Conventional design of pier structures is based on the assumption of fully rigid joints.In practice,the real connections are semi-rigid that cause changes in dynamic characteristics.In this study,quality of the joints is investigated by considering changes in natural frequencies.For this purpose,numerical and experimental modal analyses are carried out on related physical model of a pier type structure.When numerical results are evaluated,natural frequencies generally do not match the expected experimental results.Uncertainties in different aspects of engineering problems are always a challenge for researchers.The numerical models which are constructed on the basis of highly idealized scheme may not be able to represent all of the physical aspects of the physical one.For this study,determination of percentage of semi-rigid joints is considered as an optimization problem based on the numerical and experimental frequencies.Probabilistic sensitivity analysis is also used to determine the search space.A new technique of optimization problem is solved by a combination of smart particle swarm optimization(PSO)and genetic algorithms,and a complicated and efficient system for model updating process is introduced.It is observed that the hybrid PSO-Genetic algorithm is applicable and appropriate in model updating process.It performs better than PSO algorithm,considering the good agreement between theoretical frequencies and experimental ones,before and after model updating.