Foundation Definition - Is A Floating Slab on Grade a "Foundation" 1

[LTBGeek]

This is deep in some technical weeds, but a client asked me to define the floating slab on grade in a two-story building as either a "flooring" or a "foundation" (this matters due to some leasing agreements). I can see two arguments - (1) the primary building structure loads are transmitted through columns, piers, and isolated foundations, and the "building" so to speak doesn't really care if the slab on grade is there or not, so the slab on grade is really the "flooring" in the first floor - or - (2) the loading in the first floor space (retail live load, etc) is transmitted through the slab on grade to the subbase and native soil below, so therefore the slab on grade is a foundation element. Has anyone ever been asked to differentiate? I cannot seem to find a definition of "foundation" in the IBC - other thoughts on where to look? I need some ground to stand on one way or the other - though quite frankly right now, response is "I don't know".

 

[SlideRuleEra]

Has anyone ever been asked to differentiate?

Yes, part of my job was to classify assorted concrete foundation work at our new electric generating stations and explain in regulatory hearings how certain concrete work is, or is not an integral part of an essential machine. There are major tax advantages to my company for having the foundations that are part of a machine. (An example of a foundation being an acceptable part of a machine is an inertia block foundation that dampens the machine's vibration.)

Your definitions are excellent, don't expect to find written definitions... the "answer" can go either way.... depends on the interpretation of how the concrete is used. Within the bounds of truth, argue the position that benefits your client. "Don't know" should not be an option, be creative in coming up with an answer... but tell the truth.

http://www.SlideRuleEra.net

 

[dold]

We've run into this in a lease dispute before in an existing retail shell building (between our client and a landlord).

Is this a new building that you provided the design for? My argument probably wont hold water for a two story building, but one example for a single story retail might be "the slab on grade provides uplift resistance for interior columns", but this would only apply if you provided the design and can show that indeed you have considered resistance from the slab to supplement your spread footing deadload.

Are the exterior walls tied into the slab in any way? Like a CMU dock wall with dowels into the slab, for example?

 

[LTBGeek]

It is not a new building - it is a 100 year old building with a 15 year old slab on grade that was destroyed by current lessee's use (hypothetically). The slab is not turned down in any area; isolated from all foundation walls and isolated footings. While I've argued ad nauseum that the slab on grade as constructed was NEVER designed to support the loads that have been imparted on it by this particular tenant, that doesn't seem to matter to most parties - everyone is stuck on the definition in the lease of "foundation" and "flooring"................

 

[SlideRuleEra]

...I've argued ad nauseum that the slab on grade as constructed was NEVER designed to support the loads that have been imparted on it by this particular tenant...

Are you arguing with "words", or with "numbers"?
You need numbers... reverse engineer the floor (using your own reasonable assumptions for material properties, if required) to determine what loading the floor will support and at what range of values it would likely fail.
Do the same thing for loads the lessee applied to the floor.
Your client may, or may not like your answer but you now have an answer.

http://www.SlideRuleEra.net

 

[dhengr]

LTBGeek:
Can you make some meaningful and helpful distinction btwn. flooring and foundations, by the materials or nature of materials involved. Flooring is usually some kind of walking/moving surface finish, wood flooring, vinyl, cork, rubber tiles, marble, any stone tile, terrazzo, brick and the like, all have some decorative and finished, aesthetic nature; and are usually supported by some structure below them to actually carry the load over soft or low spots. They act o.k. in bearing or compression, within some limits, but not so well in tension or bending (bridging). A slab on grade is the structure to carry the loads from above to the foundation soils below, even though it may have some special architectural (attractive, special ?) finish. It distributes the loads, even heavy concentrated loads, to the soils and bridges soft spots in the soil; while most flooring needs some form of structural support because it can’t tolerate much in the way of a concentrated loading while spanning any distance.

“Foundation… That part of a building which is in contact with the soil. The foundation transfers the weight of the building or structure to the soil and is almost always of concrete. Types of foundations include Footings, Floating Foundations, Belled Piers, and Piles.” From “Illustrated Encyclopedic Dictionary of Building and Construction Terms, by Hugh Brooks, Pub. by Prentice-Hall, Inc., 1981 printing. That seems like a pretty reasonable definition, although not ALL inclusive as to a listing of foundation types, nor is it limiting or excluding any possibilities. That’s where SRE’s suggestions to you come into play. Maybe take a look at the ‘foundation’ definitions in any of our textbooks on the subject. That might give you some quotations and author’s names to hang your hat on. Furthermore, foundations or flooring by any definition must be designed for their loadings and conditions of use. One or the other, as the case may be, they are not designed for an abusive usage or loading way beyond what would normally be assumed in common designs and specs. There are a number of ways to harden, strengthen, thicken or improve a floor, structural component or foundation, but the abusive loading or conditions must be known beforehand for these added conditions to be considered and potentially accounted for in the design.

 

[Settingsun]

Set the cat amongst the pigeons and tell them it's neither. Flooring = carpet, tiles etc and foundation = soil, rock etc.

Really though, you should understand why there's a distinction for leasing and work on that basis. Seems that this may not be an engineering question?

 

[dold]

Playing devils advocate, what is the difference between a (floating) slab on grade outside of a building (sidewalk, slab inside a pavilion/picnic shelter, etc) and a (floating) slab on grade inside a building? Would this discussion even be happening if the slab in question was exterior slab/sidewalk/etc? All an interior slab does is give you a more sturdy and clean surface to put your property. You can still put your property inside a structure even if the floor is just soil. And "flooring" doesn't necessarily have to be supported. Perhaps a lawyer could argue that "flooring" can indeed bear directly on soil, in the case of wooden planks or a straw mat in a hut for example. That is taking it to the extreme but it isn't false.

 

[WARose]

A true "raft"/floating foundation is used where the soil has low allowable bearing pressure (or potential settlement issues) and (as a result) the soil is excavated down to a level where the weight of the structure is balanced by the weight of the soil removed. (I.e. you are not significantly increasing the net bearing pressure.)

I suppose a slab on grade could be constructed with this in mind.....but it's almost always done for building support.

To me, a slab on grade is a "foundation element" anyway you cut it.

 

[jayrod12]

I am of the argument that a slab on grade that only supports the loading of the floor it services, i.e. the ground floor, is considered a floor and not a foundation. If there are bearing walls for stuff above, then it would be, in part at least, a foundation.

Generally speaking, many tenant occupied commercial buildings we have constructed have the slab-on-grade omitted in each tenant space to allow the tenant to install their own sub-slab services prior to pouring. Many times, around here, the installation of the slab-on-grade in new construction is baked into the initial tenant's lease. Therefore to me, it's a floor.

 

[Ron247]

To better answer this, it would seem prudent to know how the 2 different words are then used to establish who pays for the damaged slab. That is the reason for the delineation of the terms. It is the contract that acts like there is only 2 possibilities, not the engineering world. Also, it could be both in the engineering world, not one or the other. If the slab was stamped concrete, I could easily call it flooring while also calling it a foundation since a 4,000 lb gun safe sets on it and it transfers the gun safe's weight to the soil below.

Contract items such as "Tenant is responsible for damage to any foundation" or "Tenant is not responsible for damage to any foundation" would be helpful in distinguishing the 2 terms as related to the contract. Also, who drafted the contract? That ambiguity thing.

 

[KootK]

At this stage in the process, I think that all you can say is the truth which is the following according to me:

1) The slab on grade is not required for life safety and therefore would not be considered a "structural foundation" from that perspective.

2) The slab on grade is required for the serviceable use of what is generally considered to be the permanent structure and therefore would be considered "structural foundation" from that perspective.

3) A concrete slab on grade is conventionally thought to constitute a part of the permanent structure specified by the engineer of record. Therefore, from the perspective of lease agreement intent, if not explicit wording, the slab on grade should not be damaged or altered as part of a tenant fitout etc except in extreme situations (loading dock etc) and where specifically agreed to by the owner.

4) I don't see any right thinking person considering a slab on grade to be merely a "finish". However "finish" often does encompass things conventionally considered to be "flooring".

But this is really just me imagining what I'd say as the owner's lawyer or, perhaps, as an expert witness. If you're client insists on you simply checking or unchecking a "foundation" box on a form, I'd be inclined to refuse on the basis that it's a nebulous thing and not ethical for you to imply a level of classification certainty that isn't really appropriate.

 

[Mad Mike]

I would simply argue that a foundation's purpose is to carry loads for the design life within serviceable limits- the fact that your floor fell to pieces beneath the lessee's loads would preclude it classifying as a foundation.

All the best,
Mike