Tag Archives: concrete slab on grade

Top of Barndominium Slab

Where Should the Top of Barndominium Slab Be?

Loyal reader DANIEL in OWENSVILLE writes:

“Mike,

First I want to say thanks for all that I have learned from your Blog. I am confused on a couple of points you made concerning floor height…

“Occasionally we have clients who ask why they can’t run the concrete to the top of the splash plank, as they want to use the splash plank to “screed” the concrete slab top. Using any other measure for the concrete slab top, will result in wall steel and doors not properly fitting, as well as possible interior clear height loss.”

This really is not answering the question… the building could be designed with the door openings, ceiling heights, etc. to compensate for a higher floor height/thicker floor. Request it in the design and build it to the plan.

Also, “Your new Hansen Pole Building has as the bottom horizontal framing member, connecting pressure treated column to pressure treated column, is a pressure preservative treated splash plank. The building design is such so the top of any concrete floor is set at 3-1/2″ above the bottom of the splash plank.” and, In another post you stated the splash plank rests on the finished grade. That would put the finished concrete floor only 3-1/2″ above the finished grade. And below the weep screed, rat guard, any water being shed on the outside of the sheathing, and what codes require for an occupied building.

Please explain if there is any “real” reason for not raising the interior floor to 6 inches or more above grade (as is required for a house)?”

Daniel ~

Thank you for your kind words. Certainly any building could be designed for door openings, ceiling heights, etc., to be adjusted for top of slab on grade to be at any point. This would entail leaving greater amounts of splash plank exposed on exterior beneath siding in order to prevent concrete aprons, sidewalks, driveways, etc., from being poured up against wall steel. Some people find great amounts of splash plank being exposed to be aesthetically unpleasant however. By being consistent in design, it also allows for one set of assembly instructions to be used – rather than having to rely upon making adjustments for whatever custom situation individuals (or their builders) deemed their particular case.

I went back and read through both IRC (International Residential Code) and IBC (International Building Code) codes and there is no requirement for an interior concrete floor to be at six inches or more above grade for an occupied building or a house.

From 2018 IRC R506.1 “Concrete slab-on-ground floors shall be designed and constructed in accordance with the provisions of this section or ACI 332. Floors shall be a minimum 3-1/2 inches thick.”

From 2018 IBC 1907.1 “The thickness of concrete floor slabs supported directly on the ground shall not be less than 3-1/2”

Both of these imply top of concrete floor at 3-1/2″ above ground (grade) is totally acceptable. 

Having been involved in tens of thousands of post frame buildings successfully engineer designed and approved in structural plan reviews leads me to believe how we are doing it both works and is code conforming.

For extended reading on this subject: https://www.hansenpolebuildings.com/2016/05/concrete-floor/ and https://www.hansenpolebuildings.com/2012/02/where-is-the-top-of-the-concrete-slab/.

Importance of Constrained Posts

Importance of Constrained Posts

In structural design of post frame (pole) buildings, an ability to transfer wind shear loads from roof to endwalls to ground becomes a key to cost effective design success. When sidewall columns are in a properly constrained condition (usually by attachment to a concrete slab-on-grade) shear forces are reduced by 25%. This reduction can result in smaller dimension sidewall columns, as well as a reduction or elimination of need for OSB (Oriented Strand Board) or plywood reinforced roof or endwall planes.

These savings are most often apparent in buildings with a far greater length than width, are fairly tall (especially if narrow) and/or are in high wind regions. In some cases savings from constraining sidewall columns can overcome a significant amount of costs to pour a slab-on-grade!

Savings – I now have your attention. Now I will explain constrained.

Excerpted from National Frame Building Association Post-Frame Building Design Manual (January 2015):

“5.2.4 Foundation Constraint

If a post or pier foundation is not restrained from moving horizontally at or just above the ground surface it is said to be non-constrained. Conversely, if a post or pier foundation pushes against (or is attached to) an “immovable” structural element such that the lateral displacement at some point at or just above the ground surface is essentially equal to zero, the foundation is said to be constrained. An example of a constrained post or pier foundation is one that bears against a concrete slab-on-grade.

A single post can be both constrained or non-constrained, depending on the load case. Using the previous example of a concrete slab-on-grade, and assuming that the post is not attached to the slab, if the wind loading was such that the post was pushing on the slab, the post would be considered constrained. However, if the wind were blowing in the opposite direction, the post would not be supported by the slab; hence, the post would be analyzed for that load case as non-constrained.”

In simple terms, attach sidewall columns to a concrete slab-on-grade to prevent ground surface movement. Rebar hairpins can be an effective method to achieve a constrained condition, and can be read more about here: https://www.hansenpolebuildings.com/2016/10/rebar-hairpins/.