THE CONCRETE BRIDGE MAGAZINE

SUMMER 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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C O N C R E T E B R I D G E T E C H N O L O G Y 26 | ASPIRE Summer 2018 Using Fully Bonded Top Strands in Pretensioned Concrete Bridge Girders by Dr. Bruce W. Russell, Oklahoma State University F u l ly b o n d e d t o p s t ra n d s p rovid e s ig ni fi c a n t b e n e fi t s fo r p r e c a s t , prestressed concrete bridge girders. By using fully bonded top strands, several serviceability issues are improved relevant to the engineer's ability to control stresses in the end regions and eliminate or mitigate unwanted cracking: • Harped (or deflected) strand patterns can be eliminated for most designs. • The need for debonded strands is eliminated in many designs, and, where debonding is required, the required length and number of d ebond ed strands are significantly reduced. • Girder camber is reduced in both the short term and long term. • Cracks that occur in end regions at detensioning can either be eliminated or reduced in both number and size. Many engineers, however, do not take advantage of this reasonable approach because of concerns that fully bonded top strands add tension to the bottom fiber. There are also concerns that top strands may reduce the flexural strength of the cross section. This article asserts that increases to bottom-fiber tensile stresses are small, easily overcome, and usually inconsequential; moreover, the flexural strength of the cross section is largely unaffected. It is the opinion of the author that the positive benefits of using fully bonded top strands far outweigh any adverse effects on service load stresses. Background My interest in this topic grew out of my research (while at the University of Texas at Austin) with Dr. Ned H. Burns to determine rational design guidelines for debonding strands. The overarching p rin ci p l e t h a t w e fo l l ow e d in o u r recommendations, 1-4 which was also presented in an earlier paper by Horn and Preston, 5 is that debonding strands should be minimized in number and length because the pretensioned concrete beam or girder is inherently stronger with bonded strands than with unbonded strands. While the topic of fully bonded top strands was not part of our original conclusions, a paper published in 1994 indicated that fully bonded top strands improve the efficiency of the prestressing strand pattern, and that one of those efficiencies was the elimination or minimization of the need for debonding. 6 In 1997 and 1998, Dolese Bros. Co. of Oklahoma City, Okla., hired me to redesign strand patterns—from harped to straight—for two sets of bridges. The first, the Interstate 35 (I-35) Bridge over Rollercoaster and Pine Roads in Logan County, Okla., is featured in the example in this article. In this design example, the middle span, with 103.43-ft center- to-center bearings, was constructed from American Association of State Highway and Transportation Officials (AASHTO) Type IV girders spaced at 8.2 ft. This I-35 bridge is the first bridge in Oklahoma designed to use straight strand patterns in lieu of harped strands. Also, and as part of the redesign, this bridge became the first in Oklahoma designed using the new (first edition) AASHTO LRFD Bridge Design Specifications 7 in p l a c e o f A A S H TO ' s S t a n d a r d Specifications for Highway Bridges . 8 There have been changes to the AASHTO LRFD specifications since that first edition—most notably the fact that the relatively new code provisions for prestress losses reduce the required number of prestressing strands. Another change is that the allowable compressive stress for temporary stresses is now 0.65 f' ci instead of 0.60 f' ci . Design Figure 1 illustrates the strand pattern Type IV Girder N = 40 strands Four fully tensioned top strands F pe = 1580 kips g = 8.50 in. e = 16.23 in. ES losses = 20.5 ksi –2.002 c.g.s. –2.432 0.141 -4.293 –2.002 2.878 –0.167 0.709 –F p /A –F p e /S –M g /S = c.g. Type IV Stresses at 60d b after transfer Figure 1. Calculated stresses at transfer at 60d b from end of AASHTO Type IV bridge girder with straight strand pattern and four fully bonded top strands but with no debonded strands. Note: Compressive stresses are negative, stresses shown are in ksi, and figure is not to scale. All Figures: Dr. Bruce Russell.

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