Evolution is a permanent challenge in product line engineering. Reusable assets such as software components or documents evolve continuously due to new customer requirements or technology changes. This leads to modifications or extensions of the product line?s variability models describing the reference architecture. Due to the large size of product lines, single stakeholders or teams can only maintain a small part of a system which poses additional challenges for evolution. This paper presents a tool-supported approach for building and maintaining variability models of large-scale product lines. We structure variability models into multiple model fragments of manageable size that can be created and maintained by individual teams. Model fragments can be merged semi-automatically into a variability model. We illustrate the approach with examples from ongoing industry collaboration.