The modelling and propagation of the uncertainties related to the earthquake hazard is an important issue in the seismic assessment and design of structures. Different models are available for the seismic input characterisation and for the probabilistic response evaluation. The model choice might have a significant effect on the design of structural components. Viscous dampers are passive control devices which have proven to allow reaching satisfactory levels of protection of structures under earthquake input [1,2]. Deterministic methodologies for the design of viscous dampers for a single seismic intensity level have already been proposed [3,4] but a robust optimal solution should involve reliability studies considering different intensity levels and their relevant occurrence probability [5]. In [6] a reliability-based methodology has been proposed for the optimal design of viscous damper properties. The methodology is based on a stochastic representation of the seismic input and it aims at identifying the damper viscous constants that minimize the dampers cost (related to the sum of the damper forces) while limiting the probability of exceeding a prefixed inter-storey drift ratio for the frame. The probabilistic response assessement is based on Subset Simulation, which provides an efficient mean for estimating the stocastich quantities involved in the objective function and probabilistic constraint. Although the proposed design methodology employs an efficient and advanced simulation method for the reliabilty analyses [7,8], it is computationally expensive and it cannot be employed for design practice. For this reason, in this paper two alternative approaches are proposed and analyzed, which permit to reduce the computational cost of the reliability analyses. The first approach involves a response probabilistic model, which is built via linear regressions [9]. The second approach considers a single level of the seismic hazard for the generation of a prefixed set of seismic inputs to consider during each optimisation loop. The proposed design methodologies are embedded into the Matlab toolbox OpenCossan [10] , which is connected to the opensource software Opensees [11] for the dynamic analysis.