In this article, we present OTAWA, a framework for computing the Worst Case Execution Time of a program. From its design, it provides an extensible and open architecture whose objective is the implementation of existing and future static analyses for WCET computation. Inspired by existing generic tools, it is based on an architecture abstraction layers where hooked annotations store specific analyses information. Computing the WCET is viewed as performing a chain of analyses that use and produce annotations until getting the WCET evaluation. Finally, the efficiency of the framework, in term of development productivity, is evaluated by two case studies that show some pitfalls that we are currently fixing but also the success of the approach.