1/3 Stress Increase History

[jimrcasper]

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.

 

[WillisV]

http://www.aisc.org/Template.cfm?Se...&template=/CustomSource/Epub/EJSearchForm.cfm

See article by Duane Ellifritt entitled "The Mysterious 1/3 Stress Increase."

costs $10 for non-members of AISC.

 

[WillisV]

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

 

[whyun]

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

Try

http://www.aisc.org/stressincrease

 

[stephpsu]

Hello,

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?

 

[civilperson]

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

 

[whyun]

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.

 

[stephpsu]

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.

 

[UcfSE]

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.

 

[bjb]

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?

 

[WillisV]

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.

 

[UcfSE]

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?

 

[sennafan]

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.

 

[civilperson]

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.

 

[csd72]

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.