EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Nat. Hazards Earth Syst. Sci., 13, 1903–1912, 2013 Natural Hazards Natural Hazards www.nat-hazards-earth-syst-sci.net/13/1903/2013/ doi:10.5194/nhess-13-1903-2013 and Earth System and Earth System © Author(s) 2013. CC Attribution 3.0 License. Sciences Sciences Discussions Open Access Open Access Atmospheric Atmospheric Chemistry Chemistry and Physics and Physics Discussions Open Access Open Access Evaluation and considerations about fundamentalAtmospheric periods of Atmospheric damaged reinforced concrete buildings Measurement Measurement Techniques Techniques R. Ditommaso1, M. Vona1, M. R. Gallipoli2, and M. Mucciarelli1 Discussions 1 Open Access School of Engineering, University of Basilicata, Potenza, Italy Open Access 2National Research Council of Italy – IMAA, Tito Scalo – PZ, Italy Biogeosciences Biogeosciences Correspondence to: M. Vona ([email protected]) Discussions Received: 4 July 2012 – Published in Nat. Hazards Earth Syst. Sci. Discuss.: – Revised: 2 June 2013 – Accepted: 17 June 2013 – Published: 31 July 2013 Open Access Open Access Climate Climate Abstract. The aim of this paper is an empirical estimation 1 Introduction of the Past of the Past of the fundamental period of reinforced concrete buildings Discussions and its variation due to structural and non-structural dam- age. The 2009 L’Aquila earthquake has highlighted the mis- The estimation of the fundamental period is a crucial aspect Open Access Open Access match between experimental data and code provisions value in response analysis for existing buildings and for their as- Earth System not only for undamaged buildings but also for the damaged sessment and retrofitting.Earth A reliable System estimation of the funda- mental period T is an important aspect both in classic (force- ones. The 6 April 2009 L’Aquila earthquake provided the Dynamics Dynamics first opportunity in Italy to estimate the fundamental period based design, FBD) and in more recent design procedures Discussions of reinforced concrete (RC) buildings after a strong seismic (e.g. pushover analysis, displacement-based design; see for sequence. A total of 68 buildings with different characteris- a review Masi and Vona, 2010). In the linear static or dy- Open Access tics, such as age, height and damage level, have been inves- namic method (FBD) the fundamentalGeoscientific period (predicted in a Geoscientific Open Access tigated by performing ambient vibration measurements that simplified manner or calculatedInstrumentation by analytical model) is the Instrumentation crucial parameter to define the spectral acceleration and thus provided their fundamental translational period. Four differ- Methods and Methods and ent damage levels were considered according with the def- the base shear force. initions by EMS 98 (European Macroseismic Scale), trying At present, concerning theData evaluation Systems of seismic actions on Data Systems to regroup the estimated fundamental periods versus build- single structures, most design codes (e.g. ATC, 1978; BSSC, Discussions Open Access 2003; CEN, 2005; NZSEE, 2006) provide period-heightOpen Access em- ing heights according to damage. The fundamental period of Geoscientific RC buildings estimated for low damage level is equal to the pirical expressions, usuallyGeoscientific set up with an elastic force-based previous relationship obtained in Italy and Europe for un- design in mind. The recent research into earthquake engi- Model Development neering shows thatModel the fundamental Development periods estimated by nu- damaged buildings, well below code provisions. When dam- Discussions age levels are higher, the fundamental periods increase, but merical models are often significantly different than those again with values much lower than those provided by codes. obtained when using an experimental approach (Gallipoli et Open Access Open Access Finally, the authors suggest a possible update of the code for- al., 2009, 2010; OliveiraHydrology and Navarro, 2010;and Michel et al., Hydrology and mula for the simplified estimation of the fundamental period 2010). These differences are probably due to the fact that the of vibration for existing RC buildings, taking into account dynamic properties of buildingsEarth are System evaluated using numer- Earth System also the inelastic behaviour. ical analyses based on inaccurateSciences FE (finite element) mod- Sciences els, based on too simplified models which inadequately re- produce the dynamic behaviour of real structures. Although Discussions Open Access the mass properties can be easily assessed, the geometryOpen Access and stiffness of structural and non-structural elements (when con- Ocean Science sidered) are too simplifiedOcean as well Science as the consequent damp- Discussions ing properties of the structure considered. In fact, the infill Open Access Published by Copernicus Publications on behalf of the European Geosciences Union. Open Access Solid Earth Solid Earth Discussions Open Access Open Access The Cryosphere The Cryosphere Discussions 1904 R. Ditommaso et al.: Evaluation and considerations about fundamental periods panels are not generally included in this kind of numerical Table 1. Distribution of damage levels of surveyed RC buildings. models. The inability to model correctly the structural damp- ing linked to material properties and other physical charac- Damage level teristics (e.g. opening and closing of cracks, structural and Age No. 0 1 2 3 4 non-structural elements interaction, etc.) makes numerical Buildings estimate of periods less realistic (Masi and Vona, 2010). There are recent works where special-purpose-element mod- 1946–1961 9 1 7 1 els have been developed in order to represent the actual 1962–1971 7 7 1972–1981 18 1 11 6 seismic response of the RC (reinforced concrete) structures 1982–1991 23 5 7 4 3 4 (Karayannis et al., 2011). Several key parameters influenc- 1992–2000 9 6 2 1 ing the dynamic characteristics of the buildings have been in- 2001–2009 2 2 cluded, such as infills, beam-column RC joints, and boundary conditions such as structure to soil and/or adjacent structure 68 14 10 29 11 4 interaction. Nevertheless the gap between numerical and ex- perimental results exists, and the experimental estimations of fundamental period as well as its variation due to estimates the behaviour of anti-seismic RC buildings in L’Aquila is not damage should be an important step to update the models. very different from those related to RC buildings designed The aim of this study is to estimate the fundamental period only to vertical loads, as reported in Mucciarelli et al. (2004) variation taking into account different levels of damage into and Masi and Vona (2010). This similarity is due to the use account. To this purpose, we have collected several record- of old codes of seismic design. In fact, some typical charac- ings during and after the L’Aquila (2009) seismic sequence teristics have been observed: (Mucciarelli et al., 2011; Picozzi et al., 2011). The funda- mental periods of 68 RC buildings have been estimated for – Beam–column joints without ties and ineffective an- the first time in Italy after a strong seismic sequence. The chorage of the steel bars (Fig. 1a). fundamental periods of RC buildings with different typolo- gies, structural characteristics, age, heights and damage lev- – Open hoops (Fig. 1b). els have been investigated by means of ambient vibration – Structural elements with small section and few rein- measurements. Four different damage levels were consid- forcement bars (hoops and longitudinal bars, Fig. 1c). ered with regard to the 5 damage levels defined by EMS 98 (Gruenthal, 1998). A new height–period relationship is pro- – Structural system: strong frames mainly along one di- posed for the practical purposes such as the assessment and rection (typically the longitudinal direction) while in the the retrofitting of the existing European RC moment-resistant orthogonal direction weak frames are generally present. frame (MRF) buildings. On the other hand, it is worth noting that the mechanisms of surveyed damage are not different to those surveyed in recent 2 Buildings data set and analysis Italian earthquakes on existing from RC buildings designed to resist to vertical loads only (Fig. 2). The investigated structures have been selected in L’Aquila The 68 RC buildings where measurements were under- and within the surrounding villages in order to cover a wide taken had heights ranging from 11 to 27 m, and different span of important characteristics such as damage levels so that it is possible to study the real influ- – seismic design; ence of strong ground shaking on fundamental period and to compare these results with those obtained with regard to the – design/construction age; undamaged buildings. The damage levels (DL) range from DL = 1 (non-structural damage) to DL = 4 (heavy structural – height; damage) accordingly with the EMS 98 (Gruenthal, 1998). – damage level. Figure 3 shows typical examples of the observed damage levels. Table 1 reports the distribution of buildings according The age of construction ranges from 1950 to 2000. After to age classes and damage level. It has to be noted that the se- the 1915 Avezzano earthquake, L’Aquila has been classified lected buildings are mainly grouped in the oldest age classes, as a seismic area, therefore all the RC buildings studied were and obviously the DL = 0 is mainly present in more recent designed according to the Italian seismic code enforced at the buildings. time of construction. The main characteristics of the buildings studied are sum- With regard to the seismic aspects, design details do not marized in Table 2. We also considered some particular appear different to those of RC buildings designed only to cases: two buildings with completely bare frames (Fig. 4b, vertical loads. Moreover, the surveyed damage showed that number 36 and 37 in Table 2), two buildings without stiff Nat. Hazards Earth Syst. Sci., 13, 1903–1912, 2013 www.nat-hazards-earth-syst-sci.net/13/1903/2013/ R. Ditommaso et al.: Evaluation and considerations about fundamental periods 1905 Table 2.