Factored vs unfactored vs ULS vs SLS

[Woody1515]

Hello!

I have a question regarding factored vs unfactored loads. Sometimes I come across foundation plans that call out a required screw pile capacity as an unfactored load. In Canada, we can calculate a ULS snow load and SLS snow load – the only difference between the 2 being an importance factor. When it comes to determining an unfactored required pile capacity, would you use the ULS snow load or SLS snow load? As I write this out, I’m thinking it is correct to use the ULS snow load as it is still a strength design check?

 

[jayrod12]

I've specified the unfactored loads on screw piles as well. I do it as my total load. And that is due to the fact that many of the capacities provided by the screw pile guys are allowable (i.e. tested to failure and then a safety factor is applied).

That being said, whether to use the different importance factors or not, I personally use the ULS importance factors at all times until I'm desperate to make something work out. Because for a normal importance building it means you get to reduce by 10% for your deflection calcs, when in all honesty more often than not the members are strength governed.

 

[Woody1515]

jayrod12 - that makes sense to me. I think I had SLS and unfactored confused, they are not the same thing. I would say a pile load is an ULS design requirement, so the ULS snow load would need to be used. But then you just simply add all load components with no other factors (load combinations) for the unfactored load.

 

[TLHS]

The load you should be passing is the specified load. The actual load which the item is intended to be used for.

That would generally mean that it's strength design forces, but factors haven't been applied. That gets a little... fuzzier... when you're talking about earthquakes.

 

[skeletron]

The geotechs are still lagging behind in the limit states design world up here. I think it was only recently that the CFEM started addressing ULS and SLS with resistance factors. So, for foundation design, I typically still go back to the allowable/unfactored/service loads. They should still work, theoretically, versus an ultimate limit states design. The advantage is that the unfactored loads are what pile load tests or proprietary/historical designs have been established on.