Factored vs Unfactored Pile load and Capacities 2

[Woody1515]

thread256-461138

Hi everyone,

I have asked a similar question in the past but I’m not happy with how I worded it. I will try again here...

When designing a helical pile with a geotechnical report, the geotechnical report gives ultimate bearing capacities to design the helical pile helices. Once the ultimate pile capacity has been determined, it says to factor this resistance by 0.4, which will give a factored (smaller) pile resistance value. This all makes sense to me. However, do I compare this value with the factored pile load or unfactored pile load? I have heard it is common practice (in parts of Canada) to use unfactored loads when determining a concrete strip footing size, but still is factored loads when designing the concrete reinforcement. Is it a similar process for helical piles? Use unfactored loads for helical piles when checking the bearing capacity against the factored soil resistance, and use both factored loads and factored resistance when designing the pile itself (welds, etc.)?

 

[EireChch]

To me it comes down to Ultimate Limit State (ULS) and Serviceability Limit State (SLS).

You use ULS load combinations, they can be many different load combinations of wind, earthquake, live load etc. These loads should be compared to the factored (or allowable) pile capacity, which in your case is Ultimate Capacity x 0.4 (or Ultimate Capacity / FoS 2.5).

You would use SLS load to check what your serviceability is, which is pile deflection.

 

[Woody1515]

Thanks for the response! I guess I am having troubles with determining if loading the soil over its bearing capacity is an Ultimate Limit State or a Serviceability Limit State? I know some engineers use SLS loads to check the bearing capacity of a strip footing using the ULS bearing capacity.

 

[EireChch]

Bearing capacity can be based on both shear failure and settlement limit.

I actually like to use the terminology of allowable bearing capacity when based on shear, and allowable bearing pressure when based on settlement limit.

Allowable bearing capacity is Ultimate bearing capacity / 3 (in most instances).

Your bearing capacity (based on shear), could be 200kPa, however, your bearing pressure (based on settlement) could be 100kPa.

If you are loading the soil above 200kPa then you are risking a ULS failure. This could be potentially dangerous as shear failure could cause a collapse of a structure.

If you load the soil above 100kPa, you will have more settlement than your criteria (25mm for isolated footings, 50mm for rafts which have a width of 5m or greater, these criteria differ from country to country but this is what I use). Applying a load of 120kPa for example may give you settlement of 30mm. You are then risking a SLS failure, so excessive settlement might mean a noticeable tilt in the floor, doors jamming etc. Not critical to the structures safety but just a pain in the ass and costly to fix.

 

[Woody1515]

This thread (thread507-470255) is asking similar questions. One response from skeletron said that for the piles, you take the factored column load against the factored bearing resistance to design the pile diameter. But for the strip footing, they said to use the total load against the SLS bearing capacity to determine the footing size. Why the difference between the piles and strip footing?

 

[PEinc]

Compare the factored-up column loads to the factored-down bearing capacity.
The strip footing is probably using the unfactored, service loads in order to calculate the settlement of the strip footing.
The concrete strip footing itself should be designed for factored-up wall loads with factored-down bearing capacity.

http://www.PeirceEngineering.com

 

[EireChch]

Woody that thread you linked was confusing as hell for me too. Id have to call the geotech, the Ultimate Capacity, then Ultimate resistance etc.

But I think PEinc summed it up well.

Factored Column load (which in simplest terms may be dead load x 1.3 and live load 1.5, or similar) against your factored down (or allowable) pile capacity.

For settlement calc, settlement is calculated against the actual load, unfractured.

For shear failure you would compared your factored column load against your (allowable) factored down bearing capacity.

Re the Skeleton comment, I think that is correct too. The total load (which i assume he means unfactored) should be compared against your SLS bearing capacity, which to me would be the bearing pressure that gives you 25mm settlement (or whatever your limit is).

I wouldnt call it SLS bearing capacity though, I would just say bearing pressure based on settlement. Similarly, I wouldnt say ULS bearing capacity, I would just say bearing pressure based on shear. Thats just me though.

 

[Settingsun]

Ideally, you should ask the geotechnical engineer who wrote the report for clarification. If you can't get that clarification, use the factored loads (ultimate design loads) to be safe, whilst considering whether to inform your client that lack of clear geotechnical recommendations may be costing them money.

Structural codes usually have clear definitions to differentiate between the expected failure load and the 'safe' (reduced) capacity that is used in limit state design. Some countries are fortunate enough to have this defined in their foundation codes as well (image below). When you go outside those countries, it gets confusing. Eg in this discussion "Factored Column load (which in simplest terms may be dead load x 1.3 and live load 1.5, or similar) against your factored down (or allowable) pile capacity." To me, that's the conservative route to be taken in the absence of clear recommendations, as allowable capacity means (to me) the old system of allowable stress design (without load factors), but the rest of the sentence seems to talk about limit state design.

It also seems to be the case that some geotechnical engineers don't understand that there isn't simply an 'actual' (unfactored) load anymore (never was, but there was a single design load magnitude). Load varies over time. For long-term effects like consolidation, it may be quite conservative to use the nominal live load when the sustained live load might only be ~1/3 of that. That means that the limit based on settlement (sustained load) and the limit based on shear failure (peak instantaneous load) are not directly comparable, ie you can't simply say settlement consideration gives a lower number so governs because its load is also lower. Of course, you can choose to forego construction savings (maybe minor) to avoid this design complexity, but it would be nice for this to be a choice based on common understanding between structural and geotechnical engineers, rather than because we can't speak the same language.

 

[geotechguy1]

I used to practice in Alberta and the way I saw it done was:

'factored up loads' (assuming that the loads are unfavourable to stability) compared to 'factored down' ultimate capacity for the ULS case

unfactored loads compared to computed settlement based on unfactored loads. Usually for unfactored loads we would only use the dead load.

It was common that that the SLS case would govern the sizing of footings.