Allowable Net Bearing Capacity Sustained vs Allowable Net Bearing Capacity Transient
Allowable Net Bearing Capacity Sustained vs Allowable Net Bearing Capacity Transient
(OP)
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.






RE: Allowable Net Bearing Capacity Sustained vs Allowable Net Bearing Capacity Transient
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.
RE: Allowable Net Bearing Capacity Sustained vs Allowable Net Bearing Capacity Transient
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?
RE: Allowable Net Bearing Capacity Sustained vs Allowable Net Bearing Capacity Transient
@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.
RE: Allowable Net Bearing Capacity Sustained vs Allowable Net Bearing Capacity Transient
RE: Allowable Net Bearing Capacity Sustained vs Allowable Net Bearing Capacity Transient
RE: Allowable Net Bearing Capacity Sustained vs Allowable Net Bearing Capacity Transient
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.
RE: Allowable Net Bearing Capacity Sustained vs Allowable Net Bearing Capacity Transient
RE: Allowable Net Bearing Capacity Sustained vs Allowable Net Bearing Capacity Transient
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.
RE: Allowable Net Bearing Capacity Sustained vs Allowable Net Bearing Capacity Transient