Micropile analysis via LRFD composite column analysis 1

[Jasonincharlotte]

Does anyone know why a micropile could not be structurally designed as a composite column as is done in the LRFD steel manual. This approach provides much higher structural capacities then if designed per traditional micropile methods.

 

[ishvaaag]

Except if the code mandates other thing, and sometimes, even if concrete mandates other thing, one must follow what he knows to be technically correct.

However, designing micropiles or piles have special features that the standing methods acknowledge, hence if you plan to follow your path, you should fix your attention on what differs from what normally sound technical texts are saying, and try to guess why.

Perhaps the reason of your question is the micropiles taking contagion of the soil they are in contact with and must endure bigger safety factors. Construction by segments debilitate the shaft at regular intervals, the possibilty of significant actual pressures and corrossion are there, you are not seeing what is underground, etc ... al these things undermine acknowledged capacity.

 

[jdonville]

Jason,

FHWA has a somewhat dated, but still good treatment of micropiles here:

http://isddc.dot.gov/OLPFiles/FHWA/009966.pdf

The structural design should be left to the specialty contractor that will install them, or an experienced geotechnical engineer. Naturally, the connection to the structure to be supported should be designed by the structural in consultation with the geotech/contractor.

Jeff

 

[jdonville]

Jason,

I should have added that the current AASHTO LRFD Bridge Design Specifications should specify resistance factors, geometric restrictions (spacing limits) and stress limits for the structural design of the micropile elements.

Jeff

 

[Jasonincharlotte]

jdonville and ischvaag, thankss for your responses. I am a specialty geotechnical engineer. I have used FHWA "micropile design guideline" manual and used the bridge code in my past. I am familiar with both. I ask why is it that a building column can be designed as a composite colum, meet bldg code requriements, but yet a micropile of the same cross section would be considered overstressed by FHWA or AASHTO standards. Why should we expect micropiles supporting the same load to be so much larger in cross section than a building column supporting the same load? I understand the construction differerences, but logically, I an struggling understanding why this difference ocurrs.

 

[jdonville]

Jason,

After re-reading your original post and your latest, I am assuming that you are referring to a structural shape/member encased within the grout as a composite column or a filled section, such as a concrete-filled HSS. In the case of a micropile, I would assume that the HSS would likely be round.

For the first case, AISC requires logitudinal and transverse steel to confine the structural steel core. Additionally, shear studs or another shear connector must also be added to the core member(s). I can't see anybody doing this for a micropile application.

For the second case, you would need to (at minimum) reduce the wall thickness of the HSS/pipe member to account for wear, possible reduced strength at any threaded joints, and/or long-term corrosion. See FHWA NHI-05-039 for discussion of these issues.

AASHTO LRFD 4/2008 has specific guidance on resistance factors for micropile structural design in Article 10.5.5.2.5, and design equations in Article 10.9.3.10 that appear on first blush to be significantly more permissive than those in NHI-05-039.

The limits on structural capacity of driven piles are presented (if obliquely) in Articles 6.5.4.2, 6.15.2 with good discussion on the reasons for discounting the structural capacity for driven piles - or other pile types - in the commentary. Driving stresses are in Article 10.7.8.

I would imagine that you would have no trouble using the provisions of Article 6.9.5 to analyze the structural capacity of a micropile, provided that the limiting stresses in Articles 10.5.5.2.5 and 10.9.3.10 are observed.

Sorry if this is a bit meandering.

Jeff

 

[born2drill]

Also don't forget that in most cases micropiles are field tested to 200% of the design load to ensure a geotechnical factor of safety. Of course this means that structurally the pile needs to be able to withstand that test load. So the FHWA factors of 0.47 on steel and 0.40 on grout are actually pretty close to the edge.