Double-Tensioned Steel Strands Vertical Prestress System on the Webs of Long-Span PC Box-Girder Bridges

Abstract

Using low-retraction double-tensioned steel strands vertical prestress system (DTSSVPS) to replace finish rolled rebars (FRR) a vertical prestress system could effectively enhance the vertical prestress efficiency and reduce the occurrence of diagonal cracks on the webs of long-span prestressed concrete (PC) box-girder bridges. This study will conduct a T girder test, short rectangular slab test, double-tensioned anchor (DTA) performance test as well as field tests on two bridges with DTSSVPS to investigate the immediate prestress loss, shear cracking performance of the web, and minimum biting length (d) between the anchor nut and cup. The results showed that the immediate prestress loss using DTSSVPS reduced from 20% to 40% after one tension to approximately 10% after two tensions and the prestress efficiency was much higher than FRR. Therefore, it is recommended that the design retraction value of the DTA was 2 mm. The experimental shear cracking load of the web with DTSSVPS was 1.39−1.45 times the theoretically calculated load. Following unloading, the diagonal cracks on the web were closed and the web worked in the elastic stage. Finally, based on the single-hole DTA performance test, the minimum d between the anchor cup and nut was 10 mm. The design tension force (f) carried by the unit arch length of the thread between the anchor cup and anchor nut was 130 N/mm. A formula is proposed for the design of the minimum d of a multiple hole DTA. In addition, the stress distribution on the surface of the anchor nut was investigated. This study could provide a basis for a wide application of DTSSVPS technology in the future.