THE CONCRETE BRIDGE MAGAZINE

Winter 2019

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.

Issue link: http://www.aspiremagazinebyengineers.com/i/1064399

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O ver the past 10 to 15 years, the d e s i g n a n d f a b r i c a t i o n o f prestressed concrete girders have benefited from the availability of high- performance materials, new technologies, and advanced methods for structural design. More specifically, the advent of concrete materials technology, w i d e s p r e a d a v a i l a b i l i t y o f h i g h - performance/high-strength concrete, the development and popularization of self-consolidating concrete, and the introduction of 0.6-in.-diameter strands with the potential for 300- ksi and 0.7-in.-diameter strands have created new opportunities within the prestressed concrete industry. Coupled with these advances in materials, the industry has moved forward to accept more sophisticated methods of design that allow for better optimization by removing unnecessary levels of design conservatism. As a result of these advancements, many states have developed better-optimized prestressed concrete bridge girders. In at least three states—Washington, Nebraska, and Florida—record-breaking spans have been designed and constructed. With girder lengths in excess of 200 ft, the stability of slender pretensioned concrete girders has recently become a serious concern, which necessitates additional considerations in bridge d e s i g n , f a b r i c a t i o n , h a n d l i n g , t r a n s p o r t a t i o n , e r e c t i o n , a n d construction. To address this concern, the Washington State Department of Transportation (WSDOT) has been requiring designers to evaluate stability of girders during both handling and transportation. (For more on the WSDOT views on this issue, see "Designing Precast, Prestressed Concrete Bridge Girders for Lateral Stability: An Owner's Perspective" in the Winter 2018 issue of ASPIRE ® .) Additionally, on the national level, the American Association of State Highway and Transportation Officials (AASHTO) Committee on Bridges and Structures took on this issue by adopting revisions to several articles in the AASHTO LRFD Bridge Design Specifications 8th edition at the committee's June 2018 meeting in Burlington, Vt. Fi r s t , A r t i c l e 5 . 5 . 4 . 3 a n d i t s commentar y have been revised to highlight the importance of considering girder stability. The specifications have been revised as follows: "Buckling and stability of precast members during handling, transportation, and erection shall be investigated." Commentary for this article provides the necessary background, stating that "Stability during handling, transportation, and erection can govern the design of precast, prestressed girders. Precast members should be designed such that safe storage, handling, and erection can be accomplished by the contractor. This consideration does not make the designer responsible for the contractor's means and methods for construction, as discussed in 2.5.3." S e c o n d , A r t i c l e 5 . 9 . 4 . 5 h a s been developed and added to the specifications. This article addresses the use of temporary top strands. This specification states, "Temporary top strands may be used to control tensile stresses in precast, prestressed girders during handling and transportation. These strands may be pretensioned or post-tensioned prior to lifting the girder from the casting bed or post-tensioned prior to transportation of the girder. Detensioning of temporary strands shall be shown in the construction sequence and typically occurs after the girders are securely braced and before construction of intermediate concrete diaphragms, if applicable." Additional u s e f u l d e t a i l s a b o u t p r o v i d i n g , tensioning, and detensioning temporary strands are also presented. Further, the effects of temporary strands on camber calculations and prestress losses are also acknowledged. The commentar y for the article provides a detailed discussion about how the use of temporary top strands improves the stability of girders by by Dr. Oguzhan Bayrak, University of Texas at Austin AASHTO LRFD Bridge Design Specifications: Stability of Pretensioned Concrete Girders Erecting a long-span pretensioned concrete girders. Photo: Concrete Technology Corporation. A A S H T O L R F D ASPIRE Winter 2019 | 47

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