Keywords: earthquake, damage, distress, restoration, analysis, remedial work.

A two-storey house has been constructed in the suburb of Philothei, Athens, Greece in 1933. Two extensions to the original property were carried out in the SW corner of the development in 1938 and 1947 respectively. Construction of most of the low-rise houses of these days was done on a grid of shallow beam foundations made of reinforced concrete (rc), good quality load bearing stone walls, rc floor slabs and a conventional slopping roof made of ceramic tiles. In early September of 1999 a 5.9 magnitude earthquake in the Richter scale hit Athens with epicentre only 10 to 20 Km from the centre of Athens. The tremor was quite powerful and tested most of the old and new building stock of the capital of Greece. It resulted in extensive damage to buildings and killed more than a 100 people. This near field event was responsible for medium to severe distress to a number of old low rise buildings in downtown Athens including the two-storey house in Philothei. To this end, the work described in the paper has been intended to (a) record and describe the damage to the structure (b) back-analyse and evaluate using modem computer techniques the behaviour of the structure during the event and (c) propose and study the recommended restoration of the building for future use.

The main seismic event resulted to maximum ground accelerations (recorded by a nearby station) of 0.19g and 0.13g in the two horizontal directions and 0.11g in the vertical direction. The acceleration response spectra of the event, at the range of small periods ( 0.2 sec), indicate a maximum horizontal acceleration of 800 cm/sec2, a value that is double the 400 cm/sec recommended by the latest Greek Seismic Code for soil category B. The 67 years old two-storey stone house is particularly stiff with a natural frequency of about 0.1 sec. The structure has experienced no damage during previous earthquakes and particularly the severe Athens earthquake of 1981. This is due to the fact that past earthquakes were all far field events with high accelerations at periods greater than 0.3 sec. However, the high lateral forces developed during the 1999 seismic event, mainly along the EW direction of the structure, resulted in the following damage pattern:

(a)

horizontal cracks at the perimeter of the building, some 10 to 15 cm higher than the ground floor slab

(b)

severe shear cracks (inclined at 45º) to both SW and NW corners of the first floor of the building.

(c)

open vertical cracks along of the joint where the original structure has been extended during the re-development of 1938 and 1947.

(d)

non structural cracking to infill panels and partition walls particularly in the area close the stairway.

(e)

movement of the roof ceramic tiles of the East side by some 12 cm.

Based on the observations stated above the level of distress to the structure was considered medium to severe for the ground floor of the house and a detailed analysis was requested by the owner prior to any remedial and/or restoration works. The analysis was not limited to the study of the dynamic behaviour of the structure during this particular earthquake but also tried to cover the likely seismic behaviour of the restored structure to future near and far field events. It was decided to carry out the analysis of the seismic behaviour of the structure using a simplified three dimensional model on a powerful and tested, commercial structural analysis program (STAAD III). An initial model and seismic loading were set up using known parameters of material properties and seismic input (i.e. response spectra from the actual event). Firstly a back analysis of the behaviour of the structure to the seismic input of this earthquake was carried out. This indicated a closed agreement between the observed and the predicted behaviour in terms of crack patterns, stress and displacement characteristics. Based on the confidence that was built-up from the first successful run, the model has been further developed and used to analyse the engineering significance of:

(a)

other seismic events with high likelihood of occurring during the next 50 years and may affect the structure.

(b)

the effectiveness of the remedial/restoration measures

(c)

the effect of any structural or non-structural alterations requested by the owner to the ground floor to cope with future needs.

The analysis indicated that the damage to the structure was compatible with the high horizontal forces experienced in the EW direction together with the relatively high uplift. The main aim of the restoration of the building concentrated in strengthening the weak sections of the buildings against future near and far field events. In addition a more balanced distribution of stiffness among the structural elements of the ground floor is proposed to minimize the cracking to the non-structural elements of the building during a future event (i.e. demolition of a number of original infill panel).

The three types of proposed remedial/restoration works strengthen the weak zones of the building i.e. the areas which the recent earthquake has exposed. It is believed that these works will significantly minimize the cracking levels which this structure may experience during an earthquake from the Attica peninsula with the next 50 years. It is concluded that with the use of a powerful computer program the expected behaviour even of an old building to severe dynamic loading can be predicted, the weak areas of the structure can be identified and remedial/restoration works can be employed to strengthen the structure against future seismic events. Special detailing is required to effectively treat damages zones depending on the intensity of cracking.

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