Structural Damage: Kocaeli, Turkey Earthquake, August 17, 1999
by Michel Bruneau, MCEER Deputy Director and Professor, Department of Civil, Structural and Environmental Engineering, University at Buffalo
While thousands of people died in the numerous building collapses, the damage to reinforced concrete buildings was not surprising, nor unexpected. Similar types of damage were observed to a lesser extent in many prior earthquakes throughout Turkey. However, because these other earthquakes were in more remote, less populated regions, the message from earlier reconnaissance visits apparently did not resonate to the same degree.
The predominant structural system used for buildings in Turkey consists of reinforced concrete frames with unreinforced masonry infills. This structural form is used for all building heights and occupancy, from single-story commercial to multistory residential and office buildings. Frame-shear wall interactive systems are also used in new buildings. Industrial buildings are either reinforced concrete (cast-in-place or pre-cast) or steel frame structures.
A typical reinforced concrete frame building in Turkey consists of a regular, symmetric floor plan, with square or rectangular columns and connecting beams. The exterior enclosure as well as interior partitioning are of non-bearing unreinforced brick masonry infill walls. These walls contributed significantly to the lateral stiffness of buildings during the earthquake and, in many instances, controlled the lateral drift and resisted seismic forces elastically. This was especially true in low-rise buildings, older buildings where the ratio of wall to floor area was very high, and buildings located on firm soil. Once the brick infills failed, the lateral strength and stiffness had to be provided by the frames alone, which then experienced significant inelasticity in the critical regions. At this stage, the ability of reinforced concrete columns, beams, and beam-column joints to sustain deformation demands depended on how well the seismic design and detailing requirements were followed both in design and in construction.
The damage to reinforced concrete buildings from this earthquake can be attributed to one or more of the following:
Foundation failures were observed for a large number of buildings with large settlements, and in some cases, entire structures overturned (as shown in figure at right).
A large number of residential and commercial buildings were built with soft stories at the first-floor level. First stories are often used as stores and commercial areas, especially in the central part of cities. These areas are enclosed with glass windows, and sometimes with a single masonry infill at the back. Heavy masonry infills start immediately above the commercial floor. During the earthquake, the presence of a soft story increased deformation demands very significantly, and put the burden of energy dissipation on the first-story columns. Many failures and collapses can be attributed to the increased deformation demands caused by soft stories, coupled with lack of deformability of poorly designed columns. This was particularly evident on a commercial street where nearly all buildings collapsed towards the street.
Soft first stories caused many residential and commercial buildings to collapse
Strong beams and weak columns
Most frame structures have strong beams, remaining elastic, and weak columns suffering compression crushing or shear failure. In many cases, relatively deep beams were used with flexible columns, contributing to the strong-beam weak-column behavior.
Lack of column confinement and poor detailing practice
Most of the structural damage observed in frame buildings was concentrated at column ends. Unfortunately, confinement reinforcement virtually did not exist in these members, making them unable to maintain the required ductility. A number of detailing deficiencies were observed in the damaged structures. This included lack of anchorage of beams and column reinforcement, insufficient splice lengths, use of 90o hooks, poor concrete quality, less than full height masonry infill partitions, and frequent combinations of many of the above. These errors were often compounded by geometric irregularities such as eccentric beam-to-column connections that induced severe torsion in short perpendicular stub beams.A number of buildings directly sitting on the fault were also destroyed by the relative movements of the fault. It is noteworthy that an industrial complex being constructed 100 feet from the fault had very well confined columns with damage limited to spalling and large residual displacements.
Steel, being by far the most expensive construction material in Turkey, has been used rather sporadically in construction; only industrial structures rely on steel for their lateral load resistance. Some were damaged by this earthquake. A few collapsed. Typical causes for collapses include failure of anchor bolts at column bases and structural instability under overturning forces. Other evidence of damage include fracture of brace connections, buckling of braces, and local buckling in concrete filled steel hollow pipes used in wharves.