Footing design combinations for overturning & bearing stress to Australia Standard 1

[Ey_2]

I have a relative simple question in regards to footing design
Given the allowable bearing pressure. What combinations do you use to check the stability when lateral loads and moments are involved?
For gravity I can do G+Q but I don't know what load combinations do I use for wind & earthquake?

Or do you times a factor to get ultimate bearing pressure? If so, where can I find clauses that allow us to do so?

 

[Settingsun]

I was suggesting (very tentatively) that you could use the current ULS action then divide the resulting pressure by 1.6 as an allowable-design load on the footing.

 

[Ey_2]

Thanks. I am not familiar with US code but I just checked it and found the load combination D + L + E for allowable soil-bearing pressures. Is the E here service load? If so, why use 1.2D + E + L for ultimate as well?

 

[r13]

Ey2,

Just a quick answer - service load (non-factored) combination is used to size the footing; the footing (strength) design is based on the factored load combination.

 

[Ey_2]

I get this one. But my question is for the load itself because there are ultimate earthquake, ultimate wind, service wind. And look like the example is using the ultimate one?

 

[Ey_2]

Like this? (G+0.3Q+Eu)/1.6 for earthquake stability check when allowable bearing pressure is given?

 

[human909]

Not sure why people are defining Ultimate Load Combinations for a service stress design.

Not sure why you are considering stability and overturning as a service stress design. If your building falls over then you have bigger issues than serviceability.

 

[Ey_2]

As far as my understanding is concerned, the strength of the footing (bending and shear) is checked under ultimate load combination no matter what pressure is given. But for the footing size, it should be different. When ultimate bearing pressure is given, ultimate load combinations should be used but when allowable bearing pressure is given we don't have to use ultimate load combinations. And not using ultimate load combination doesn't mean the building will fall.

 

[human909]

And not using ultimate load combination doesn't mean the building will fall

Don't forget uplift.

Your own post (second post) list stability as a ultimate limit state.

 

[Ey_2]

My second post simply means I couldn't find the corresponding stability load combinations for allowable bearing pressure because the ones list on the code are ultimate combinations, which is for ultimate bearing pressure not allowable one

 

[r13]

Ey2,

Another angle to look at this matter. In general, the allowable bearing pressure is derived to ensure the settlement will be within the allowable/acceptable settlement tolerance, if it is not exceeded, so it can be seen as a state of service limit. At the ultimate stage, the settlement can be large, however, the failure will not occur, simply because the ultimate soil bearing strength has not exceeded, remember that qu = qa*FOS, and FOS ≥ 2, while ratio of factored load vs service load lies in the order of 1.5 - 1.6 (check me, if I am wrong).

 

[Ey_2]

Agree with the settlement.

So to make my question easier to understand. I will take earthquake as an example. I want to know how large the footing is adequate. If ultimate bearing pressure is given. I can use load combination G+0.3Q+Eu to check. But if allowable bearing pressure is given, which is lower than ultimate one for sure, what load combination do I use here? Using the same one "G+0.3Q+Eu" will lead to a larger footing size.

 

[human909]

So far there has not been a satisfactory answer to the original question IMO. (RAPT is probably one of the more knowledgeable engineers who has posted, but his post seems to miss the mark on the stability requiring ultimate loads). Many of the structures I have been involved in designing, getting enough uplift resistance is critical to avoid overturning. This is most certainly an ULS and 0.9G+Wu is what I used and it would be hard to justify much else.

Part of the problem is that there remains a difference in terminology between geotechnical and structural engineers and geotechnical engineers often don't provide the ultimate capacity nor the FOS. This isn't a new problem in my experience and a quick google brings up this from Engineers Australia, it is pretty clear that this is an issue within the industry:

https://www.engineersaustralia.org....it_state_design_in_foundation_engineering.pdf

So you are hardly alone in your question Ey_2. A good geotechnical report should give you ultimate bearing capacity, but in practice they rarely do.

 

[r13]

The answer hinges on the local practice, and conservatism. My previous explanation is the prevalent practice in the US, which is meant to minimize the construction cost, while maintain a level of confidence on the structural integrity at the ultimate stage (no collapse). However, in no way it is the most conservative approach, but an economical and reasonable one.

If both the allowable and ultimate bearing strengths are provided, IMO, the service load combination should be used to size the footing (against the allowable bearing pressure); then impose the factored load on the footing to ensure the resulting pressure is less than the ultimate bearing pressure, and design the footing (shear, bending) using the resulting pressure. Again, this is a practice acceptable in the US for now, not to say you can do this way without local scrutiny. I suggest to talk to a senior engineer on this, and let us know the result/his (her) comment to help others.

 

[human909]

For what it is worth. Geotechnical engineers allowable bearing pressures are generally excessively conservative. This is highlighted in the paper linked and I have experienced this in my own work. But in general I supply unfactored, uncombined loads to others who are doing the foundation design.

 

[Settingsun]

I've now looked up the old loading code. I didn't work in the permissible stress era but think the following are what would have been done:

G + Q < allowable

G + Q + [Wu/1.5 OR Eu/1.4] < 1.33*allowable

The 1.5 factor was given in the 1989 wind code for permissible design, and the 1.4 factor in the 1993 EQ code for permissible. Both were primarily limit state codes but with some transition clauses. I don't think reduced live load was done; the capacity side was adjusted instead.

That may be more conservative than limit state design, and that was one reason limit state design came about as pointed out in the slideshow that human909 linked to.

 

[captain_slow]

@OP,

I am a little late as the discussion seems to have reached a reasonable conclusion. I will however say that in my experience (Melbourne, Australia) application of ULS combinations to allowable pressures obtained from geotechnical reports is common practice.

@human909,

Sincere thanks for the slide deck - very informative and I have been thoroughly entertained on this Saturday evening.

 

[human909]

I will however say that in my experience (Melbourne, Australia) application of ULS combinations to allowable pressures obtained from geotechnical reports is common practice.

Yes I too have often seen it applied in Melbourne and across Australia but I would argue that it is not good engineering because it is not methodologically consistent and is far too conservative. I also seen highly experienced structural engineers throw geotechnical reports with excessively low allowable bearing pressures in the bin and just rely on soil reports and their own experience.

@human909,

Sincere thanks for the slide deck - very informative and I have been thoroughly entertained on this Saturday evening.

You are welcome. I found it informative too. It helped clarify to me that I'm not crazy, and that there really is a disconnect between what structural engineers calculate and what geotechnical engineers usually provide.

 

[captain_slow]

@human909,

I have seen piling contractors throw geotechnical reports "in the bin" and adopt their own set of soil parameters "based on prior experience". At some point our field got so good at putting failsafe systems in place that we moved on from adjusting our understanding to fit reality towards adjusting reality to fit our understanding.

I didn't mean to say that application of ULS actions to allowable soil pressures is good and/or efficient, I simply stated that it is common practice in Melbourne. Most people and checkers accept it because it's inherently conservative.

 

[r13]

The approach addressed by captain is what I meant "local practice and conservatism". It is conservative, but not overly, if the ultimate stage is so close to be reality.

 

[human909]

Yeah I realise you weren't endorsing it.

But the point I was making is that being too unnecessarily conservative especially when it is inefficient is poor engineering. And it does seem like the inconsistencies between our foundation design and ULS design is an issue being raised by some.

Good further discussion here by some of the same characters of the previous slideshow presentation:

https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.547.7585&rep=rep1&type=pdf

(I'm learning. My area of practice is fairly niche so I don't really deal with foundation design.)