Structural modelling issues in seismic performance assessment of industrial steel buildings

The paper presents part of a study on the probabilistic seismic performance as-sessment of an existing industrial facility featuring a steel structure as the main building. The building is located in central Italy and is composed of structures having different construction ages. One portion was designed and built in the early 1970s and expanded in the late 1970s. In the early 1990s, a new portion was built close to the former but separated by means of gaps. All the structures are very similar in concept, though they are different in detailing and structural behavior. Six models were built up for the existing structures, four two-dimensional (2D) and two three-dimensional (3D) models. The four 2D models represent four distinct frames, one per each of the two main plan directions of the two structures (the older, i.e. the 1970s’, and the newer, i.e. the 1990s’). The two 3D models are variants of a model of the newer building: the difference is the modeling options of roof truss joints. Non-linear dynamic analyses of the six structural models were carried out. Ground acceleration records were specifically selected consistent with disaggregation of seismic hazard at the site. The proba-bility distribution of demands was then investigated, considering both global and local failure modes. This paper, in particular, presents and discusses results for only the global response assessment, in terms of peak transient and residual roof drifts. The structural modeling issues are discussed with a focus on the effects that different modeling options may have on assess-ment of seismic fragility and risk. The main conclusions drawn are: (i) roof truss failure might occur in the form of instability of a group of members, a failure mode – triggered by local out-of-plane truss vibrations and P-Delta effects – that can be only captured by a 3D model; (ii) including residual drifts, along with peak transient values, in the evaluation of the probability of failure can be of utmost importance if the structure is relatively stiff (thus expe-riencing small peak drift) but weak (thus easily experiencing inelasticity). The first conclusion above is specific for industrial steel buildings but applicable to all those characterized by a roof truss with the possibility of out-of-plane joint displacements, while the second conclusion appears to have general validity for the seismic risk assessment of structures.