CODEN (USA): IJCRGG, ISSN: 0974-4290, ISSN(Online):2455-9555 Vol.10 No.8, pp 146-153, 2017
Abstract: Many existing structures located in seismic regions are not adequate because of current seismic design codes3. In addition, a number of major earthquakes during recent years have underscored the importance of mitigation to reduce seismic risk. Seismic retrofitting of existing structure is one of the most effective methods of reducing this risk6,7. However, the seismic performance of the structure may not be improved by retrofitting or rehabilitation unless selecting the appropriate technique. Therefore, the requirements of rehabilitation of various retrofit techniques must be taken before selecting retrofit schemes. In the present study, a beam column joint from an existing G+3 storey office building is considered for analytical study. The CFRP and GFRP sheets of varying thicknesses in beam column joint were modeled and compared with the model without FRP. The most effective and economical retrofit material is identified. The analytical study for the model has been done by using ANSYS software and the results are discussed. Key words : Beam Column Joint, CFRP, GFRP, Seismic Retrofitting, ANSYS.
A large number of existing buildings in India are severely damaged during earthquake forces and the number of such deficient buildings is growing rapidly1. Retrofitting of any existing building is an important mission and requires more skill. Retrofitting of RC building is mainly difficult due to complex performance of the RC composite material2-4 . The behaviour of the buildings during earthquake not only depends on the member size and amount of reinforcement, but to a greater amount on the placing and detailing of the reinforcement. There are three main sources of deficiencies in a building, which have to be taken into consideration by the retrofitting Engineer.
Inadequate design and detailing. Degradation of material with time and use. Damage due to earthquake or other catastrophe.
The three sources, suggest a retrofit scheme to make up for deficiencies and demonstrate that the retrofitted structure will able to safety resist the future earthquake forces expected during the lifetime of the structure.
Four types of fibers are normally used for retrofitting of beam column joints.
Glass fiber reinforced polymer sheets Carbon fiber reinforced polymer sheets Basalt fiber reinforced polymer sheets Aramid fiber reinforced polymer sheets
Fiber glass reinforced plastics use textile grade glass fibers. These textile fibers are different from other forms of glass fibers used to deliberately trap air, for insulating application. Fiber mats are web-form non-oven mats of glass fibers[6]. Mats are manufactured in cut dimensions with chopped fibers, are in continuous mats using continuous fibers. Chopped fiber glass is used in processes where lengths of glass threads are cut between the ranges between 3 and 26 mm.
Fig 1 Testing of beam column joint using GFRP sheets.
Carbon fibers are made when poly-acrylo-nitrile (PAN) fibers, pitch resins and rayon are carbonized by means of thermal pyrolysis and oxidation at high temperatures[3]. Carbon fibers are manufactured in diameters
production processes. Further production process including weaving and braiding into carbon fabrics, cloths and mats rather than glass that can then be used in actual reinforcements.
Fig 2 Testing of beam column joint using CFRP sheets. Table 1 Properties of FRP Sheets
Properties | CFRP | GFRP |
---|---|---|
Elastic Modulus (KN/mm2) | 240 | 73 |
Tensile Strength (N/mm2) | 4900 | 3400 |
Fiber Weight (g/m2) | 200 | 350 |
Density (g/cm3) | 1.7 | 2.6 |
Thickness (mm) | 0.017, 0.117, 0.217 | 0.067, 0.077, 0.087 |
Ultimate Strain (%) | 1.55 | 4.5 |
A beam column joint is modeled by using ANSYS. The simple linear static analysis is performed for the following cases of study
Case 1: Model without FRP. Case 2: Model with CFRP. Case 3: Model with GFRP.
Coarse meshing has been preferred for the beam column joint.
Element type for various structural elements are discussed below in Table 2.
Table 2 Ansys Element Type
Element Type | ANSYS Element |
---|---|
Concrete | SOLID 65 |
FRP composite | SOLID 46 |
Steel Reinforcement | LINK 8 |
The material properties are assigned in ANSYS analysis are discussed below in Table 3.
Table 3 Material Properties Assigned in ANSYS
Material properties | CFRP | GFRP |
---|---|---|
Elastic Modulus (MPa) | 240 x 103 | 73x 103 |
Poisson Ratio | 0.3 | 0.28 |
Ultimate tensile strength (MPa) | 4900 | 3400 |
Bulk modulus (MPa) | 20000 | 92308 |
Shear modulus (MPa) | 92308 | 17578 |
. In the analysis, the concentrated load of 10 KN is applied at the free end of Cantilever Beam. The end of the column is fixed and top of the column is made free for analysis.
The following are the results obtained from ANSYS software.
3.1 Model without FRP
Fig3 Typical view of Equivalent Von Mises Fig 4 Typical view of Normal Stress in Y Axis Stress
Table 4 Deformation, Stress and Strain Results of Model Without Wrapped with FRP
LOAD = 10KN
Properties | Results |
Total deformation | 0.96 |
(mm) | |
Equivalent (von | 40.48 |
mises) stress | |
(MPa) | |
Normal stress in | 29.26 |
Y axis (MPa) | |
Normal elastic | 0.00032 |
strain |
3.2 CFRP Model 1
Fig 5 Typical view of Equivalent Von Mises Fig 6 Typical view of Normal Stress in Y Axis Stress
3.3 CFRP Model 2
Fig 7 Typical view of Equivalent Von MisesStress Fig 8 Typical view of Normal Stress in Y Axis
3.4 CFRP Model 3
Fig 9 Typical view of Equivalent Von MisesStress Fig 10 Typical view of Normal Stress in Y Axis Table 5 Deformation and Stress Results of Model Wrapped with CFRP Laminates
LOAD = 10KN
Name of CFRP | Thickness (mm) | Total deformation (mm) | Equivalent (vonmises) stress (MPa) | Normal stress in Y Axis (MPa) |
---|---|---|---|---|
Model 1 | 0.017 | 8.03 | 62.35 | 19.26 |
Model 2 | 0.117 | 1.42 | 50.93 | 27.28 |
Model 3 | 0.217 | 0.94 | 50.30 | 27.12 |
3.5 GFRP Model 1
3.6 GFRP Model 2
3.7 GFRP Model 3
LOAD = 10KN
Name of GFRP | Thickness (mm) | Total deformatio n (mm) | Equivalent (vonmises) stress (MPa) | Normal stress in Y Axis (MPa) |
---|---|---|---|---|
Model 1 | 0.067 | 0.96 | 50.87 | 27.50 |
Model 2 | 0.077 | 0.96 | 50.65 | 27.51 |
Model 3 | 0.087 | 0.96 | 50.57 | 27.49 |
Table 7 Normal Elastic Strain Results for CFRP and GFRP
Name bf FRP | Normal Elastic |
Strain | |
CFRP Model 1 | 0.00013 |
CFRP Model 2 | 0.00014 |
CFRP Model 3 | 0.00014 |
GFRP Model 1 | 0.00015 |
GFRP Model 2 | 0.00016 |
GFRP Model 3 | 0.00015 |
Table 8 comparison of Results of CFRP and GFRP Model with the Model Without Wrapped with FRP
Model name | % Increase in Equivalent (von-mises ) stress | % Increase in Normal Stress in Y axis |
---|---|---|
CFRP model 1 | 54.035 | 51.92 |
CFRP model 2 | 25.82 | 7.274 |
CFRP model 3 | 24.24 | 7.895 |
GFRP model 1 | 25.68 | 6.416 |
GFRP model 2 | 25.15 | 6.342 |
GFRP model 3 | 24.94 | 6.454 |
From the study it is concluded that many existing structures that were built according to past design codes and standards are often found vulnerable to earthquake damage due to inadequate detailing, under estimated earthquake loads or material deterioration by time etc., the high cost of new construction and historical importance of older buildings has led building owners to renovate rather than replace the existing structures. Most retrofitting techniques will result in increase in stiffness and slightly increase in mass which causes in a return a shorter period. Shortening in period of vibration often results an increase in strength and stiffness of retrofitted structure. Based on the ANSYS modelling and analysis carried out on the beam column joint model the following conclusions are drawn.
Thus it is concluded that, the model retrofitted with CFRP of 0.017 mm thickness is more effective in taking concentrated load than other model analysed above.
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