Back analysis of a building collapse under snow and rain loads in Mediterranean area
Abstract. At the end of February 2018 the Mediterranean area of Montpellier in France was struck by a significant snowfall that turned into an intense rain event caused by an exceptional atmospheric situation. This rain-on-snow event produced pronounced damages to many buildings of different types. In this study, we report a detailed back analysis of the roof collapse of a large building, namely the Irstea Cévennes building. Attention is paid to the dynamics of the climatic event, on the one hand, and to the mechanical response of the metal roof structure to normal loading, on the other hand. The former aspect relies on multiple sources of information that provide reliable estimates of snow heights in the area before the rain came into play and substantially modified the snow quality. The latter aspect relies on detailed finite element simulations of the mechanical behaviour of the roof structure in order to assess the pressure due to snow cover loading which could theoretically lead to failure. By combining the two approaches, it is possible to reconstruct the most probable scenario for the roof collapse. As an example of building behaviour and vulnerability to an exceptional rain-on-snow event in the Mediterranean area of France, this detailed case study provides useful key points to be considered in the future for a better mitigation of such events in non-mountainous areas.
Isabelle Ousset et al.
Status: final response (author comments only)
RC1: 'Comment on nhess-2022-93', Anonymous Referee #1, 18 Apr 2022
- AC1: 'Reply on RC1', Isabelle Ousset, 25 May 2022
- AC2: 'Reply on RC1', Isabelle Ousset, 12 Jul 2022
RC2: 'Comment on nhess-2022-93', Miroslav Sýkora, 14 Jun 2022
- AC4: 'Reply on RC2', Isabelle Ousset, 12 Jul 2022
- AC5: 'Reply on RC2', Isabelle Ousset, 12 Jul 2022
RC3: 'Comment on nhess-2022-93', Anonymous Referee #3, 20 Jun 2022
- AC3: 'Reply on RC3', Isabelle Ousset, 12 Jul 2022
Isabelle Ousset et al.
Isabelle Ousset et al.
Viewed (geographical distribution)
The article describes the collapse of a large hall with a flat roof in southern France after heavy snowfall turned into rain. The analysis of such cases is very valuable on the one hand to check the design snow loads and on the other hand to identify structural weak points of a structure. The study was carefully prepared, but unfortunately no quantitative data was available on important input parameters such as the amount of snow load at the time of the damage or the condition of the structure before its collapse. Therefore, the main statements of the article are somewhat speculative. In order to finally answer the question what was the exact cause of the collapse of the building, the collapsed structure would have to be investigated in more detail (e.g. material technology tests). In several building collapses, such as the ice rink in Bad Reichenhall, material deficiencies were partly responsible.
The beginning of chapter 1 Introduction has only a small relation to the main content of the article. Dynamic effects of gravitational natural hazards on a building have completely different consequences than static effects such as snow load. I recommend making only a reference to snow loads in the introduction.
The collapse of the building would need to be described in a bit more detail:
The building and the supporting structure would also need to be described in a more precise way:
The assumed snow depths and snow densities appear to be somewhat speculative:
The calculated bearing capacity of the supporting structure seems to be rather high:
Section 4.3 is not directly related to the damage analysis presented and is somewhat speculative. Measurements of the temporal development of snow loads in the Mediterranean region are practically non-existent. I recommend to omit this chapter. Instead, to give further hints on how such rain on snow events can be better managed on a flat roof (slope of roof? arrangement of drainage? emergency outlets?). Further, it would be helpful to indicate what information would be useful for a more complete damage analysis in future (e.g. photos immediately after damage? Snow depth measurements? Snow load measurements? ...).
Line 42/43: deficient building, better: insufficient material strength?
Line 53: Determining the ultimate bearing capacity of a building is similar to or more difficult than determining the possible snow load.
Line 56: What is the AROME numerical model?
Line 64: What changes are expected about the characteristic snow loads…. not clear what is the meaning.
Lines 79 – 95: Add precipitation data.
Lines 97: Why is the rain-on-snow event exceptional? Return period of event?
Fig. 4: add the location of the collapsed structure
Line 129: Explain why 30 – 35 cm were chosen (see Fig. 4: 20-30 cm). Give some reference values for new snow density: 250-350 kg/m3 seems to be too high.
Line 134: The rain on snow event should be discussed in more detail. How much water can the snowpack absorb? What can be the density of a wet snowpack?
Fig. 5: How was snowfall measured? Where is the meteo station Lavalette?
Line 145: (nearly) flat – the slope of the roof is in a rain-on-snow event very decisive. What means nearly? 1°? 3°?
Line 154: The drainage system should be explained in detail.
Line 157: central part of the structure: where is that? Indicate the location on a Fig. e.g. 11; The western and eastern facades were “heavily” (not hardly) damaged.
Line 161/162: was there an element that was clearly the weakest?
Tab. 1: Steel quality of the different structural members?
Line 178: “dead” weight
Line 204: better snow load not snow pressure
Tab. 3: Better snow load instead of pressure value
Line 208-209: …by construction the applied pressures. Difficult to understand.
Line 232: snow load on the ground was estimably 30-35 cm – how was the snow load on the roof? Was a shape factor of 0.8 applied? Was there wind during the snow fall event which reduced the snow height on the roof?
Line 255: difficult to understand: the highest height scenario…
Line 278: which were the detected structural weaknesses?
Line 285: “a maximum range of span to be on the safe side”…difficult to understand: If the planned geometry of a building is taken into account in the design of the load-bearing structure, a structural failure should not occur. In connection with the drainage, the roof pitch and the sags in the service state would need to be checked.
Line 334: Is there some evidence that the drainage openings were blocked by ice? With temperatures around 6° C might be hardly the case?