Pile Foundations

[TimNY]

Given:

"Pile caps shall be of reinforced concrete, and shall include all elements to which piles are connected, including grade beams and mats. The soil immediately below the pile cap shall not be considered as carrying any vertical load. The tops of piles shall be embedded not less than 3 inches (76 mm) into pile caps and the caps shall extend at least 4 inches (102 mm) beyond the edges of piles. The tops of piles shall be cut back to sound material before capping."

Would you entertain a foundation where the engineer has proposed using both the capacity of the piles and the grade beams in determining the capacity of the foundation?

For the sake of simplicity say he needs 10,000 lbs of bearing. 8000 are carried by the pile and 2000 are carried by the grade beam resting on undisturbed soil.

Tim

 

[Mark K]

Assuming that the text in quotes is from the geotechnical report it would appear that what the engineer is proposing is inconsistent with the geotechnical report. Note the inconsistency in the plan check comments.

 

[TimNY]

Assuming that the text in quotes is from the geotechnical report it would appear that what the engineer is proposing is inconsistent with the geotechnical report. Note the inconsistency in the plan check comments.

Hi Mark,

That's actually the code section from the IBC. So the pile cap includes the 10 feet of grade beam extending in either direction? I mean, that's what it says.. But.. Really?

Tim

 

[Paul Sweet]

I was always taught that grade beams were intneded to carry loads between piles or piers, and not to transfer loads to the soil.

If the soils are highly compressible, the piles would prevent the building from settling enough to transfer much load to the soil

If the soils are highly expansive, there is usually a void below the grade beam to keep the soil from lifting the building up.

If the soils are adequate to carry significant loads, piles shouldn't be necessary.

 

[tmurray]

I would want the structural engineer to consult with the geotechnical engineer and get a letter from both.

 

[Mark K]

If this is the code provision then what they are proposing is a code violation. It would be useful to see the code section reference.

Many building officials seem to assume that problems can be resolved by a letter from the engineer. It is not always clear what this letter should address but the implication seems to be that the building official sees the letter as a way to pass responsibility to the design professional.

The building official's responsibility to determine code compliance is independent of the design professionals responsibilities. Thus statements by the design professional cannot excuse the building official from the need to determine compliance.

The building code assumes that the building official either has the expertise to determine that the submission is in compliance with the code or has to ability to obtain the necessary assistance. The registered design professionals are expected to provide the information that the building official or his consultants need to determine that the submittal documents comply. Normally this consists of submission of calculations or technical data in addition to the drawings and specifications.

Thus I contend that the building code does not allow the building official to require a letter from the design professional. The building official can require additional calculations and technical data to allow him to determine code compliance.

 

[Phil]

Mark,

In the 2012 and 2009 IBC, it is 1810.3.11. In the 2006 IBC, it is section 1808.2.4. Paul gives a good example of why it is not good to idea to use grade beam bearing capacity with pile foundations.

 

[TimNY]

If this is the code provision then what they are proposing is a code violation. It would be useful to see the code section reference.Many building officials seem to assume that problems can be resolved by a letter from the engineer. It is not always clear what this letter should address but the implication seems to be that the building official sees the letter as a way to pass responsibility to the design professional.

The building official's responsibility to determine code compliance is independent of the design professionals responsibilities. Thus statements by the design professional cannot excuse the building official from the need to determine compliance.

The building code assumes that the building official either has the expertise to determine that the submission is in compliance with the code or has to ability to obtain the necessary assistance. The registered design professionals are expected to provide the information that the building official or his consultants need to determine that the submittal documents comply. Normally this consists of submission of calculations or technical data in addition to the drawings and specifications.

Thus I contend that the building code does not allow the building official to require a letter from the design professional. The building official can require additional calculations and technical data to allow him to determine code compliance.

I neither assumed nor required anything. I was simply soliciting the experiences of other who have experience with pile foundations.

Phil has the right sections; it's listed under Soils and Foundations>Pier and Pile Foundations.

I have all the geo and engineering data. The discovery of 2000psf soils by the the geo engineer where 3000+ are all but guaranteed is what kicked this off. The structural PE has designed a solution utilizing both capacities.

The issue is if the definition specifically excludes this method, it leaves me no discretion to accept the design under this section. It leaves me staring at the alternative designs section, which I try to avoid when I can.

I was hoping somebody here had tackled a similar situation.

Tim

 

[Mark K]

If the Owner has not asked for variation based on IBC Sections 104.10 or 104.11 you are just looking at whether the design complies. Section 104.10 kicks in only if there is a major problem if the project were to comply with the building code. No mention was made as to what the geotechnical report recommended.

Unless the engineer can show that the project cannot be built if he must comply with the code Section 104.10.

It is not the plan checker's job to find solutions.

My expectation is that they can find a solution that complies with the code. I doubt that the geotechnical engineer will propose such a design approach since if you need the drilled priers for gravity loads the upper soil will not be effective.

In cases where the drilled piers are not needed to resist gravity loads but are needed to resist seismic or wind uplift it might make sense to share bearing and pile capacities but in these situations I would expect that they will be able to comply with the code.

 

[tmurray]

Many building officials seem to assume that problems can be resolved by a letter from the engineer. It is not always clear what this letter should address but the implication seems to be that the building official sees the letter as a way to pass responsibility to the design professional.

Sorry Mark, we use limit states design here so all we need to confirm are the engineer's assumptions are valid (usually a letter).

 

[Mark K]

While in the United States we use slightly different terminology our concrete codes are also based on ultimate strength capacity.

My understanding is that the Canadian legal and regulatory system is different so the expectations of what is acceptable may be different. I do not see how a letter can provide the information that allows the building official to verify compliance. Thus it appears that the only purpose of the letter is to verify that the engineer and the geotechnical engineer agree.