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 34 | ASPIRE Spring 2016 Alternatively, and especially in D-regions, a designer may use STM in designing and detailing a reinforced concrete member. The use of STM allows a designer to simplify internal stress paths by using a simple truss model. In this way, a designer can propor tion the longitudinal and transverse reinforcement and ensure that the specified compressive strength of concrete is sufficient and that reinforcing bar anchorage requirements are met. To illus trate the simplicity and transparency of STM, let us use the design of an inver ted-tee beam as an example. For brevity, I will focus on hanger reinforcement design rather than all aspec ts of the s truc tural design of an inverted tee. Figure 1 includes an STM model for a rectangular beam that is loaded on its compression chord, and Figure 2 is an STM model for a ledge- loaded inverted-tee beam. Since both beams are of equal length and the design loads act at the same locations along the beam length, both beams have identical bending moment and shear force diagrams. The flexural and shear designs resulting from these diagrams would be identical, except for the fact that if legacy design methods The 2016 Interim Revisions to the 7th edition of American Association of State Highway and Transportation O f fi c i a l s ' A A S H TO L R F D B r i d g e Design Specifications place a greater e m p h a s i s o n d elin e a t ing di s t u r b e d r e gio n s ( D - r e gio n s ) o f r e in fo rc e d c o n c r e t e m e m b er s f ro m t h e b e a m regions (B-regions). Whereas the use of legacy design methods is appropriate in B - r e gio n s , t h e A A S H TO L R F D s p e ci fi c a t io n s r e c o m m e n d t h e u s e of the s tru t-and-tie method (STM) in D-regions as an alternative with a s trong physical basis to the more p r e s c r i p t i v e o r e m p i r i c a l l e g a c y methods. To the extent it relates to STM, and in an effort to emphasize the importance o f s im p li ci t y a n d t ra n s p a r e n c y in s t ru c t u ra l d e s ig n , in t hi s a r t i c l e I will focus on the s truc tural d esign o f r e in fo rc e d c o n c r e t e b e a m s . I n broad terms, the design of reinforced concrete beams can be accomplished either by following the rules of legacy d e s ig n m e t h o d s o r b y u s ing ST M . L e g a c y d e s ig n m e t h o d s , s u c h a s sec tional design methods, have long b e e n u s e d in d e s ig ning r e in fo rc e d concrete beams. In such designs, axial load, shear force and bending moment diagrams are drawn first. Following the development of these diagrams, critical sec tions a l o n g t h e l e n g t h o f a b e a m a r e identified. By using the forces present at critical s ec tions, thos e s ec tions are designed to have sufficient axial, flexural and shear s trength to meet the appropriate code requirements. In addition, all postulated failure modes have to be considered and the member has to be detailed so as to preclude all failure modes such as reinforcing bar anchorage failure and bearing failure. SIMPLICITY AND TRANSPARENCY IN STRUCTURAL DESIGN by Dr. Oguzhan Bayrak, University of Texas at Austin Strut-and-tie method Figure 1. Longitudinal strut-and-tie model of a rectangular beam with compression chord loading. All Figures: Oguzhan Bayrak. 150 k 150 k 71 212 150 150 150 212 300 300 50 350 0 Rectangular Cap Elevation Cross-Section A B 100 k 100 k 100 k 100 k 100 k 100 k 150 k 150 k 71 212 150 150 150 212 300 300 150 350 100 Elevation Cross-Section Inverted Tee Cap A B Figure 2. Longitudinal strut-and-tie model of a inverted-tee beam with tension chord loading.