How to obtain differential settlement for in foundations 1

[ONENGINEER]

I have run some settlement analysis of soil and understood that I have a total of 37 mm of settlement under a 1.7 m x 4 m spread footing. There are 3 similar footings 10 m c/c. Soil profile is similar for all. Can I calculate the potential differential settlement?

Would the differential settlement be with regards to one footing itself or with regards to one footing against the other footing.

 

[ishvaaag]

As usally understood, differential settlement is measured as between udifferent footings, no between different assumptions of settlement at the same footing (even if a range of settlements may be properly aduced to obtain own settlement and differential settlements with other footings).

In the downloadable document for foundations of the spanish CTE you can see what restrictions are expected here to be respected respect differential settlements between different points at the foundation.

 

[Ron]

ONEENGINEER....your handle notes that you are a geotechnical engineer. This is basic to geotechnical engineers.

Differential settlement is the DIFFERENCE in settlement between any two foundation elements, typically between any two column footings.

So to answer your first question, yes...you can calculate differential settlement between the footings; however, if you calculated 37mm of settlement and you do the same calculations with the same footing size and loads, resting on the same soil profile, you should get the same estimate of settlement...37mm. That would indicate that all would settle the same amount or in other words, you would have UNIFORM settlement, not differential settlement. If the soil profile or other parameters are different, then you might get a different settlement value, thus differential settlement between the two elements that you compare.

Not sure what you are evaluating, but 37mm is quite a bit of settlement. If you have that much differential settlement in 10m, it will induce a lot of stress in the structure and will severely crack masonry.

 

[ONENGINEER]

There are three of these footings for a gantry crane. I got from Ron's response that if the foundations are similar in load and soil condition, the differential settlement would be zero (of course practically some but insignificant). If that be the case no matter if it is 37 mm or even more, the structure should not be damaged (even if it was masonry). I hope my understanding is correct.

(I am a little confused as often in geotechnical practice they say the settlement is .. 25 mm and differential settlement assumed 3/4 of it i.e. 19 mm and cannot recall where the 3/4 comes from.)

Thanks.

 

[Ron]

No...it does not mean the structure wouldn't suffer damage...just one particular manifestation of damage...that being masonry cracking between the columns. You can have lots of damage even with uniform settlement.

For a gantry crane on three footings, I'd be concerned about uplift as well. 37mm of settlement for a gantry crane, in my opinion, is excessive...uniform or differential.

 

[ONENGINEER]

The 37 mm calculated fopr one footing. The subsoil is pretty uniform. Two of the foundations are slightly lower load and give a settlement of 32 mm. Therefore I understand that my total settlement is 37 mm and may be the differential setttlement may be in the order of 5 mm?

 

[jdgengineer]

I'm not a geotechnical engineer, but I'd assume that the differential settlement is calculated based on their understanding of how uniform the soil underneath the footing really is. Even in "uniform" soil I'm sure there is some variation is the expected soil properties. Depending on this level of variation, I would assume that different ranges of differential settlement can be inferred. Something that may be very variable (i.e. expansive clay) would have a much larger expected differential settlement than rock.

I'm not an expert though, and if you aren't familiar with the calculations to determine expected differential settlement, I'd find someone who is that can help you.

I would think the typical 3/4" inch of differential settlement across the foundation is a CYA statement in a lot of geotechnical reports when they expect the differential settlement to be reasonably low.

 

[ToadJones]

A gantry crane on three separate foundations?
How does that work? I'm curious

 

[MiketheEngineer]

My guess is one footing wouldn't move at all and the other 37mm - so your differential could potentially be 37mm - not good....

 

[charliealphabravo]

The settlement is also a function of the uniformity of the loads. I think with a crane that load uniformity will not be a given and so some educated assumptions will need to be made with respect to the possible load distributions and the effect on differential settlement...

Just talking from the back seat here.

 

[shobroco]

I'm with Toad on this one: every gantry I've seen has one foundation.