Factored vs Unfactored Pile Loads and Pile Capacities

[Woody1515]

Hi all,

I have a job where we have been asked to design a screw pile foundation for a weigh scale. Factored pile loads have been provided by others. The geotechnical report gives unfactored soil properties. Typically, the CFEM says to use a 0.4 resistance factor on the capacities. Is this necessary when comparing to factored loads? Or should this 0.4 resistance factor only be used when comparing with unfactored loads? Seems like a lot of safety factors.

Thanks in advance!

 

[phamENG]

You really shouldn't be comparing factored loads to unfactored (service) capacities. There are ways to "convert" them, but there are a lot of assumptions to be made. Your best bet is to go back to the "others" and ask them for an load case breakdown. That way you can combine them as needed. I'm not familiar with the CFEM, so I can't speak specifically to the application of its factors.

 

[GeoEnvGuy]

The pile capacity from the geotechnical report should provide an ultimate bearing capacity and serviceability capacity. These values should be factored based on the level of understanding and soil variability. As per the tables in the CFEM typically 0.4 with a standard investigation.

The structural design needs to consider the dead and live load combinations. Typically the more permanent live loads and dead loads are utilized to be within the serviceability capacity. The worst case loading combination needs to be within the ultimate bearing capacity.

 

[PEinc]

You need to use factored loads along with the appropriate resistance factors on the various material strengths, including the soil.

http://www.PeirceEngineering.com