Collaborative project delivery models (CPDMs) are increasing in popularity as infrastructure con-struction projects become more complex and grow in scale and scope. CPDMs build on highlevels of trust, information exchange and formal contractual relations, creating complex relation-ships between actors in the project network. The literature emphasizes the procurement phaseand contractual aspects of CPDMs; however, few studies explicitly focus on the role of socialrelationships in CPDM projects. Social network theory has recently been introduced in the archi-tecture, engineering and construction industry industry to study relationships between networkactors, but the role of social ties within CPDM project networks is still unclear. Through twoqualitative case studies, we have analyzed the role of social ties in projects applying a CPDMcontract. The empirical evidence of 41 semi-structured interviews and observations points toaspects that affect the development of social ties in CPDMs, such as initial project setup, projectidentification, perception of actors, resource sharing and shared space, as well as the importanceof formal and informal ties for supporting collaborative project practices. The evidence showshow social ties develop into negative or positive bonds affecting the level of collaboration. Thisstudy contributes by emphasizing the interplay between project networks using CPDMs andsocial ties.