THE CONCRETE BRIDGE MAGAZINE

SUMMER 2011

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/306186

Contents of this Issue

Navigation

Page 61 of 63

60 | ASPIRE , Summer 2011 A A S H T O L R F D A r e c e n t i n t e r i m r e v i s i o n t o S e c t i o n 3 o f t h e A A S H T O L R F D B r i d g e Design Specifications introduced a second fatigue limit-state load combination. This a r t i c l e d i s c u s s e s t h e i n t e n t o f t h e t w o fatigue limit states. T h e n e w fa t i g u e l i m i t s t a t e i s Fa t i g u e I , w h e r e t h e f a c t o r o n l i v e l o a d i s 1 . 5 . F a t i g u e I I i s t h e p r e - e x i s t i n g f a t i g u e l i m i t s t a t e w h e r e t h e f a c t o r o n l i v e l o a d r e m a i n s a t 0 . 7 5 . T h e c o n c e p t o f t w o f a t i g u e limit states was intr oduced s o d e s i g n e r s w o u l d b e t t e r understand fatigue design. It does not change fatigue designs or the pr opor tions of bridge members. Fatigue damage does not a c c u m u l a t e s i g n i f i c a n t l y d u e t o a r e l a t i v e l y s m a l l n u m b e r o f h e a v y t r u c k s b u t m o r e d u e t o t h e v a s t n u m b e r o f t r u c k s o f m o r e t y p i c a l w e i g h t . T h u s , t h e f a t i g u e l i m i t - s t a t e f a c t o r o n l i v e l o a d i n c l u d e d i n t h e L R F D S p e c i f i c a t i o n s s i n c e t h e f i r s t e d i t i o n wa s l e s s t h a n o n e , s p e c i f i c a l l y 0 . 7 5 . F u r t h e r, t h i s l o a d factor is not applied to the H L - 9 3 v e h i c l e a n d l a n e s u p e r p o s i t i o n , b u t o n l y t o t h e d e s i g n t r u c k w i t h a f i x e d r e a r- axle spacing of 30 ft. The factor ed str ess r a n g e f r o m t h i s l o a d f a c t o r a n d t r u c k r e p r e s e n t s t h e m o s t t y p i c a l t r u c k . T h i s f a c t o r e d s t r e s s r a n g e i s u s e d t o d e s i g n b r i d g e d e t a i l s t o e x h i b i t a f i n i t e fa t i g u e l i f e b a s e d u p o n t h e a v e r a g e d a i l y t r u c k t r a f f i c ( A D T T ) . T h i s p r e - e x i s t i n g fa t i g u e load factor has been assigned to the new Fa t i g u e I I l i m i t - s t a t e l o a d c o m b i n a t i o n . T h e l i v e - l o a d fa c t o r o f 0 . 7 5 wa s d e r i v e d a s t h e r o o t - m e a n - c u b e o f t h e s t r e s s ranges experienced by a bridge detail. M a n y b r i d g e d e t a i l s e x h i b i t a f a t i g u e t h r e s h o l d s u c h t h a t i f a l l a p p l i e d s t r e s s r a n g e s a r e k e p t b e l o w t h i s t h r e s h o l d v a l u e o f s t r e s s r a n g e , t h e d e t a i l w i l l n o t c r a c k b u t w i l l t h e o r e t i c a l l y e x h i b i t i n f i n i t e f a t i g u e l i f e . T h e n e w Fa t i g u e I l i m i t - s t a t e l o a d c o m b i n a t i o n i s i n t e n d e d t o r e p r e s e n t i n f i n i t e - l i f e f a t i g u e d e s i g n . The Fatigue I load factor on liv e load of 1.5 r epr esent s the str ess range due to the heaviest tr uck that needs to be consider ed for fatigue. It is not the a bsolute heaviest tr uck. The liv e-load load factor of 1.5 was deriv ed as the 1 in 10,000 gr eatest str ess range experienced by a bridge detail. The addition of a second fatigue limit- state load combination is not a r ev elation on fatigue design, but mer ely a r e-writing o f t h e e x i s t i n g f a t i g u e p r o v i s i o n s , f o r b o t h c o n c r e t e a n d s t e e l , t o e x p l i c i t l y a c k n o w l e d g e i n f i n i t e - l i fe fa t i g u e d e s i g n . Pr evious to the interim r evision of adding Fatigue I, the factor of two r epr esented by t h e F a t i g u e I l o a d f a c t o r d i v i d e d b y t h e F a t i g u e I I l o a d fa c t o r wa s i m p l i c i t l y i n c l u d e d i n t h e f a t i g u e - r e s i s t a n c e p r o v i s i o n s w h e r e a p p r o p r i a t e . T h u s , f a t i g u e d e s i g n h a s n o t c h a n g e d , b u t i n f i n i t e - l i f e design has become explicit w i t h t h e d e s i g n e r n o w k n o w i n g w h e n d e s i g n i n g f o r i n f i n i t e o r f i n i t e l i f e u s i n g t h e F a t i g u e I a n d F a t i g u e I I l i m i t s t a t e s , r espectiv ely. W i t h t h e l o a d s i d e o f t h e f a t i g u e l i m i t - s t a t e functions discussed a bov e, a f u t u r e a r t i c l e w i l l i n v e s t i g a t e t h e r e s i s t a n c e s i d e o f t h e f a t i g u e l i m i t - state functions specifically for concr ete bridges. by Dr. Dennis R. Mertz the Fatigue Limit states, Part 1 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_Sum11.indb 60 7/1/11 10:00 AM

Articles in this issue

Links on this page

Archives of this issue

view archives of THE CONCRETE BRIDGE MAGAZINE - SUMMER 2011