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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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 , Spring 2013 A A S H T O L R F D T h e A A S H T O L R F D B r i d g e D e s i g n Specifications currently includes the following six different procedures to estimate the shear resistance of concrete members: a. A r t i c l e 5 . 8 . 3 . 4 . 1 — S i m p l i f i e d Procedure for Nonprestressed Sections b. Article 184.108.40.206.2—General Procedure c. Article 220.127.116.11.2 reference to Appendix B 5 — G e n e r a l P r o c e d u r e fo r S h e a r Design with Tables d. A r t i c l e 5 . 8 . 3 . 4 . 3 — S i m p l i f i e d P r o c e d u r e f o r P r e s t r e s s e d a n d Nonprestressed Sections e. Article 5.8.6—Shear and Torsion for Segmental Box Girder Bridges f. Article 5.6.3—Strut-and-Tie Model Procedures a, c, and f of the six procedures were discussed in the Winter 2013 issue of ASPIRE™. These procedures were included in the first edition of the LRFD Specifications published in 1994. Procedures b, d, and e were added over the years since the original edition of the LRFD Specifications and are reviewed herein. Procedure b is the current basic sectional m o d e l i n t h e L R F D S p e c i f i c a t i o n s . I t r e p r e s e n t s a r e f i n e m e n t o f t h e i t e r a t i v e m o d i f i e d c o m p r e s s i o n f i e l d t h e o r y (MCFT) of Procedure c. In Procedure b, a f a c t o r i n d i c a t i n g a b i l i t y o f d i a g o n a l l y cracked concr ete to transmit tension and shear, β , and the angle of inclination of d i a g o n a l c o m p r e s s i v e s t r e s s e s , θ , a r e directly calculated. In the author's opinion, Procedur e b is the pr eferr ed procedur e to estimate shear resistance when a sectional m o d e l i s a p p r o p r i a t e . A g a i n , s e c t i o n a l models are based upon the assumption that the reinforcement required at a par ticular section depends only on the separated values of the factored section force effects (moment, a x i a l l o a d , s h e a r, a n d t o r s i o n ) a n d d o e s not consider the specific details of how the force effects are introduced into the member. Procedure c in Appendix B5 remains only so that software written using the previous tabularized values of β and θ, while perhaps yielding slightly different solutions, remains code compliant and can be used to load rate bridges designed with the table values. The newness of the MCFT and its perceived complication due to its iterative nature, as presented in the first edition of the LRFD Specifications, led to a National Cooperative Highway Research Program (NCHRP) project to find a simpler estimate of shear resistance. This NCHRP project resulted in Procedure d. Pr ocedur e d is mor e in line with that of the American Concr ete Institute's (ACI's) a p p r o a c h , w h e r e i n t h e n o m i n a l s h e a r resistance provided by the concrete is taken as the lesser of the resistance associated with the two types of inclined cracking: flexure- s h e a r c r a c k i n g a n d w e b - s h e a r c r a c k i n g for which the associated resistances are V ci a n d V cw , r e s p e c t i v e l y. T h i s p r o c e d u r e wa s dev eloped concurr ently with Procedur e b, the refined MCFT with direct calculation of β and θ. Procedure d appears less accurate for bridges than Procedure b. In the author's opinion, the simplicity of Pr ocedur e d is no longer needed as Procedure b is just as simple. Finally, Procedure e was brought into the LRFD Specifications from the AASHTO Guide Specifications for Design and Construction of Segmental Concrete Bridges b e c a u s e o f t h e s e g m e n t a l - b r i d g e c o m m u n i t y 's r e a c t i o n t o t h e n e w n e s s o f M C F T a n d their inexperience with MCFT. Slowly, the segmental-bridge community is warming to Procedure b for segmental bridges as well. The AASHTO Technical Committee T-10, C o n c r e t e D e s i g n , i s b e g i n n i n g a n e f fo r t t o r e o r g a n i z e a n d r e a s s e s s t h e c o n c r e t e d e s i g n p r o v i s i o n s o f S e c t i o n 5 o f t h e LRFD Specifications. Most likely, these six variations in estimating shear r esistance o f c o n c r e t e m e m b e r s w i l l u l t i m a t e l y b e consolidated. by Dr. Dennis R. Mertz AAsHto LrFd: shear resistance, Part 2 Six Procedures to Estimate Shear Resistance • Article 18.104.22.168.1—Simplified Procedure for Nonprestressed Sections • Article 22.214.171.124.2—General Procedure • Article 126.96.36.199.2 reference to Appendix B5—General Procedure for Shear Design with Tables • Article 188.8.131.52.3—Simplified Procedure for Prestressed and Nonprestressed Sections • Article 5.8.6—Shear and Torsion for Segmental Box Girder Bridges • Article 5.6.3—Strut-and-Tie Model AspireBook_Spr13.indb 48 4/1/13 11:02 AM