Substation foundations using LPile

[structural-eng]

If Lpile is supposed to use working loads, which makes sense to me because foundation movement usually governs the design, should the NESC 250B case be considered without the overload factors? It seems like 250B is an ultimate load case given the OLF and I've seen published documents that refer to it as an ultimate stress design. I have a dead end structure where the ultimate 250B load is higher than the ulitmate ASCE (sim 250C) load but if I look at unfactored loads the ASCE is higher than the NESC 250B case. Everything is likely to be controlled by deflection including the steel framing for the dead end so I really have 2 questions. Which load should be considered for the foundation design using Lpile if pier movement is going to govern and should the steel deflections be checked using the working load for both the NESC 250B and the ASCE or should the deflection be calculated using NESC ultimate loads and ASCE working loads?

 

[dauwerda]

should the NESC 250B case be considered without the overload factors?

Yes, for determining foundation size to resist axial, shear, overturning loads you should consider loads without the overload factors (service level loads). However the actual concrete/reinforcing design should consider the load case with overload factors (strength level loads).

Which load should be considered for the foundation design using Lpile if pier movement is going to govern

You should consider all load cases with loads combined at service levels and design for the governing load case. In the case presented above it sounds like the ASCE load case will control the design.

should the steel deflections be checked using the working load for both the NESC 250B and the ASCE or should the deflection be calculated using NESC ultimate loads and ASCE working loads?

Deflection is a service condition and should be checked using service level loads - per the client/utility specification. For deadend type structures this can get a little more complicated because the actual wire tensions, which typically control both strength and deflection design can change drastically based on loading/weather conditions. Most utilities determine line tensions at different weather cases using PLS-CADD or Sag-ten software. Typical weather cases include the NESC 250B, 250C, and 250D rules and sometimes include a condition with extra heavy ice as well as a "normal everyday loading". The "normal everyday loading" is usually with ambient temp of 60deg and low to no wind (this will change based on geography/location and utility preferences) - this results in fairly low tensions compared to some of the other weather/load cases. Many utilities will specify this "normal everyday load" to be used as the load for determining deflections and checking against the allowable deflections. When this "normal load" is not used I often see the wind (250C or ASCE) load designated as the deflection design load. However, I have also seen some utilities (not many) that specify the 250B loads be used at service levels (without load factors) for the deflection design.

 

[structural-eng]

Thank you for the response. You have confirmed my understanding of how to use the loads relative to design for both strength and deflection.