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/657823
A A S H T O L R F D 48 | ASPIRE Spring 2016 T he discussion of the agenda items that the American Association of State Highway a n d Tr a n s p o r t a t i o n O f f i c i a l s ( A A S H T O ) Subcommittee on Bridges and Str uctures (SCOBS) adopted at their 2015 annual meeting begun in the last issue continues in this column. The agenda items represent revisions and additions to the 7th edition of the AASHTO LRFD Bridge Design Specifications. This column reviews the 2015 concrete-structures agenda items, which became the 2016 Interim Revisions, that have been published and are now available from AASHTO. The remaining agenda items related to concrete structures, agenda items 6 through 9, follow. Agenda Item 6 The 2013 interim revisions extended the minimum yield strength of reinforcement for use in selected design articles to 100 ksi. This included its use in reinforced-concrete bridge decks. However, the use of a yield strength of 100 ksi in connection with Equation 184.108.40.206- 1, the crack-control equation that limits bar spacing, results in unrealistically close bar spacing in bridge decks. Agenda item 6 adds that bar spacing need not be less than 5 in. for control of flexural cracking where higher strength reinforcement is used. In addition, it restores commentary removed with the 2005 Interim Revisions that suggests, for calculation purposes, a value of d c not greater than 2.0 in. plus the bar radius may be used. Agenda Item 7 Agenda item 7 increases the compressive stress limit at transfer in Article 220.127.116.11.1 to f ci ' 0.65 for pretensioned and post-tensioned concrete components, including segmentally constructed bridges. While previous research 1 suggests that the concrete compressive stress limit at transfer for prestressed concrete components can safely exceed f ci ' 0.65 , concrete in the precompressed tensile zone subjected to compressive stresses at transfer greater than f ci ' 0.65 can experience microcracking, leading to unconser vative predictions of the external load required to cause cracking. 2-5 Agenda Item 8 A g e n d a i t e m 8 r e p r e s e n t s a c o m p l e t e rewrite of the strut-and-tie method (STM) of Article 5.6.3. In addition, concrete members are delineated as being composed of beam regions (B-Regions) and disturbed regions ( D - Re g i o n s ) w i t h s p e c i f i c a t i o n s fo r t h e d e s i g n o f e a c h . T h e p r o p o s e d p r o v i s i o n s t a k e a d v a n t a g e o f a n e x t e n s i v e r e s e a r c h effor t sponsored by the Texas Depar tment o f Tr a n s p o r t a t i o n a t t h e U n i v e r s i t y o f Texas-Austin, 6,7 which involved a thorough examination of previous tests, primarily of deep beams; additional large-scale, deep- beam tests at University of Texas-Austin; and a comparison of current AASHTO provisions and those which have been used in Europe for many years. The most significant changes in the proposed provisions are • e l i m i n a t i o n o f d i s t r i b u t e d r e i n f o r c e m e n t i f a n a s s o c i a t e d efficiency factor is used, • u s e o f s i m p l e c o n c r e t e e f f i c i e n c y f a c t o r s s i m i l a r t o t h o s e i n t h e fib ( I n t e r n a t i o n a l Fe d e r a t i o n fo r Structural Concrete) Model Code for Concrete Structures, • u s e o f t h e e x i s t i n g A A S H T O confinement factor to increase the usable concrete strength where there is clear distance on all sides of a bearing plate or load plate, • provision of expanded design rules to size the nodes in the STM, • use of a single panel truss model for shear span-to-depth ratios less than 2.0, • elimination of principal tensile strain as a criterion for nodal capacity, and • elimination of a separate strut capacity check away from the nodes. Agenda Item 9 The revisions to Chapter 5 of the AASHTO L R F D B r i d g e D e s i g n S p e c i f i c a t i o n s i n Agenda item 5, discussed in the last issue, include the term "equilibrium density" as determined by ASTM C567 in the definition of lightweight concrete. Agenda item 9 adds the term "equilibrium density" to Article 8.2.3 of the AASHTO LRFD Bridge Construction Specifications for consistency. References 1. Hale, W. M. and B. W. Russell. 2006. "Effect of Allowable Compressive Stress at Release on Prestress Losses and on the Performance of Precast Prestressed Concrete Bridge Girders." PCI Journal. Precast/Prestressed Concrete Institute, Chicago, Ill., Vol. 51, No. 2, March- April 2006. 2. Birrcher, D. B. and O. Bayrak. 2007. Effects of Increasing the Allowable C o m p re s s i v e S t re s s a t R e l e a s e o f P re s t re s s e d C o n c re t e G i rd e rs . Technical Report 0-5197-1, Center for Transportation Research, Bureau of Engineering Research, University of Texas at Austin. 3. Birrcher, D. B., O. Bayrak, and M. E. Kreger. 2010. "Effects of Increasing A l l o w a b l e C o m p r e s s i v e S t r e s s a t Prestress Transfer." ACI Structural Journal. American Concrete Institute, Farmington Hills, Mich., Vol. 107, No. 1, January-February. 4. Heckmann, C. and O. Bayrak. 2008. Effects of Increasing the Allowable Compressive Stress at Release on the Shear Strength of Prestressed Concrete Girders. Technical Report 0-5197-3, Center for Transportation Research, B u r e a u o f E n g i n e e r i n g Re s e a r c h , University of Texas at Austin. 5. Schnittker, B. and O. Bayrak. 2008. Allowable Compressive Stress at Prestress Transfer. Technical Report 0-5197-4, Center for Transportation Re s e a r c h , B u r e a u o f E n g i n e e r i n g Research, University of Texas at Austin. 6. Birr cher, D. B., R. G. Tuchscher er, M. R. Huizinga, et al. 2009. Strength a n d S e r v i c e a b i l i t y D e s i g n o f Reinforced Concrete Dee p Beams. Technical Report 0-5253-1, Center for Transportation Research, Bureau of Engineering Research, University of Texas at Austin. 7. Larson, N., E. F. Gómez, D. Garber, et al. 2013. Strength and Serviceability D e s i g n o f Re i n f o rc e d C o n c re t e Inverted-T Beams. Technical Report 0-6416-1, Center for Transportation Re s e a r c h , B u r e a u o f E n g i n e e r i n g R e s e a r c h , U n i v e r s i t y o f Te x a s a t Austin. by Dr. Dennis R. Mertz 2016 Interim Changes Related to Concrete Structures, Part 2