SEISMIC PERFORMANCE OF SINGLE STORY PRECAST
In reinforced concrete industrial precast structures one of the most common seismic 4 damage is the collapse of the cladding panels because of the failure of the panel to structure 5 connections. This damage is caused by the interaction between the panels and the structures, which 6 is usually neglected in the design approach. The present study aims at investigating this interaction. 7 Nonlinear dynamic analyses are performed on several structural models in order to take into 8 account both the panel to structure interaction and the roof diaphragm. According to the analyses 9 results, if the current European single story precast buildings stock is considered, panels stiffness 10 significantly influences the overall structural behavior under seismic actions and the failure of the 11 connections occurs at very low intensity values. The progressive collapse of the panels is also 12 simulated in order to evaluate the redistribution of seismic demand in the columns during the 13 earthquake. In the final part, fragility curves are evaluated in order to generalize the dynamic 14 analyses results. 15 Author keywords: precast structures, cladding panels, seismic response, dynamic analyses, 16 incremental dynamic analysis, fragility curves 17 Introduction 18 Connection systems are the crucial points in the seismic performance of precast reinforced concrete 19 (RC) structures. During recent earthquakes in Europe (Belleri et al. 2015, Belleri et al. 2014, 20 Magliulo et al. 2014) several existing precast structures showed severe damage at the connections. 21 One of the most common damage was the failure of the external cladding panels; it caused many 22 injuries and casualties as well as significant economic losses due to the interruption of the 23 industrial/commercial activities. The panels failure was caused by the connections fracture and it 24 can be explained by the design approach. Indeed, the cladding panels are commonly considered as 25 1 Senior Lecturer, University of Greenwich, Email: M.Ercolino@greenwich.ac.uk 2 Assistant professor, University of Naples Federico II (corresponding author) E mail: gmagliul@unina.it 3 Full professor, University of Naples Federico II, E mail: gamanfre@unina.it nonstructural elements and the interaction with the structure under seismic loads is neglected. 26 According to Eurocode 8 (CEN, 2005) the panel to structure connections are dimensioned only for 27 forces in the out of plane direction of the panels. These forces are both seismic actions deriving 28 from the panel self weight and wind actions. However, under dynamic actions the panel to structure 29 interaction can occur and the connections could fail since they are carrying forces that were not 30 considered during the design phase, e.g. in plane actions. 31 In the last decades several research studies were developed on the panel to structure interaction 32 under seismic loads. In these works the effect of the panels was investigated in order to evaluate 33 both the dynamic properties (e.g. frequencies and damping ratios) and the structural seismic 34 performance (e.g. drift ratios, displacement demand) However, most of these research studies 35 concerned some typical U.S. buildings, i.e. steel high rise buildings, as extensively reported in Hunt 36 and Stojadinovic (2010) In the last years the scientific community recognized the need of a 37 systematic study on the structure cladding panel interaction in RC precast buildings, typically 38 implemented in Europe for industrial and commercial activities. Recently, an European research 39 project was conducted on this topic: the SAFECLADDING project, aimed at studying the effect of 40 cladding panels on precast industrial buildings. In the framework of this project, a first study was 41 developed by Biondini et al. (2013) in this work the authors investigated the effect of vertical 42 panels by assuming the interaction at the level of the panel to panel connections. Since the 43 numerical forces in the connections were too high if the interaction between the panels was 44 considered, the authors proved the efficiency of an innovative dissipative connection between 45 vertical panels. In the framework of the same project, experimental campaigns were performed on 46 some typical panel to structure connection systems, in order to assess their seismic performance, 47 e.g. failure modes, strength and deformability (Zoubek et al. 2016) The experimental results were 48 used in order to perform a systematic seismic fragility study, described in Babic and Dolsek (2014) 49 In this research work, the authors performed 3D nonlinear analyses on several precast structures and 50 they defined the fragility curves for twelve industrial precast building classes. These classes were 51 defined by varying the type of non structural components (vertical panels, horizontal panels and 52 masonry infills) the geometrical configuration, the design approaches and the panel to structure 53 connection systems. According to the outcomes of this study, the authors concluded that the effect 54 of non structural components should be taken into account in both the design and the seismic 55 assessment of such buildings since the overall safety of the structures decreases if these components 56 are considered in the models. In this study the connection models are assumed from the 57 correspondent tested connections and the roof is modeled as flexible in its own plane. Despite the 58 noteworthy effort in the last years, other investigations are necessary in order to delineate more 59 general conclusions on the panel influence for precast structures as well as on the design of precast 60 structures with cladding panels. Indeed, during the design phase some simplified assumptions are 61 generally necessary (e.g. flexible or rigid roof hypotheses without modeling the roof elements in 62 the structural models) If the cladding panels have to be considered in the structural analysis, a 63 detailed model of the connections is required (e.g. based on experimental results) which should be 64 generalized by extensive experimental campaigns and/or numerical studies. At this aim, Magliulo et 65 al. (2015) proposed a simplified model of precast structure with cladding panels. In this model the 66 panel to structure interaction is assumed at the beam to panel connections (vertical panels) the 67 nonstructural elements are considered as elastic elements and the connections are fixed constraints 68 between the structure and the panel. By performing an extensive parametric study, the authors 69 evaluated the first periods of single story precast buildings by means of linear modal analyses in 70 two limit cases: negligible panels effect (bare structures) and not negligible panels effect (structures 71 with panels) In this last case, a significant effect of the panels on the structural dynamic behavior 72 was demonstrated and the simplified period formula of Eurocode 8 was compared with the analysis 73 results in order to verify its capability. 74 The presented study investigates the cladding panel effect on the nonlinear seismic response of a 75 reference precast industrial building. The adopted case study is a new building, designed according 76 to Eurocodes. The seismic assessment of the structure is performed by means on nonlinear dynamic 77 analyses in OpenSees (McKenna and Fenves, 2013) program. Several structural models are 78 considered by varying the assumptions on both the roof stiffness and the occurrence of panel to79 structure interaction. The results of models with the cladding panels demonstrate the significant 80 effect of the panels on the seismic behavior of these buildings; moreover, they justify the typical 81 connection failure under dynamic actions. In order to improve the reliability of the numerical 82 results, a model with the cladding panels is developed that is able to simulate the panel collapse 83 during the seismic load. If the panels collapse is simulated, the results show a significant difference 84 in the structural response in terms of both connection failure and forces distribution. In the final part 85 of the paper, the seismic vulnerability of the structures is evaluated by taking into account all the 86 above cited modeling assumptions in order to define general conclusions to lead the designer to 87 choose the approach for such buildings. 88 Case study structure 89 The investigated case study is a precast industrial building (Figure 1) It consists of precast 90 columns, fixed at the base by means of socket foundation and connected at the top by secondary 91 girders and principal beams in the two horizontal directions. The principal beams support the roof 92 elements. The columns height is equal to 9m (29.5 ft) the width of the six X bays is 12m (39.4 ft) 93 and the width of the two Z bays is 19m (62.3 ft) The horizontal elements are designed by 94 considering only the vertical loads (permanent and variable loads) they consist of prestressed TT 95 roof elements, prestressed principal beams in the transversal direction (Z direction in Figure 1) and 96 secondary girders in the longitudinal direction (X direction in Figure 1) Dowel connections are 97 usually installed between the principal beam and the columns as well as between the roof elements 98 and the principal beams. The girder to column connection usually consists of bolted steel joints. 99 Vertical reinforced concrete precast cladding panels are connected to beams or girders, as described 100 in section "Model with cladding panels" 101 The columns are designed according to Eurocode 8 (CEN, 2005) assuming a peak ground 102 acceleration equal to 0.168g (return period of 475 years and soil type "B" and ductility class 103 "DCH" The behavior factor is equal to 4.5, as indicated in Eurocode 8. The columns have square104 shaped cross sections (80cm x 80cm 31.5 inches x 31.5 inches) reinforced with 22mm diameter