Remaining Fatigue Life
Remaining Fatigue Life
(OP)
We're performing a fatigue analysis of an early 50's riveted, non-composite bridge. We're following AASHTO Standard specs and the 1989 Guide Spec for Fatigue Evaluation.
From our analysis, most of the girders for the existing condition have zero fatigue life. When we analyze the bridge assuming a new composite deck our fatigue problems for the most part disappear. girders; we're finding that most of the girders have an infinite life.
Someone has raised the point that the new condition cannot have an infinite life - we can get philosophical and say that since the world will end it can't be infinite life - but rather the remaining life is somewhere between zero and infinity.
However, the way I interpret AASHTO the clock is reset after the retrofit. In the early 80's I did fatigue analyses on some bridges in New England using the client's guidelines, which predated the AASHTO guide spec. Their guideline had a method for determining remaining fatigue life after performing a retrofit. However, this method is not in AASHTO.
Has anyone run into this situation? Any thoughts on the matter?
Thanks
From our analysis, most of the girders for the existing condition have zero fatigue life. When we analyze the bridge assuming a new composite deck our fatigue problems for the most part disappear. girders; we're finding that most of the girders have an infinite life.
Someone has raised the point that the new condition cannot have an infinite life - we can get philosophical and say that since the world will end it can't be infinite life - but rather the remaining life is somewhere between zero and infinity.
However, the way I interpret AASHTO the clock is reset after the retrofit. In the early 80's I did fatigue analyses on some bridges in New England using the client's guidelines, which predated the AASHTO guide spec. Their guideline had a method for determining remaining fatigue life after performing a retrofit. However, this method is not in AASHTO.
Has anyone run into this situation? Any thoughts on the matter?
Thanks






RE: Remaining Fatigue Life
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RE: Remaining Fatigue Life
Keep in mind that cyclic loading is cumulative across a vibration spectrum. If you have vibration, you induce stress, even at low amplitude. If you have cyclic structural loading (which obviously you have)the amplitude is high but the frequency is low. Either case presents a material performance conundrum....in the end, you resort to engineering judgment based on your best information and document such in the event of a failure in the future.
I would also characterize and categorize each member for its failure criticality and establish the inspection protocol for such. Realizing that most states have established inspection protocol and frequency of inspections, you just have to impress upon them the need to increase the frequency and supplement their protocol for the site specific conditions.
RE: Remaining Fatigue Life
What we're debating internally is more philosophical. We know the fatigue life isn't zero because the bridge is still standing but on the other hand AASHTO is vague (more like silent) about whether a retrofit negates all previous fatigue cycles. So claiming infinite life may not be accurate either, even though the retrofit reduces the stress range.
The AASHTO formula for fatigue life is somewhat comical in the sense that it can give a negative fatigue life, which would mean that the bridge failed before it was built.
RE: Remaining Fatigue Life
Just because it's standing doesn't mean there aren't a ton of flaws and cracks you cant detect. Just because it has zero fatigue life doesnt mean the bridge is coming down.
RE: Remaining Fatigue Life
Another option is to estimate the cumulative damage to date and the additional damage expected over the next X number of years with the retrofit using the Palmgren-Miner rule. If the expected total damage is small compared to the cumulative damage you can reasonably assert that the retrofit will greatly reduce the likelihood of future fatigue cracks.
I'm not familiar with the 1989 Guide Spec, but I've found "A Fatigue Primer for Structural Engineers" by Fisher, Kulak and Smith to be very helpful.
RE: Remaining Fatigue Life
One possibility - although impractical and probably impossible - is to create a time history of various types of trucks that may have crossed over the bridge during the past 50 years. It's been done before but on a much shorter time scale.
RE: Remaining Fatigue Life
I would not make any sort of assessment on the remaining life of this structure. Anything you say will be misleading. I did a case study on an old riveted bridge failure over a large river from the 30's. There are four other nearly identical bridges built within the same 2 years downstream that have not failed or fractured. It's really a roll of the dice and inspection is the only way to mitigate IMHO.
RE: Remaining Fatigue Life
what would a 1/1000 yr earth quake do to your bridge ? presumably it's designed for a 1/100 year quake/storm ?
RE: Remaining Fatigue Life
If the notion of infinite life continues to cause agita perhaps, the best remedy is to suggest re-issuing the report with a statement that the fatigue life is within the life span of the new deck. In the 80's I did some fatigue analyses based on Miner's equation but AASHTO doesn't recognize this method. If I can change my analysis to the LRFD specifications my problems disappear because rivets are a Category C rather than D; the fatigue life would be infinite from day one.
Regarding inspection, the owner's policy is to perform 100% hands-on inspection of fracture critical and fatigue prone details during biennial inspection. As I mentioned earlier, to recommend increasing the frequency of the inspection could open a can of worms - if it's done here then it has to be done everywhere applicable (as a side note, the Federal government is looking to increase the time between inspections nationwide to save money.)
Fatigue is a strange bird. Several times in the 90's I inspected a bridge - 5 span continuous steel girders that with integral steel box beam pier caps, the roadway is curved but the girders were set on chords - that was retrofitted for fatigue damage. Cracks developed at the ends of the cross frame plates, which at the time of original construction weren't connected to the girder flanges, and cracks continued to develop after the retrofit, which involved connecting the plates to the flanges. The classic repair - didn't work; go figure. We weren't hired to do any analysis but I suspect the retrofit should have been - if it wasn't - designed taking into account the curvature of the structure.
rb - regarding the earthquake. Back in the 40's no one in this area would even think about an earthquake. Several years ago we did a seismic vulnerability assessment, which indicated that the bridge could be in trouble - primarily due to it being founded on Type E soil with moderate potential for liquefaction.
However, after the August east coast earthquake no damage; but that's not to say it isn't vulnerable.