Observations after past earthquakes

Many valuable data about the dynamic behaviour of a certain structural system can be gained from observations after the earthquakes. There have been several earthquakes in Europe that gave insight into the response of RC precast structures. From one strong earthquake to another, the field inspections and reports showed different responses, from good behaviour on the one hand, to total disasters on the other.

For instance, the 1976 Friuli earthquake in Italy pointed out the relatively good behaviour of prefabricated buildings (Fajfar et al., 1978; EERI, 1976). However, the frequency of the ground motion was relatively high, while the predominant structural periods of relatively flexible RC precast structures are usually around one second or even above (Kramar, 2008). According to EERI (1976), most damage after the Friuli earthquake could be attributed to the collapse of the roof system due to the lack of connections between the beams and columns, relying only on friction.

Satisfying behaviour of precast structures, in general, was also observed during the 1979 Montenegro earthquake (Fajfar et al., 1981), apart from some collapses in Port of Bar. In contrast, there have been catastrophic collapses of prefabricated structures observed after the 1988 Spitak Earthquake in Armenia that created distrust of precast construction in general. Fischinger et al.

(2014) have pointed out that the large panel precast structures performed well (see Figure 2.1).

Therefore, any generalised conclusions about the good or bad performance of precast structures should not be drawn. However, during the Spitac earthquake, the industry suffered long-term business interruptions because many industrial facilities either collapsed entirely or were severely damaged (EERI, 1989).

Figure 2.1: Large panel precast structure standing among the rubble of the precast frames that caused a tragedy during the Spitak 1988 earthquake (Fischinger et al., 2014)

Slika 2.1: Visoka montažna panelna zgradba stoji praktično nepoškodovana poleg ruševin AB montažne okvirne konstrukcije, ki so bile med drugim vzrok za tragedijo med potresom leta 1988 v Armenskem mestu Spitak (Fischinger et al., 2014)

Damage of precast buildings was reported after the 1977 Vrancea earthquake in Romania (Tzenov et al., 1978) and during earthquakes in Turkey, that is, the 1998 Ceyhan, 1999 Kocaeli and 1999 Düzce earthquakes (Saatcioglu et al., 2001; EEFIT, 2003; Arslan et al., 2006; Dogan et al., 2010).

Three common types of structural damage were observed: flexural hinges at the base of columns, axial movement of the roof girders, leading to pounding at columns or unseating of the girders, and failure of the roof girders in an out-of-plane direction (Dogan et al., 2010). The probable reason for a considerable number of collapses and substantial damage after these earthquakes might also be the strong low-frequency content of the ground motions.

More recently, the 2009 L’Aquila earthquake in Italy demonstrated that the behaviour of the main precast structures, that is, the columns and roof elements, was, in general, good. The majority of the damage observed during this event can be attributed to the failure of non-structural parts, explicitly to the failure of heavy cladding panels (Toniolo & Colombo, 2012). Failures occurred with different types of connections and buildings concerning both vertical and horizontal panels.

These observations confirmed the reliability of the design of the structures according to seismic provisions and, at the same time, pointed out the inadequate design of fastening systems for cladding panels. Fastening systems often have not even been analysed for the effect of seismic loading because the main response mechanisms were not known.

In 2012, a series of earthquakes hit the Emilia Romagna region in Northern Italy, causing much damage, followed by many field inspections (Bournas et al., 2013; Liberatore et al., 2013; Magliulo et al., 2014; Savoia et al., 2017). This industrial region with intense economic activity was only classified as seismic since the year 2003, meaning that most of the structures were not designed for seismic loading (Daniell and Vervaeck, 2012). However, some of the newly designed buildings have also suffered substantial damage, which could be, as in the case of Turkish earthquakes, related to the relatively strong energy content of the second shock in the low-frequency range.

Most industrial buildings in the affected area were designed for gravity loads only, with the lack of adequately designed connections between precast elements (Bournas et al., 2013). For this reason, many roof failures due to unseating of the main beams from the columns have been reported.

Approximately 75% (Bournas et al., 2013) of precast industrial buildings designed without seismic provisions in the area of the Emilia earthquake suffered damage and detachment of the wall cladding panels. Bournas et al. (2013) even claim that the number of cladding connection failures was not significantly reduced in the newly designed buildings. Examples of the collapses in the Emilia Romagna earthquake are shown in Figure 2.2.

Many authors (Toniolo & Colombo, 2012; Bournas et al., 2013; Belleri et al., 2015; Savoia et al., 2017) thought that the main reason for the failure of panels was insufficient displacement capacity of the cladding-to-structure connections in the direction parallel to the panel plane, which led to the overturning of façade panels. Namely, the cladding fastening systems were often not designed according to seismic provisions. Only the forces acting in the direction perpendicular to the panel plane, calculated based on the panel mass, were considered (CEN, 2004).

Figure 2.2: (a) Failure of the horizontal RC cladding panels and (b) the damaged cladding connections during the earthquake in Emilia Romagna

Slika 2.2: (a) Porušitev horizontalnih armiranobetonskih panelov in (b) poškodovani fasadni stiki med potresom v Emiliji-Romanji

The L’Aquila Earthquake provided evidence of displacements up to ± 150 mm at the top of some buildings (Toniolo & Colombo, 2012). This could have a relevant impact on the cladding-to-structure connections, which must accommodate large displacements in a longitudinal, that is, in-plane, direction.

Leading causes for the damage and collapses of precast buildings observed during the past earthquakes can be summarised as:

- Failure of the columns: Inadequate confinement and detailing of the hoops led to buckling of the longitudinal bars and substantial damage to precast columns.

- Unseating roof elements: Connections between beams and columns in older buildings designed without seismic provisions relied only on friction. Mechanical connections between the columns and the roof girders were not included in the design.

- Failure of the peripheral cladding panels: Because the cladding connections were designed only for out-of-plane forces, the insufficient deformation capacity of the fastening systems in the longitudinal direction parallel to the panel plane led to the failure of panels. Mostly in older buildings, failure of the panels occurred also due to the failure of the main structural system.

The first two of the listed reasons for damage in precast structures were more evident during the earthquakes that occurred in the past. Formerly even the main precast structure, that is, columns and roof elements, was not designed according to seismic provisions. As the code provisions are improving, the behaviour of the newly designed precast structures on their own was relatively good during the recent earthquakes (e.g. L’Aquila earthquake), and the issue related to the inappropriate seismic response of the façade system was more exposed.