Positive Moment Cracking in Diaphragms of Simple-Span Prestressed Girders Made Continuous

Abstract

Precast prestressed girder bridges can be made continuous for live load if the deck and diaphragm are cast with sufficient positive and negative moment reinforcements. The continuity eliminates costly joints and enhances seismic performance, structural integrity and overall durability of the structure. If diaphragm is poured with sufficient negative moment reinforcement before the deck is cast, continuity may also apply to the dead load of the slab. Although, connection of the girders at the diaphragm varies from state to state, it generally consists of bent bars or bent strands. Also, a short length of the girder may be embedded into the diaphragm. The continuity connection is a doubly reinforced section, which requires a time-dependent analysis including differential shrinkage, creep due to prestressing and dead loads, and temperature effects. These time-dependent effects can result in considerable positive restraining moments at the supports, which can in turn crack the diaphragm or pull the girder out of the diaphragm. These positive moment cracks are not only unsightly, but may also result in durability issues for the bridge. Furthermore, it questions the integrity of the continuity connection. The paper examines the extent of positive moment cracking based on field observations, time-dependent analysis, and previous studies.