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/295658
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 at www.aspirebridge.org. A A S H T O L R F D by Dr. Dennis R. Mertz 2014 Interim Changes Related to Concrete Structures T he American Association of State Highway and Transportation Officials (AASHTO) Subcommittee on Bridges and Str uctur es (SCOBS) conv ened their annual meeting in Portland, Ore., this past June. During the meeting hosted by the Oregon Department of Transpor tation, SCOBS considered and adopted five agenda items specifically related to concrete structures. After several years of dev elopment, Technical Committee T-10, Concrete Design, moved Agenda Items 6, 7, 9, 10, and 11 to subcommittee ballot in Portland. These agenda items represent revisions and additions to the AASHTO LRFD Bridge Design Specifications. This column reviews the 2013 concrete-structures agenda items, which are integrated into the 2014 seventh edition of the Specifications. A g e n d a I t e m 6 r e s o l v e s c o n f l i c t i n g r equir ement s and terminolog y r egar ding t h e d i s t r i b u t i o n o f l o n g i t u d i n a l t o r s i o n reinforcement in box girders as specified in Articles 18.104.22.168.3 and 22.214.171.124. It clarifies that the calculated value of reinforcement is the total area of reinforcement to be distributed around the outer-most webs and top and bottom slabs of the box girder. The American Segmental Bridge Institute (ASBI) assisted T-10 in developing this ballot item, although the provisions in Article 5.8.3 apply to all cellular concrete cross sections. The minimum transverse reinforcement requirement for segmentally constructed post- tensioned box girders was unified with that of all other concrete cross sections of Article 126.96.36.199 in Agenda Item 7 . Based upon new research, this agenda item removes the previous specified exemption for segmental p o s t - t e n s i o n e d b ox e s a n d i n c r e a s e s t h e minimum r einfor cement by making it a function of concrete strength. Agenda Item 9 addr esses a desir e to i n c r e a s e s p a c i n g o f s h e a r c o n n e c t o r s t o facilitate accelerated construction of concrete bridges with precast concrete deck panels. It revises Ar ticle 188.8.131.52 by increasing the maximum limit on the spacing of non-welded shear connectors to 48 in. or the depth of the member, whichever is smaller. However, an exception is made for cast-in-place box girders where the original limit of 24 in. is maintained. The revisions are based upon the findings reported in NCHRP Report 584, Full- Depth Precast Concrete Bridge Deck Panel Systems. Article 184.108.40.206, which specifies the shear resistance of segmental concrete bridges, is revised by Agenda Item 10 to allow Article 5.8.3—Sectional Design Model for shear resistance of concrete members to be applied to segmental concrete bridges. The traditional segmental concrete bridge shear-resistance provisions taken from the AASHT O Guide Specifications for Design and Construction of Segmental Concrete Bridges continue to be an acceptable alternative. This revision increases the permitted load-carrying capacity of some segmental bridges; howev er the service limit-state check for shear in Article 5.8.5—Principal Stresses in Webs of Segmental Concrete Bridges, is retained as a check on serviceability. A g e n d a I t e m 1 1 p r i m a r i l y c l a r i f i e s existing Article 220.127.116.11.4b, regarding crack control behind intermediate post-tensioning anchors through tieback reinforcement, with additional commentary including illustrative figures. Also, the required length of the tie-back reinforcement is shortened. Interim revisions to the AASHT O LRFD Bridge Design Specifications are considered annually by SCOBS. Their next meeting is scheduled for June 22 through 26, 2014, in Columbus, Ohio. ______________ Dr. Dennis R. Mertz is professor of civil engineering at the University of Delaware. Formerly with Modjeski and Masters Inc. when the LRFD Specifications were first written, he has continued to be actively involved in their development. The PCI State-of-the-Art Report on Seismic Design of Precast Concrete Bridges Seismic design of precast concrete bridges begins with a global analysis of the re- sponse of the structure to earthquake loadings and a detailed evaluation of con- nections between precast el- ements of the superstructure and substructure. Because modeling techniques have not yet been implemented for jointed details, the focus of this report is on procedures for the evaluation of system response and the detailing of connections for emulative behavior. Seismic analysis procedures are discussed with the pri - mary emphasis on force- based analysis procedures. Displacement-based analysis and computer modeling are also discussed. Relevant seis- mic design criteria of early years are summarized along with the current criteria of the AASHTO Specifcations, Cal- trans criteria, Japan's bridge design, and the New Zealand Bridge Manual requirements. This ePubulication is avail- able on line at http://www.pci.org/epubs The PCI State-of-the-Art R e p o r t S t ate - o f - t h e -Ar t R e p o r t o n S e i s m i c D e s i g n o f Pre c a s t Co n c re te B r i d g e s S D - 0 1 - 1 3 F i r s t E d i t i o n AspireBook_Spr14.indb 48 3/21/14 2:21 PM