ACI supersede IBC - Gravel Footings 1

[kylesito]

I put both elements of this question in the title hoping to get feedback from those that might have familiarity with either topic.

A subcontractor on a project is proposing the use of gravel foundations for a 4 story wood framed building we are doing. These gravel foundations are intended to work in conjunction with precast concrete foundation walls which are used on residential buildings governed by the IRC (3 stories or less). Because these buildings are 4 stories, they fall under the IBC which doesn't make mention of gravel foundations and after a call to ICC committee are not approved for use in IBC governed buildings.

The subcontractor has proposed that since their foundation walls are an "engineered system" as defined by the ACI code which therefore supersedes the IBC and should therefore be permitted.

I have two concerns:
1. I have no experience with a system, any system, that claims precedence over the IBC unless very explicitly done (and I don't know of any that do)
2. I don't have confidence in gravel foundations under a 4 story building for several reasons such as: differential settlement possibilities between the wall footings and interior column footings, in plane shear loads, and wall chord forces.

Does anyone know of this reference about the building code?

Has anyone designed a 4 story or larger foundation on gravel footings?

PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[briancpotter]

It's my understanding that precedence works the other way around - the code doing the referencing takes precedence. So the IBC, which incorporates ACI 318 by reference, takes precedence over ACI in the event of conflict. It's just as how a state building code which incorporates the IBC by reference, takes precedence over the IBC itself. So in your situation, ACI wouldn't take precedence unless explicitly done so by a state code, as you mentioned.

However, it's unclear what you mean by "not approved". Do you mean that per the ICC committee, gravel foundations aren't allowed? Or that they just haven't specifically approved them? If it's the first, they are out, no question. But if it's the second, the subcontractor may be correct.

Brian C Potter, PE
Simple Supports - The history and practice of structural engineering.
ConstructionPic - Send annotated jobsite photos.

 

[kylesito]

The code official we talked with at the ICC office said they were "not approved". I simply took that to mean "not allowed" but I see your point. I am not sure what the intention of his statement was.

PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[Ron]

Under the IBC, if a "system" is proposed and it deviates from the statement of the code, it should have an evaluation report that assesses its equivalence to the code mandate.

If there is a choice between two systems, the one with the more stringent requirements prevails. See Chapter 1 of the IBC under alternate materials and methods.

 

[dcarr82775]

The IBC certainly trumps ACI no question. However, if the Geotechnical report states a gravel foundation is acceptable and provides the necessary design parameters I don't see why it would not be allowed. The IRC is largely prescriptive whereas the IBC is much more open ended requiring more leeway for actual engineered designs (in most but not all instances) as opposed to a prescriptive one.

 

[Ron]

ACI 318 is integrally woven into the IBC. They are not separable. There are certain provisions of ACI 318 that have been superceded by the IBC, but in most cases the IBC makes the provision more stringent, not less so.

A gravel foundation does not do the following:

It provides no uplift resistance so if your building is subject to wind loads, you can't meet the requirement for a continuous tensile tie from the foundation to the roof. Separate anchorage would be required.

If you are in a seismic zone of any import, you have no tangible mass in the foundation that can assist the structure in any way.

If you are in an area of soils that could migrate into the gravel (and almost any soil will...some worse than others), you have a settlement potential that is not clearly predictable.

If you are in an area of high rainfall or flood potential, you have a tremendous undermining potential that could place the structure at jeopardy for stability.

Personally, I would not allow this. Further, it is not an "approved" system and therefore does not meet the requirements of Section 104 of the IBC.

 

[msquared48]

Are these gravel filled geopiers?

Mike McCann
MMC Engineering

 

[kylesito]

To answer some questions:

The geotech was not aware they wanted gravel foundations at the start of the project so did not make that part of his report. We have stipulated since the start of this discussion that they be brought back on board to approve before we consent.

No, these are not geo piers. Those would be a different situation. These are literally trenching a strip, as you would for a wall footing of concrete, and filling it in with gravel then compacting.

Ron, great list. Many are concerns we have had but it's great getting another engineers consensus on the same issues.



PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[msquared48]

Other than to possibly save $$$ and time, why would the contractor want to use this type of foundation?

Like Ron, I would never do that here from what I can imagine.

Mike McCann
MMC Engineering

 

[kylesito]

Yes. To save money (prevailing wage for onsite masons or rod busters vs shop rates) and schedule (the precast guy says he can do a foundation start to finish in a week. The owner is holding the contractor to a cut throat GMP and a very abbreviated schedule which has left them with supposedly no other option than to use this product. However, we have never agreed to using this and have tried for months to get the "engineering data" the precast guy is using to justify their system but they and the contractor have ignored us completely. Now we are starting to get answers to our questions and finding out that it was all based on this product.

We are skeptical to say the least but are being squeezed from both sides to approve this.





PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[JAE]

If it was one BIG piece of gravel (i.e. a huge boulder) that the entire frame sat on - then bolting your structure to it would be OK..

But I'm being (or trying to be) silly.

Ron's list is perfect.

 

[McSEpllc]

Independent of the minimum requirements of codes, when your engineering judgment waves a flag, I recommend to honor it. The GC’s motivation is maximizing profit, yours to assure life safety and the building’s function.

Eric McDonald, PE
McDonald Structural Engineering, PLLC

 

[msquared48]

I would very strongly consider removing yourself as the engineer of record. It can be done...

Mike McCann
MMC Engineering

 

[miecz]

Seems to me that the most knowledgeable party in this scenario is the Geotechnical Engineer. As the Engineer of Record, my stance would be that the Geotech must approve the foundation system.

 

[dcarr82775]

Wow. I didn't think a trench filled with gravel was what gravel foundation is. I wouldn't use it that for much of anything.

 

[msquared48]

miecz...

That may be OK for bearing, assuming it is not eventually undermined, but any uplift resistance needed to resist wind or seismic forces will just not be there as Ron stated unless concrete deadmen are also provided. I guess these would blow the contractor's budget... WOW!

This is very plainly just a very bad idea.

Mike McCann
MMC Engineering

 

[hokie66]

To extend on what Mike said, this is very very plainly just a very very bad idea.

 

[JAE]

My guess would be that this is a very very bad idea.

 

[hokie66]

Just curious, being someone who has never before heard of a "gravel footing", what goes between the gravel and the wood?

 

[msquared48]

air... and sky lifts...

Mike McCann
MMC Engineering