Long term avalanche risk quantification for mapping and the design of defense structures is done in mostcountries on the basis of high magnitude events. Such return period/level approaches, purely hazardoriented,do not consider elements at risk (buildings, people inside, etc.) explicitly, and neglect possiblebudgetary constraints. To overcome these limitations, risk based zoning methods and cost-benefit analyseshave emerged recently. They combine the hazard distribution and vulnerability relations for the elementsat risk. Hence, the systematic vulnerability assessment of buildings can lead to better quantify the riskin avalanche paths. However, in practice, available vulnerability relations remain mostly limited to scarceempirical estimates derived from the analysis of a few catastrophic events. Besides, existing risk-basedmethods remain computationally intensive, and based on discussable assumptions regarding hazard modelling(choice of few scenarios, little consideration of extreme values, etc.). In this thesis, we tackle theseproblems by building reliability-based fragility relations to snow avalanches for several building types andpeople inside them, and incorporating these relations in a risk quantification and defense structure optimaldesign framework. So, we enrich the avalanche vulnerability and risk toolboxes with approaches of variouscomplexity, usable in practice in different conditions, depending on the case study and on the time availableto conduct the study. The developments made are detailed in four papers/chapters.In paper one, we derive fragility curves associated to different limit states for various reinforced concrete(RC) buildings loaded by an avalanche-like uniform pressure. Numerical methods to describe the RCbehaviour consist in civil engineering abacus and a yield line theory model, to make the computations asfast as possible. Different uncertainty propagation techniques enable to quantify fragility relations linkingpressure to failure probabilities, study the weight of the different parameters and the different assumptionsregarding the probabilistic modelling of the joint input distribution. In paper two, the approach is extendedto more complex numerical building models, namely a mass-spring and a finite elements one. Hence, muchmore realistic descriptions of RC walls are obtained, which are useful for complex case studies for whichdetailed investigations are required. However, the idea is still to derive fragility curves with the simpler,faster to run, but well validated mass-spring model, in a “physically-based meta-modelling” spirit. Inpaper three, we have various fragility relations for RC buildings at hand, thus we propose new relationsrelating death probability of people inside them to avalanche load. Second, these two sets of fragilitycurves for buildings and human are exploited in a comprehensive risk sensitivity analysis. By this way,we highlight the gap that can exist between return period based zoning methods and acceptable riskthresholds. We also show the higher robustness to vulnerability relations of optimal design approaches ona typical dam design case. In paper four, we propose simplified analytical risk formulas based on extremevalue statistics to quantify risk and perform the optimal design of an avalanche dam in an efficient way. Asensitivity study is conducted to assess the influence of the chosen statistical distributions and flow-obstacleinteraction law, highlighting the need for precise risk evaluations to well characterise the tail behaviour ofextreme runouts and the predominant patterns in avalanche - structure interactions.