Work in socio-technical systems (STS) exhibits dynamic and complex behaviors, becoming difficult to model, evaluate and predict. This study develops an integrated soft computing approach for nonlinear risk assessment in STS: the functional resonance analysis method (FRAM) has been integrated with fuzzy sets. While FRAM is helpful to model performance variability in qualitative terms, the assessments are usually subjected to a high degree of uncertainty. This novel approach is meant to overcome the subjectivity associated with the qualitative analyses performed by experts' judgments required by FRAM. For demonstration purposes, the approach has been applied to model a waste recycling process for construction materials. The results show how the approach allows assessing and ranking critical activities in STS operations.