Case study on structural health assessment for existing reinforced concrete building

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Introduction Cities located in Nepal of high seismic hazard have large numbers of buildings, which are constructed of reinforced cement concrete resisting moment frames with infill walls that share common characteristics and seismic vulnerabilities. Collapses of such structures during recent earthquakes have killed about 3000 people. The main reason of collapse of such buildings is constructing illegal additional storeys for business purpose. All cities in Nepal are categorized for type of buildings and legality of maximum floors that should be constructed within range of cities by government, but lack of supervision of government, monopoly of contractor and using of low-quality materials several buildings are constructed within Kathmandu valley and in other major cities. Secondly, the strong column and weak beam principle is not followed in mostly constructed RC building being one of the causes of damage and collapse of buildings during earthquakes. Structures get dilapidated with time for which repairs and reconstruction are not feasible so, retrofitting is the efficient approach which can be adopted to combat the defiance [1]. Retrofitting is a process of regaining the strength of deteriorated structural components of existing structures to the sufficient level of safety and protection against seismic hazards. It is carried out to reduce vulnerability of damage to the existing structure due to any natural disaster or seismic activity [2]. The inadequacy in strength of structural elements may be result of higher design load, poor workmanship, design errors or construction deficiencies, deterioration and corrosion or modification of structural system. Also, various findings show that old engineering structures do not correspond to the new seismic codes resulting difference on design requirements now and then. Proper solution implying retrofitting of concrete and masonry structures can be carried out by following techniques: 1. Previously building had been modeled by Pranay Ranjan and Poonam Dhiman - 4 storey and approximately 2000 sq. feet building in STADD PRO Software to design RC, FRP and SFRC Jacketing of failed columns of an existing building and to compare suitability of these three methods of retrofitting. Alexander Sichko, Halil Sezen had studied twelve experimental columns of differing interface conditions and compared across various coefficient values to determine which value best matches the observed performance. Basically, fiber-reinforced polymers (FRP), RC jacketing using highperformance concrete (HPC) is also highlighted [3]. Catalin Baciu et al. presented classic and modern retrofitting technologies for industrial buildings. They also explains the single storey industrial building retrofitting, using four different intervention options and concluded that all the retrofitting methods presented in paper lead to a more resistant structure, reducing the seismic risks: lateral displacements decrease, while ductility, bending moment and shear force capacities significantly increase and so on [4]. Shamim A. Sheikh and Jingtao Liu presents fiber-reinforced polymer (FRP) jackets research on square and circular column of size 305 and 356 mm respectively with its length 1473 mm. They explain brief results on the evaluation of the seismic behavior of reinforced concrete columns laterally confined by FRP. Here, the specimens were tested under lateral cyclic displacement excursions also at the same time it is subjected to a constant axial compression to simulate seismic forces and the results include the evaluation on amount of FRP-confinement. 2. Different types of retrofitting techniques can be illustrated as follows. Concrete jacketing (Figure 1). By placing reinforcing steel rebar around its periphery, concreting is widely adopted, called concrete jacketing. It is for the enlargement of existing structural members like columns and beams. This method increases the member stiffness and its size [5]. Steel jacketing (Figure 2). Using various steel angles, channels, and bands, jacketing of columns and beams is done by this technique. Fiber reinforced polymer jacketing (Figure 3). It is a modern technique for enhancing strength, and this technique base on composite materials such as carbon and glass fiber reinforced polymer. Using these technique high-strength sheets retrofitting of structures can be done easily [5]. Figure 1. Concrete jacketing Figure 2. Steel jacketing Figure 3. Fiber reinforced polymer jacketing Using the above different technique of retrofitting, following advantages and disadvantages can be obtained: Advantages: - higher sale price and lower operational cost; - better return on investment and better rental income; - improves building quality and reduces risks; - greater building durability, survivability and functionality; - improves indoor environmental quality and save energy. Disadvantages: - increase in dead load and chances of erosion are high; - high installation cost; - if any indication of corrosion in the reinforcement, this technique is not useable; - bonding between concrete and steel plates may not be proper; - the production of dust causes health hazards to the public (Figure 4). Figure 4. Different techniques of retrofitting [6] Statement of the problem and objectives The commercial building considered for this research is situated in Biratnagar, Nepal. This building was initially designed to be built up to G+3 storey in approx. 328.63 m2 and building have 3 bay in Y direction and 5 bay in X direction. The foundation is 6 feet below the soil and isolated footing is taken in account. The model building has a staircase and lift system with size 1.6 m by 2.5 m. There are altogether 24 columns in every floor with rooms having size 4.5 m by 4.7 m, 4.0 m by 4.7m and so on. Seminar hall, lobby, and guest room occupy other space of the building. The dimensions of columns are 0.5×0.5 m, where as that of beams are 0.30×0.55 m and secondary beam are 0.3×0.4 m. Slab has a thickness of 6-inch, i.e. 0.15 m. For the initial purposes, (G+3) storey building with storey height 3 meter for all floor, with plan 24×13 m is taken and load was applied as per IS code 456:2000 to show that no column failed for 4 storey, i.e. the building passed the design from ETABS 2017. As the construction phase grows client changes the initial design idea and thus 2 storeys are need to be added over the G+3 storey building and construct a new G+5 storey building. Thus new G+5 storey building was then modeled in ETABS 2017 with initial considerations resulting 52 number of columns failed the design due to increase in load with added storey thus where retrofitting comes in account. The main aims of this paper are: - to identify different types of retrofitting technique that are suitable for the structures by studying the na-ture of failure on structures; - to assess a building for its structural data types like grade of concrete used, reinforcement details and dimensions etc.; - to analyze the structure according to IS 1893:2016 and to check its functionality, and its structural be-havior; - to design the structure with proper retrofitting options for satisfying the design codes and serviceability; - to identify the deficiency in structural elements such as beams and columns. Methodology The six-storied existing commercial building including 2 more stories after previous design was modelled on ETABS 2017 with load patterns and cases as per IS 875 and IS1893:2016 for a building with commercial purposes for the study (Tables 1-3). Structure analysis of the structure was carried out using ETABS 2017. Also, three dimensional models were prepared and the existing building was designed as per the client requirements. The deficient in reinforcement was calculated for columns that need to be reinforced. Over stressed (o/s) columns were used for the design of RC jacketing whereas columns with reinforcement deficient were used in design of CFRP. Furthermore IS 15988[8]; 2013 code was used for the design of RC column jacketing and ACI 440.2R-08/ACI-318-05[9] was used design of CFRP for strengthening of RCC column due to deficiency in longitudinal steel reinforcement. The methodology we used can be shown in Figure 5. Figure 5. Methodology flowchart The 3D model diagram of building in ETABS can be seen in Figure 6 whereas the plan of first floor and the layout of column is shown in Figures 7 and 8 respectively. Figure 6. 3D modelling of building in ETABS Figure 7. First floor, plan of building Figure 8. Column layout of building Table 1 Silent features of building, description of model buildings Parameters Data Unit Remarks Building type Commercial Plan Regular Number of storey G+5 Storey height 3 m Footing type Isolated Footing depth 6 feet Infill wall thickness 230 mm Imposed load on roof 1 KN/m2 Imposed load on regular floor 3 KN/m2 Floor finish load 1 KN/m2 Lift load 10 KN/m2 Staircase load 4 KN/m2 Wall load 8.4 KN/m2 Size of column 500×500 mm×mm Size of beam 300×550 mm×mm Secondary beam 300×400 mm×mm Slab depth 150 mm Grade of concrete, fck 25 MPa IS 456:2000 Grade of steel 500 MPa IS 456:2000 Specific weight of RCC 25 Soil type Soft IS 456:2000 Seismic zone V IS 456:2000 Zone factor 0.36 IS 456:2000 Importance factor (I) 1.5 IS 456:2000 Response reduction factor, R 5 IS 456:2000 Time period of vibration 0.655 sec IS 1893:2016, T = 0.075h0.75 Spectral acceleration coefficient Sa/g 2.50 IS 1893:2016 Horizontal seismic coefficient Ah 0.135 Poisons ration concrete 0.2 Modulus of elasticity infill, Em 5310 MPa Modulus of elasticity concrete, Ec 25 000 MPa IS 456:2000, Ec = 5000√

About the authors

Kushum Prasad Adhikari

Pokhara University

Email: kushumprasadadhikari@gmail.com
ORCID iD: 0000-0002-0157-8758

Bachelor in Civil Engineering, School of Engineering

Pokhara Metropolitan City-30, Lekhnath, Kaski, Federal Democratic Republic of Nepal

Govind Prasad Lamichhane

Pokhara University

Email: gplamichhane@pu.edu.np
ORCID iD: 0000-0002-6820-5331

PhD in Technical Sciences, Associate Professor of the Faculty of Science and Technology

Pokhara Metropolitan City-30, Lekhnath, Kaski, Federal Democratic Republic of Nepal

Kshiteez Lamichhane

Pokhara University

Email: kshitizlami@gmail.com
ORCID iD: 0000-0002-1614-4469

MSc. in Structural Engineering, School of Engineering

Pokhara Metropolitan City-30, Lekhnath, Kaski, Federal Democratic Republic of Nepal

Krishna Ghimire

Pokhara University

Author for correspondence.
Email: krishnaghimire030@gmail.com
ORCID iD: 0000-0001-7450-0321

MSc. in Structural Engineering, School of Engineering

Pokhara Metropolitan City-30, Lekhnath, Kaski, Federal Democratic Republic of Nepal

References