Earthquake Engineering Center at the Department of Civil Engineering, N-W.F.P. University of Engineering and Technology, Peshawar, Pakistan
An earthquake of Magnitude Mw = 7.6 occurred on October 8, 2005 at 08:50 am local time causing damage and casualties over an area of 30,000 km2 in the N-W.F.P. province of Pakistan and parts of Pakistan-administered Kashmir. The main event was followed by more than 978 aftershocks of Magnitude Mw = 4.0 and above, as of October 27, 2005. The epicenter of the main earthquake was located at a latitude of 34° 29? 35? N and longitude of 73° 37? 44? E. The focal depth of the main earthquake was determined to be 26 km (USGS). This was the deadliest earthquake in the recent history of the sub-continent resulting in more than eighty thousand casualties, two hundred thousand injured, and more than 4 million people who have been left homeless. The adverse effects of this earthquake are estimated to be larger than those of the tsunami of December 2004.
The major cities and towns affected are Muzaffarabad, Bagh and Rawlakot in Kashmir and Balakot, Shinkiari, Batagram, Mansehra Abbotabad, Murree and Islamabad in Pakistan. Almost all the buildings, mainly stone and block masonry laid in cement sand mortar with RC slabs or GI sheet roofing, collapsed in the areas close to the epicenter. In regions approximately 25 kms away from the epicenter nearly 25% of the buildings collapsed and 50% of the buildings were severely damaged. The structures in the affected region are primarily unreinforced stone, concrete block and brick masonry, and reinforced concrete frames with concrete block or brick masonry infill panels.
Performance of Unreinforced Stone Masonry Buildings
A significant number of casualties and injuries in the affected region was associated with the complete collapse of single story unreinforced stone masonry buildings. The stone masonry walls consisted of irregularly placed undressed stones mostly rounded that were laid in cement sand, mud mortar or even dry in some cases. Features of construction which appear to be responsible for widespread collapse of buildings are:
- Stone masonry buildings were more common in the villages (75% of the building stock) than in the cities (15% of the building stock). The quality of mortar and stones used and the level of workmanship were very poor due to the poor economic situation of the people. The most commonly used mortars consisted of 1 part cement to 10 part sand. The approximate crushing and shear strength of such mortar is 300 Psi and 5 Psi respectively. The rounded and smooth stones in addition to the poor quality of mortar rendered a very loose bond between the stones which made the structures extremely vulnerable to earthquake forces.
- No horizontal bond beams were provided at the levels of plinth, or roof. Lintel beams were provided only above the openings and were not run continuously along the perimeter of the walls.
- No vertical members of concrete or wood were provided in the walls and therefore the collapse of a particular portion of the wall progressed in an uninterrupted manner to other portions of the walls and buildings.
- In some cases, certainly due to economic constraints, the stones were observed to have been laid even dry (no mortar at all) and the gaps were filled by small pieces of stones, leaving the walls extremely vulnerable to horizontal ground shaking.
Performance of Unreinforced Solid Concrete Block Masonry Buildings
Concrete block masonry buildings with 6 inch thick walls were widely used in the cities (about 60% of the buildings) and villages (about 25% of the buildings) in the affected area. Solid concrete blocks 6 inches thick, 6 inches wide and 12 inches long were laid in cement sand mortar. The collapse of these block masonry buildings in urban areas (more than 60%) was responsible for the major portion of deaths and injuries in the cities. The most probable reasons for failure were observed to be:
- Poor quality of concrete used for fabrication of blocks, rendering low strength blocks.
- Poor quality of mortar.
- Inadequate thickness of walls (6 inch) which were the main shear resisting elements.
- No integrity of the wall in the transverse direction
- Weak connections at corners
Performance of Unreinforced Brick Masonry Buildings
By and large brick masonry buildings performed relatively better than the stone or concrete block masonry buildings. Unreinforced single and two story brick masonry buildings, with RC slabs as roofing, comprise 25% of the total building stock of the cities near the epicenter. It was observed that only 30 % of these building collapsed, while the rest suffered only slight damage. The brick masonry buildings were only constructed by well-off people because the unit cost of brick masonry was higher than that of other forms of masonry in the area. It was observed that along with better workmanship, good quality mortar was used in the construction of brick masonry buildings.
However no evidence of either bond beams or other earthquake resistance improvement techniques were found in such buildings. It is also worth mentioning that 4.5 inch thick brick masonry walls collapsed or were badly damaged in almost all buildings of the affected areas, which in some cases resulted in the collapse of the entire building.
Performance of Reinforced Concrete Framed Buildings
For the past 15 years, reinforced concrete frame buildings have been increasingly used for the construction of government offices, colleges, hospitals, hotels, markets, and residential buildings. Many concrete buildings completely collapsed and many more were seriously damaged by this earthquake. The key reasons for these failures are:
- The failure mode of the structures reveals that most of the structures were designed with strong column-weak beam connections. In severely damaged buildings, columns were observed to have cracked at the beam-column intersection. Figure 7 shows the formation of a plastic hinge in one of the columns of a building. Inclined cracks were also found at the midheight of some reinforced concrete columns. In these structures, beams were found to be intact and undamaged. Infilled 4½"-thick masonry walls in these structures were severely damaged.
- Primary factors contributing to the failure of reinforced concrete frame structures include deficient design for seismic forces, improper length and location of column splices, improper spacing and anchorage of lateral ties in columns, and poor quality of concrete.
- The maximum strength of concrete in these buildings was about 2,000 psi.
Landsides and Liquefaction
Landslides of enormous magnitude also occurred at various locations in the affected area. Landsliding is responsible for some casualties and for the blockage of roads which badly hampered the rescue and relief efforts. A significant portion of the mountain range to the north of Muzaffarabad city was lost due to landsliding. Rivers in the city of Muzaffarabad were flooded with material due to landsliding. No reports of liquefaction have been received so far.
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