Micropile Design - LRFD Factors 1

[JohnnyB_]

Hello -

I am working on designing a grade beam foundation supported by micropiles. The foundation will be supporting light residential loads and the loads have been calculated using LRFD load combinations. The contractor is choosing to use micropiles as opposed to overexcavation. These two methods of design have been proposed due to the expansive shale at the build site.

I am using Publication No. FHWA NHI-05-039 to design the micropiles. In the publication there are a few design equations and examples but it does not look like they are incorporating any factors for ASD/LRFD design. As an example the equation for the allowable compression (Pc) load is attached. I have highlighted the grout portion of the equation in yellow and the steel portion of the equation in green. In order to get the design strength (ΦcPc) should I be multiplying the different portions of the equation by the appropriate Φ factors, (Φ concrete compression = 0.85 and Φ steel compression = 0.9)?

 

[EngDM]

In the past when specifying micropiles we have simply provided loads and the design of piles was by pile supplier. Is this an option here?

I haven't sized these, and I apologize that I cannot be of more help but perhaps this is an easier "out" for you.

 

[JohnnyB_]

The problem is, the person installing the micropiles is a single guy and does not have an engineering background. He installs a lot of micropile in the area but I don't feel comfortable with him doing the design under my stamp.

 

[EngDM]

The problem is, the person installing the micropiles is a single guy and does not have an engineering background.

I see. When I've used micropiles it was for large commercial projects and the supplier had their own engineer retained. If you are stamping the piles then I guess you're kinda stuck looking into this.

 

[driftLimiter]

Specify test loading as demonstrated in FHWA. The geotech should be able to help with a preliminary casing size and depth. The test results can be used to justify a given capacity.

 

[PEinc]

JohnnyB, can you ask the structural engineer for unfactored service loads instead of factored loads. You will need service loads if you need to calculate settlement. This is for a building, not a highway. IBC has a different ASD formula than AASHTO/FHWA.

Look for a download of FHWA-SA-97-070, Micropile Design and Construction Guidelines, Implementation Manual, June 2000. It has LFD (not LRFD) design info.

http://www.PeirceEngineering.com

 

[JohnnyB_]

Thank you PEInc for the feedback.

 

[AskTooMuch]

I've checked and seen a lot of micropile/helical pile design from helical pile vendor. They normally use Lpile.

Anyway the structural engineer gives them foundation loads from ASD load combination and helical piles engineer consider this "unfactored". They will design their helical piles adding 2.0 Safety Factor for compression and tension to make it allowable pile capacity.

This is no different if you reverse it. You will be given allowable pile capacity of driven piles per pile and you decide how many piles you need by comparing it to your allowable load combination reaction.

 

[JohnnyB_]

Thanks for all of the feedback everyone!