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/306189
56 | ASPIRE , Fall 2011 A A S H T O L R F D T he fatigue limit states of Article 5.5.3 of the AASHTO LRFD Bridge Design Specifications include two distinct checks: one for metallic reinforcement in members subjected to tension, the other for concrete of fully prestressed concrete members subjected to compression. The fatigue limit-state function for metallic reinforcement in concrete members subjected to tension is: γ(∆) ≤ (∆) where: γ = load factor specified for the Fa t i g u e I l o a d c o m b i n a t i o n discussed in Part 1 of this article (ASPIRE™, Summer 2011) ∆ = force effect, live load stress range due to the passage of the fatigue truck, as specified in LRFD Article 184.108.40.206 (∆) = c o n s t a n t - a m p l i t u d e f a t i g u e t h r e s h o l d f o r t h e m e t a l l i c reinforcement being considered As discussed in the previous article, the constant-amplitude threshold is a threshold value of stress range below which the metallic r einfor cement will not crack during the expected life of the bridge. In this case, metallic reinforcement in a concrete member is said to theoretically exhibit an infinite fatigue life. The metallic reinforcement to be checked for fatigue includes nonprestressed reinforcing bars, prestressing strands, and welded or mechanical splices of reinforcement. It is of paramount importance to point out that fully prestressed concrete components designed to have an extreme fiber tensile stress due to the Service III Limit State within the tensile stress limit specified in LRFD Table 220.127.116.11.2-1, are specifically exempted from the fatigue check of their metallic reinforcement. Further, for reinforced concrete members, fatigue needs be considered only in regions where the permanent compressive stress is less than the maximum tensile live-load stress resulting from the Fatigue I load combination; in other words, only if the Fatigue I live-load stress overcomes any permanent compression due to dead load and prestressing. reinforcing Bars For nonprestressed reinforcing bars, the constant-amplitude fatigue threshold specified in LRFD Article 18.104.22.168 is: (∆) = 24−0.33 where = m i n i m u m l i v e - l o a d s t r e s s resulting from the Fatigue I load combination, combined with the more severe stress from either the permanent loads or the permanent loads, shrinkage, and creep-induced ex ternal l o a d s ; p o s i t i v e i f t e n s i o n , negative if compression. For welded wire reinforcement without a cross weld in the high-stress region: (∆) = 24−0.33 For welded wire reinforcement with a cross weld in the high-stress region: (∆) = 16−0.33 For flexural reinforcement, the high-stress region is one third of the span on each side of the section of maximum moment. Prestressing tendons For prestressing tendons not satisfying the exemption above, the constant-amplitude fatigue thresholds specified in LRFD Article 22.214.171.124 are: (∆) = 18.0 ksi for radii of curvature in excess of 30.0 ft. (∆) = 10.0 ksi for radii of curvature not exceeding 12.0 ft. For radii of curvature between 12.0 and 30.0 ft, linear interpolation is permitted. Finally, for welded or mechanical splices, the constant-amplitude fatigue thresholds are given in LRFD Table 126.96.36.199-1. Concrete The fatigue limit-state function for concrete of fully prestressed concrete members subjected to compression is specified in LRFD Article 188.8.131.52 as: Fatigue I compressive stress + ½(effective prestress + permanent loads) ≤ 0.40 f c ′ Previously, this compressive stress-limit check was not explicitly specified as a fatigue check. A future article will discuss the determination of ∆, the force effect, live load stress range due to the passage of the fatigue truck, in more detail. by Dr. Dennis R. Mertz the Fatigue Limit states, Part 2 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. Book_Fall11.indb 56 9/29/11 12:00 PM