.42626 447.235 372.5842 60.885 156.85946 104.573 169.2651 3 6.67 121.77 169.95875 113.3058 174.1359 182.655 182.65815 121.7721 179.0067

Failure mode Flexural Shear Shear Flexural Shear Shear Flexural Flexural Flexural

3.6.4 Ductility index F When the Ductility Index F for each column is calculated according to its failure mode, it is necessary to consider the strength margin for shear failure and deflection angle. The deflection angle is considered as follows: the maximum deflection angle of column to the deformable length cRmax, the deflection angle of column at the yielding cRmy, the plastic deflection angle of column cRmp, the deflection angle of story at flexural yielding modified by the clear height (ho) and standard height (ho) Rmy, the deflection angle of story at ultimate flexural strength Rmu, the deflection angle of story at the ultimate shear strength Rsu, and the deflection angle at story yielding Ry. The F index results are shown in Table 19. Table 19. Ductility index F (Hoop spacing 200mm). The strengths of members (hoop spacing of 200mm) Frame

Y1

Y2

Y3

34

cQmu

(kN)

cQsu (kN)

Failure mode

162.73

291.39

331.94

Flexural

2

325.46

345.34

344.95

Shear

1

488.19

395.49

357.97

Shear

3

223.61

312.02

336.81

Flexural

2

447.23

383.22

354.70

Shear

1

670.84

447.24

372.58

Shear

3

60.89

104.57

169.27

Flexural

2

121.77

113.31

174.14

Flexural

1

182.66

121.77

179.01

Flexural

Story

N

3

cRmax

cRmy

Rmy

Rsu

1/50

1/129

3/722

1/50

1/129

3/722

1/241

1/50

1/129

3/722

1/250

1/50

1/129

3/722

1/50

1/129

3/722

1/250

1/50

1/129

3/722

1/250

1/50

1/150

1/150

1/50

1/150

1/150

1/50

1/150

1/150

-

-

-

cRmp

cRmu

Rmu

8/741

1/173

-

-

-

1.0

-

-

-

1.0

1/129

3/722

-

-

-

1.0

-

-

-

1.0

1/329

0

13/376 3/103 13/527

F 1.2

1.0

1/50

1/50

2.6

1/50

1/50

2.6

1/50

1/50

2.6

3.6.5 Basic seismic index of structure Eo 3.6.5.1 Strength index C The strength Index C is calculated using the follow Eq. (9), this equation consider the lateral load capacity of the structure, supported by the vertical member’s cQmu (kN), and the maximum weight of structure in each floors ƩW (kN). Values are calculated in table 14.

𝐶=

𝑐𝑄𝑚𝑢 Ʃ𝑊

(9)

Where 𝑐𝑄𝑚𝑢 is the ultimate lateral load- carrying capacity of the vertical member, Ʃ𝑊 he weight of the building including live load for seismic calculated by the story concerned.

3.6.5.2 The effective strength factor The effective strength factor indicates the ratio of the restoring force at the ultimate deflection angle of the first group (R1) to the ultimate strength. The practical calculation method is as follows, the effective strength factor is calculated using the ratio of R1 to Rmy where Rmy, is the deflection angle at yielding. As for the effective strength factor of the shear column, it is modified by the inverse number of the margin for shear failure (ultimate flexural strength/ ultimate shear strength). The effective strength factors with hoop spacing of 200mm are calculated in Table 20. Table 20. Effective strength factor (hoop 200mm). Effective strength factors (hoop spacing of 200mm) S.

Frame

ƩW (kN)

Y3 3

Y2

1/150 389

Y1

Y2

1/150 778

Y1

Y2 Y1

3/722 3/722

Y3 1

3/722 3/722

Y3 2

Rmy

1/150 1167

3/722 3/722

cQmu (kN)

cQsu (kN)

C

F

104.57

169.27

0.27

312.02

336.81

291.39

1st group F1=0.8

F1=1.0

1.0