THE CONCRETE BRIDGE MAGAZINE

SUMMER 2014

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications and it was consistent with the experimental results showing the connection could tolerate some cracking, the provision was retained as Article 5.14.1.4.5. Given that calculation of the diaphragm stresses is complex, there was a desire for a simple rule. At the time, the Tennessee Department of Transportation had a provision requiring the girders be aged 90 days before continuity was established. This was adopted as Article 5.14.1.4.4, which requires that the engineer provide a positive moment connection with a strength of 1.2 Mcr and specify in the contract documents that the girders are to be at least 90 days old when continuity is established. The reasoning given in the commentary is that by 90 days, 60% of the creep and 70% of the shrinkage in the girder is theoretically gone. The behavior of the system will be dominated by differential shrinkage of the deck so the possibility of positive moment cracking significant enough to affect continuity is very low. In effect, the provisions and commentary of Article 5.14.1.4 give the designer four options: 1) Provide a positive moment connection with a strength of 1.2 Mcr and require the girders to be at least 90 days old at the time continuity is established. Experience in Tennessee shows there is no reason to specify a minimum age longer than 90 days unless some unusual situation suggests that significant upward camber may occur after 90 days. 2) Provide a positive moment connection with a strength of 1.2 Mcr and use the provisions of Article 5.4.2.3, with ktd = 0.7, to establish the minimum age at which continuity can be established (commentary). 3) Use the provisions of Article 5.14.1.4.5 and consider the bridge continuous if the net stress at the bottom of the diaphragm from superimposed permanent loads, settlement, creep, shrinkage, temperature gradient, and 50% of live load is compressive. 4) Calculate the actual restraint moments and determine the degree of continuity from the analysis (Article 5.14.1.4.2). If the connection does not provide full continuity, the effect of partial continuity must be considered as required in Article 5.14.1.4.5. References 1. Oesterle, R. G., J. D. Glikin, and S. C. Larson. 1989. Design of Precast-Prestressed Bridge Girders Made Continuous, National Cooperative Highway Research Program Report 322, Transportation Research Board, National Research Council, Washington, DC. 2. Miller, R. A., R. Castrodale, A. Mirmiran, and M. Hastak. 2004. Connection of Simple-Span Precast Concrete Girders for Continuity, National Cooperative Highway Research Program Report 519, Transportation Research Board, National Research Council, Washington, DC. 3. Newhouse, C. D., C. L. Roberts-Wollman, and T. E. Cousins. 2007. “Comparison of Strands and Bars for Positive Moment Connection of Continuous Precast Concrete Bulb-Tee Girders,” PCI Journal, May/June. The editors of ASPIRE™ thank J. P. Benard, chief engineer with Bayshore Concrete Products, for raising the topic. In addition, the editors thank Dr. Richard A. Miller of the University of Cincinnati for preparing this explanation and Dr. Dennis Mertz and Dr. Carin Roberts-Wollman of Virginia Tech for their input. ADVERTISEMENT: -------------------------------------- HILMAN ROLLERS Move the Heavyweights! www.hilmanrollers.com 12 Timber Lane, Marlboro, NJ 07746 USA Phone: (732) 462-6277 Fax: (732) 462-6355 e-mail: sales@hilmanrollers.com

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