THE CONCRETE BRIDGE MAGAZINE

WINTER 2018

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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Boral Resources is the only fly ash marketer with operations coast to coast. No one offers more ash sources or more solutions to ash quality and supply reliability issues. When you think about fly ash for concrete, think about Boral Resources. Boral Material Technologies & Headwaters Resources have combined to form Boral Resources America's largest fly ash marketer… …and a whole lot more 56 | ASPIRE Winter 2018 can also be tackled by using an effective modulus approach; it is typically a very small loss component for modern, low- relaxation strands. As an aside, it is important to note that the AASHTO LRFD specifications allow for the calculation of prestress "gains" due to transient loads; however, if such "gains" are calculated, stress checks in accordance with Section 3 of the AASHTO LRFD specifications is appropriate. In doing so, the stress "gains" experienced by the strands will be coupled with the precompression losses in the concrete that surrounds the strands. In this context, and to be clear about the requirements included in the AASHTO LRFD specifications, we must recall the revisions made to the specifications in the 2016 interim. The commentary provided for article 3.4.1 of the 2016 interim gives a clear explanation on when/where to use load factors of 0.8 and 1.0. For readers' convenience the table adapted from the specifications (Table 3.4.1-4 of the 2016 interim) is included in this article as the table above. As discussed in this article, a designer can choose from several paths when running stress calculations. While all techniques are in compliance with AASHTO LRFD specifications, they may or may not be permitted by a particular state in new designs. For load- rating purposes, the benefits of a more refined analysis are clear, and, as we move forward, running such a refined analysis may help us better understand the actual bridge behavior. References 1. Brice, Richard, Bijan Khaleghi, and Stephen J. Seguirant. 2013. "Evaluation of Common Design Policies for Precast, Prestressed Concrete I-Girder Bridges." ASPIRE Winter 2013: 10-11. 2. AASHTO (American Association of State Highway and Transportation Officials). 2017. AASHTO LRFD Br i d g e De s i g n Sp e c i f i c a t i o n s . 8 t h e d . Wa s h i n g t o n , D C : A A S H TO (and earlier editions as appropriately referenced within the article). 3. Wa s s e f , W. G . , J . M . K u l i c k i , H.H.Nassif, A.S. Nowak and D.R. Mertz 2014. Calibration of LRFD Concrete Bridge Design Specifications for Serviceability, report on NCHRP 12-83 (in progress), Transportation Research Board, National Research Council, Washington, DC. Component Load Factor, γ LL Prestressed concrete components designed using the refined estimated of time- dependent losses in conjunction with taking advantage of transient elastic gains 1.0 All other prestressed concrete components 0.8 Table 1: Load Factors for Live Load for Service III Analyses (Adapted from Section 3 of AASHTO LRFD Bridge Design Specifications 2 ) Layered Section Approach -1053 Strain Compatability Approach -1045 Traditional Approach Transformed Section *:Stresses at top and bottom fibers, psi -1048 Traditional Approach Gross Section -1081 668 416 428 410 Increasing levels of refinement, and hence, complexity The implications of refinement in analysis for bottom fiber stress calculations. Figure: Nathan Dickerson.

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