Iowa Department of Transportation
Iowa Highway Research Board
Brent Pharesbphares@iastate.edu email >
Bridge Research Engineer, BEC
About the research
It is common practice to put additional longitudinal reinforcement (b2 bars) over intermediate supports to resist any negative moment induced by the superimposed dead loads and live loads on bridges. However, little research has been conducted on the performance of the additional negative reinforcing steel. Requirements for the termination of the additional negative moment reinforcing steel have largely been based on engineering judgement, previous performance, and existing practice.
The main objective of this research was to evaluate the effect of different amounts of b2 bar on resisting the negative moment over the pier on a continuous prestressed concrete girder bridge when it is subject to the live load-generated moment and secondary moment. To achieve this objective, a live load field test was performed on a bridge designed with different amounts of b2 bars to allow for comparison of the varying levels of negative moment reinforcement present.
A full-scale finite element model was developed and validated against the field-collected data to study b2 bar performance subjected to live loads. An evaluation was performed utilizing an analytical approach by calculating the time-dependent secondary moment using mRESTRAINT and loading the beam-line finite element model with the maximum negative moment.
It was found that the negative moment induced by the live load and secondary moment does appear through the service life of the bridge. The high differential shrinkage rate between the fresh deck concrete and the girder concrete is the main source of the negative moment over the supports. The magnitude of the secondary moment was found to be highly influenced by the time when the continuity is established.
The results also indicated that the additional longitudinal reinforcing steel provides minimal effect on resistance to the negative moment prior to the formation of deck cracking, regardless of whether the negative moment was induced by live loads or by the secondary moment. The current design approach determines the b2 bar requirement for the strength level based on the live load, while it may be necessary to include the secondary moment in the design.