Dear Readers,
This special publication combines issues 2 and 3 of the current volume and it is motivated by the recent sequence of shocks that affected the Emilia-Romagna region of Italy during the month of May 2012. We have been able to release it in a relatively short timeframe thanks to the efforts of the contributing authors that submitted their reconnaissance reports in a record time without forsaking thoroughness of observation and depth of analysis, considering causes, effects and indicating possible remedies, to the observed damage and disruptions.
The events underlined again some of the problems well known to the whole community but at the same time have revealed new lessons that require renewed attention. It is worth mentioning that the nature of the seismic excitation appeared characterized by a significant component of vertical acceleration as well as a large spectral displacement demand for long periods. The significant vertical acceleration level and its effects highlighted, once again, its critical and under-researched role in inducing damage. Two were the evidences that, even though well known to the scientific comunity, most dramaticaly captured the pubblic attention: the extent of liquefaction occurrences and the vulnerability of industrial buildings. Both topics are addressed in this issue. The classification of the region as a low seismicity zone does not justify however the incidences of poor engineering judgment and lack of common sense, that emerged in some cases. In Emilia Romagna, as in other Italian regions, historic buildings form a consistent portion of the urban centres. The survey of both heritage and historic buildings in the many small towns of the hit region, indicated a generic good performance with the exception of cases where wall connections were poor and low material quality and lack of maintenance was evident.
Of particular interest was also the after-earthquake approach of the agencies involved in the immediate emergency management and planning for reconstruction, in clear contrast with what observed after L’Aquila’s earthquake. The opening paper suggests thoughts on this subject that should guide us in our future work of prevention, conservation and reconstruction.
Have a good read.
Month: July 2012
SOIL LIQUEFACTION PHENOMENA OBSERVED IN RECENT SEISMIC EVENTS IN EMILIA-ROMAGNA REGION, ITALY
Significant and widespread liquefaction effects, which caused panic of inhabitants and damage to buildings and infrastructures, were observed in various areas of Emilia-Romagna region, Italy, during the seismic events of May 20 and 29, 2012, with magnitude respectively of Ml 5.9 and Ml. 5.8. In Italian seismic literature these phenomena represent an interesting case study for a number of reasons: for the exceptional impacts, for the size of the area in which liquefaction effects were observed, for the amount of damage produced, for the rarity of soil liquefaction Italian case histories. Immediately after the earthquake of 20 May an extensive field reconnaissance was conducted through the natural and built environment to capture as quickly as possible surface evidences of liquefaction and to document the extent and the severity of damage. The most significant and widespread liquefaction impacts were found in the two settlements of San Carlo and Mirabello. With the aim of a better understanding of the observed scenarios, a detailed investigation program, including geophysical surveys and geotechnical testing (soundings, cone penetration tests, seismic cone penetration tests, cross-hole and down-hole tests as well as many cyclic laboratory tests), was planned. These investigations are now in progress and will be object of forecoming technical notes. This paper summarizes the observation of soil liquefaction effects made during field investigations and presents a first interpretation, based on available information on ground shaking and soil conditions, of the factors that may have contributed to determine the low liquefaction resistance of soils.
THE EMILIA ROMAGNA, MAY 2012 EARTHQUAKE SEQUENCE. THE INFLUENCE OF THE VERTICAL EARTHQUAKE COMPONENT AND RELATED GEOSCIENTIFIC AND ENGINEERING ASPECTS.
The 20 – 29 May 2012 earthquake events occurred along Emilia Romagna segment of Po Plain valley (Pianura Padana), a NW – SE oriented basin along the Southern tectonic boundary of Adria plate, with Northern Apennines. Major earthquake sequence events’ focal mechanisms, evince the activation of low angle thrust structures, separating the earthquake activity on at least 2 distinct fault planes, with similar slip characteristics. Both planes seem to be on the same seismogenic source, on the Western part of 1570 events’ source, interpreting the delimitation of the seismic activity of the May – June 2012 earthquake sequence. Widely observed secondary effects were categorized with respect to spatial distribution, density and trend, to: i) individual liquefaction cases, ii)Strictly oriented liquefactions and lateral spreading due to exposure of unconformities to surfacial liquefaction-prone geological formations and iii)extended liquefactions of confined liquefiable formations (at a depth of≈10m) and lateral spreading along river strike. This depth resulted in less surfacial deformation and differential subsidence, as well as very limited structural damage. Only fences, pavements and other light weight structures have been heavily damaged, deformed and dislodged. The incurred structural damage is extremely heavy for the churches and the cultural heritage, as well as for the industrial infrastucture of the region, besides the death toll and injuries. The available strong motion records are not adequate in order to explain in a rational and reliable way the above mentioned heavy damage. Based on years’ experience in post earthquake investigations and on results of numerous full scale tests of structures on shaking tables, the authors concluded that the main reasons of the damage is the extremely high vertical ground shaking (of the order of 1.0 g) in combination with the moderate horizontal motions. Due to this very high value of the vertical ground shaking, impact phenomena are observed as well as total or partial loss of friction (due to the loss of gravitational forces).The incurred damage are grouped in general categories possessing similar characteristics. The majority of the observed damage of churches is mainly due to collapse of their roofing systems. This occurred due to resonance-like vertical vibration of the timber trusses of the roofs. This was proved by analysing representative dimensions of timber trusses with the overburden mass and, it was found that their fundamental periods are between 0.06 and 0.10 sec. On the other hand, a predominant period of the vertical component is, according to the recorded first event of 20th May 2012, 0.06 to 0.07 sec. Thus, intensive up and down motions are induced in the whole trusses and in their horizontal lower beams resulting in dislocations from their supports on the walls. On the other hand, it is well known that the arrival time between P (vertical) and S (horizontal) waves in epicentral regions is quite small. This resulted in a convolution between vertical and horizontal ground motions. The top of the walls, where are the supports of the trusses, were significantly displaced as free standing vertical cantilevers due to the loss of connection with the truss. In this way the trusses totally lost their supports and collapsed. In examining the debris one may guess what structure between the two (truss or wall) collapsed first. A proof of the dominance of the vertical component might be based, among others, on the response of bell towers that in the majority of the cases were not significantly damaged and on the fact that the mode of collapse is inside the ground plan of the building. Quite similar observations are also valid for the damaged industrial facilities. The natural vertical periods of the roofs are also, between 0.06 and 0.10 sec. The constructed supports are from functional point of view quite similar to those used for the support of the timber trusses in the churches (just simply supported cantilevers). Nevertheless, by using the conventional bolts in the supports, it is not certain whether or not the situation would be ameliorated. In order to prevent the effects of the high frequency impact type of the vertical ground motion a kind of absorbing elastomeric devices could be used in the supports of beams and trusses besides the conventional anchoring systems. Also, a kind of base isolation devices, that should not depend on friction at the base of the structures might be used. Finally, the partial collapse of the ceramic factory is attributed to the rhythmic phenomena of destruction. After the present communication was completed, it has just been released the important and technically very interesting vertical strong motion record of the second event, presenting a peak ground acceleration of the order of 0.9 g, a fact that comes in full agreement with the findings and arguments exposed in the present paper.
ASSESSMENT OF THE MOST DAMAGED HISTORIC CENTRES OF THE REGION EMILIA ROMAGNA DUE TO THE EARTHQUAKE OF THE 20th AND 29th OF MAY 2012
The Emilia Romagna earthquake of the 20th of May 2012 has damaged some of the historic centres closer to the epicentre of this event. Aftershocks and a second earthquake on the 29th of May 2012 registered in the same region have caused around 30 dead people, 350 of injured people and several damage types to historic assets. In this paper the typical features of the residential constructions in the areas damaged by these earthquakes are introduced and the seismic vulnerability of the Comune of Cento is carried out by using the FaMIVE approach. The same area has also been surveyed by the Italian Civil Protection to assess the safety of the buildings with the aid of AeDES form. The damage scenario observed on site, the safety judgements conducted by AeDES and the forecasted vulnerability obtained by FaMIVE will be illustrated for some building clusters selected in the Comune of Centro.
SEISMIC DAMAGE ASSESSMENT OF SCHOOL BUILDINGS AFTER 2012 EMILIA ROMAGNA EARTHQUAKE
In the paper the usability checks performed on precast RC gyms and masonry school buildings, located in the Municipalities of San Pietro in Casale (BO), Bomporto (MO) and Bondeno (FE) after the seismic events occurred on 2012 May 20th and 29th, are reported and discussed.
First, the main features of surveyed buildings have been identified. Later on, on the basis of the detected damages, their seismic deficiencies have been recognized. Finally, the indication of some appropriate simple interventions for retrofitting studied buildings has been given.
THE SEISMIC BEHAVIOUR OF ANCIENT MASONRY BUILDINGS AFTER THE EARTHQUAKE IN EMILIA (ITALY) ON MAY 20TH AND 29TH, 2012
The seismic sequence that occurred in Emilia Region on May 2012 caused serious damage to specific building types (e.g. industrial sheds designed without any aseismic prescriptions) and cultural heritage buildings. The distinctive features of the seismic motion, recorded also very close to the epicentre, are a significant spectral displacement demand at long periods and a very high vertical component. These are very critical for structures characterized by long periods, as churches or bell tower, or for standing out elements, as pinnacles or battlements. The paper presents a preliminary damage assessment of traditional masonry constructions (both ordinary and monumental), made by solid brick masonry with lime mortar. Observed damage has been interpreted through simplified mechanical models, by using the displacement-based approach. It is worth noting that the behaviour of ordinary residential buildings was quite good, thanks to the good connection between masonry walls and the localization of cracks in the spandrels, which guaranteed high displacement capacity.
DAMAGES AND COLLAPSES IN INDUSTRIAL PRECAST BUILDINGS AFTER THE EMILIA EARTHQUAKE
The present paper describes the collapses in precast industrial buildings caused by the 2012 Emilia earthquake. This kind of buildings, not designed with seismic criteria, has been found very vulnerable for several reasons, such as the absence of connecting systems between the various precast monolithic elements, the inadequacy of connection systems of external precast walls to the bearing elements (columns and beams), the interaction between non-structural walls and structural elements, and the inadequacy of the foundations.
THE EMILIA-ROMAGNA EARTHQUAKE: DAMAGES TO PRECAST/PRESTRESSED REINFORCED CONCRETE FACTORIES
Immediately after the seismic event (started on May 20th, 2012) which struck the Padana Flat area between the municipalities of Ferrara, Modena, Reggio Emilia, Bologna (Emilia-Romagna Region), Mantova (Lombardia Region) and Rovigo (Veneto Region), an ENEA team of experts (Maurizio Indirli, Bruno Carpani, Elena Candigliota, Alessandra Gugliandolo, Francesco Immordino, Giuseppe Marghella, Anna Marzo, Giuseppe Nigliaccio, Alessandro Poggianti, Maria-Anna Segreto) supported the Italian Civil Defense, in order to perform prompt investigations regarding the safety evaluation of different typologies of structures (bridges, industrial factories, residential houses, etc.) made by various kinds of materials (masonry, reinforced concrete, precast/prestressed reinforced concrete, mixed). General information is given in several preliminary reports [among them, Decanini et al., 2012). In particular, this article is devoted to the behaviour analysis of precast/prestressed reinforced concrete (p/p. r.c.) construction, widely used in the affected area since the second half of the last century, trying to deepen some crucial aspects of the matter.