Assessing the needs for intermediate diaphragms in prestressed concrete girder bridges

Abstract

Reinforced concrete Intermediate Diaphragms (IDs) are still being used in prestressed concrete (PC) girder bridges in Louisiana. Some of the advantages of providing IDs are disputed and also use of IDs increases the cost and time of construction. There is no consistency in the practice of providing IDs among various states and codes of practice and overall there is a lack of clarity on the effectiveness of IDs and their needs in prestressed concrete bridges. The objectives of this research are to assess the need of reinforced concrete (RC) IDs in PC girder bridges and to determine their effectiveness and also to search for an alternative steel diaphragm configuration which would be as effective as RC IDs and could replace them if necessary. Systematic parametric studies for various bridge configurations, which are representative of an entire range of bridge geometries with different parameters, are analyzed through simplified and solid finite element models, which were already calibrated under live loads. This study was carried out on right and skewed bridges which are simply supported and continuous. A reduction factor which could be multiplied by a load distribution factor to account for the influence of the diaphragm in load distribution was developed. To assess the effectiveness of various diaphragms in protecting the girders against the lateral impact and to determine the design forces in the steel bracing members during construction of deck, a finite element analysis was carried out using 3-D solid models. The results from the parametric studies indicated that several parameters such as skew, span length, spacing, stiffness of diaphragm and girder have different levels of influence on the effectiveness of diaphragms in live load distribution for bridges. Correction factors were developed which could quantify the ID influence on load distribution. Results from various studies indicated that a steel diaphragm section can possibly replace the RC steel diaphragms.