تقاضای لرزه ای برای ساختمان بتنی تقویت شده کم ارتفاع در منطقه اسلام آباد-راولپندی (پاکستان) Seismic Demand for Low-Rise Reinforced Concrete Buildings of Islamabad–Rawalpindi Region (Pakistan)

1 Introduction Pakistan lies in an earthquake-prone region and the occurrence of major earthquakes in the northern region of Pakistan area is result of the continuing subduction of the Indian Plate under the Eurasian Plate at a rate of 40 mm/year [1]. The convergence and collision of these two plates caused folding and thrusting of the upper crustal layers, which resulted in the formation of many important thrusts and many active strike slip faults, particularly in the north and north-east region. These important thrusts include Panjal Thrust, Main Mantl Thrust (MMT), Main Boundary Thrust (MBT), Riasi Thrust, Salt Range Thrust (SRT) as shown in Fig. 1. The seismicity along the collision boundary is relatively shallow, and most earthquakes have a depth up to 50 km [2]. More recently, the devastating Kashmir earthquake in 2005 (Mw = 7.6) [3] occurred due to rupture along the “Muzaffarabad fault”, which is located along the northern most part of the Riasi Thrust (Fig. 1). The earthquake had a focal depth of 26 km and was located 10 km north-east of the Muzaffarabad city and 105 km north-northeast of Islamabad (Fig. 1). This earthquake caused severe damage in the Kashmir and the adjoining areas of Khyber Pakhtunkhwa (KPK) Province of Pakistan and left approximately 73,000 people dead, more than 70,000 wounded and 3.3 million displaced [4]. It is estimated that damages incurred were well over US$ 5 billion [4]. The earthquake ground motion was recorded at three different stations of Abbottabad, Murree and Nilore (Fig. 1), approximately located at 48, 64 and 100 km, respectively, from epicentre. The PGA values from these stations were 0.231, 0.078, 0.026 g, respectively [5]. The 5% damped elastic response spectrum from the Abbottabad record showed a wide range of high amplifications over the period range of 0.4–2 s with the highest amplification of 4. Since Nilore is located in Islamabad, the maximum amplification from the spectrum was found to be 3. This record is recommended to be used with care since the horizontal peak ground acceleration (PGA) is overestimated by attenuation relations, whereas vertical PGA at Nilore is well predicted (Durrani et al. [6]). The most probable reason for this overestimation of horizontal PGA was linked to raft foundation size where the instrument is anchored (Durrani et al. [6]). In the past, a series of seismic zoning maps [7–9] were prepared for Pakistan and upgraded with the passage of time due to additional data and knowledge, resulting in variations in the seismic zonation between the maps of different times. Since no recurrence intervals of the various intensities were specified, demand in each zone could be not coupled with a probability of exceedance (POE). After the Kashmir earthquake, many probabilistic seismic hazard studies have been conducted by different organizations and researchers [10– 13] which predicted higher seismic hazard as compared to previous seismic zoning maps, especially for the northern region of Pakistan. As a result, it is now accepted that much of the building structures in the region are not adequately designed for the seismic hazard perceived today, and hence there is a need to identify appropriate seismic forces for design of new structures and to carry out an independent vulnerability assessment of the existing building stock. Moreover, seismic forces, time period, displacement, etc. can change signifi- cantly because of the soil–structure interaction (SSI) effects depending on soil stiffness, foundation type, size, method of analysis, etc.