DEVELOPMENT OF BRICK BUILDING SYSTEMS FOR IMPROVED EARTHQUAKE PERFORMANCE

Abstract

Studies of seismic performance of brick build ings have revealed their inadequacy to resist earth quake shocks due to their heavy weight, poor tensile strength, low shearing resistance, lack of proper bonding between the shear walls and the cross-walls and poor workmanship. Systematic dynamic studies do not appear to have been carried out for investigating their realistic seismic capabilities. The strengthen ing methods as recommended in some of the earthquake regulations of the countries are meant for conventional brick buildings to improve their performance with little increase in the overall cost. But a dynamic evaluation of the specification is still wanting. The aim of this thesis is to fill this gap to some extent as well as to examine a new possibility of saving such buildings from the damaging influence of earthquake on the principle of vibration isolation. The following studies are made: 1. Seismic Response Analysis of Conventional Buildings A typical multistoreyed brick building is chosen here for seismic response analysis. A number of variables representing the physical properties of the structural system, namely, number of storeys from one to four, wall thickness in various storeys from 1 (iii) to li brick thick and damping from 5 to 15$ of critical value are considered. Shear beam type multidegrees freedom system is taken to represent these buildings mathematically in which the masses of the floors end walls are assumed as lumped at the floor levels, the floors are assumed as rigid diaphragms and the 'Pier Method* is to use to derive the storey stiffness. The restoring force versus lateral deflec tion characteristics are assumed to be linear in each storey. Both shearing and bending deformations are considered to take place in the piers. Two accelero grams are used for computing dynamic response of the buildings : (a) Longitudinal component of Koyna earth quake of December 11, 1967 recorded close to the epicentre of the shock and having high acceleration pulses and high frequency contents and (b) North- South component of El Centro shock of May 18, 1940 recorded at about 50 km from the epicentre and having relatively lower acceleration peaks and frequency con tents. Runge-Kutta fourth order method is used for computing the seismic response. Overturning and tor sional effects have been included in the determination of timewise net stresses in the piers and their capa bilities have been examined for resisting earthquake shock. From this study the critical sections for providing reinforcing have been identified and the minimum amount of necessary steel has been estimated. (iv) 2. Brick Buildings with Sliding Joint at Plinth Level To investigate the ground motion isolation feasibility, a new system is considered in which a clear smoothened surface is created at plinth level just above the damp-proof course and the superstructure simply rests at this level and is free to slide except for the frictional resistance. Pilot tests carried out on 2/4 scale models showed very large reduction in the roof acceleration as compared with conventional fixed base case under given shake table motion indicating a definite possibility of- earthquake isolation. The seismic res ponse of one storey sliding type buildings is worked out through a two-mass mathematical model treating the frictional resistance as rigid plastic. The various parameters involved in the analysis are: time period, roof-base mass ratio, viscous damping and coefficient of friction. The seismic response of the system is computed using the same acceleograms and the same numerical techniques as for the conventional system. This study leads us to a concept of 'frictional response spectra* in which the spectral quantities of a sliding mass-spring-dashpot system are plotted against the undamped natural period for various coefficiants of friction and mass ratios. These spectra clearly show the reduction of response of sliding system as compared with the conventional buildings. (v) 3. Large Model Shake Table Tests on Conventional and Sliding Buildings Eight half scale single storeyed brick building models are tested under base shocks so as to study their behaviour upto ultimate failure when cons tructed with different strengthening arrangements or sliding base arrangement. Their relative competence to withstand severe shocks is throughly examined. The outside dimensions of these models are 2.17 m x 1.75 m in plan and 1.60 m high above the plinth level with a 7.5 cm reinforced concrete slab roof. The tests were performed on a specially made railway wagon shake table facility in group of four models at a time. The eight models were of the following types? (a) Conventional Fixed Base Types - One each, unstrengthened, in mud and cement mortars; one unstren gthened in cement mortar but with lintel band; One strengthened in cement mortar with lintel band and vertical steel at corners and jambs; and another simi larly strengthened in cement mortar but with plinth band in addition. (b) Sliding Types - One each in mud and cement mortars having lintel band. These tests show that unstrengthened brick buildings of conventional cons truction are not only weak but inadequate in energy absorption and that models with horizontal ring beam at lintel level and vertical reinforcement at critical (vi) sections achieve strength and toughness both. The models with sliding permitted at base, again show a significant reduction in response and adequate behaviour upto very high base accelerations. As such, sliding arrangement shows great promise for adoption in actual building construction as a measure of earthquake safety. The following main conclusions are drawn: Once a brick building cracks, its stifiness, strength as well as damage threshold acceleration go on reducing and at a faster rate as the extent of damage increases. Reinforcing the brickwork at critical sections both in vertical and horizontal directions is a must for achieving adequate plateau of strength and ductility. The critical sections are identified and an estimate of required steel given for moderate and severe seismic zones. A sliding joint created at plinth level between foundation and superstructure could be used as an effective means of isolating the base motion.