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/297006
Editor's NotE If you would like to have a specific provision of the AASHTO LRFD Bridge Design Specifications explained in this series of articles, please contact us a www. aspirebridge.org. 48 | ASPIRE , Summer 2013 A A S H T O L R F D T h e A m e r i c a n A s s o c i a t i o n o f S t a t e Highway and Transportation Officials ( A A S H TO ) S u b c o m m i t t e e o n B r i d g e s a n d S t r u c t u r e s c o n s i d e r e d a n d a d o p t e d f i v e agenda items specifically related to concrete structures at their annual meeting hosted in July 2012 by the Texas Depar tment of Transpor tation in Austin, Tex. Technical Committee T-10, Concrete Design, developed A g e n d a I t e m s 3 5 t h r o u g h 3 9 o v e r t h e past several years and moved them to the subcommittee ballot for consideration in Austin. The agenda items represent revisions and additions to the AASHTO LRFD Bridge Design Specifications. This column reviews the 2012 concrete-structures agenda items, w h i c h h a v e b e c o m e t h e 2 0 1 3 I n t e r i m Revisions. A r t i c l e 5 . 8 . 2 . 8 o f t h e g e n e r a l s h e a r p r o v i s i o n s a n d A r t i c l e 5 . 8 . 6 . 2 o f t h e s e g m e n t a l l y c o n s t r u c t e d b r i d g e s h e a r p r o v i s i o n s r e q u i r e d t h a t t h e e f f e c t s o f inclined flexural compression or tension to be considered. Many times, the effect may be beneficial rather than detrimental. To simplify design, Agenda Item 35 revises both of these articles to require their effect be considered where it is "detrimental (increase in shear load) but may be ignor ed if the effect is beneficial (decrease in shear load)." In the design of segmentally constructed bridges, Article 184.108.40.206 of the Specifications was not clear how much of the web depth an inclined tendon had to traverse in order for the ver tical component of prestressing t o e f f e c t i v e l y r e d u c e t h e a p p l i e d s h e a r. Agenda Item 36 clarifies this by inserting modified language from the AASHTO Guide Specifications for Design and Construction of Segmental Concrete Bridges. The revised article states that the vertical component of inclined tendons shall only be considered t o r e d u c e t h e a p p l i e d s h e a r w h e r e t h e t e n d o n s e x t e n d t h r o u g h t h e w e b d e p t h , engage both the flexural compression and flexural tension zones and are anchored or fully developed by anchorage, deviators, or internal ducts located in the top or bottom 1 / 3 of the webs. Agenda Item 37 extends the provisions for dev elopment of pr estr essing strand in Ar ticle 5.11.4 to "normal-weight concrete with specified concrete compressive strengths up to 10.0 ksi at transfer ( f ci ' ) and up to 15.0 ksi for design ( f c ' )." This extension is based upon research repor ted by Ramirez a n d R u s s e l l i n t h e N a t i o n a l C o o p e r a t i v e Highway Research Program (NCHRP) Report 603, Transfe r, De velopment, and Splice Length for Strand/Reinforcement in High- Strength Concrete. Agenda Item 38 r esult s fr om NCHRP Report 679, Design of Concrete Structures Using High-Strength Steel Reinforcement by Shahr ooz et al., which concludes that reinforcing steel with specified minimum y i e l d s t r e n g t h s o f u p t o 1 0 0 k s i c a n b e s u c c e s s f u l l y u s e d i n n o n s e i s m i c b r i d g e applications for both incr eased corr osion r e s i s t a n c e a n d h i g h e r y i e l d s t r e n g t h . A value of yield str ength, f y , not exceeding 100 ksi was found to be permissible without r e q u i r i n g s i g n i f i c a n t c h a n g e s t o t h e LRFD specifications or, more critically, to t h e d e s i g n p h i l o s o p h y a n d m e t h o d o l o g y prescribed therein. Some limitations to this increase were identified. This agenda item extends the minimum yield strength for use in design to 100 ksi for most nonseismic applications. Where higher yield strengths are not permitted by the specifications, the yield strength defaults to the existing values. Appendix D5 defines the ar ticles wher e a minimum yield str ength up to 100 ksi is permitted. NCHRP Report 603, cited in the discussion of Agenda Item 37 above, also addresses d e v e l o p m e n t o f d e fo r m e d r e i n fo r c e m e n t . Based on the research, Agenda Item 39 revises Ar ticles 5.11.2 and 5.11.5 to allow these provisions to apply to development and splice lengths of deformed r einfor cement in tension for normal weight concrete with s p e c i f i e d c o n c r e t e c o m p r e s s i v e s t r e n g t h s u p t o 1 5 . 0 k s i p r o v i d e d t h a t m i n i m u m transverse reinforcement is provided along the development and splice lengths. by Dr. Dennis R. Mertz 2013 interim Changes related to Concrete structures Decrease in Shear Load Hammerhead Pier Increase in Shear Load Beam Beam Decrease in Shear Load Article 35: Conditions that decrease and increase shear load. Figure: PCI. Book_Sum13.indb 48 7/1/13 7:03 AM