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Title: Design of Continuity Diaphragms Following New AASHTO Provisions
Contact: Bernard L. Kassner
Project No.: 122312
Target Date: 12/31/2026
Simply supported, prestressed concrete beams made continuous for live loads are more efficient in carrying live loads, have greater moment capacity, and provide a redundant structural system compared to those beams that remain simply supported in service. However, this jointless system requires a more complex design that takes time-dependent moment redistribution into account, as well as restraining moments due to thermal gradients and differential shrinkage. Prior to 2021, AASHTO allowed a simplified design of the continuity diaphragms for the spans, provided that the beams were at least 90 days old and the diaphragms could resist 1.2 times the cracking moment. However, AASHTO recently excluded this approach. Instead, engineers must calculate a
time-dependent restraint moment for every diaphragm, regardless of the age of the beams at the time they become continuous with beams from adjacent spans. Furthermore, the stress in the positive moment reinforcement must be less than 36 ksi, for the purposes of mitigating any cracking occurring in that region. 

Unfortunately, the revised code offers minimal guidance for calculating the restraint moments, although AASHTO commentary does refer to two publications. However, there are flaws in at least one of the example problems in those publications. Furthermore, this same example is rather simplistic. Thus, the first objective of this research is to answer whether the 90-day waiting period for the now-defunct simplified provisions can be reinstated or even shortened. If that is not the case, the next objective is to develop the best design approach and accompanying examples with correct time-dependent calculations for unique, variable scenarios (including straight, skewed, curved/chorded, and offset beams) to aid engineers in designing the reinforcing details for the positive moment connection at the continuity diaphragm. These objectives will be achieved through analytical modeling and large-scale testing.

     Carin Roberts-Wollmann
     Ioannis Koutromanos
     Bernard L. Kassner

  • Structures