Also, the accelograms recorded near the area indicate two major subevents (Papageorgiou, A. 1999, MCEER), estimated to be 30 km. apart from each other. This increased the effective duration of the earthquake, resulting in increase of damage to the structures.
Figure-5)
Typical accelogram from the Izmit earthquake (Kandilli
Observatory, Turkey 1999)
Performance of Residential Buildings
The most common residential structure types in the country are 2 to 5 story low-rise buildings. The primary material used in constructing these structures is concrete, reinforced with straight, or in some occasions ribbed bars. Infill walls are made of unreinforced brick walls, the use of shear walls are quite rare. The floor plans are usually symmetric, incorporating 2 to 4 apartment units on each floor. A typical residential building in the area is shown below.
Figure-6)
Typical residential building in Izmit area, damaged after the earthquake. The
brick walls and reinforced concrete columns are exposed at the corners. (USGS,
1999)
Observations conducted by several research teams indicate common characteristics in the buildings that have been severely damaged or failed, some of which are,
- Poor structural detailing: Many of the buildings in the area collapsed as a result of failure in beam-column connections. This was primarily due to the poor detailing in the structural design that led to insufficient amount of reinforcing and bracing in critical parts of the structure. Figure-7 shows a perfect example of a column with almost no bracing at all.
Figure-7) Lack of bracing caused the damage of many buildings in the area. (USGS, 1999)
- Lack of shear walls: Many of the buildings the region were not built with
shear walls. In many instances, the unreinforced masonry walls resisted the
lateral loads (Bruneau, M. , MCEER 1999). The resulting failure is also referred
to as the “pancake effect”, in which the frame elements cannot resist
lateral loads, collapsing the building at once. Figure-6 shows a typical example
of such failure.
- First floor failure: The design of buildings constructed on the main streets were modified in order to create more open spaces in the first floor, in order to accommodate shops and stores. This was a fatal mistake, most of these buildings collapsed on their first floors, usually damaging the floors above it as well.
- Inadequate foundation support: Areas like Duzce and Golcuk were constructed on lakebeds and shorelines, where the soil is mainly composed of weak sand and silt. This resulted in topping and settling of buildings in these areas. According to observation done by the EERI team, more than 60 percent of multistory buildings in liquefaction areas were severely damaged or collapsed.
-Damage due to inundation and tsunamis: The earthquake caused a downthrown of 2m on the northern part of the ridge (Kandilli Observatory) that resulted in damage to the coastal buildings in the region. Several tsunamis are also reported, but their effects on structures are not clearly known.