Mechanical and bond behavior of L‐shaped utility tunnel joints using large‐spaced U‐shaped stirrup lap splices

Abstract

U‐shaped stirrup lap‐spliced connection (USLSC) is a mature and reliable reinforcement connection technique; optimizing its spacing improves concrete compaction and joint performance. This study investigated bottom L‐shaped joints in utility tunnels with stirrup spacings of 200, 400, and 600 mm. Four full‐scale specimens—one ordinary and three USLSC—were tested under static loading to evaluate load capacity, failure mode, and ductility. Theoretical models for stirrup and bond stresses were developed to examine the effects of haunch height and spacing. Results showed that all specimens failed in compression‐bending (concrete crushing), while LX‐3 also developed splitting cracks (bond failure), indicating sufficient capacity and connection reliability; however, ductility decreased with greater spacing. Theoretical analyses further indicated that larger spacing increased bond stress along the anchorage, but a greater haunch height mitigated this trend, and the failure mode shifted from compression‐bending to bond failure.