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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A P R O F E S S O R ' S P E R S P E C T I V E 42 | ASPIRE Spring 2017 The eighth edition of the American Association of State Highway and Tr a n s p o r t a t i o n O ff i c i a l s ' A A S H T O LRFD Bridge Design Specifications, with substantial revisions in regard to applying the strut-and-tie model (STM), will be published in 2017. This edition of the AASHTO LRFD specifications places an increased emphasis on designing concrete structures with STM and clearly delineating B-regions from D-regions. Article 5.7, devoted to the design of B-regions, states the following: Where it is reasonable to assume that plane sections remain plane after loading, regions of components may be designed for shear and torsion using either the sectional model as specified in Article 5.7.3 or the strut-and-tie method as specified in Article 5.8.2. This implies that STM can be used in lieu of the sectional design provisions. It is also worth noting the introductory sentence in Article 5.8, which is devoted to the design of D-regions: Refined analysis methods or strut-and-tie method may be used to determine internal force effects in disturbed regions such as those near supports and the points of application of concentrated loads at strength and extreme event limit states. By these excerpts, it is obvious that all components of a concrete bridge can be designed using STM. Most certainly, bridge substructure components, such as pile caps and bridge bents, are prime candidates for design by STM. The introduction of loads by bridge beams and geometric discontinuities seen in some bent caps, such as inverted- tee caps, render such bridge elements as D-regions almost in their entirety. Following this line of thought, and starting about two years ago, I began teaching bridge substructure design by STM in our concrete bridge design class at the University of Texas. My teaching efforts, in this regard, greatly benefited f ro m d e c a d e - l o n g S T M re s e a rc h , development, and implementation efforts funded by the Texas Department of Transportation. 1,2 The starting point that I use in my bridge design class relates to some field problems and performance issues encountered in designing D-regions by using sectional design methods. It is true that in many cases the use of legacy sectional methods produces reasonable bridge bent designs. It is also true that there have been a number of cases in which the use of small pot bearings and associated stress concentrations u n d e r n e a t h t h o s e b e a r i n g s h a v e created field issues. Similarly, I am aware of cases in which large quantities of stirrups used in the bent caps were not providing the benefits calculated in sectional designs because direct strutting of the load from bearings to supporting columns was the primary load-transfer mechanism. Providing clear explanations of the observed field problems and how those problems could have been avoided by using STM proved to be a great starting point in my classes. The following excerpt, taken from the commentary of the AASHTO LRFD specifications (C5.8.2.1), serves to let the designer know about some of the aforementioned performance issues in a concise manner. Tr a d i t i o n a l s e c t i o n - b y - section design is based on t h e a s s u m p t i o n t h a t t h e reinforcement required at a particular section depends only on the independent values of the factored section force effects V u , M u , and T u and does not consider the manner in which the loads and reactions are applied which generate these sectional forces. The STRUT-AND-TIE MODEL by Dr. Oguzhan Bayrak, University of Texas at Austin Concrete bridge design by STM Students in the Ferguson Structural Engineering Laboratory at the University of Texas at Austin observe a reinforcing bar cage being placed into a form. Photo: David Birrcher and Robin Tuchscherer.

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