We present an analysis of the stability of urban trees under the effect of wind pressure. The modeling is led using the yield design theory from a cinematic approach ('by the exterior'). The family of ruin mechanisms retained are very simple block rotation, as in-situ observations show. We develop entirely the mechanical modeling leading to the definition of an assistance tool for the preventive maintenance. Methods to evaluate the calculation parameters are presented. To make this evaluation the most objective, we derived it from data measured on the tree and approximations based on established results in the literature. This approach is then used to build the software 'CILVA' which is presented in another paper. Full Text (PDF format, 380000 bytes)