Forensic and progressive collapse analysis of a RC building impacted by the 1998 Sarno landslides

Many existing reinforced concrete (RC) buildings, especially those designed before modern codes and guidelines, may be vulnerable to local failures that may trigger progressive col- lapse. This work focuses on the collapse-triggering mechanism and structural robustness of a five-storey RC building that partially collapsed under the 1998 Sarno, Italy, landslides. Simulated design was performed based on technical codes and practice rules in use at the time of the building construction. A nonlinear fibre-based model was developed in OpenSees and subjected to sensitivity analysis, identifying a plausible range of collapse-triggering scenarios.

A back-analysis procedure, supported by post-event photographic evidence and forensic observations, allowed identifying the most critical column whose failure was used as collapse trigger in a robustness assessment of the building based on pushdown analyses. Five different approaches were investigated, each representing a distinct assumption on loading history and boundary conditions. One of those approaches allowed evaluating robustness starting from the actual stress state reached when the local failure occurred. Analysis results highlight the limited redistribution capacity of the structure and typical vulnerability factors of gravity-designed RC buildings. It is also shown how pre-existing loading conditions can influence residual capacity and robustness of RC buildings to mudflows.

In recent decades, the impact of landslides on the built environment has increasingly drawn attention due to frequent events causing significant damage and collapse of reinforced concrete (RC) buildings. Particularly, debris-flow type landslides represent a severe hazard, as they are characterised by high velocities and large masses that can induce localised failures in structural elements, potentially triggering global collapse. Recent research has mainly focused on assessing structural vulnerability through fragility models, examining the interac- tion between debris-flow parameters - such as flow depth and impact velocity - and building characteristics, including infill-wall configurations and structural typologies (e.g., Parisi & Sabella, 2017; Miluccio et al., 2020).

However, limited attention has been given to the struc- tural response following localised damage induced by debris-flow impacts, despite its critical importance for robustness assessments aimed at preventing disproportionate or progressive collapse. Structural robustness – defined as the ability to limit damage propagation after the failure of one or more load-bearing elements – is especially relevant for gravity-load designed (GLD) RC buildings, which often have poor detailing and limited redundancy. Several stud- ies have assessed structural robustness through nonlinear static analyses – often referred to as pushdown analyses (PDA) – to evaluate the residual capacity of structural systems and damage propagation after local failure (e.g. Brunesi & Parisi, 2017; Scalvenzi et al., 2021).

Nonetheless, most of these approaches are threat-independent, employing notional removal scenarios without reference to specific hazards. The present study adopts a threat-dependent approach, starting from a detailed back-analysis supported by photographic evidence and fo- rensic observations to identify the critical column failure scenario from an actual collapse event. A five-storey RC building that collapsed due to impact of one of the 1998 Sarno land- slides was analysed using fibre-based nonlinear static models developed in OpenSees. After identifying the most likely initial failure mechanism, structural robustness was assessed through multiple pushdown analyses performed under various loading sequences.

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IF CRASC ’25 ha posto al centro del confronto tecnico ingegneria forense, crolli, affidabilità e consolidamento strutturale, riunendo a Napoli esperti del settore per analizzare cause dei dissesti, responsabilità tecniche e soluzioni avanzate per la sicurezza del costruito, tra ricerca, pratica professionale e ambito giudiziario. All'interno interviste e video delle relazioni.
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Case study

On May 5th and 6th, 1998, a series of rapid debris flows struck the Pizzo d’Alvano area in Southern Italy, causing over 170 damaged buildings and 160 fatalities. The case-study build- ing was located in one of the hardest hit zones, i.e. the Episcopio district of the municipality of Sarno. The structure was a five-storey reinforced concrete (RC) residential building located near the slope toe that partially collapsed under the impact of the flow (Errore. L'origine riferimento non è stata trovata.).

A three-dimensional numerical model of the building was developed in OpenSees, assuming a fixed-base condition since soil-structure interaction was not taken into account. The first underground floor was excluded from the model as its structural contribution was considered negligible for the purposes of impact simulation. Infill walls were also omitted in the capacity model, resulting in a bare frame structural system. Floor systems were modelled using diag- onal, linear elastic, equivalent truss elements. The numerical model was developed using nonlinear fibre-based elements with displacement-based formulation.

Two different consti- tutive models were used for concrete and steel: a zero-tensile strength material was assigned to both confined and unconfined concrete, and a uniaxial bilinear model with kinematic hardening was used for reinforcing steel. Geometric nonlinearities – e.g., large displacements, rotations, and P–Delta effects – were included using a co-rotational transformation. A fine discretisation of the elements was adopted specifically for the columns expected to be im- pacted by the landslide. For the structural assessment, mean material properties were used, with reference values based on experimental data from Italian existing RC buildings (e.g. Verderame et al., 2001; Masi et al., 2019). The adopted values included mean compressive strength fcm = 22 MPa for concrete and mean yield strength fym = 220 MPa for steel reinforcement.

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