## ULS and SLS

## ULS and SLS

(OP)

Could someone please explain how to evaluate bearing capacity using ULS? (I'm in Canada, if that makes a difference.) I have tried finding answers and reading reports, but I never clearly understand how ULS and SLS are calculated.

1) Is ULS a factored ultimate bearing capacity? If so, why do some geotechnical reports mention "Factored ULS"? Isn't that double factoring?

2) Is ULS the same as the allowable bearing capacity? Is ULS calculated as ULS = q_ult/FS where FS = factor of safety?

3) How do you determine SLS to limit settlement to x mm?

Thank you.

1) Is ULS a factored ultimate bearing capacity? If so, why do some geotechnical reports mention "Factored ULS"? Isn't that double factoring?

2) Is ULS the same as the allowable bearing capacity? Is ULS calculated as ULS = q_ult/FS where FS = factor of safety?

3) How do you determine SLS to limit settlement to x mm?

Thank you.

## RE: ULS and SLS

How I see it, you have 3 bearing capacities:

1. What you get out of the bearing capacity equation (this is the largest value)

2. A factored bearing capacity (usually value 1. times 0.3-0.5) - this is what you compare to structural loads i.e. Factored up structural load must be less than factored down bearing capacity

3. An allowable bearing capacity (1. divided by 3 or 4) - in modern practice you really shouldn't be using this any more.

Unfortunately the terminology isn't really agreed on and some people will call 1. 'Ultimate limit state bearing capacity' and 2. 'factored ultimate bearing capacity'. I prefer to call 1. Geotechnical Ultimate Bearing Capacity and 2. Factored Bearing Capacity.

## RE: ULS and SLS

I am a Structural Engineer primarily but I also have a lot of geotechnical field testing/reporting experience so that shapes my interpretations

There's demand, and there's capacity

Ultimate Limit State and Serviceability Limit State are demand terminologies

ULS > SLS in general terms as that's the obvious intention of calculating these loads

SLS is day-to-day type loads - dead load + occupancy live loads, moderate winds and earthquakes, etc

ULS is the "oh shit" load case - house crowded to the brim, mega earthquake, hurricane, whatever

When undertaking design verification we obviously need to show that our capacity exceeds our demand

In NZ we begin with 'Geotechnical Ultimate Bearing Capacity' (GUBC) which can be calculated in various ways but is general estimated from Scala penetrometer correlations

This gives us a number - say 300kPa - that is the theoretical ultimate bearing capacity of the soil

We then modify this according to the parameters of the foundation (Strip vs Pad, width, bearing depth, etc) to get a design Ultimate Bearing Capacity

This is the theoretical 1:1 capacity to demand ratio number

Now we apply reduction factors

Common practice here is to use a strength reduction factor (phi factor) of 0.5 for gravity, maybe higher for earthquake

So 300kPa * 0.5 = 150kPa - compare this directly against your ULS bearing demand

For serviceability we deviate from LRFD to working stress and use a FOS of 3 - as you put in your post, we divide 300kPa by 3 = 100kPa

We then compare this against our SLS bearing demands

The basis for using working stress for SLS is the answer to your 3rd question, sort of - the guy who did all the testing in the 70s to calculate the Scala penetrometer calculations did a bunch of work to conclude that a FOS of ~3 was sufficient to limit deflections to 25mm

So, 50 years later, we haven't shaken that demon as no one has bothered to investigate any more

Seems to work though