HELICAL PIERS finding worse case allowable for axial loads

[lcrdesignink]

Given a soils report that states use end bearing capacity of 10,000psf for 15' depth take that time 8" shaft area and find 3360 lbs divide by 0.5 from equation 18-4 ibc 2009 this is the Pa allowable axial design load? Contractor uses chance piers the show a allowable of 36.7kips from using cohesian and muliplyer of Nc = 9 with blow count of 86? total loads on piers are around 10000lbs two story concrete basement wall with brick veener. am I missing something ?

 

[lcrdesignink]

should read multiply by 0.5 from IBC not divide

 

[JAE]

8" shaft area?
Do you mean 8" diameter shaft?

Typically helical anchors use between 8 to 14 inch diameter plates and sometimes use multiple plates.

I'm not sure you can equate an allowable bearing pressure from a soils report for end bearing with the pile capacities developed by helical piling.

(If the 10,000 psf is an allowable pressure - not sure you take another 0.5 factor on it as typically allowable pressures already have safety factors of 2 or 3 depending on soil)

 

[lcrdesignink]

Yes 8" diam area published is 0.36 sqft but my question is the soils eng does have approx 1.4 S.F. I know they will take alot more with the torque equation but how do you get around the checks per IBC 2009 eq 18-4 check 1. sum of the areas of the helical bearing plates times the ultimate bearing capacity of the soil or rock comprising the bearing stratum. Pa = 0.5xPu for the allowable axial design loads.

 

[JAE]

But your "allowable" bearing pressure of 10,000 psf is not Pu (which is an ultimate pressure) is it?

 

[lcrdesignink]

No I realized that after I posted and read what I posted they assume you always read the report as allowable. Question this does not meet the 2.0 S.F. required by deep foundation elements 1810.3.3.1.7? Also use the 10k times the area of pier published .36 sqft and you get 3600lbs this is still no where near the 37k given by the pier co and the soils is saying the pier co is off on coheasian by at least a multiplier of 10? Do you take this as one of those values for published and tested and ignore the checks for IBC section 1810?

 

[JAE]

Typically for helical piling I don't use the IBC at all as these are usually specified as a proprietary structural product with a peformanced-based specification.

Thus, the helical pile company is required to provide certification or warranty that their product will perform - (i.e. provide x kips of capacity) with a designated safety factor.

I also wonder if 15 feet of depth is not what you'll end up with as many times helical piling go as deep as necessary to develop the required axial capacity.

 

[lcrdesignink]

Thanks for the help allot that's what we were coming to the conclusion. One thing that really had us concerned was the area is famous for the piers torqing out to soon and not going far enough into the bedrock below the clay. in this case soils recommends to predrilled if that is the case and they grout the shaft this would fall into the category of using frictional and end bearing.

 

[a2mfk]

Agree with JAE. We work with helicals fairly often and these are proprietary and their results are based on testing, as well as a formula that converts installation torque to a bearing capacity. However, you want to work with a geotech and soil testing to make sure you get to the soil strata that you want to bear on. As you stated, clay can be very difficult for them to advance the helical through, and you can get misleading torque numbers, so drilling at least through the clay is an option to consider.

 

[RFreund]

Look at AB Chance's "Technical design manual" (you may have to request this) and this will clear most of this up. They usually use Terzaghi's equation for ultimate bearing capacity.
Basically for non-cohesive soils, if you have an effective friction angle of the soil you can find "Nq" for your non-cohesive bearing factor then multiply this by the overburden pressure. So basically they are using the ultimate bearing capacity and then applying a suggested safety factor, usually 1.5 - 2.0.

For cohesive soils Nc=9 is recommended.
cohesion = c=N/8 where N = blow count. However this is given as an estimate, it is recommended other means be used to determine cohesion.
Bearing = Q = Area * c * Nc





EIT

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