SEISMIC ASSESSMENT AND RETROFITTING OF HISTORICAL BRICK MASONRY BUILDINGS

Abstract

There are large number of historical brick masonry monuments in India which are required to be preserved against various natural hazards, earthquake is the most critical amongst them. Historical masonry structures were designed for gravity loads and there were no earthquake resistant provisions in those times and hence they are very vulnerable and are required to be safeguarded against seismic forces. Numerous failures of such monuments have been reported in literature due to earthquakes. An exhaustive literature survey on assessment of historical and important buildings has been undertaken with respect to various construction techniques, modelling issues including material properties, failure mechanisms, seismic strengthening and retrofitting etc. In order to safeguard the historical marvels, retrofitting measures are required from future catastrophic seismic events. Therefore there is a need for developing a cost efficient and easily executable retrofitting strategy inclusive of latest materials compatible with historical brick masonry structures. Most of the historical brick masonry buildings were constructed in lime mortar therefore as a case study a massive historical brick masonry building constructed during the period 1923 A.D -1929 A.D in lime mortar has been selected. The building is massive having dimensions 305.83 m x 83.43 m, which may require massive effort in modeling and numerical analysis of the complete building. The building consists of 6 museum halls, large working offices, a convocation hall and two huge towers. A sub-structuring method has been adopted including all important aforementioned structural parts. For seismic safety of historical masonry buildings, vulnerability assessment and seismic retrofitting need to be undertaken. Therefore masonry vulnerability assessment survey 7 of the building is carried out for identifying the locations of distress. A detailed survey has identified damage in masonry, cracking in domed vaults of the building at ground floor and cross vaults of first floor. Cracking in arches, separation of diaphragm from walls and out of plane cracking behavior of walls are also found at first floor. From the observed damage patterns, it has been concluded that all the damages in the building are due to earthquake induced seismic forces but needs to be further validated by numerical analysis. Numerical assessment of vulnerability depends upon the estimation of material properties. Therefore, material properties were required to be evaluated for characterizing lime mortar brick masonry. Testing on insitu material was not permitted because of damage to the historically important structure. Therefore testing of lime mortar has been devised. Lime mortar grades used are as per Indian Standard - 2250 (1981) and designated as MM3, MM2 and MM1.5 grade having lime: sand ratio in proportion of 1:1, 1:2 and 1:3 respectively. Uniaxial Compression Testing and Cyclic Shear Testing have been carried out in the laboratory using aforementioned three different mortar grades. Modulus of elasticity, Poisson ratio, and shear modulus have been evaluated using testing data. Uniaxial compression testing data has also provided stress-strain relationships for lime mortar masonry used for material modeling in numerical simulation. Through cyclic shear testing material properties such as load displacement relationships, cumulative dissipated energy, ductility factor, hysteretic damping and shear modulus are evaluated. SPT and MASW testing have also been conducted which provided information that the foundation soil is stiff having sufficient shear resisting capacity. Dynamic System Identification of the building has also been carried out to evaluate the present state of the building in terms of frequency parameter. Test data has been processed using ARTeMIS Extractor and Matlab softwares. Based on the processed data fundamental frequency of the building has been evaluated. A method to evaluate seismic vulnerability assessment of massive historical buildings has been suggested based on macroelements. The response of the macroelements 8 together define the response of whole structure. The numerical simulation of the massive building is carried out in parts. The complete building has been sub-structured into 5 macroelements namely, (i) Convocation Hall; (ii) Museum Hall ; (iii) Tower Portion; (iv) Corridor (v) Tower. During the process of developing mathematical model softwares namely SOLIDWORKS and AUTOCAD have been used. Meshing and analysis have been carried out using ANSYS14.5 software with element types SOLID 186, SOLID 187; beam elements: BEAM 188 and BEAM 189; surface elements: SURF 154 for FRP ; and contact elements CONTA 174 and TARGE 170. The stress-strain relationships evaluated from testing have been used to define multilinear isotropic hardening material model available in ANSYS 14.5. Mathematical model for laminates have been developed using surface elements namely SURF 154 in ANSYS 14.5. The building is a huge masonry structure with most of the walls as squat thick walls having adequate inplane stiffness to resist inplane and out of plane seismic forces. Apart from these walls there are few huge walls which are more vulnerable against out of plane action as compared to inplane seismic forces. Therefore, piers having high slenderness ratio are critical load carrying structural elements needed to be safeguarded. Seismic vulnerability of historical masonry is very tricky as in most cases retrofitting interventions are not allowed as it may spoil it from architectural point of view. Therefore, retrofitting material and techniques should be acceptable in such cases. Most vulnerable components are found to be the tall and wide masonry walls in convocation hall and museum halls. The corridor masonry piers, arches and vaults are also found to be vulnerable. Vulnerability is further increased by the presence of initial cracks. Piers are wrapped using FRP, GFRP and CFRP laminates upto a height of 700 mm from floor level. Numerical simulation of the retrofitted building has been carried out considering completely bonded behavior of laminates to masonry. Wrapping is done at ground floor only. 9 Dead load, live load and seismic forces are considered for vulnerability analysis and for these loads (i) Gravity load analysis; (ii) Modal analysis; (iii) Response Spectrum analysis; (iv) Nonlinear Static pushover analysis of the building are carried out. Results for gravity load analysis are tabulated as compressive stresses, for modal analysis frequencies and mode shapes are evaluated. First 50 modes have been considered to evaluate the seismic response. Site specific response spectrum is taken to represent earthquake occurrence of 2500 years return period for historical buildings. Nonlinear static pushover analysis has been carried out for identifying the locations of distress. The lateral force for nonlinear static pushover analysis is worked out using response spectrum analysis. Results for response spectrum analysis and nonlinear static pushover analysis are tabulated as shear stresses, bending compression and bending tension stresses for both perpendicular directions of lateral load application. Results are given for unretrofitted and retrofitted buildings. Base shear and displacement responses are also evaluated for individual frames in two perpendicular directions for identifying possible damage locations. This study has presented a detailed study of a historical building with respect to its typology, construction techniques, material used, modeling issues, response analysis and suggesting suitable retrofitting strategy. A method to evaluate the response of massive historical buildings is suggested based on individual response of macro-elements. The technique has been successfully used to identify the location of distress and in suggesting retrofitting strategy of massive historical buildings.