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

 

[hokie66]

Air? Especially after the termites have had their fill?

 

[Ron]

Verily, verily is say unto thee.....this ain't a good idea.

 

[miecz]

hokie66,

I thought the op said the building had precast concrete foundation walls. He never said how thick or how deep these walls were. I still think this is the geotech's call.

 

[BigH]

Just wondering if the precast walls have a "strip footing" stub cast at the base of the wall? - i.e., projections beyond the wall by 6 or 8 inches each side that, then, in effect act as a footing - like a precast T-beam. The "gravel" trench might very well be put in to improve the soil that is there . . . just wondering . . .

 

[hokie66]

miecz,
I see, on rereading. In that case, they would be precast concrete footings. Gravel foundations perhaps, but not gravel footings, in my lingo at least. Acceptance of any system is not something I leave wholly to a geotechnical engineer.

 

[dhengr]

Hokie:
I think the idea of the gravel footings came from the Permanent Wood Foundation concept, years ago. At least, that’s the first time I saw it, and they do call them gravel or crushed rock footings. It was meant as a footing immediately under a 2x10 or 2x12 PT sill plate. You dug a 16 or 18" wide trench, 12 or 14" deep, filled it with compacted crushed rock and set the found. wall sill plate right on top of the crushed rock. It was also intended as part of the perimeter and under floor drainage system, and was tied into the exterior perimeter drain tile system. I always thought that they might as likely collect (attract and hold) water, and eventually fill with fines and be ineffective. They weren’t using any geotextile fabric wrapping in the early years. They could be built in winter without worrying about cold weather conc. practices. And, they performed best on sites with soils which drain well, sand or other granular soils. Some crazy contractors or builders expanded that concept to digging a trench filling it with a couple feet of rock and pouring an unformed stem wall on the rock for houses, etc. I’ve never seen it used for four stories, the most was two above grade floors and a basement wall which used the PT wood found. details. I’ve been involved in 6 or 8 cases over the years on this building technique, and those turned out poorly because the builder had no idea what he was really doing and used bad details, improper materials and poor waterproofing methods. Of course differential settlement could be a problem, for lack of the stiffness a poured conc. wall or a conc. blk. wall would offer.

 

[focuseng]

The one precast foundation wall system I am familiar with does not allow this as part of its ESR and strictly specifies the footing should be designed in accordance with the IBC. Gravel footing are only allowed in the IRC. Don't know the MFR you are dealing with but the ESR is more about the panels than the footings. The gravel footing tables cited in the ESR only go up to 3 story buildings as well (consistent with IRC design limitations).

http://www.superiorwalls.com/downloads/reports_files/ESR-1662XiWallSystem-Nov2012.pdf

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MAP

 

[XR250]

I think Frank LLoyd Wright was the first to use them.
I would not go 4 stories. The foundation is the last place you want to try to save money as it is the hardest and most expensive to fix later.

 

[dik]

CSA S406, a Canadian Standard for Preserved Wood Foundations permits wood sleepers on a gravel bed. Sleeper must be wider than the foundation wall stud to project beyond the sheathing.

Dik

 

[kylesito]

Just getting back to civilization after a long, much needed, break for the 4th.

Great feedback on everything. Focuseng had the right product that we are dealing with. It is a precast concrete wall beneath the wood sill plate that rests on a trench filled with gravel that they call a gravel footing. From what I have learned, Superior Walls licenses their product to individual suppliers who are responsible for their own engineering/installation data. It would seem then that some suppliers rely strictly on what is proposed in the codes and some, like who we are dealing with, apparently think their product is 'superior' to the codes (pun intended).

While I was out, it was decided that they would continue along the track of using this product. We are not going to specify it at all though nor are we going to approve it in any way. If they (contractor) choose to install this product, they must negotiate with the owner directly to find another EOR and we will absolve ourselves from it. We will provide a complete design based on what we believe in and it's completely on the owner to choose which route he proceeds with.

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]

Excellent choice... Whew!

Mike McCann
MMC Engineering

 

[kylesito]

It's still a dicey situation. It's extremely frustrating to have to put so much effort into disassociating ourselves from the responsibility for a project. That's not what we train to do nor is it in our make up as engineers to make decisions that way. Playing the CYA game is no fun.

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

 

[XR250]

Everytime I hear "value engineering", I want to disassociate myself from the project.
It always turns into a mess and a bunch more liability for the EOR. The savings are not likely what they think when all is said and done.

 

[charliealphabravo]

The IBC is customized and adopted by the state legislature and basically has the strength of law. Specific sections of the ACI standards are empowered by reference in the IBC. The IBC claims precedence over any differences. Having said this, the IBC is not really a rigorous design code and it defers heavily to the ACI (and other) technical standards...much in the same way that the code now defers almost entirely to ASCE-7 for loads.

I recall that there is language at the front of the IBC which recognizes the engineer's latitude and prerogative for alternate methodologies, materials, standards, etc. I don't recall the red tape involved in exercising this however.

 

[TopKnot]

I've seen these footings work surprising well, but I'd never do it on anything other than 2 story or less, and it would have to be for someone that was really desperate to save money. You would need fabric around the gravel footings to reduce migration of fines, but significant prolonged loading will cause this to settle. Terrible idea for four stories. Your bottom plate will be destroyed by crushing perpendicular to the grain. Triple 2x8 treated bottom plate?



_________________________
Tony Krempin, PE
TopKnot Engineering

 

[kylesito]

Tony, this is my experience as well. The salesman for the product has insisted that their product "gets used all over the place" yet most of what we have looked at has all been 2 and sometimes 3 stories. Not a single four story application. Great comment on the fabric around the footings and I will keep it in mind. Not sure I follow you though about the bottom plate...that's a function of 4 stories more so than the precast walls, right?

Excel...I know what you mean. Value engineering certainly doesn't apply to how the EOR views it.

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