Stability Loading Combination

[Lily_Linton]

Hello Everyone,

I do checking for stability of foundation as per 0.9G (Sec. 4.2.1 AS 1170.0). Now, I will be designing a simple retaining wall and I thought that the load combination at section 4.2.1(b) can be used since there's a lot of destabilization force present. But my checker told me to just use 0.9G for stability.

Question, when are we to use the section a and section b of for this?



Thank you very much

 

[ShawThing]

Hi Lily_Linton

In respect to dead load (G) you would use part (a) when the load is resisting the other forces requiring to be considered, ie. the overturning forces of a retaining wall or in a roof situation when you have uplift forces due to wind.

You would use part (b) when the combination of forces can act together in the same direction, ie. when combined with a live load or downward forces due to wind etc.

Hope I have explained this adequately and it helps.

Cheers.

 

[Settingsun]

The wording of the clause is confusing at best. The commentary explains it clearly if you can get a copy.

Part A (0.9G) applies to *parts* of the dead load that are stabilising the structure. Part B (1.2G) applies to parts of the dead load that are destabilising.

The example in the commentary is a cantilever beam. The cantilever part is destabilising so the load on it is 1.2G + 1.5Q. The backspan is stabilising so the load on it is 0.9G.

 

[Agent666]

If you have access to AS 4678, it does a pretty good job of explaining the general concept of the separate effects of stabilising/destabilising loads with respect to retaining structures, see screenshots below.

Though the rest of the standard in the way it asks you to meddle with the basic properties before calculating fundamental geotechnical properties always seemed a bit of a weird way to go about things.

Ignore the different load factors in your case, but keep in mind AS4678 is able to be used to justify the use of an 'ultimate limit state method' as explained in the commentary to AS/NZS1170.0 so that you can apply a load factor of 1.0 (see 4.2.3(f)(i)), this can justify a lower than 1.5 factor on the lateral earth pressure in some cases.

 

[Agent666]

Not sure if you are in NZ or Australia, but the general approach that seems to be considered/taught at University (at least at Auckland University) in NZ is to simply factor up the lateral earth pressure by 1.5, and take a factor of 1.0 on everything else acting vertically.

In NZ we don't have any retaining code, so within reason you can take any approach you like provided it satisfies first principles.

EDIT : -This actually reminds me that the NZBC (NZ Building Code) has some examples of wall design, have a look here.

Unfortunately MBIE have produced some guidance that seems to be a mix of ultimate limit states design and working stress (which I don't fully agree with), so its pretty confusing out there. Their design examples go completely in the face of those in the NZBC. The MBIE guidance suggests your surcharge is some weird loadcase, which in no way produces the maximum overturning. I have queried their advice previously and the answer I got was 'it was looked at by others & peer reviewed', there wasn't any attempt to answer the specific queries questioning what they came up with at all and that it doesn't make sense based on what people have been taught for the last 20 years at Univ of Auckland by the likes of Mick Pender, etc.

Basically I don't think anyone is going to question what you do if you follow what was taught to you at university I guess. I've been doing that for near on 20 years and I rarely use 0.9G anymore, if anything footings are always deeper/heavier than detailed/designed. I vaguely recall something about going to 0.9G actually can for a heavier wall result in lower bearing soil stresses, dending on the magnitude of axial vs lateral loads. I guess look at the sensitivity of these types of things and make an informed decision around which way to go. All load cases have an '&' for a reason as opposed to a 'plus', if it doesn't produce a worse load effect then don't use it as part of the combination.