EXPLORATION OF DIFFERENT RETROFITTING OPTIONS FOR RC FRAME BUILDINGS IN KATHMANDU

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1 EXPLORATION OF DIFFERENT RETROFITTING OPTIONS FOR RC FRAME BUILDINGS IN KATHMANDU Rajani PRAJAPATI 1, Hima SHRESTHA 2 and Ramesh GURAGAIN 3 1 Structural Engineer, National Society for Earthquake Technology-Nepal, Nepal rprajapati@nset.org.np 2 Director, Earthquake Engineering Research and Training, National Society for Earthquake Technology-Nepal, Nepal 3 Deputy Executive Director, National Society for Earthquake Technology-Nepal, Nepal Rajani Prajapati 1, Hima Shrestha 1 ABSTRACT Various study reports on the seismic vulnerability of Nepal have revealed that more than 60% of the buildings in Kathmandu valley are unsafe and extremely vulnerable to the large impending earthquake (IX MMI intensity). National Society for Earthquake Technology-Nepal (NSET) has been involved in earthquake vulnerability assessment of hundreds of private and public buildings in Nepal for the past two decades which shows that most masonry buildings and more than 36% of RC frame buildings are noncompliant to construction standards with seismic consideration. Recent Gorkha Earthquake 2015 has already shown the vulnerability of existing buildings in Kathmandu valley. Many buildings got severely damaged even at MMI VI intensity Earthquake or below. Since reconstruction of all unsafe buildings is not possible economically, retrofitting of these buildings could be better option to make them seismically safe. NSET has been doing retrofitting of many masonry and RC Frame type school, residential and commercial buildings. This paper mainly focus on retrofitting of RC Frame buildings. It presents different retrofitting techniques explored and implemented along with their feasibility for retrofitting of RC Frame buildings such as RC jacketing of frame; Steel profile jacketing of frame; Addition of reinforced masonry wing walls; Strengthening of infill masonry. Keywords: Vulnerability assessment, retrofit, RC Frame Buildings, Kathmandu, jacketing 1. INTRODUCTION Past seismic records of Nepal show that many destructive earthquakes have occurred throughout the country, claiming thousands of lives and property. In 1934AD, a strong earthquake shook the Kathmandu Valley, destroying 20 percent and damaging 40 percent of the buildings in the valley. Many earthquakes have since followed causing further damage to the buildings of Kathmandu Valley, warranting the fact that future destructive earthquakes are unavoidable. (KVERMP, 1998) Through development and rapid urbanization, the seismic vulnerability of Kathmandu valley has increased a great deal over the past century. With an increase in population to

2 October 2015, Kathmandu, Nepal about 1 1/2 million, construction of new buildings and infrastructure has increased at an extremely fast rate. The majority of this new infrastructure especially that with buildings of poor construction, does not meet basic seismic requirements hence increasing the seismic vulnerability of Kathmandu. If an earthquake of similar intensity to that of 1934AD ( IX MMI Intensity) were to occur in modern day Kathmandu Valley, the lost estimate study of the valley reveals that as many as 60 percent of all buildings in the Valley are likely to be damaged heavily, many beyond repair. (KVERMP, 1998) National Society for Earthquake Technology-Nepal (NSET) has been involved in earthquake vulnerability assessment of hundreds of private and public buildings in Nepal for the past two decades. Assessment results show that most masonry buildings and more than 36% of RC frame buildings assessed were also found non-compliant. The problem lies with the configuration of the building as well as lack in strength and ductility. Recent Gorkha Earthquake 2015 has already shown the vulnerability of existing buildings in Kathmandu valley. Many buildings got severely damaged even at MMI VI intensity Earthquake or below claiming thousands of life and huge property loss. Since reconstruction of all unsafe buildings is not possible economically, retrofitting of these buildings could be better option to make them seismically safe. NSET has been doing retrofitting of many school buildings under different projects; and many residential buildings and commercial buildings upon request of clients. Though retrofitting of both masonry and RC Frame buildings have been done, this paper mainly focus on retrofitting of RC Frame buildings. This paper presents different retrofitting techniques explored and implemented for retrofitting of RC Frame buildings. RC jacketing of frame; Steel profile jacketing of frame; Addition of reinforced masonry wing walls; Strengthening of infill masonry walls are the most common methods of retrofitting used to retrofit of RC Frame buildings along with their feasibility. 2. SEISMIC RETROFIT Seismic retrofit of a structure is the correction of the major weakness in the structure relating to seismic performance. It refers to a process of enhancing the structural capacities such as strength, stiffness, ductility, stability and integrity of a building to mitigate the effect of future earthquakes. The need of seismic retrofitting of building arises under two circumstances: (i) earthquake damaged buildings and (ii) earthquake vulnerable buildings that have not yet experienced severe earthquakes. Due to the lack of standards and guidelines for retrofit design, problems in designing and implementation of retrofitting works are faced by engineers. Seismic behaviour of a structure can be enhanced by adopting different retrofitting strategies. The choice of the optimal retrofitting strategy depends on good understanding of the dynamic behavior of the building, cost of the chosen retrofit strategy and also on the future use of the building. In structures where many of its members do not have adequate strength and ductility, an effective way to retrofit the structure would be to add new lateral load resisting elements. Additions of infill walls, shear walls and braces are examples of global retrofit strategies. Improving regularity and mass reduction can also be categorized under global retrofit strategy. In structures where only few members lack adequate strength or ductility and the structure has got sufficient level of strength or ductility at the global level, local retrofit New Technologies for Urban Safety of Mega Cities in Asia

3 strategy is adopted. Strengthening of individual beams, columns, joints and walls are examples of local level retrofit strategy. Seven strategies have been identified to retrofit building. They are: Improving regularity, Strengthening, Increasing ductility, Softening, Damping, Mass reduction and Change in use. 3. RETROFIT DESIGN OF RC FRAME BUILDINGS For purpose of strengthening RC Frame Building, at first the deficient frame members and joints are identified during the detailed evaluation of the building. Depending upon the deficiencies identified, retrofitting strategies either to increase strength, ductility or a proper combination of both is employed. 3.1 Strengthening Options Various options are available for retrofitting of the deficient RC building. RC Jacketing of Columns and Beams, Steel Profile Jacketing of beams and columns, Addition of Infill wall, Addition of Shear Wall, Addition of Wing walls, Strengthening of infill masonry walls, Addition of Steel Braces, Retrofitting of Shear Walls, Addition of Frames, Fiber Reinforced Polymer Wrapping, Reduction of Irregularities, Reduction of mass, Energy dissipation devices and base isolation techniques Addition of Buckling restraint braces are the most common methods. NSET has retrofitted a large number of school buildings, some residential buildings and few commercial buildings. Even though many retrofitting techniques are available, only few of them are feasible for retrofitting of residential buildings considering technical difficulties and socio-economic condition and of Nepal. Among the different retrofit options, mostly used four retrofit options are presented here: RC Jacketing of Columns and Beams, Steel Profile Jacketing of beams and columns, Addition of Wing walls and Strengthening of infill masonry walls RC Jacketing of Columns and Beams RC Jacketing involves placement of new reinforcement and concrete overlay around the existing beam and column member. It increases the flexural strength, shear strength as well as ductility of the column. Size of columns, size of beams, quantity of longitudinal reinforcement, transverse reinforcement can be increased in deficient columns and beams as per design requirement. After RC jacketing of beams and columns, their size increase, minimum 100mm from all faces. This retrofitting technique is suitable only for medium rise school buildings and commercial buildings. It is not feasible for low rise residential building where there is space limitation. Such size increased columns and beams also reduces the aesthetic value of the residential buildings. NSET has retrofitted few School buildings using this technique. Exploration of Different Retrofitting Options for RC Frame Buildings in Kathmandu

4 October 2015, Kathmandu, Nepal Figure 1: Different typical sectional details of RC column jacketing Photo 1: RC Column jacketing New Technologies for Urban Safety of Mega Cities in Asia

5 3.2.2 Steel Profile Jacketing of beams and column Steel profile jacketing of beams and columns is the process of encasing these members with steel plates. It is basically used for remedy of inadequate shear strength and also to provide passive confinement to frame members. As the steel plates cannot be made continuous through the floor slab and frame members, it cannot be used for enhancement of flexural strength of the columns in which the beams are of same size of columns or beams are flushed with columns. Steel jacketing can also be used to strengthen the region of faulty splicing of longitudinal bars. Considering the space limitation in residential building, this method is more feasible than RC column jacketing as there is no much increase in column and beam sizes and their stiffness. NSET has used this technique to retrofit residential buildings. Photo 2: Steel profile jacketing to increase flexural strength in column Addition of Wing Wall The lateral strength of existing columns can also be increased by adding walls segments on each side of the column and reinforcing them or reinforcing the existing walls surrounding the column. These walls are called wing walls. Wing walls generally have thickness considerably less than the width of adjacent column. While designing wing walls, adequate connection to existing structure needs to be ensured. Wing walls should be anchored to all beams and at foundation level also. The wing wall will shorten the clear span of the beam creating large reversal moment in the beam at the face of the wing wall. If the beam is not detailed for the reversible moment at that location, strengthening of the beam or make other provision is required to ensure that beam does not fail. In RC and Steel profile jacketing of columns, walls surrounding the columns needs to be demolished to carry out the retrofitting work. But this method includes less interventions compared to RC and Steel profile jacketing of columns since walls need not to be demolished to carry out retrofitting work. Exploration of Different Retrofitting Options for RC Frame Buildings in Kathmandu

6 October 2015, Kathmandu, Nepal Photo 3: Strengthening of wing wall in RC Frame Building Strengthening of infill masonry walls In RC building, specially non-engineered residential buildings built years ago, columns are very lean and non-ductile. Longitudinal reinforcements provided are highly insufficient and stirrups are provided are large spacing about 600mm- 800mm c/c. In such condition, the contribution of RC Frame is very less. But 230mm infill masonry are provided around most of the columns. Such buildings can be retrofitted using the contribution of infill masonry. The existing RC columns and beams will act as masonry wall confining elements. Reinforcing the unreinforced masonry walls through Splint and Bandage method or Full Jacketing method as per requirement are the better options for such types of buildings. Another advantage of this retrofitting technique is that it not only strengthen the RC Frame, but also ties all the masonry walls to RC frame, hence preventing it s out of plane failure as well. Splint & Bandage: In this method, steel reinforcing bars are provided at critical locations of the wall. The reinforcing bars along with 40 mm thick micro-concrete overlay are provided in the form of vertical and horizontal elements. Vertical elements are known as splints and horizontal elements are known as bandage. Splints are provided at wall corners, wall junctions and adjacent to openings of the wall. Splints resists in plane forces. Bandages are run all around the walls a plinth, sill, lintel, floor and roof levels of the building. Bandages integrate various walls together and prevent out-of-plane collapse of walls. Splint & bandages are provided on both faces of the wall. Splint and Bandages at inner and outer walls are tied together at places by steel wires (G.I. wires) at 450mm interval and anchor bars at 450mm alternatively Jacketing of walls: Steel reinforcement mesh of vertical and horizontal bars are applied on both faces of the wall surface. The mesh is tied at places by steel wires (G.I. wires) at 450mm interval and anchor bars at 450mm alternatively. 40 mm thick layer of microconcrete overlay is applied on to the wall. The jacketing is designed to resist both inplane and out-of-plane forces. Jacketing helps to basket the walls and thereby increasing integrity and ductility. New Technologies for Urban Safety of Mega Cities in Asia

7 Figure 2: Splint and Bandage technique Figure 3: Jacketing on both faces Photo 4: Strengthening of infill masonry wall through Splint and Bandage method Exploration of Different Retrofitting Options for RC Frame Buildings in Kathmandu

8 October 2015, Kathmandu, Nepal 4. CONCLUSION For retrofit of RC Frame buildings, RC Jacketing of Columns and Beams, Steel Profile Jacketing of beams and columns, Addition of Wing walls and Strengthening of infill masonry walls have been found to be most feasible options technically and economically. REFERENCES Agrawal, P. and Shrikhande, M., Earthquake Resistant Design of Structures. Prentice-Hall of India Private Limited, New Delhi. ATC-40, Seismic Evaluation and Retrofit of Concrete Buildings, Volume 1 and 2. Applied Technology Council, California. Central Public Works Department, Indian Building Congress in associate with IIT Mandras, Handbook on Seismic Retrofit of Buildings. Narosa Publishing House, New Delhi. FEMA-273, NEHRP Guidelines for the Seismic Rehabilitation of Buildings. Federal Emergency Management Agency, Washington D.C. FEMA-274, NEHRP Commentary On the Guidelines For The Seismic Rehabilitation Of Buildings. Federal Emergency Management Agency, Eashington D.C. FEMA-356, Prestandard and Commentry for the Seismic Rehabilitation of Buildings. Federal Emergency Management Agency, Washington D.C. JSCE, Guideline for Retrofit of Concrete Structure (Draft). Rai, D.C., IITK-GSDMA Guidelines for Seismic Evaluation and Strengthening of Buildings. Department of Civil Engineering, Indian Institute of Technology, Kanpur, Kanpur. Sugano, S. Seismic Retrofit of Existing R/C Buildings in Japan (Lecture Note). International Institute of Seismology and Earthquake Engineering, Building Research Institute, Ministry of Construction, Japan. Tomazevic, M., Earthquake-Resistant Design of Masonry Buildings. Imperial College Press, 57 Shelton Street, Covent Garden, London. New Technologies for Urban Safety of Mega Cities in Asia