seismic retrofitting of reinforced concrete buildings case studies · 2020. 4. 24. · case study...

Seismic Retrofitting of Reinforced Concrete Buildings – Case Studies

Introduction

This chapter deals with a few case studies in which the applications of the most common retrofitting schemes are employed to improve the efficiency and proficiency of either the seismically deficient vulnerable buildings or earthquake damaged buildings. In view of the mixed and complex seismic responses of retrofitted structures, heterogeneous nature of different constructions alongwith the strain dependent elastic properties of various materials hamper to bring a complete justification of the application of analytical studies. A sound qualitative basis of experimental studies or the experience of retrofitted structures during future earthquake will exactly judge and reveal the success of retrofitted structures. Since we have a considerable dearth of experience and experimental data on the behaviour and response of retrofitted structures, the case studies presented here are based on the experience obtained by the others. Incidentally, two major earthquakes of March 14 and September 19, 1979 hit a large number of reinforced concrete buildings in Mexico. Some of them were retrofitted whose efficacy came to be actually judged by the reoccurrence of an earthquake in the same region in 1985. Similar experience has been initially obtained from Turkey earthquake, 1988 in which a large number of buildings were damaged and retrofitted. This proved to be a good learning opportunity about the behaviour of the retrofitted structures. A few available case studies presented in this chapter serve as good instances for a better understanding of conventional retrofitted schemes. Some of the studies referred here are based on advance technological devices like base isolation and supplemental dampers. The information regarding suitability, effectiveness, test results of the analytical or experimental recommendations are based on the studies and experience obtained by

individual authors as expressed in the published work.

Methodology for Seismic Retrofitting of RC Buildings

A brief outline procedure followed for seismic assessment and retrofit works for a reinforced concrete building has been described here. This procedure has been

adopted by the inspection team for retrofitting of reinforced concrete buildings in Turkey after the Adana – Ceyhan earthquake in Southern Turkey on June 27, 1998 (Sucuoglu, Gur and Glkan, 2000). The survey team recommended 120 moderately damaged reinforced concrete residential apartment buildings in Ceyhan. The procedure of the method employed for 3 –9 storey building stock may be followed as:

Visit to the actual site with all documentation of buildings should be made and all structural dimensions and details should be verified. If necessary, reinforcement has to be checked on selected elements by rebar locator with some non-destructive testing (NDT) and by stripping concrete cover. Foundations should be inspected by excavating trenches at one or two exterior footings.

An intense investigation has to be made regarding the existing concrete quality by taking 1-3 core specimens from each building and taking rebound hammer readings on a large number of structural elements calibrated with the core test results.

Inspection of each structural and architectural element for damage should be done and the observed damage grade (none, light, moderate or heavy) on the structural and architectural plans should be accordingly marked.

Three-dimensional linear elastic model of the existing building should be prepared and subjected to code specified vertical and lateral loads. The modulus of elasticity on concrete is to be reduced in accordance with material test results.

The method for temporary shoring of damaged elements in buildings should be recommended. The damaged structures should be shored for vertical loads and braced for 25% of the estimated lateral loads and taking into account the live loads that will exist during construction. The most commonly used elements have been timber elements, steel elements, and tubular scaffolding (Iglesias, 1989).

The buckling of longitudinal reinforcement, rupture of ties and crushing of concrete is often observed in columns of damaged building. In that case, the original geometry of columns is recovered by the use of hydraulic jacks.

The seismic retrofit strategy for the building after considering all aspects should be recommended.

The upgraded building is analyzed under code specified loading and its compliance with the code is verified.

For selected buildings, capacity spectrum method is employed to assess the seismic performance of the retrofitted building.

Case Study 1: Seismic Retrofitting of RC Building with Jacketing and Shear Walls

Source The Mexico Earthquake of September 19, 1985 – Typical Cases of Repair and Strengthening of Concrete Buildings

M. Jara, C. Hernandez, R. Garcia, and F. Robles

Earthquake Spectra, Vol. 5, No. 1, 1989

Typical Features of the Building

Number of Stories - Eight stories with basement

Year of Construction - 1966

Lateral load Resisting System - Reinforced concrete frames

Floor system - two-way slab with beam

Foundation - Grid foundation with retaining walls around the perimeter

Typical floor plan and elevation shown in Figure 1

Features of Damages in Mexico earthquake, 1979

Minor cracks in beams and columns

Retrofitting Techniques Employed

Addition of concrete shear wall in axes 2 and A

Addition of masonry wall in axes 5

Behaviour of Retrofitted Building in Mexico Earthquake, 1985

Severe damage such as spelling of the concrete cover and buckled bar at the interface of the walls and beam-column joints

Main reinforcement in the columns located at the ground floor buckled and crushing of the concrete core

Most damaged columns were the columns adjacent to the added walls

Damage attributed to the inadequate connection between the added walls and original frame connection and the poor quality of the concrete

Retrofitting Techniques Employed after Mexico earthquake, 1985

Minor cracks - Repaired by injecting epoxy resins

Buckled longitudinal reinforcement, broken ties, and crushed concrete – Replacement of new reinforcement welded with the existing bars and new additionally closed ties were placed, concrete with low shrinkage properties were placed

Severely damaged columns adjacent to added walls – Retrofitted with encasing in concrete with appropriate longitudinal and transverse reinforcement, existing surface should be chipped and cleaned of all loose materials. The surface was moistened before the new concrete was placed.

Other columns – Retrofitted with wire mesh and a cover of 50mm of shotcrete

Damaged concrete wall added after 1979 earthquake – Demolished and replaced with new concrete walls with 200mm in thickness

Wall with slight damage – repaired by injecting epoxy resins and by increasing their thickness to 200mm.

Added new walls along the axes 2, 5, 6, E and A.

Foundation – The foundation grid was encased to permit the anchorage to the new longitudinal reinforcement. Additionally, the grid was connected to the retaining walls located around the perimeter to ensure monolithic behaviour.

Expected Performance Static and dynamic analysis was performed on the original undamaged building, match to the distribution of the damage observed accordingly.

Retrofitted building has been analyzed with the assumption of monolithic behaviour between old and new material.

Results indicate no additional piles to the foundation

Figure 1: Plan and elevation of the building

Case Study 2: Seismic Retrofitting of RC Building with Steel Bracing and Shear Wall

Source Seismic Retrofit of a RC Building: A Case Study Enrique DEL VALLE CALDERON, Douglas A. FOUTCH, Keith D. HJELMSTAD, Eduardo FIGUEROA – GUTIERREZ and Arturo TENA - COLUNGA

Proceedings of Ninth World Conference on Earthquake Engineering, Tokyo-Kyoto, Japan (Vol. VII)


Number of Stories – Twelve, acting as Hotel building

Year of Construction – 1927

Lateral Load Resisting Systems - Non-ductile reinforced concrete frames

Floor system - Cast-in-place concrete joist beam construction with 2.5-inch concrete slab

Foundation system - Mat foundation (2.4m thick) on concrete friction piles.

Typical floor plan and elevation shown in Figure 2.

Features of Damages in Michoacan earthquake, 1979

Extensive damage to first four stories in transverse direction.

The spandrel beams and columns in Frame 1 and 5 experienced diagonal cracking over much of their length in the first floor. In addition, the beam – column joints of these frames suffered severe cracking and spalling.

The medium column in the fourth storey of Frame 3 suffered cracking and crushing.

The foundation performed well


Cracked beams and columns – repaired with epoxy injection

The columns of Frames 1 and 5 – encased in steel through the fourth storey level

Frame 1 and 5 – Braced steel frames were attached on the outside of the building in E-W direction.

The columns of the frames and diagonal bracing at the first level were fabricated steel boxes. The other bracing members were made from 2 channels placed toe-to-toe with gusset plate between them. New footing and piles were placed under the columns of the new frames and were attached to the original foundation to ensure monolithic action. A 1.3 wide section of the floor slab was strengthened at each location where the new steel frame was attached to existing structure.

Insertion of new infill reinforced concrete shear walls

in N-S direction.

The walls have been placed in the 1-2 and 4-5 bays of frames A and C for the full height of the building. Nails were inserted into

the existing masonry walls.

Expected Performance A three-dimensional response spectrum analysis has been conducted on the structure using the ground motion measured in Mexico City. Results indicate that the steel braced frames attached to the building strengthened it, and they stiffened the structure, moving its natural period away from the predominant ground period of 2.0 sec.

The retrofitted building performed well and suffered no damage during the Mexico earthquake 1985, even though it was located in the vicinity of several collapsed buildings and was located in the part of the city that experienced the strongest ground shaking


Case Study 3: Seismic Retrofitting of RC Building with Steel Bracing

Source Forced Vibration Studies of an RC Building Retrofit with Steel Bracing Keith D. HJELMSTAD, Douglas A. FOUTCH, Enrique DEL VALLE, Ruth E. DOWNS Proceedings of Ninth World Conference on Earthquake Engineering, Tokya-Kyoto, Japan (Vol. VII)


Number of Stories – 12-storey reinforced concrete condominium apartment building

Building Details- Plan size 10.8 x 17.45 m and height is 28.2m above the foundation level, including penthouse

Lateral Load Resisting Systems - Moment resisting RC frames

Floor system - Racticular waffle slab 5 cm thick with 35 cm deep ribs

Foundation system - Mat foundation (15 cm thick) underlain by deep, slender stiffening beams (140 cm x 40 cm N-S and 140 cm x 30 cm E-W) located along the column lines. The stiffening beams are supported on concrete friction piles

Typical floor plan and elevation shown in Figure 3


The building suffered extensive damage at the fourth storey columns due to pounding against an adjacent four-storey building located approximately 5 cm north of the this building.

The building also experienced large inter-storey deformations of its frame; resulting in damage to the exterior walls (both longitudinal and transverse). In addition, the longitudinal and transverse partition walls were badly cracked at several levels.

No indications of the foundation failure were observed


Diagonal steel bracing was added to the central bay of frames 1, 2 and 3 in the transverse direction.

The cross-braces were fabricated by continuously welding of two angles together toe-to-toe to form a structural box. The columns of the three braced bays were encased in a steel lattice composed of angles at the corners and diagonal flat plates. This encasement provided the additional strength necessary to carry the increased axial forces anticipated in the columns of the braced bays. These forces result from the additional overturning moment attracted to the braced bays. Special steel collars were fabricated and placed at the top and bottom of each column to facilitate the attachment of the steel cross-braces. These collars were grouted and bolted to both the original concrete columns and the adjoining slab to smooth out the transfer of forces between stories.

Insertion of new reinforced concrete infill walls of 4 cm thickness to all bays of the exterior longitudinal

frames.

The reinforcement ratio of these walls was about 0.64% in both horizontal and vertical directions. The steel braces and reinforced concrete walls increased the weight of building approximately 3%. No additional piles or other foundation modifications were required largely because the weight of the structure was essentially unchanged. Besides, failure of foundation was not observed following the 1979 earthquake.

Expected Performance

The retrofitted building performed well and it suffered only minor structural damage during the 1985 Mexico earthquake, even though the intensity of shaking was much greater than in 1979.

Forced vibration studies of the building was carried out, the test result indicates that the steel-bracing scheme used to retrofit the building was an important factor in its better structural behaviour during the 1985 earthquake.

Free vibration test results show an increase in average stiffness in N-S direction by approximately 50%, which is also verified by the shape of the response spectrum. Lateral displacements were controlled and pounding against adjacent building was reduced.


Case Study 4: Seismic Retrofitting of RC Building by Complete Jacketing of Frames

Source The Mexico Earthquake of September 19, 1985 – Typical Cases of Repairs and Strengthening of Concrete Buildings




Number of Stories – Four storey with basement, ground floor and three upper floors act as a warehouse

Typical Features – Corner building

Year of Construction – 1959

Lateral load Resisting System - Reinforced concrete frames

Floor system - two-way slab with beam

Foundation - Mat foundation with retaining walls around the perimeter

Typical floor plan and elevation shown in Figure 4. Features of Damages in Mexico earthquake, 1985

Severe damage at second floor level columns

Damage consists of cracks more than 1 mm in width, loss of material and buckled bars

The façade walls suffered extensive cracking Short column effect

Excessive splicing of the longitudinal reinforcement at the same section


Concrete Jacketing - Both beams and columns

Expected Performance Static analysis was performed taking into account the torsional effects

Retrofitted building was analyzed with the assumption of monolithic behaviour between the old and the new material.


Case Study 5: Seismic Retrofitting of RC Building with RC Shear Walls and Jacketing of Columns

Source The ADANA – CEYHAN Earthquake of June 27,1998: Seismic Retrofit of 120 R/C Buildings

Haluk SUCUOGLU, Turel GUR and Polat GULKAN

12th World Conference on Earthquake Engineering, 2000


Number of Stories - 8-storey reinforced concrete apartment building.

Building Dimensions - Floor area 245 m2 and storey height is 3.0m above the foundation level, including penthouse.

Design and Construction -1984

Lateral Load Resisting Systems - Moment resisting RC frames. A structural wall around the elevator.

Floor system - Concrete slabs in the first six stories and joist slabs in the top two stories.

Foundation system - Strip foundation in both the orthogonal directions

Features of Damages in Adana-Ceyhan (Turkey) earthquake, 1998

Building under moderate damage category.

Extensive damage was observed in beams especially between the first and fifth floors.


Infilling of appropriate frame bays by in-situ reinforced concrete shear walls with proper anchorage to the existing frame designed for these shear walls (Figure 5).

Damaged columns or columns lacking required vertical load carrying capacity are jacketed. Where feasible, use of composite reinforced polymer fabric is recommended.

In the selection of seismic retrofit scheme, closing exterior window openings, intervention with the existing piping system and limiting architectural functions are avoided as much as

possible.

Expected Performance Free vibration test results indicate the lowest mode vibration periods of the original (as built) building are calculated as 0.85 s (torsion), 0.68 s (translation in the short direction) and 0.65 s (translation is long direction). In the damaged state, these periods become 1.09, 0.87 and 0.84 respectively. After adding the shear walls periods are reduced to 0.65s (torsion), 0.50 (translation in the long direction) and 0.43 s (translation in short direction).

Naturally, the reduction in natural vibration periods

after seismic retrofit is due to increase in the stiffness of buildings. The study indicates that the mean increase in lateral stiffness for retrofitted building is roughly

1.1

2

xT

T

k

k

meanr

e

e

r

kr = Stiffness after retrofitting ke = Stiffness before retrofitting Tr = Time period after retrofitting Te = Time period before retrofitting

Figure 5: Strengthening schemes applied to the building (darker shading shows

newly added RC walls)

Case Study 6: Seismic Retrofitting of a Soft Storey RC Building Retrofitted by Adding RC Frames

Source The Mexico Earthquake of September 19, 1985 – Typical Cases of Repair and Strengthening of Concrete Buildings




Number of Stories – Eight stories consisting of ground floor with seven upper floors that act as a housing building.

Typical Features - Soft storey, Mixed construction masonry with reinforced concrete


Lateral Load Resisting Systems –Masonry bearing walls except at the ground floor. Columns are only at the ground floor.

Floor System - Waffle slab at the first level and beam-block slab at the other levels

Foundation system - Grid and slab with friction piles located under each column



Severe damage occurred in masonry walls

Foundation of the columns at the first level suffered no damage

Principal failure direction was east-west due to irregularities in plan and insufficient area of walls in east-west direction


Adding of reinforced concrete frames over the existing column in the ground floor along axes 1, 3, 4 and 6

Adding two concrete shear walls from first level to the upper storey were placed in axes 3 and 4

The existing masonry walls were retrofitted using wire mesh and 30mm of mortar

The cover of the existing columns was removed to permit the continuity of the new longitudinal reinforcement. The dimensions of the existing columns were increased

The monolithic behaviour between the new frames and the floor system was provided by eliminating part of the floor system so that the new reinforcement of the frame was cast together with slab

Foundation – The foundation grid was encased to permit the anchorage to the new longitudinal reinforcement

Expected Performance Four new concrete frames with concrete walls were analyzed by taking into account the torsional effects

Retrofitted building was analyzed with the assumption of monolithic behaviour between old and new material.

The results indicate that the bearing capacity of the existing foundation was considered sufficient to resist the forces induced by the new structure


Case Study 7: Seismic Retrofitting of RC Building by Steel Bracing and Infill Walls

Source The Mexico Earthquake of September 19, 1985 – Typical Cases of Repairs and Strengthening of Concrete Buildings




Number of Stories – Six stories consisting of a basement, ground level with five upper floors that act as an office building.


Lateral Load Resisting Systems - Reinforced concrete frames

Floor system - Waffle slab

Foundation system - Mat foundation with retaining walls around the perimeter, friction piles were placed under the mat foundation



No significant damage during the earthquake. Only minor damage to non-structural walls

Foundation performed well


Although there is no significant damage but the owner of the building decided to retrofit it for future events and to eliminate the damage in nonstructural elements

Steel bracing in transverse direction. Bracing consists of angle sections welded together forming a box section

Infilled masonry walls were reinforced to stiffen the structure in the longitudinal direction. Wire mesh and shotcrete were used to strengthen the walls.

Expected Performance Static analysis was performed to verify that the upgraded structure could resist the code loads

The bracing frames were designed in such a way that they would carry all the lateral loads while the existing structure was considered to carry all the vertical loads.

Figure 7: Plan and elevation with strengthening schemes of the building

Case Study 8: Effect of Shear Wall Location on Response of Retrofitted Reinforced Concrete Building

Source Effect of Shear Wall Location on Response of Retrofitted Multi-Storied Building

S.K. Thakkar, Pankaj Agarwal, and Debasis Sinha

12th Symposium on Earthquake Engineering, IIT Roorkee


Number of Stories - Fourteen stories (G + 13) in Zone IV

Building Details – Five blocks, Central block retrofitted,

Lateral load Resisting System – Ordinary Moment Reinforced Concrete Frames

Floor system – RC beam slab construction, thickness of slab 20 cm

Foundation system – Considered fixed at base above the raft foundation

Seismic Evaluation of Building

3D linear dynamic analysis of building indicates that capacity demand (C/D) ratio of majority of elements (beams and columns) are less than 1


Addition of concrete shear wall

Two alternative locations of shear walls considered

Adding of shear wall in external frame and in internal frame as shown in Figure 8

Expected Performance Provision of shear walls in external frames are more effective than to provide shear wall in internal frame however it is more effective for reducing base shear.

Storey drifts can be significantly decreased by addition of shear wall.

Figure 8: Two different retrofitting schemes by shear wall

Case Study 9: Seismic Retrofitting of RC Building by Seismic Base Isolation

Source Passive Control of Structures for Seismic Loads Ian G Buckle

12th World Conference on Earthquake Engineering, 2000

Latest Advances in Seismic Isolation

William H. Robinson

Eleventh World Conference on Earthquake Engineering

Retrofitting of Historical Building by Seismic Base Isolations

Sarvesh Kr. Jain and S.K. Thakkar

Workshop on Earthquake Disaster Preparedness, Roorkee

Typical Features of the Buildings

Name of Buildings – New Zealand Parliament House and Library, both are historical buildings

Year of Construction –1899 and 1922 respectively

Lateral Load Resisting Systems – Seismically vulnerable un-reinforced masonry


Seismic isolation chosen over conventional strengthening techniques to maintain the historic fabric of the building

The isolation system comprises 145 lead-rubber bearings, 230 high-damping rubber bearings and 42 sliders.

Installation of the isolators required strengthening of basement walls and columns, and the provision of floor diaphragms.

The retrofit involves re-piling the building with lead rubber bearings and rubber bearing in the supports, as well as cutting a seismic gap in the 500mm thick concrete wall.

Figure 9 shows the strengthening of foundation walls below NZ parliament House and location of isolators.

Expected Performance The effect of the isolation is calculated as increasing the fundamental period from a value of 0.45 seconds to 2.5 seconds

During an earthquake the building will be able to move in any direction on a horizontal plane up to distance of 300mm.

The total cost for the restoration and seismic retrofit of these two buildings was approximately US$90 million.

Figure 9: Strengthening of foundation walls below NZ Parliament House and location of isolators

Case Study 10: Seismic Rehabilitation of a Soft-Storey RC Building by Viscous Damper

Source Seismic Rehabilitation of a Non-Ductile Soft-Storey Concrete structure Using Viscous Damper

H. Kit Miyamoto and Roger E. Scholl

Eleventh World Conference on Earthquake Engineering


Number of Stories - 4-storey building consisting of ground floor with three upper floors.

Typical Features: Historical building, soft/weak storey structure in E-W direction, ground floor is used as commercial/retail space, and the 2nd floor and above is single occupancy apartments.


Lateral load resisting systems - No lateral resisting elements in North & South elevation of the building at the ground floor, except for 16” square light RC frame, Non-ductile soft/weak storey structure in the East – West direction, Non-ductile reinforced concrete frames at the first level, and conventional shear walls or braces at levels 2,3, and 4.

Floor system - Cast-in-place concrete joist beam construction with 2.5-inch concrete slab.

Features of Possible Damages

Three dimensional time history analysis of the original building was performed and the result indicates that the concrete columns at the ground floor level were overstressed in bending and shears due to excessive deflection and the lack of ductility detailing and strength. This type of adverse behaviour could cause total collapse of the superstructure.

At present, there is no significant damage in the structure.


Since the building is a National Registered building, only limited options for retrofitting were considered so that they should not affect the appearance of the landmark hotel, maximizing the retail/commercial area at the ground floor, avoiding disturbance to tenants living in apartment on the upper floors and of course should be cost effective. The finally selected retrofitting schemes are

Steel moment frames with fluid viscous dampers (VDs) at the ground floor. The steel moment frames were designed to provide stiffness, strength, and redundancy to the existing lightly reinforced concrete columns. VDs were provided to control drift at the first

floor and to keep steel moment frames in the elastic range. VDs were attached to the top of the steel Chevron Braces and were strategically located to meet the above requirements.

Expected Performance Dynamic analysis was performed on two different mathematical models of the retrofitted building. One was a simple 2 dimensional stick model and the other was a complex 3 dimensional finite element model. The analyses revealed that installing VDs and moment frames at the first level reduced drift at all levels to the desired performance. In addition, using VDs is cost effective and also maintain the historical appearance and commercial utilization requirement of the building.

Figure 10: Typical visco damper assembly elevation

References

1. Sucuoglu, H., Gur, T. and Gulkan, P. (2000). “The ADANA – CEYHAN Earthquake of June 27,1998: Seismic Retrofit of 120 R/C Buildings” 12th World Conference on Earthquake Engineering, New Zealand.

2. Iglesias, J. (1986). “Repairing and Strengthening of Reinforced Concrete Buildings Damaged in the 1985 Mexico City Earthquake“, The Mexico Earthquakes- 1985 – Factors Involved and Lessons Learned, Proceedings of the International Conference, Mexico City, Mexico, September 19-21, 1986. (Edited by Michael A. Cassaro and Enrique Martinez Romero), ASCE Publication.

seismic retrofitting of reinforced concrete buildings case studies · 2020. 4. 24. · case study...

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