Fatigue Failure In Industrial/Commercial Building
Fatigue Failure In Industrial/Commercial Building
(OP)
I'm a junior engineer looking to get a little clarification on when to be calculating for fatigue failure in Commercial Applications. In school we learned about fatigue in more mechanical situations where we have significant force being cycled quite often but when it comes to building design in a lot of application/design manual it seems to be neglected completely.
Now assuming a building is loaded to capacity at least twice a day (will discuss actuality later) for 365 days a year and a design life of 50 years. We would be looking at a cycle of 36500. According to a simple fatigue failure curve of steel that would give us a reduction ration of approximately 0.6-0.7 roughly.
Now when calculating live load from the building code does this already assume a factor of safety so that the structure is never loading to that limit and is more in the endurance limit of the steel? Just assuming we use 100psf for example which is then factored to 150psf that would assume in a 10x10ft area we have 75 people weighing 200 lbs (nearly impossible).
If anyone has any insight or design guides on when I should/shouldn't design for fatigue it would be very helpful.
Now assuming a building is loaded to capacity at least twice a day (will discuss actuality later) for 365 days a year and a design life of 50 years. We would be looking at a cycle of 36500. According to a simple fatigue failure curve of steel that would give us a reduction ration of approximately 0.6-0.7 roughly.
Now when calculating live load from the building code does this already assume a factor of safety so that the structure is never loading to that limit and is more in the endurance limit of the steel? Just assuming we use 100psf for example which is then factored to 150psf that would assume in a 10x10ft area we have 75 people weighing 200 lbs (nearly impossible).
If anyone has any insight or design guides on when I should/shouldn't design for fatigue it would be very helpful.






RE: Fatigue Failure In Industrial/Commercial Building
RE: Fatigue Failure In Industrial/Commercial Building
RE: Fatigue Failure In Industrial/Commercial Building
Second, I'd've thought your allowables were derated so this shouldn't be a problem.
Third, assuming 2 limit load cycles per day seems awfully severe. It'd make more sense to say your everyday loading is maybe 60% of limit (ie design) which offsets the reduced allowable.
Fourth, I don't know if you included a safelife factor in determining your allowable discount. If you want a service life of 36,500 cycles, then you should have a fatigue life of between 100,000 and 365,000 cycles (depending on safelife factor.
another day in paradise, or is paradise one day closer ?
RE: Fatigue Failure In Industrial/Commercial Building
As you observed in your OP, it's mainly because the odds of the framing seeing that full live load again and again (enough to be in a fatigue situation) are typically pretty low.
It's not a bad thing to be on the lookout for though.......especially if you are designing framing (getting beyond the situations I mentioned above) where a large amount of human traffic is expected every day.
RE: Fatigue Failure In Industrial/Commercial Building
1) Most commercial building elements spend most of their time being relatively lightly stressed.
2) Commercial buildings generally do not seem to suffer from fatigue issues in practice, based on experience.
I remember reading an article once about prestressing anchor bolts in industrial applications (utility poles etc) to ameliorate fatigue issues. They cited a handful of instances where it made sense prestress. Once of them was "anchors subject to cyclic tension as a result of wind induced overturning loads". You know, kinda like every commercial building braced frame ever.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Fatigue Failure In Industrial/Commercial Building
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Fatigue Failure In Industrial/Commercial Building
RE: Fatigue Failure In Industrial/Commercial Building
Building components, with a few exceptions, are almost always loaded zero-max-zero or x-max-x. Think of a cable stay on a bridge- it is installed with some preload, and bears that tension all the time, plus added tension if you drive a big truck (or whatever) across the bridge. That cable stay, throughout its life, will NEVER be loaded in compression. Most components of a building are loaded in a similar mode. A floor joist in a house, when dead and live loads are in place, is subject to some bending load 'x'. Unless something very, very bad (or weird) happens to the structure, that joist will never experience a bending load of -x.
Assume you have a sample part subject to some sinusoidal load, such that the amplitude between maximum and minimum loading is constant. As the mean load moves away from zero in either direction (meaning that the minimum load approaches zero), the entire fatigue curve stays the same shape but moves up the y-axis, meaning that for any number of cycles the fatigue limit stress approaches the yield stress of the material.
A perfect real-world example of this is shot peened shafts. Consider two rotating shafts subject to some bending load, one simply machined and one machined and shot peened. In the first shaft, the load on the outer fibers is sinusoidal and the mean loading is zero- the load is fully reversed. In the shot peened shaft, the load on the outer fibers is also sinusoidal, but the mean loading is NOT zero. The load in the tension area is the tension load due to bending, minus the compressive load the shot peening process left behind. The load in the compression area is the compressive load due to bending PLUS the compressive load due to shot peening. The amplitude between the tension and compression load values is the same as in the first shaft, but the mean is shifted away from zero, and any millwright will tell you that the shotpeened shaft will last longer.
This is in addition to everything KootK just said, but in the case of most components of a modern structure, it seems to me that the ways in which the structure handles loads pushes the true, real-world fatigue limit stress high enough that other failure modes will tend to dominate.
RE: Fatigue Failure In Industrial/Commercial Building
DaveAtkins
RE: Fatigue Failure In Industrial/Commercial Building
Failure due to fatigue of building structures refereed to in section 4.3 and designed for serviceability in accordance with Article 4.1.3.6 is in general, unlikely except for girders supporting heavily used cranes on which Article 4.1.5.11 provides guidance.
RE: Fatigue Failure In Industrial/Commercial Building
I wonder if the bridge guys have some insight?
RE: Fatigue Failure In Industrial/Commercial Building
I suspect that some parts of some buildings are stressed beyond this level, but without some significant vibration load or something similar, you'd never get enough cycles built up in the part's history for fatigue to be a real problem.
I'd also bet that if we got a bridge guy sucked into this thread he or she would be able to give us some additional insight.