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One-third stress increase

One-third stress increase

One-third stress increase

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?

RE: One-third stress increase

JAE, Here is my understanding on the 4/3 increase in allowable stress.  Please keep in mind that I haven't seen the article you've mentioned but certainly would be interested in viewing it.

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

Qshake - that makes good sense.  At least its a rational basis for the number.

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

I remember the article as well.  I believe that it was called "The Mysterious 1/3rd Increase" and I think it was in an AISC Journal .....sometime in the late 70s but I haven't found it.  Regarding the 1/3 rd increase, I agree with it.  I don't believe that you should design a structure with the same safety factors (ASD) or reduction factors (LRFD)for loads that might be experienced once or twice in the life of a structure.  I believe the survival of the 1/3rd increase (despite having little rational basis)is that no one has yet to find a direct link between poor performance of structures and using the 1/3rd 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

My thougt's
.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

The article "The mysterious 1/3 stress increase" by Duane S. Ellifritt can be found in the 2nd Quarter 1977 AISC Engineering Journal.

RE: One-third stress increase

Hi, ctruax

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

for what its worth I happen to agree with Austim on the temporary works issue.  Too often those loads exceed what the engineer expects on the jobsite...which is amazing considering the number of large equipment used in constructing our works.  Moreover, stupidity factors into the mix way too often.  I've been around awhile having worked in construction early in my career and now design...I'm still looking for an appropriate factor for stupidity!

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

Qshake,  I always welcome support for my own views - thanks.

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

All good comments, eureka, austim.  I'm afraid to wander too far from the original post....but...

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

JAE - I have a copy of the article "The Mysterious 1/3rd Stress Increase" somewhere in my archives if you still need it.   Let me know.

RE: One-third stress increase

Ya!  If you can fax it that would be great.


RE: One-third stress increase

Just mark it "Attn:  Structural Dept."

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

Thanks, TE

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