Allowable Net Bearing Capacity Sustained vs Allowable Net Bearing Capacity Transient

[oengineer]

I am using a spreadsheet to design drilled piers and I noticed that when you enter the Allow. Net Bearing Pressure (Qn Sustained) value the spreadsheet multiplies it by 1.5 to obtain a Allow. Net Bearing Pressure (Qn Transient) value. It then goes on to calculate the Allow. Gross Bearing Pressure (Qg Sustained) & Allow. Gross Bearing Pressure (Qg Transient) values. I am curious regarding the 1.5 factor applied to the Allow. Net Bearing Pressure (Qn Transient). Is anyone familiar with the multiplication factor to the Allow. Net Bearing Pressure (Qn Transient)? My concern is that it is adding greater strength to the pier than it should. In ACI 336.3R-93 Design and Construction of Drilled Piers, it mentions "Factors of safety may vary from 1.5 to 5 for side friction or end bearing, depending on the subsurface conditions, structural loads, and degree of confidence in the subsurface parameters." Is this sentence related to Using a 1.5 factor for Allow. Net Bearing Pressure (Qn Transient)? If the 1.5 is the factor of safety, I am not sure why it would be multiplied into the Allow. Net Bearing Pressure (Qn Transient)? Any comments/suggestion is appreciated.

 

[jayrod12]

Just a guess, but I would say it's saying for transient loading (I.E. short term) there is an increase in allowable bearing pressure. If I remember my soils classes correctly, this is due to the fact the pore water pressures would provide increased bearing capacity for short term loads, but for sustained loads the pore water pressure would relieve itself and you'd be back to relying solely on the soil strength.

However, if you're unsure, your better off neglecting the increased capacity.

And perhaps it's best not to use someone else's spreadsheet that you don't understand fully. Or ask the creator of the spreadsheet.

 

[Shotzie]

There is some good discussion here: [URL unfurl="true"]http://www.eng-tips.com/viewthread.cfm?qid=193640[/url]

Are you using factored or unfactored loads? The 1.5 factor could be there in order to make your soil capacity comparable to a factored live load?

 

[oengineer]

@Shotzie - I am using unfactored loads. The 1.5 factor only effects the load combinations that include wind loads.

@jayrod12 - I looking into neglecting the increased capacity, and in some cases it makes a big difference. If I am unable to determine why it is there I will remove it. Hoping others might have some insight.

 

[txeng91]

A lot of geotechnical engineers reduce their safety factor from 3 to 2 when determining bearing capacity for dead+sustained loads as opposed to dead+short term loads, so your bearing capacity will increase by 50% for wind loads. However, I would not use this increase unless you have a geotechnical report that says you can do it.

 

[oengineer]

@dnlv - I just found some information mentioning that usually 1.333 ~ 1.50 are allowed for transient load. So I believe this is what you are referring to.

 

[Nor Cal SE]

Per International Building Code Section 1806, you are allowed to apply a 1.33 short-term-load increase factor on allowable soil bearing pressure when using the Table 1806.2 allowable soil values. These values tend to be conservative, intended for use when no geotechnical report for a site exists.

In the event you have a geotech report for the site, I would check whether it has any verbiage about an allowable increase for short-term loads. In my experience, 90% or more of geotechnical reports will state that the 1.33 increase is acceptable, usually stated one or two lines after the allowable soil value is listed.

I'm not sure whether I've seen a larger allowable increase, such as 1.5, recommended by an geotechnical engineer's report or any other code/reference.

 

[oengineer]

@Nor Cal SE - I believe your are referring to the sentence "An increase of one-third is permitted when using the alternate load combinations in Section 1605.3.2 that include wind or earthquake loads." found at the bottom of Table 1804.2 of IBC. So it is okay to apply 1.33 based on IBC to the Allowable Net Bearing Capacity Transient, since the Wind load is the Transient load? I have seen a soil report where they use 1.5 factor of safety for Transient Loads.

 

[Nor Cal SE]

In my judgment, yes, it is acceptable to apply that 1.33 increase. However, if you have a soils report and are using allowable values from that report, then I recommend confirming that the report itself allows the 1.33 increase.

Using the IBC line you've referenced as justification is technically only valid if also using the conservative allowable soil values in the adjacent IBC table, if that makes sense. But from the numerous geotech reports I've seen over the years, almost all of them seem to allow this same increase on their report-prescribed values. But it's worth the double-check.

And yes, wind definitely counts as a "short-term" or "transient" load, as does seismic.

 

[oengineer]

@Nor Cal SE - I see your point regarding applying the 1.33 soil bearing increase for the transient loads. In theory since the values given in the table are low conservative soil bearing values, the use of the 1.33 should not be viewed as a non-safe design since the allowable soil bearing values provided by a soil report will be greater. I will look into confirming the 1.33 increase.