Difference between Allowable Bearing capacity Service and Ultimate?

Question

When we calculate the dimension of a foundation we always use the allowable SERVICE bearing capacity (lets call it qs) and all the necessary checks are based on this value qs.

My question is why does the geotechnical report calculates the ultimate bearing capacity (lets call it qu)?

In my own research I stepped upon an article that says that when the geotechnical engineer calculates the ultimate bearing capacity it calculates the ultimate load by multiplying the service loads with a safety factor (lets call it Fs) now for a structural engineer (such as i am) we use different load combinations to calculate the ultimate load (especially when we have seismic and wind) in a simple case for example the most famous load combination in the ACI Code is 1.2D + 1.6L if in a certain case the Live Load is >>>> than the Dead Load the use of a simple Fs will not work to calculate the ultimate load combination.

Is that Explanation for this topic is feasible ? and what do you have other suggestion to the difference of qs and qu ? and where do we use each one?

Thank You in Advance.

Answer 1

Let me raise an example:

By Ultimate Capacity, my slab thickness required is 80mm for a particular span X meter. Which my slab would not fail to carry the load into the beam and column.

By Serviceability Capacity, my slab thickness required is 150mm for a particular span x meter. Which the deflection may not exceed the desired displacement.

So, one hand I have a sagging slab which looks like it is going to fall anytime, even hit my head when I am standing right at the middle underneath.

On the other hand I have a rather smooth and flat slab which look nice and gives me confident that it wont fall on my head & still have some space above me when I am standing underneath.

In short,

In Limit State Design, designers have to access both ultimate capacity and serviceability of a structure to serve its purpose!

Answer 2

Limit state design generates a certain envelope of factored applied forces which are compared to their corresponding factored capacities. As long as the factored applied load is less than the corresponding factored capacity load all is good.

Example:

F(uls) < R(uls) F(sls) < R(sls)

All is good carry on with your design.

F(uls) < R(uls) F(sls) > R(sls)

Problem with sls, need to redesign component for sls loading.

Factors of safety is a term used for working stress design and was the method of design used prior to the adoption of limit state design.

I entered university just as Limit State Design was adopted in Canada so I had no exposure to working stress design. When I first got out it was difficult for me to under understand calculations that were being performed by senior engineers that were not taught limit state design or had not been using. One of the strangest things I found was for about 1-2 decades after graduation, Geotechnical reports would not contain ULS capacities or refer to their results in limit state design terminology. Its never, the factored SLS bearing capacity is XXX or unfactored bearing capacity. Having said all that, its usually the the SLS state that will govern your design, but you need to check both.