I am using a spread sheet to design drilled piers and I noticed that when you enter the Allow. Net Bearing Pressure (Qn Sustained) value the spread sheet 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)? Any comments/suggestion are appreciated.
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