International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Retrofitting of High Rise RC Building using CFRP to Resist Earthquake Effect
Theint Theint Thu Soe, San Yu Khaing
Abstract - Moderate and severe earthquakes have struck different places in the world by causing severe damage to reinforced concrete structures. Upgradation to higher seismic zones of several cities and towns in the country has also necessitated in evolving new retrofitting strategies. Retrofitting of existing structures are the major challenges that modern civil engineering field is facing these days. In this paper, twelve storey reinforced concrete building is proposed in seismic zone 2B (case A) originally. Superstructure of high-rise building is designed with ACI 318.99 by using ETABS (Extended Three Dimensional analysis of building) software as a structural tool. Required seismic load data is considered base on UBC 1997 code. When the structure is considered prone to higher seismic risk and is analyzed in seismic zone3 (case B), some members in the structure can become seismically deficient. So, these members must be strengthened in order to satisfy the condition given in zone3. Strengthening of deficient beams and columns are made by the use of ‘‘Sika Curbodur Composite Strengthening Systems of FRP Analysis Software’’ to increase flexural, shear and confinement strength of reinforced concrete structures based on FIB, Bulletin No.14. Design and analysis obtained from ETABS software are used as input data in FRP software.In this study, FRP design is made only critical beams and columns for upgrading of the building for lateral loads.
Index Terms— Moderate and severe earthquake, Retrofitting, Sika Carbodure software
I.INTRODUCTION
Among the natural hazards, earthquakes have the potential for causing the greatest damages to structures. Beams and columns, being the lateral and vertical load resisting members in RC structure are particularly vulnerable to failures during earthquakes and hence their retrofit is often the key to successful seismic retrofit strategy. Seismic retrofitting is an effective method of reducing the risks for existing seismically deficient structures. Basic methodology of strengthening mechanisms can be classified into two fundamental approaches. They are local modification of structural components and global modification of the structural systems.
Global modification, also termed as structural-level retrofit includes addition of new structural wall, steel braces, base isolators etc. However, member-level retrofitlocal modification is a much more cost effective method thanthe earlier one since it involves selecting and strengthening only
Manuscript received Oct 15, 2011.
Theint Theint Thu Soe, Department of Civil Engineering, Mandalay Technological University, (e-mail: ). Mandalay, Myanmar, 09-401565096
San Yu Khaing, Department of Civil Engineering, Mandalay Technological University, Mandalay, Myanmar, 09-2132780 (e-mail: ).
the weak and deficient components of the wholestructure.It includes addition of steel jackets, FRP materials etc for the confinement of column and flexural and shear strengthening of beams. Though bonding with steel plate is proved to be successful to some extent, steel as a strengthening material has some certain limitations. Among these are low corrosion resistance, difficulty in handling at construction site becauseof its excessive size and weight and lack of durability. These problems associated with using steel plates as a retrofit method have led to invent new rehabilitation and strengthening techniques. Among these techniques fiber-reinforced polymer (FRP) composites as retrofit materials has gained much notable success in recent years. This paper focuses on the recent progresses in retrofitting of RC columns and beams using various FRP retrofitting schemes with a view to improve the seismic performance of the deteriorated structure.[8]A fiber reinforced polymer system consists of fibers typically made of carbon, glass, or aramid and a polymer adhesive that can be an epoxy, polyester, or vinyl-ester. An effective FRP system is one where the fibers and adhesive are working together so that the fibers can take on a portion of the load from the original structure through an adhesive bond. FRP materials are lightweight, non- corrosive, non-magnetic and exhibit high tensile strength. Therefore, FRP composites were selected to be used in retrofitting of the building.[5]
II.FRP reinforcement systems
It's possible to choose between several systems of reinforcement that differ for the type of fiber, for the resins and also for the application techniques.
The fibers used in civil engineering are carbon fibers (CFRP), glass fibers (GFRP) and aramid fibers (AFRP). The systems available are Wet Lay-Up Systems (cured in-situ),Pre-cured Systems (prefabricated) in a way.
The installation procedure for Wet Lay-Up Systems (cured in-situ) and Pre-cured Systems (prefabricated) in a way.
A. Wet Lay-up Type (sheets or fabrics)
1)preparation of concrete substrate with "Putty-Filler" resins;
2)application of "Primer" resins on concrete substrate;
3)application of "Saturating" resins;
4)application of fabric sheet;
5)application of another layer of "Saturating" resins;
6)application of "Protective Coatings" resins.
For multiple layers repeat points from 3 to 5. Besides more than 5 layers of cured in-situ fabrics are not recommended to apply unless proved by experimental (exception: confinementapplications). Unless otherwise specified, this may be done before the previous layer has cured.
B. Prefab or Pre-Cured Strips or Laminates
1)application of a primer;
2)adhesive application to the concrete;
3)adhesive application to the laminates;
4)positioning of the laminate;
5)verification of the bond line.
For multiple layers repeat points from 1 to 5. Besides more than 3 layers of pultruded strips are not recommended to apply unless proved by experimental.[9]
III. Case Study Building
The case study building is twelve srorey residential RC building with an overall height of 138ft .Typical storey is 10 ft high and ground floor is 12ft high. The building is rectangular in shaped and is 90ft long by 64ft wide. The building is SMRF system. It is located in seismic zone 2B. Figure 1 provides a typical floor plan.
Figure 1. Typical floor plan of case study building
A. Material Properties of the structure
Analysis property data
Weight per unit volume of concrete= 150 pcf
Modulus of elasticity = 3122 ksi
Poisson’s ratio = 0.2
Coefficient of thermal = 5.5×10-6 in/in per degree F expansion
Design property data
Reinforcing yield stress, fy = 50,000 psi
Shear Reinforcement yield stress , fys = 50,000 psi
Concrete compression = 3000 psi
strength (cylinder), f′c
B. Load Consideration
- Gravity Load: Design data for dead load and live load which are used in the structural analysis are as the following Table I.
Table I. Design Data for Dead Load and Live Load
Dead Load / Live LoadUnit weight of concrete / 150 pcf / Live load on residential / 40 psf
9"thk. brick wall weight / 100 psf / Live load on roofing / 40 psf
4.5"thk. brick wall weight / 50 psf / Live load on stair case / 100 psf
Superimposed dead load / 25 psf / Live load on landing / 100 psf
Weight of elevator / 2 tons / Weight of water / 62.4pcf
- Wind Loads: Data for wind loads are as follow;
Exposure type = B
Basic wind velocity = 80 mph (mile per hour)
Effective height for wind load= 130 ft
Method used= Normal Force Method
Windward Coefficient= 0.8 inward
Windward Coefficient= 0.5 outward
Wind importance factor= 1
- Earthquake loads: Data for earthquake load which are used in structural analysis are as followed Table II;
Table II. Data for Earthquake Load
(case A) / (case B)Type of seismic zone / zone 2B / Type of seismic zone / zone 3
Seismic zone factor, Z / 0.2 / Seismic zone factor, Z / 0.3
Response modification factor, R / 8.5 / Response modification factor, R / 8.5
Seismic importance factor, I / 1 / Seismic importance factor, I / 1
Soil profile type / SD / Soil profile type / SD
Seismic coefficient, Ca / 0.28 / Seismic coefficient, Ca / 0.36
Seismic coefficient, Cv / 0.4 / Seismic coefficient, Cv / 0.54
C. Members Size for Proposed Building
The proposed building is analyzed and designed in seismic zone 2B by the use of ETABS software. The stories drift, overturning moment, resistance to sliding and torsional irregularity are checked for safety of the proposed building. The design results of beam sections are B1(10×10), B1(a)(10×10), B2(10×12), B2(a)(10×12), B2(b)(10×12), B3(10×14), B3(a)(10×14), B4(10×16), B4(a)(10×16), B5(12×16), B5(a)(12×16), B5(b)(12×16), SB1(10×10), SB2(10×12), SB2(a)(10×12), SB3(10×14), CB(10×12) and the design results of column sections are (12x12), (14x14), (16x16), (18x18), (20x20), (22x22).
IV. Members Required to Retrofit
The propose building which is designed in zone 2B(case A) is considered prone to high seismic risk and is analyzed in strong zone (Zone 3)(case B) by the use of ETABS software. The structure has deficiencies in the member′s reinforcement or detailing and some are failed. Initially, before retrofitting the building, the check of reinforced steel area is executed for every columns and failed beams. Reinforced steel area can be checked with the two conditions. When the deviation of necessary steel area and provided steel area for column is less than 5%, the retrofit is not required. When the deviation of necessary steel area and provided steel area for beam is less than 10%, the retrofit is not required. [7]
The following tables III, IV, V list the columns and beams required to retrofit when the seismic is considered in zone 3.
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Table III. Comparison of Longitudinal Steel Area (for top) in Failed Beams
Level / Location / Size (inxin) / Case A / Case B / RemarkProvided Steel Area, As(in2) / Required Steel Area, As(in2)
Left and Right / Middle of Span / Left and Right / Middle of Span
1F / 5, B-C / 12x16 / 2.36 / 0.88 / 2.79 / 0.66 / yes
5F / 5, B-C / 10x16 / 2.08 / 0.88 / 2.32 / 0.55 / yes
5, G-H / 10x16 / 2.08 / 0.88 / 2.34 / 0.55 / yes
9F / 5, D-E / 12x16 / 1.19 / 0.88 / 0.66 / 0.17 / no
5, E-F / 12x16 / 1.19 / 0.88 / 0.66 / 0.17 / no
Table IV. Comparison of Stirrup Steel Area in Failed Beams
Level / Location / .Size (inxin) / Case A / Case B / RemarkProvided Steel Area, As(in2) / Required Steel Area, As(in2)
Left and Right / Middle of Span / Left and Right / Middle of Span
1F / 5,B-C / 12x16 / 0.055 / 0.049 / O/S / 0.047 / yes
5F / 5,B-C / 10x16 / 0.055 / 0.031 / O/S / 0.041 / yes
5,G-H / 10x16 / 0.055 / 0.031 / O/S / 0.041 / yes
9F / 5,D-E / 12x16 / 0.055 / 0.055 / O/S / 0.042 / yes
5,E-F / 12x16 / 0.055 / 0.055 / O/S / 0.042 / yes
Table V. Comparison of Longitudinal Steel Area in Columns required to Retrofit
Location / Level / Size (inxin) / Case A / Case B / RemarkProvided Steel Area As(in2) / Required Steel Area As(in2)
C2 / GF to 1F / 20×20 / 7.9 / 9.726 / yes
5F to 6F / 16×16 / 3.52 / 3.821 / yes
9F to10F / 12×12 / 3.52 / 4.115 / yes
C4 / GF to 1F / 16×16 / 3.52 / 4.47 / yes
5F to 6F / 14×14 / 2.64 / 3.095 / yes
6F to 7F / 14×14 / 2.64 / 3.31 / yes
GF to 1F / 12×12 / 2.64 / 3.83 / yes
3F to 4F / 12×12 / 2.64 / 3.341 / yes
D5 / 4F to 5F / 14×14 / 4.8 / 5.526 / yes
E5 / 5F to 6F / 14×14 / 4.8 / 5.204 / yes
6F to 7F / 12×12 / 2.64 / 3.574 / yes
F2 / GF to 1F / 18×18 / 9.48 / 9.823 / yes
3F to 4F / 16×16 / 6.32 / 6.999 / yes
4F to 5F / 14×14 / 4.8 / 5.615 / yes
5F to 6F / 14×14 / 4.8 / 5.306 / yes
F5 / 6F to 7F / 14×14 / 4.8 / 5.611 / yes
G2 / GF to 1F / 20×20 / 7.9 / 8.102 / yes
G4 / 3F to 4F / 16×16 / 3.52 / 4.185 / yes
6F to 7F / 14×14 / 2.64 / 3.113 / yes
7F to 8F / 14×14 / 2.64 / 3.302 / yes
GF to 1F / 12×12 / 2.64 / 3.843 / yes
GF to 1F / 12×12 / 2.64 / 3.325 / yes
I7 / GF to 1F / 12×12 / 2.64 / 2.935 / yes
9F to10F / 12×12 / 2.64 / 2.847 / yes
V. Sika Carbodur FRP Analysis Software
The software package FRP-Analysis may be employed as a user friendly, simple and reliable design tool for the selection of FRP dimensions to provide flexural strengthening, shear strengthening or confinement of reinforced concrete sections. These three topics are treated in the guideline, which present the theoretical basis of the calculations.
The equations used in this program are given in the fib Bulletin No. 14, July 2001: "Design and use of Externally Bonded FRP Reinforcement for RC Structures”.[5]
VI. Types and Properties of FRP used for Strengthening
The fiber type used in strengthening for the proposed building is carbon fiber. Properties of FRP used for Sika Carbodur Composite Strengthening Systems are provided in Table VI and VII.
Table VI.Mainly Shear Strengthening or Confinement Products (Flexible Sheets)Product name / Elastic modulus / Ultimate tensile strain / Width / Thickness
Ef (kN/mm2) / εfu (-) / bf (mm) / tf (mm)
Sika Wrap Hex-230C / 231 / 0.017 / 300/600 / 0.12
Table VII.Mainly Flexural Strengthening Products (Plates)
Product name / Elastic modulus / Ultimate tensile strain / Width / Thickness
Εf (kN/mm2) / εfu (-) / bf (mm) / tf (mm)
Carbodur S512 / 165 / 0.017 / 50 / 1.2
Carbodur S1012 / 165 / 0.017 / 100 / 1.2
VII. Flexural Strengthening
RC‐members, such as beams, columns and floor slabs may be strengthened in flexure using FRP EBR bonded to their tension zones, with the direction of the fibers parallel to that of high tensile stresses (Figure 2). [10]
Figure 2.Flexural strengthening of a beam
A. Design Results of beams from FRP Software for flexural Strengthening
The procedure for flexural strengthening of beams, B5(b)(12x16) is now calculated as follow. The other beams are calculated similarly, results can be seen in Table VIII, Table IX and Table X.
- Input Data: The input data are presented in figure 3.
Figure3. Graphical view of the software input data
Bending moment during strengthening Mo (due to gravity load) and required design moment after strengthening are obtained from analysis result of ETABS software.
- Results: The software gives the following results in figure 4.
Figure 4. Graphical view of the software results
- Input of FRP Dimensions: The dimensions of FRP materials used are input and the number of FRP layers required is obtained which are presented in figure5.
Figure 5. Graphical view of the software results
B. Check for Ductility
The ratio of neutral axis depth is compared with the maximum value provided by EC2.The checking result is shown in figure 6.
Figure 6. Graphical view of software checked result for ductility
C. Bond Check
If the tensile force Nfd,A carried by each strip does not exceed Nbd,maxthat , can be carried by the total number of strips, then the bond check is verified, that is failure of the anchorage is not expected, provided that the appropriate bond length lbd will be available. The checking result is shown in figure 7.
Figure 7. Graphical view of software checked result for bond length
Table VIII. Design Result of Beams for Flexural Strengthening
Floor / Location / Size (inxin) / Type of FRP / Mo (kNm) / Msd(kNm) / No of layer / Thicknesstf (mm)1F / 5, B-C / 12x16 / Carbodur S1012 / 67.97 / 161.9 / 1 / 1.2
5F / 5, B-C / 10x16 / Carbodur S512 / 67.71 / 134.96 / 1 / 1.2
5, G-H / 10x16 / Carbodur S512 / 67.68 / 135.48 / 1 / 1.2
Table IX. Design Result of Beams for Flexural Strengthening
Floor / Location / Size (inxin) / Design moment at section A (kNm) / Maximum bond length, lbd,max(mm) / Required bond length, lbd,A (mm)1F / 5,B-C / 12x16 / 87.128 / 218 / 60
5F / 5,B-C / 10x16 / 69.647 / 218 / 39
5,G-H / 10x16 / 69.647 / 218 / 39
Table X. Design Result of Beams for Flexural Strengthening
Floor / Location / Size (inxin) / Maximum ductility / Ductility1F / 5,B-C / 12x16 / 0.45 / 0.21
5F / 5,B-C / 10x16 / 0.45 / 0.193
5,G-H / 10x16 / 0.45 / 0.193
VIII.Shear Strengthening
Shear strengthening of RC‐members with FRP may be provided by bonding the external reinforcement with the principle fiber direction as parallel as practically possible to that of the maximum principal tensile stresses. The effectiveness of the FRP EBR is then maximized. It is normally most practical to bond FRP EBR for shear strengthening with the principle fiber direction perpendicular to the member’s axis (Figure 8). [10]
Figure 8. Shear strengthening of a beam
A. Design Results of Beams from FRP Software for Shear Strengthening
The procedure for shear strengthening of beams, B5 (b) (12x16) is now calculated as follow. The other beams are calculated similarly, results can be seen in Table X.
- Input Data: The input data are presented in figure 9.
Figure 9. Graphical view of the software input data
To input the value of additional shear, the first step is to evaluate shear resistance of the unstrengthened beam section. The nominal shear strength, equal to the sum of the contributions of the concrete and steel, can be calculated by the following equation (1) and (2), (ACI-318-99).
ΦVn= φ(Vc +Vs) (1)
ΦVn= 0.75 (2bwd +) (2)
So, the value of additional shear is the different between frame member analysis of design shear force (in Z3) and nominal shear resistance of beam (in Z2). The equation (3) is given the following.
Vfd= Vu - ΦVn (3)
- Results: The software gives the required FRP thickness results in figure 10.
Figure 10. Graphical view of the software results
- Input of FRP Dimensions: The thickness of single FRP layer used is input and the number of FRP layers required is obtained which are presented in figure11.
Figure 11. Graphical view of the software results
Table XI. Design Result of Beams for shear Strengthening
Floor / Location / Size (inxin) / Number of layer required / Applied FRP thickness (mm)1F / 5,B-C / 12x16 / 1.00 / 0.12
5F / 5,B-C / 10x16 / 1.00 / 0.12
5,G-H / 10x16 / 1.00 / 0.12
9F / 5,D-E / 12x16 / 1.00 / 0.12
5,E-F / 12x16 / 1.00 / 0.12
IX.Strengtheningof Columns
The performance of RC‐columns under axial load, bending and shear can be significantly enhanced by confining the columns with external FRP EBR (Figure 12). It increases the concrete’s compressive strength, ductility, shear strength and provides a higher resistance against buckling of the steel reinforcement in compression.[10]
Figure 12. Confinement of column
A. Design Results of Columns from FRP Software for Confinement
The procedure for confinement of columns, (C2, GF-1F) is now calculated as follow. The other columns are calculated similarly, results can be seen in Table XI.
- Input Data: The input data are presented in figure 13.
Figure 13. Graphical view of the software input data
Requirements
To input the value of mean strength after strengthening fcc, from ETABS software, concrete mean strength of failure column of 28.89N/mm2 is increased until target required mean strength is obtained. Here, mean strength after strengthening, fcc of column C2 (GF to 1F) is 32.34 N/mm².
- Results: The software gives the required FRP thickness results in figure 14.
Figure 14. Graphical view of the software results
- Input of FRP Dimensions: The thickness of single FRP layer used is input and the number of FRP layers required is obtained which are presented in figure 15.
Figure 15. Graphical view of the software results
Table XII. Design Results of Columns for Confinement
Location / Level / Type of FRP / Required mean strength (N/mm2) / Number of layer required / Applied FRP thickness (mm)C2 / GF-1F / Sika Wrap Hex-230C / 32.34 / 2 / 0.12
5F-6F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
9F-10F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
G2 / GF-1F / Sika Wrap Hex-230C / 32.34 / 2 / 0.12
I7 / 5F-6F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
6F-7F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
C4 / GF-1F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
3F-4F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
4F-5F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
5F-6F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
6F-7F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
G4 / GF-1F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
3F-4F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
4F-5F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
5F-6F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
6F-7F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
F2 / GF-1F / Sika Wrap Hex-230C / 32.34 / 2 / 0.12
3F-4F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
6F-7F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
7F-8F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
D5 / GF-1F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
F5 / GF-1F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
E5 / GF-1F / Sika Wrap Hex-230C / 32.34 / 1 / 0.12
9F-10F / Sika Wrap Hex-230C / 36.48 / 2 / 0.24
X. Discussionand Conclusion
Seismic retrofitting has now become a crucial issue. Recent occurrences of earthquakes in different parts of the world have clearly demonstrated the urgency of repairing seismic deficient structures. Design guidelines and recommendations should be made more readily available to ensure more rapid and effective applications of FRP as a seismic material. More research needs to be conducted addressing issues related to mechanics, design, and durability of FRP retrofitted reinforced concrete to ensure a proper use of FRP composites in seismic retrofitting applications. In this study, a case study is designed for example.
This study is concentrated on retrofitting of twelve story reinforced concrete building. The proposed building is located in zone 2B and is designed and analyzed for this zone by using ETABS software. Seismic retrofitting is made to improve the seismic resistance capacity of the building required for higher seismic force in zone3 (case B). Externally bonded FRP reinforcement (FRP EBR) is used for strengthening of deficient beams and columns in shear, flexural and confinement respectively by using FRP analysis software. From the link of between two softwares, FRP analysis and designed software input data such as cross section geometry, mean strength of concrete, additional shear are obtained from ETABS software. Durability, corrosion resistance, low weight, high resistance and ease of installation are some of the factors which favor the use of FRP reinforcement. In this study, Sika warp Hex 230C (carbon fiber type) of Sika products materials is used for strengthening of beams in shear and for strengthening of columns in confinement. This study recommend for strengthening of beams in flexural using Carbodur S512 and Carbodur S1012. In this study, minimum thickness of FRP with the less number of layers is used for strengthening of members. Because of FRP debonding is affected by the stiffness of the bonded sheet or strip. With increasing stiffness, it is easier for debonding to occur. From this study, like this structure, it is hoped that it can be shared some benefits of the structural knowledge to the further research.