One-third stress increase
One-third stress increase
(OP)
I've recently been serving on a committee in my city to review the IBC 2000 code for adoption. One interesting aspect of the code is the elimination of the 1/3 stress increase that is typically used for transient loads.
I remember reading an article, published in some engineering journal, about the stress increase. The writer had spent a great deal of time looking for the source of the 1/3 stress increase and couldn't find it. He found plenty of people who understood the rationale behind it, but NO ONE who could name the orignator of it, nor the specific quantitative reasoning for it (i.e. why not 1/4?)
Does anyone know when/where/why it was first introduced?
I remember reading an article, published in some engineering journal, about the stress increase. The writer had spent a great deal of time looking for the source of the 1/3 stress increase and couldn't find it. He found plenty of people who understood the rationale behind it, but NO ONE who could name the orignator of it, nor the specific quantitative reasoning for it (i.e. why not 1/4?)
Does anyone know when/where/why it was first introduced?





RE: One-third stress increase
When considering the overstrength of a member, concrete or steel, and using the load factor method with all applicable load factors equal to 1.0 the overstrength can be estimated at 1.25 to 1.3 times the nominal member strength. With me so far? Going back to Mechanics of Materials, its like comparing the actual member strength to the demand from the load except now we're looking at the threshold of the inelastic range thus the term "overstrength".
Well, that all good for engineers familar with LFD but what about those of us who learned ASD? (Incidentally, I've since learned a number of new methods and continue to learn more each day!) Now back to ASD. Well, it turns out that by increasing the allowable by a factor of 1/3 you'll wind up with essentially the same value.
So then I suppose the next question is who or what determined that the threshold of elastic/inelastic range to be 1.25 to 1.3 of the actual nomial capacity. It was probably an empirical number based on many many strength test.
Any comments or other information?
RE: One-third stress increase
I need to attempt to find that article because I remember the writer had done a pretty exhaustive search/interview process to determine the source of it. I think the article may have been in an early AISC Steel Journal but I'm not sure. It would have been somewhere in the years 1970 to 1985.
RE: One-third stress increase
Alot of engineers, academics, researchers, etc. like to knock around the 1/3rd increase. But I think it's one of those time tested "rules of thumb" that engineers developed through personal experience and common sense before the existance of calculators and computers. Until someone can develop a better method (more efficient, faster, easy to compute)of incorporating a similar stress increase to account for infrequency of loading, then I think it's use should be continued.
RE: One-third stress increase
.9 Permanent
1.0 consistent
1.25 7 days construction period
1.33 3 months of snow
2.00 Impact
All points on an exponential curve of
time/mass vs creep
RE: One-third stress increase
RE: One-third stress increase
I really don't like your 1.25 for 7 day construction period (unless it is intended to include combination of DL+LL+W+temp, and the 1.25 is no more than the code allowance for such load combinations anyway).
I have always argued that construction temporary works (and permanent works during the construction phase) are more likely to receive their full design load (and quite often more than that) than so-called permanent work. In addition, there is an increased risk of unintended loads (eg full concrete skips impacting cofferdam struts).
If anything, I insist on LESS than 1.0, to allow for the unexpected things that occur on construction sites.
RE: One-third stress increase
RE: One-third stress increase
You can never make any thing fool proof. Never under estimate the ingenuity of a fool or a stupid person.
RE: One-third stress increase
Nowadays I am sorely tempted to add yet another factor if the contractor uses a formalised "Quality Assurance" system.
From the sidelines it has sometimes appeared to me that QA is considered to be satisfied if the paperwork is complete, regardless of the stupidities that remain undetected on site. Hence we see an excessive concentration on office paper at the expense of reduced inspection effort on site.
RE: One-third stress increase
My own experience in the field concentrated less on what I needed to fill out and more on what engineering principles did I learn that I can apply in this case and how can I communicate those to the worker. I did a fair job, this I know because, for the most part, once the worker heard and understood the the explanation he usually didn't mind so much doing what he was asked to do in the first place. Even after "I've been doing it this way since before you were in diapers".
Anyway, after reading your post AUSTIM, I couldn't help but think that too often our inspection does rely on the paperwork and not the continuous, on site, in-your-face inspection that was prevalent when I was just starting out. Oh well, just a thougt.
RE: One-third stress increase
RE: One-third stress increase
402-493-7951
RE: One-third stress increase
RE: One-third stress increase
I've been unable to find my copy of article "The Mysterious 1/3rd Stress Increase".
But if you go to the following link you can get a copy of the article for $15.00.
Alternatively, you can order all past issues of the AISC Engineering Journal on CD-ROM for $150.00.
Here is the link: http://www.aisc.org/ejsubjects.htm
You can find the article listed under the "EARTHQUAKE" heading near the middle of the list.
Best Regards
RE: One-third stress increase