Soli bearing capacity 3

[Mohamed Maher]

Dear all

Soli bearing capacity for isolated footing or any footing should be checked under working or ultimate loads?

Thanks
Mohamed Maher

 

[Lomarandil]

Traditionally, soil bearing capacity has been described as an "allowable" limit to be compared to working loads.

Over the last decade (roughly, depending on region), some geotechs have begun providing soil bearing capacities meant to be used with LRFD/factored loads. It's still less common in many parts of the world, but happens more and more. (For instance, it is my impression that European geotechs use LRFD methods more often now).

If in doubt, talk to the company who provided the geotechnical report.

 

[HTURKAK]

If you mean Net soil bearing capacity check, you would use service loads ( in general G+Q + S ) ..Regarding the load factors , you should look to the local bldg . code.

Regarding the definition of bearing capacity, you may search the web , and one of the possible outcomes,

https://theconstructor.org/geotechnical/bearing-capacity-of-soil-calculation/11996/

Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[Mohamed Maher]

Thanks alot

 

[EireChch]

You should compare your ultimate load (so working load with factors) to your allowable capacity (your ultimate/fos of 3 normally.

Working loads are only used in serviceability assessment, which is a settlement check.

 

[dik]

Here... both ULS and SLS. Geotech reports list both service and ultimate values.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[AskTooMuch]

Something to take note here is when it comes to load combination with wind load.
You should take note if you are double dipping. Geotech usually gives Sustained, Transient capacity.

 

[EireChch]

Dik but is this for LRFD approach, no?

For traditional bearing capacity assessment, ultimate is calculated using terzaghi's classical bearing capacity equation. Allowable is ultimate / 3. Ultimate loads should be compared to the allowable value.

Service loads should be compared to service values, service values should be calculated based on your settlement calculation and adopted settlement limit.

How are your service values calculated?

As you know the terminology around bearing capacity is very location specific however i think the best terminology is "net allowable bearing capacity, based on shear failure" and "net allowable bearing capacity, based on settlement criteria". I have used bearing "pressure" instead of capacity for the settlement criteria but this also gets misconstrued.

I dont deal with LRFD etc so maybe different terminology is specified/required there.

Asktoomuch - geotechs give sustained, non-transient capacity you mean? For transient load we typically allow the 33% increase in capacity. There is debate over the merits of this but thats another thread!

 

[ALK2415]

For concrete members design always use Factored loads (Ultimate loads Design)
For Bearing Capacity (BC) use service loading (un-Factored loads), since we imbed the F.S inside B.C. equitation (from 3 to 5)
So, in all cases factors are imbedded in design of structure parts or their supporting medium (soil)

 

[dik]

LRFD or a variation of it has been common in Canada for at least 20 years. We were the first class in Engineering at the U of M to use limit states only; working stress other than for historical reasons was not mentioned... and that was back in 1965.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[EireChch]

Hmmm I have never seen unfactored loads compared to allowable capacity.

Excluding LRFD, EC7 factored approaches, the traditional way was 1.3 x Dead load + 1.5 x Live load = your total factored load. This is compared to allowable bearing capacity....

 

[Greenalleycat]

Convention here is to use a bastardized miz of approaches for typical residential and light commercial designs

Geotech provides a single bearing capacity, say 200kPa,and strength reduction factors, typically 0.5 for gravity
Ultimate loads, say 1.2G + 1.5Q, are compared to the strength reductionn factor x bearing i. E. 0.5*200=100kPa

Serviceability loads are done on a working stress basis, usually a FOS of 3 on ultimate. So 200/3 compared to G+0.7Q or G+Q depending on what the designer decides to do
No explicit settlement calcs are done, it just seems to work

 

[BigH]

There is a problem in my view of using "capacity" with respect to settlement issues. Greenalleycat states that the qult/3 for FOS works well for serviceability . . . this may not be the case though. I prefer as EireChch alluded to is the word "pressure" in serviceability situations. There will be structures that limit settlements to, say 10 mm. Using qult/3 may be too high - settlement calculations are needed at least to be in the right ballpark as such calculations are fraught with errors. So for me "capacity" is for shear and "pressure" is for settlement.

 

[Greenalleycat]

@BigH I agree that this approach has some flaws - however, for this sector of the market (residential/light commercial) it is widely accepted practice
I think the reality is that the ultimate bearing capacities given by the geotechs are usually quite conservative, so the FOS of 3 for serviceability is already more like a 3.5+
Additionally, it's common practice to check G+Q vs qult/3 - when does the full Q ever occur?
Finally, unless there is an isolated pad-type foundation, the methods used to check bearing are usually very simplified and conservative
When doing point load checks under a point load on a foundation I will usually just use a load spread of 45 degrees, ignoring the spanning capacity of the foundation in distributing the load beyond that

Settlement is likely to be a significant issue where either there is uneven loading across a foundation or uneven soil types (broadly speaking of course) leading to differential movement
Both of those should be considered by designers already, so this removes a significant portion of the risk