Lateral Pier Analysis

[jcshw]

Hi,

I'm trying to model some piers with lateral loads on them. My office does not have L-Pile, but we have Bentley software. Has anyone used STAAD or Elements to model piers with lateral loads?

We are trying to get a good maximum number from our geotech to check the spring reactions against. We have three soil layers - the first has a k of 150pci and a lateral bearing capacity of only 400psf. If we are modeling a 24" diameter pier with springs every 2ft, then the maximum allowed deflection to reach the 400psf is only .019". Something seams fishy.

Any advice on this method or other good methods to get good moment, shear, and deflection results would be really appreciated. Thanks!

 

[ishvaaag]

Most likely you are using too weak springs; geotechnicals use to be very conservative when giving a RELIABLE value. Then, most structural designers have been designing foundations for the actual (er, service) loads for years and the allowables, even if conservative most surely not AS conservative as when tests began to proliferate. But that is a requirement of reliability, when the properties have high dispersion.

In the end, I have read an article describing works of the people involved in the foundations of the tallest skyscraper in Dubai and learnt there that the settlements were in accord with calculations ... once "tuned" to what there seen (i.e., regged, between ourselves) so if those called to the more exacting tasks need to reckon reality, most of we other structural designers may well continue asking for the use of probabilistic (i.e., average -and not 95% of the times available) values for these things. Except that your client or you want, of course, but the use of probabilistic values has been the staple of foundations for decades; for ... if the geotech says you (it has happened to me) your attached houses with whole mat foundations will get by their k factor a settlement of 8 inches in 1 soil they were not able to sink the penetrometer 3 m and the upper soil is not sausage, and you have never seen 1 inch or so in your whole life, you soon start to see excessive conservatism (that in that case they had to admit).

In short, to the structural designer, to the developer and even for the user -if they not want to pay far more dear foundations- except for life safety, an issue that I am not debating here, what is of interest in these things is the average behaviour of the building.

So you are asking ... might not be that there should be some other spring constant there, or a bigger strength in place? Whilst you reach your conclusion, go for conservative values fitting your understanding of compressibility and strength.

For example, once converted to the units I am accustomed to, a Vertical modulus of subgrade reaction would mean a normal soil, whereas for such vertically looked at layer a bearing capacity of 400 lbf/sf would mean some EXTREMELY poor weak soil and so inconsistent with the spec for compressibility. And since we are comparing related values, the same should happen when looked at horizontally.

 

[ishvaaag]

errata, no too weak springs. Too weak soil. As in the last paragraph explained.

 

[bksstructural]

You can download COM624P from the FHWA website. It is the precursor to LPILE and it is free. Its a bit archaic to use, but it should serve your purpose.

JE Bowles in his Foundation Analysis and Design text book discusses using a spring model to analyze lateral loads on piles. I haven't looked at it in a long time, but I seem to remember having to double the spring stiffness to account for the soil on the back side of the pile.

 

[dcarr82775]

How do you get 150 pci relating to 400 psf. Soil springs would normally be in kli of klf, what spring stiffness are you using? Also, for cohesionless soils stiffness increases with depth to some reasonable upper bound, whereas cohesive soils are more or less constant with depth.

In most geotech books you can find tables given ranges of modulus of subgrade reaction for types of soil. You can convert the MSR to a spring stiffness.

COM624P works good. Also the Navy Facilities(NAVFAC) Design Manual 7.2 has some easy to follow hand analysis methods using charts. You can download it for free. I have found that it doesn't relate bad with Lpile, but I have only used that method on problems with one type of soil.