Abstract
In modern structural codes, the reference value of the snow load on roofs is commonly given as the product of the characteristic value of the ground snow load at the construction site multiplied by the shape coefficient. The shape coefficient is a conversion factor which depends on the roof geometry, its wind exposure, and its thermal properties. In the Eurocodes, the characteristic roof snow load is either defined as the value corresponding to an annual probability of exceedance of 0.02 or as a nominal value. In this paper, an improved methodology to evaluate the roof snow load characterized by a given probability of exceedance (e.g., p=0.02 in one year) is presented based on appropriate probability density functions for ground snow loads and shape coefficients, duly taking into account the influence of the roof’s geometry and its exposure to wind. In that context, the curves for the design values of the shape coefficients are provided as a function of the coefficient of variation (COVg) of the yearly maxima of the snow load on the ground expected at a given site, considering three relevant wind exposure conditions: sheltered or non-exposed, semi-sheltered or normal, and windswept or exposed. The design shape coefficients for flat and pitched roofs, obtained considering roof snow load measurements collected in Europe during the European Snow Load Research Project (ESLRP) and in Norway, are finally compared with the roof snow load provisions given in the relevant existing Eurocode EN1991-1-3:2003 and in the new version being developed (prEN1991-1-3:2020) for the “second generation” of the Eurocodes.
Highlights
Accepted: 22 March 2021The collapse of roof structures under extreme snow loads can have catastrophic consequences, as confirmed by the failures of large-span lightweight structures that have recently occurred in Europe [1,2,3]
That criticism is based on the suspicion that the values of partial factors given in the Eurocodes as well as in most other international standards, like those issued by the International Organization for Standards (ISO), the American Society of Civil Engineers (ASCE), often underestimate the effects of climatic action uncertainties in lightweight structures
The reference roof snow load for structural design is generally defined as the product of characteristic ground snow load, characterized by an annual probability of exceedance p = 2% and a nominal or notional roof conversion factor known as the shape factor
Summary
The collapse of roof structures under extreme snow loads can have catastrophic consequences, as confirmed by the failures of large-span lightweight structures that have recently occurred in Europe [1,2,3]. In modern building codes for structural design, roof snow loads are usually derived from the ground snow loads at the site [6] by means of suitable conversion factors. The shape coefficient, which depends on the exposure, the slope, and the thermal conditions of the roof, usually varies in the interval of 0.4–1.1 [6] This high variability should be duly considered in establishing design loads for roofs [8]. The joint pdf s and the design conversion factors (shape coefficients) to be used for structural design are determined in such a way that the reference values of roof snow load and ground snow load are associated with the same probability of exceedance (e.g., p = 0.02 in one year). Specific studies show that future trends of wind velocity due to climate change are not so significant as to modify exposure conditions [18,19]
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