RESPONSE MODIFICATION FACTORS FOR REINFORCED CONCRETE BUILDINGS IN PAKISTAN

Abstract

Response modification factors in building codes are an essential part of all seismic building codes. These factors are used in the denominator of the base shear equation in the simplified method of seismic design to reduce the base shear in order to encompass non-linear seismic resistance potential of buildings. The selection of response modification factor for a specific class of structural system has, therefore, serious implications on the safety and economy of structural designs. Keeping in view the importance of response modification factor, a large number of researchers around the world, have made attempts to evaluate this factor for different types of structural systems. In all the cases, however, these factor have been evaluated considering a particular design code and local construction practices. Therefore, these factors are not universal and hence cannot be used in every region. Response modification factors that are part of building code of Pakistan have been adopted from an American code. Variation and uncertainties in construction practices and quality of construction in Pakistan are different from those in the US which renders the response modification factors inapplicable to design of buildings in Pakistan, until verified. There is a need for development of response modification factors for forthcoming edition of building code of Pakistan that are reflective of the true seismic building performance in Pakistan. This research was carried out to compute response modification factors for Special Moment Resisting Frame (henceforth referred to as SMRF) buildings in Pakistan This research work was initiated through a building stock survey that was conducted in five main cities of Pakistan to study contemporary construction practices. Several disparities were found to exist between design specifications and construction practices. These included reduced compressive strength of concrete as compared to specified xi compressive strength, haphazard and increased spacing of ties and stirrups, improper location of lap splices and smaller diameter of reinforcement as compared to nominal diameter, amongst others. Experimental models of RC beam-column connections were built incorporating these deficiencies. These models were then subjected to reverse cyclic loading in quasi-static experiments. The experimental results obtained from quasi-static tests were used to calibrate numerical models of the connections and were consequently integrated into numerical building models. Numerical simulations were carried out on reinforced concrete frames to compute response modification factors. Response modification factor was computed for a total of 256 twodimensional hypothetical reinforced concrete buildings based on experimental response of beam-column connections. The set of hypothetical frames considered varying span lengths and number of storys. The frames were designed based on BCP-SP 07 and analyzed using non-linear pushover analysis for computation of response modification factor. The disparity between design specifications and actual construction was accounted for in non-linear constitutive law for structural members derived from the experimental work. The computed response modification factors showed a wide dispersion with building span lengths, number of storys and detailing deficiencies. As a general trend, the values of response modification factors decreased with increasing period of vibration of the buildings and increasing span lengths. Based on the extensive experimental and numerical work, response modification factor value of 8.5 is recommended for RC SMRFs for standard quality control environments, where a full compliance between design specifications and construction practices is ensured. The overall range of the computed response modification factors for fullycode- compliant SMRF buildings is from 6.0 to 12.0 which signifies a large scatter in the values based on varying span lengths and building heights. This necessitates the use of period-dependent response xii modification factors in the forthcoming editions of the Building Code of Pakistan. For buildings with construction deficiencies pertaining to concrete quality and reinforcement detailing, the values of response modification factors range from 4.0 to 6.1 for buildings with varying span lengths and height. The determination of quality control environment and subsequent use of response modification factor is left to the discretion of design engineers. It is, however, emphasized that a judicious selection of response modification factor, based on ground realities, should be made by design engineers to avoid a compromise on the safety and economy of structural designs. A minimum requirement of 20.db should be adopted in the building code for proportioning the column depth in exterior beam-column connections to ensure hinging in beams rather than joints.