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1/3 Stress Increase History

1/3 Stress Increase History

1/3 Stress Increase History

With the 1/3 stress increase being phased out, I began to wonder exactly why it came about. So far the research I have done still has not given a definative answer on how the stress increase came about. Doe anyone know the history of the 1/3 stress increase or a good reference on the topic? Thank you in advance for your assistance.

RE: 1/3 Stress Increase History

Oops..Article was published in 4th quarter of 1977

RE: 1/3 Stress Increase History

More recent article from Modern Steel Construction 2003 October issue also discuss this topic.

Try www.aisc.org/stressincrease

RE: 1/3 Stress Increase History


Dealing with the 1/3 stress increase, I have been working on a project which has been peer reviewed. The project is  in IBC 2003. The reviewer has told me they have taken a 4/3 increase on the allowable bearing pressures in the geotech report. I scoured the IBC looking for where you can do that, but found nothing, however the reviewer insists his design "savings" due to smaller foundations with the 4/3 increase is code compliant.

Have any of you seen anything like this allowed in the code?


RE: 1/3 Stress Increase History

Look in the geotech report. Some reports give one third increase in allowable soil pressures for loads due to wind or seismic events.

RE: 1/3 Stress Increase History

stephpsu, one third increase for allowable bearing pressures resulting from load combinations including transient loading such as wind or seismic is typically stated in the geotechnical report.

RE: 1/3 Stress Increase History

I have encountered that before as well, but it is not in this particular report. I am going to call the geotechnical engineer and see what he says.

RE: 1/3 Stress Increase History

I've used the 1/3 increase before for loads including wind, but only with the geotech's permission after speaking with him/her. I don't typically use that increase anywhere else.

RE: 1/3 Stress Increase History

I know this thread is a little old, but I just came across it.

Is anyone aware of a failure being attributed to use of the 1/3 allowable stress increase?

RE: 1/3 Stress Increase History

No.  Neither am I aware of a failure being attributed to a lot of different code provisions that have been revised and re-written based on more recent research.  

RE: 1/3 Stress Increase History

That begs the question: Is it really necessary to update as often as happens or have we just been lucky that these things haven't experienced a design load event?

RE: 1/3 Stress Increase History

stephpsu, Please look at foot note d. on table 1804 of IBC 2003 for explicit permission to increase presumptive allowable soil stresses.  I do not think there is a comparable statement that the geotech is free to include such provisions, but it seems to be implied that the modification of allowable stresses for seismic load is valid.

RE: 1/3 Stress Increase History

Very few foundations have bearing pressure as the limiting parameter.  Settlement is usually paramount.  When bearing can be safely increased, it is allowed since the consolidation is a long term phenomena depending on moisture
movement and the duration of maximum loads due to wind or seismic event is short.

RE: 1/3 Stress Increase History

It would be good to have a geotechs comment on this but I believe civilperson is correct.

Allowable bearing pressures for clay soils are usually chosen on acceptable settlement criteria.

Not sure if this applies to sands/gravels though.

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