The "Emilia" earthquake: an overview of damages and collapses in industrial precast buildings

Transcription

1 The "Emilia" earthquake: an overview of damages and collapses in industrial precast buildings M. Savoia, N. Buratti & V. Ligabue DICAM University of Bologna, Italy L. Vincenzi DIEF University of Modena and Reggio Emilia, Modena, Italy 2015 NZSEE Conference ABSTRACT: A series of earthquakes struck the Emilia region, in Northern Italy, in May Two main mainshocks, characterized by moment magnitudes of 6.1 and 6.0 were identified. The area struck by the earthquakes was not considered as seismic until a few years ago: only in 2001 an update in the Italian seismic hazard map assigned to that region a low to moderate seismicity, becoming mandatory for designers only in For this reason, most of the buildings that experienced the earthquake were designed without seismic criterion. As the region is one of the most productive in Italy, a high concentration of industrial precast buildings can be found and the ground-shaking resulted particularly severe for these structures. In the paper, an overview of the damages and collapses caused by the earthquake on precast industrial buildings is given. Extensive field surveys took place immediately after the shocks, enabling the authors to observe and collect a vast amount of data on damages of these buildings. They were very vulnerable for several reasons. The absence of connections between the various precast monolithic elements was the main cause of most collapses. The inadequacy of connection of external precast walls to the bearing elements and the interaction between non-structural walls and structural elements were also critical aspects in many cases. For recently designed industrial buildings, the inadequacy of columns and foundations was responsible of very serious failures. Finally, some collapses were related to the overturning of metallic shelves not designed to sustain horizontal actions. 1 INTRODUCTION A series of strong earthquakes struck the Emilia region, in Northern Italy, in May 2012 (Fig. 1). Two main mainshocks, featuring similar energy, were observed. The first mainshock with moment magnitude, M w = 6.1, struck on May 20th while the second mainshock with M w = 6.0 struck on May 29th. The balance of this seismic sequence was of 17 causalities, hundreds of injured, severe damage to historical and cultural heritage buildings, and industrial facilities. No seismic criteria were mandatory for structural design until the last decade. Only in 2001, the updated Italian seismic hazard map assigned to that region a low to moderate seismicity. The hazard map was formally adopted in 2005, becoming mandatory for designers after For these reasons, almost all the industrial buildings that experienced the earthquake ground motions had been designed without any seismic criterion. The May 20th earthquake caused the collapse of several reinforced concrete precast buildings in the industrial areas of S. Agostino, Bondeno, Finale Emilia, S. Felice sul Panaro, while the May 29th earthquake was particularly severe for industrial buildings in Mirandola, Cavezzo and Medolla. Some estimates indicate that, in the industrial areas closest to the epicenters, almost 70% of reinforced concrete precast buildings collapsed or were severely damaged by the two earthquake sequences. Issues and collapses related to precast buildings were reported by many authors after past earthquakes (Iverson and Hawkins 1994; Ghosh and Cleland 2012; Toniolo and Colombo 2012), but the extent and the severity of the collapses observed after the Emilia earthquake are unprecedented in Italy. Paper Number P-55

2 The present paper describes the damage and the collapses that were observed by the authors during the field surveys that took place in the zones struck by the earthquake. The main causes of the aforementioned collapses were related to the vulnerabilities of precast buildings not designed with seismic criteria. In most cases, it was possible to identify the reasons of the collapses, in relation with the usual design criteria for non-seismic zones adopted in the region. The loss of support of roof elements from beams and of beams from column supports, due to the lack of mechanical connections between various precast monolithic elements (columns, beams, slab elements, cladding panels) was the main cause of most collapses, even if, the large displacements causing the fall of precast beams from the column supports, were in some cases amplified by other phenomena, such as the insufficient strength of columns or the large rotations experienced by the foundations, being both (columns and foundations) not designed for horizontal seismic actions but for wind actions at most. In many cases, the interaction with non-structural walls, such as masonry infills or non-structural reinforced concrete elements, had strong negative effects and was the cause of partial collapses of buildings. Furthermore, several failures involved reinforced concrete precast cladding panels, because of the inadequate anchorages on the bearing elements (columns and beams). Finally, some collapses were related to the overturning of shelves in warehouses or automated storage facilities, not designed for horizontal actions due to earthquakes. The surveys documented in the present paper, together with those produced by other research groups, were the starting point for the definition of a document, Guidelines for local strengthening interventions and global retrofitting of precast concrete industrial buildings not seismically designed (in Italian), issued two weeks after the earthquakes (June 2012) by the Seismic safety of industrial precast buildings Working Group of ReLUIS Consortium. 2 FEATURES OF THE GROUND MOTION Several ground-motion recording stations of the Italian strong-motion network recorded the groundshaking during the 2012 earthquake sequence (Gorini, Nicoletti et al. 2010). Furthermore, after the first mainshock several additional temporary recording stations were installed. In the following, the ground motions produced by the two mainshocks, and registered by the station closest to the epicentre (Mirandola) will be briefly analysed. Site classification data are not available for all the recording stations but, a class C site, according to the Eurocode 8 classification (v s,30 = m/s), can be reasonably assumed. The peak ground motion parameters are reported in Table 1 together with the Arias intensity. Pseudoacceleration (PSA) response spectra have also been computed and are depicted in Figure 2. For both the mainshocks similar horizontal pseudo-accelerations were recorded at the Mirandola station, in the 0.5 s -1.0 s period range, in spite of the different source-to-site distance. On the other hand, the May 29th ground-motion shows larger accelerations around T =1.5 s. In this case, near-source effects may Figure 1. Geographical position of the region interested by the earthquakes (left) and positions of the epicentres (right). The color scale indicates the earthquake dates. 878

3 have contributed to the spectral shape. Vertical pseudo-acceleration response spectra show very large acceleration values, up to 3g, for the May 29th recording while the accelerations for the May 20th event are smaller. This difference is obviously dependant on the different source-to-site distance and on the fast attenuation of vertical ground-motion components. 3 TYPES OF PRECAST BUILDINGS IN THE REGION The typical layout of a single-storey industrial building in the region struck by the earthquake main shocks is composed of a series of basic portal frames. Each frame has two columns with moment-fixed connections at the foundations and a pin-joined roof beam. Beam-column connections are typically friction-based supports without any connection device. In some cases, neoprene pads are placed at the column-beam and column-base interfaces. The stability of the structures and their strength with respect to horizontal actions were based on the cantilever behaviour of the columns. In order to simplify the presentation, the reinforced concrete precast buildings will be roughly divided into two main categories: Type 1: precast structures built in the 70 s and in the 80 s; Type 2: precast structures built after the 80 s. Following the technical evolution of precast technology, these structures were in fact very different in dimensions and spans. They were designed according to different criteria and experienced different damage and collapse mechanisms, the most recurrent ones will be described in the next sections. Most of the Type 1 industrial buildings have precast beams with spans around m and the distance between frames is about 6 10 m. The precast beams have a sloped pane (SI beams), and the columns are quite slender, with square cross-sections with cm edges. These buildings typically have masonry infills, along both short and long walls, providing stability against horizontal loadings due to wind (the only action considered at the time of construction), and RC columns were designed for gravity loads only. No beam-column connectors are present. The beams typically have either no or little restraints against the out-of-plane movements, especially when both transverse and longitudinal beams are supported at the top of the column. Table 1. Main features of the ground-motions recorded at the Mirandola station. Earthquake R epi (km) 20 May May 4.1 Component PGA (cm/s 2 ) PGV (cm/s) PGD (cm) Ia (cm/s) Max Horizontal Vertical Max Horizontal Vertical Figure 2. Pseudo-acceleration response spectra for the horizontal (left) and vertical (right) ground-motion components of recorded at the Mirandola station. 879

4 More recent type 2 precast buildings have frames with longer spans (up to 30 m), often in both directions (load bearing beams and slab) and normally adopt I-shaped beams with straight profiles. In order to reach significant spans in the slab direction, different kinds of prestressed elements are adopted: TT-shaped beams, beams with a constant section with a V-shaped symmetric or asymmetric profiles, and wing contour roofing elements. In the latter case, between the structural elements there are curved tiles made of glass or transparent polycarbonate for purpose of lighting the interior of the building. When the latter solution is adopted, quite commonly in the last 20 years in large industrial buildings (spans longer than 20 m), the roof plan is of course highly deformable. RC columns have very large cross-sections (edges up to cm) and must bear both vertical and horizontal loadings, because the curtain walls are externally fixed to the columns. Curtain walls can be either made of horizontal or vertical cladding panels. The panels are normally connected by means of mechanical devices to the columns and, when necessary, to the upper beams. 4 DAMAGES AND COLLAPSES IN TYPE 1 INDUSTRIAL BUILDINGS 4.1 The structural behaviour for regular buildings In precast industrial buildings built in the 70 s and 80 s, the reinforced concrete structures are very flexible under horizontal loads, due to the small cross-section of columns, typically designed for gravity loadings only. Accordingly, minimum longitudinal steel reinforcement in columns was typically adopted. The stability of these buildings against horizontal loadings due to wind (the only action considered at the time of construction) is due to the presence of masonry infills. Therefore, the effect of the in-plane stiffness of the curtain walls in both longitudinal and transverse direction is very important because, in general, it is much larger that the column stiffness. In some cases, accurate detailing was designed in order to allow a good connection between masonry walls and RC columns. In buildings with regular curtain walls along all sides of the building and the roof sufficiently rigid in its own plane to transfer horizontal actions to the wall panels, the strength of the latter was in general sufficient to resist the horizontal forces induced by the ground motions; in most cases little damages only were observed in the masonry walls (Fig. 3a). In few cases only, this kind of building collapsed. For the building depicted in Figure 3b, the roof was very deformable in its own plane due to the presence of non structural plastic roof elements, and the horizontal seismic forces acting in the transverse direction with respect to the longitudinal axis of the building were not effectively transferred to the two short facade masonry walls, that can be seen still undamaged in the picture. 4.2 The role of irregularities in external masonry walls Type 1 industrial buildings with irregular curtain masonry walls, because of strip-windows just under the precast beams, were often subject to severe damages (Fig. 4). This was probably the most frequent cause of failure in some industrial areas hit by the earthquake. In fact, the interaction of the wall with the precast columns caused the loss-of-support on the beams, often at the end frames because of short-column effect. When the roof oscillates in the transverse direction, the interaction with the facade wall actually reduces the clear span of one of the two front columns, the one moving towards the wall, making it much stiffer than the opposite one and then all the other interior columns. Hence, most of the total horizontal seismic force at the level of the roof mass was transferred to two columns only (at the opposite faces of the building, and then the ratio between horizontal force (due to the seismic action) and the vertical loading (due to gravity loading) at the beam column support overcame the friction resistance, leading to the fall of the beam not restrained by mechanical connectors. When the loss of support failure did not take place cases of either flexural or shear failure of the columns were observed DamageS and collapses in Type 2 industrial buildings 880

5 The failure mechanisms that interested Type 2 industrial buildings are various and more complex with respect to Type 1 buildings. In many cases, partial failures can be related to the fall of the roof elements, not restrained to the precast beams, even without any evident damage of the vertical columns. Especially in the case of roofs with precast elements alternated with strip windows, the roof in plan deformability and the significant relative displacements between the beams caused the loss of support (Fig. 6). The loss of support failure of the roof elements was often located in zones corresponding to in plan irregularities of the buildings, such as a variation in the number of spans of the frames (Fig. 7a). In the case of the multi-storey building of Figure 7b, the loss of support to precast roof elements was due to the irregularity of the building. The greater lateral stiffness of the right part of the building with respect to the left part caused the transfer of a large horizontal force, which overcame the friction over the beam supports at the building depth change. Many collapses were related to the failure of some internal columns, and in these cases very large portions of the building were involved. The building in Figure 8a is particularly representative of this type of failure. Significant damage was observed at intermediate columns after the 20th May earthquake, and the building fully collapsed during the following earthquakes. This structure, designed considering vertical and wind loads only, was particularly vulnerable to seismic actions. The intermediate columns had smaller resisting bending moments than the external columns, being the former designed for vertical loads, and the latter also for wind forces. Furthermore strip windows, among the roofing elements, made the roof deformable and therefore the horizontal force acting on intermediate columns was approximately double with respect to the side columns. Being lateral columns stronger than the internal ones, and stiffened by the external vertical cladding walls clamped on the reinforced concrete foundation beam, often no damage was evident from outside (Fig. 8a). Figure 3. (a) Precast RC building with regular masonry cladding walls, only slightly damaged by the earthquake; (b) Collapsed building, the only remaining elements are the two short facade masonry walls. Figure 4. Collapses caused by irregular masonry walls due to strip windows. 881

6 Some complete collapses of industrial buildings are related with base rotations of the vertical columns. Even if a certain identification of the causes would require more investigations, the complete absence of any cracking in the columns indicates, as the most plausible cause, a rotation of the foundation (Fig. 8b). This conclusion is also supported by the large usage, after the 90s, of pocket foundations, with the precast column fixed into the pocket with grout or concrete. In some cases, because of the absence of seismic regulations for design in the region, pocket precast elements were simply supported over larger cast-in-situ foundations, with the verification against overturning ruled by the acting vertical loadings and only the wind action as horizontal force. Moreover, the behaviour of the columns during the earthquake ground motions was elastic (no cracks were observed) because their cross sections were, in general, oversized in order to reduce the drift under wind actions. Therefore the bending moment transferred to the foundations overcame the overturning moment of these latter, producing permanent rotations. 4.3 Failures of external cladding walls Several failures of external cladding walls occurred, especially in the case of horizontal panels (Fig. 9a). A visual inspection always showed the lack of appropriate connectors for supporting the panes. The connectors were designed for resisting the horizontal forces perpendicular to the cladding panels (e.g. due to the wind pressure), but were not adequate for the large horizontal relative displacements occurred during the earthquakes. This type of failure was particularly common among the topmost cladding panels because the columns displacements are larger at the top of the building. Some collapses of vertical panels not clamped in the foundation and with a sandwich structure were also observed (Fig. 9b). Figure 5. Shear (a) and (b) flexural failures of the facade columns due to the interaction with irregular masonry walls. Figure 6. Loss of support failure of roof elements without any visible damage in the columns. 882

7 5 DAMAGE IN WAREHOUSES AND AUTOMATED WAREHOUSES Extensive damage was observed in automated warehouses that contained shelves not designed for seismic actions. The large masses of the items stocked in the shelves and the large spectral acceleration at the middle long periods due to site response produced extensive collapses on this type of structures. Economic losses were extensive and sometimes the collapse of large shelves lead to the failure of the buildings, as in Figure 10. In the first case the lack of lateral force resisting systems is evident. It is also interesting to notice that the weight of the items stocked was four times larger than the weight of the building and it values more than 20 times larger. 6 CONCLUSIONS The main causes of the failures are related to the lack of seismic design requirements in the region until A preliminary classification of the main collapses documented can be the following: Loss of support of the roof elements from the main beams; Loss of support of the main beams from the column supports; Loss of lateral stability of high main beams; Failures of the cladding panel connections; Damage for the loss of stability of the items stocked in the warehouses. In several cases, the following factors contributed to the aforementioned failures: the interaction of portal frames with irregular masonry infill panels; the failure of intermediate columns exhibiting large displacements due to formation of plastic hinges at the base; the absence of a rigid-diaphragm behaviour of the roof; the rotation of the foundations, mainly in the case of pocket precast foundations not anchored to the cast-in-situ foundation. Figure 7. (a) Failure of roof elements, often located in irregular regions of the buildings; (b) Failure due to the loss of support of roof elements related to the irregularity in elevation of the building. Figure 8. Complete collapses of industrial buildings related to (a) the failure of the internal columns, (b) large base rotations of the columns. 883

8 Figure 9. (a) Collapse of horizontal precast cladding panels; (b) Collapse of precast cladding panels non properly restrained at the base. Figure 10. (a) collapse of a cheese warehouse in the province of Mantova and (b) collapse of an automated warehouse containing ceramic tiles in S. Agostino. 7 ACKNOWLEDGEMENT The financial support of the ReLUIS project of the Italian Department of Civil Protection (Reinforced concrete WP2 seismic capacity of precast structures and intervention techniques). 8 REFERENCES Ghosh, S.K. & Cleland, N Observations from the february 27, earthquake in Chile, PCI Journal, 57(1): Gorini, A., Nicoletti, M., Marsan, P., Bianconi, R., Nardis, R., Filippi, L., Marcucci, S., Palma, F. & Zambonelli, E The Italian strong motion network, Bulletin of Earthquake Engineering, 8(5): Iverson, J.K. & Hawkins, N.M Performance of Precast/Prestressed concrete building structures during Northridge Earthquake, PCI Journal, 39(2): Toniolo, G. & Colombo, A Precast concrete structures: The lessons learned from the L'Aquila earthquake, Structural Concrete, 13(2):