Structural health monitoring study of the ZOIL building using earthquake records

Abstract

Accelerograms from 11 earthquakes recorded over a period of 25 years (1980–2004) in a 12-story office building in Skopje, North Macedonia, are used to describe changes in its vertical velocity of wave propagation and frequencies of vibration and infer about possible loss of stiffness due to damage. The building was built in 1976 and is a rare example of an instrumented structure on the territory of former Yugoslavia, one of the most seismically active regions in Europe. It is cast-in-place reinforced concrete structure, with 12 levels above ground at its highest point and two levels below ground (level 0). The velocities of vertically propagating waves in the building are identified by fitting a uniform and a two-layer shear-beam model using a waveform inversion algorithm, which fits propagating pulses in band-pass filtered impulse responses. The analysis reveals permanent loss of stiffness caused by the Gnjilane earthquake of 2002 (M= 5.1 at R= 48 km). The observed changes in equivalent shear-wave velocity are about 10%, which translates to 19% change in stiffness. No structural damage was observed in the building following the Gnjilane earthquake. The observed change in wave velocity was likely caused by damage of the nonstructural elements and possibly by microcracks in the structural concrete. Unfortunately, the analog seismic monitoring system, which is currently not maintained, has not been replaced by a modern digital system and opportunities to record significant response have already been lost.