The Building Code Highway

To view the figures and tables associated with this article, please refer to the flipbook above.

Reprinted with the permission of the Post-Tensioning Institute (PTI).

Engineers-in-training coming out of school have little idea of how to apply building code to what they are taught in school. On top of that, many senior engineers, those that would even call themselves “experts,” also do not understand the building code. With this lack of understanding comes a corresponding lack of knowledge regarding how to apply the numerous other standards referenced in the building code. The Post-Tension Institute (PTI) slab-on-ground standard (DC10.5) is just one of those many referenced standards that exist in the main building code.
To best understand the building code, specifically the International Building and Residential Codes (IBC and IRC aka “I Codes”), think of the codes like a highway. The highway itself is marked with exit signs that branch off in paths to other standards (these are referenced in Chapters 35 and 44 for the IBC and IRC respectively)—and even some loops that lead back from one code or one standard to another. Sometimes municipalities will unconditionally adopt the I Codes by reference. Other times, the municipalities will amend the I Codes as necessary to achieve the path through the codes that make the most sense for them. The same may also be true for entire states, like California for example, where the state may adopt and amend the I Codes as a statewide code.

Amendments to the I Codes are much like road closures or detours. In those cases, exits from the code to other standards are closed, and/or specific paths through the code are eliminated or changed. Researching amendments, whether municipal or state, is the important first step before beginning your journey into and through the I Codes. This first step provides a route with the closures and detours in mind so that we do not get lost along our journey.

The flow chart in Figure 1 helps illustrate the route through the I Codes. While both the IBC and IRC have references to other standards, this flow chart includes only those pertinent to the discussion for establishing a path to the PTI and American Concrete Institute (ACI) standards.
The arrows in the flow chart indicate a one-way direction on the path through the code. To arrive at the IRC, the municipal or state codes delegate the scope directly or as typically done in Chapter 1 of the IBC. The exit ramp to the IRC is necessary in Chapter 1 of the IBC so that the scope of any IBC references does not take place before arriving in the IRC. Once you arrive in the IRC, it gives some alternate reference standards, but in IRC Chapter 3, direction is given that two triggers exist to leave the IRC other than the alternate reference standards:

As of 2024, both the IBC and the IRC contain exit ramps to the PTI DC10.5 document. Before the 2024 edition of the I Codes, the only way to access the PTI DC10.5 standard was to go through the IBC. With the return path from the IRC to the IBC for portions of the structure that are non-compliant with the IRC, this does not appear to be an issue at first. However, the IBC exit ramp to the DC10.5 document occurs in Chapter 18 under subsection 1808.6.2 for slab-on-ground foundations within section 1808.6, “Design for Expansive Soils.” The remainder of IBC Chapter 18 does not address slab-on-ground foundations on stable soils.

While the IBC has not and still does not mention stable soils, the 2024 IRC Chapter 5 Section R506.2 references the proper title of the DC10.5 document, “Standard Requirements for Design and Analysis of Shallow Post-Tensioned Concrete Foundations on Expansive and Stable Soils.”
Before the release of DC10.5-19, the title of the DC10.5-12 reads “PTI DC10.5-12: Standard Requirements for Design and Analysis of Shallow Post-Tensioned Concrete Foundations on Expansive Soils.” The DC10.5-12 predecessor, DC10.1-08, reads “PTI DC10.1-08: Design of Post-Tensioned Slabs-on-Ground.” Both the DC10.5-19 and DC10.5-12 documents make the following statement regarding their predecessor, “This standard is based on PTI DC10.1-08. Refer to this document and the commentary on this standard for background and interpretational information that clarifies its application.” Chapter 2 of PTI DC10.1-08 deals with stable soils. As such, practicing engineers have used the PTI standards for many years on stable soils as the standard of practice to design PT Slabs on stable soils, but the IBC technically does not explicitly state it.

The resulting inconsistency regarding stable soils that persists today in the IBC leaves an opening for assertions that the IBC does not reference the PTI for stable soils. While this interpretation disagrees with the document that the IBC references, it is something that needs to be corrected.

Once we exit the IBC or IRC and enter into the destination of the PTI DC10.5 standard, there is no reason to go back or to access other standards, unless specifically instructed to do so within PTI. In the case that the PTI does not address specific structural criteria, the standard should indicate a path for the designer to return to the originating standard (IBC/IRC) for those criteria. So using the highway analogy, we made it to DC10.5 town, but they may not have all the things necessary to stay there and may indicate a return to the last exit. In the case of the PT slab-on-ground, concrete strength and durability are two of the requirements needed. So the question is – does the PTI standard have that information?

Within DC10.5-19 in Chapter 10.0 “Materials,” the standard requires that concrete shall have a minimum compressive strength of 2500 psi (17 MPa) at 28 days. Moving on to Section 10.4 “Durability,” the required compressive strength increases to 3000 psi (21 MPa) for concrete exposed to freezing and thawing or to deicing chemicals.

The standard continues to guide the designer on necessary measures for the PT slab-on-ground when the site has various levels of soil sulfates and soil chlorides. These measures include changing the cement type and minimum compressive strength for sulfates and protecting the tendons and conventional steel from chlorides. Therefore, at high sulfate levels, like freezing and thawing, the minimum 28-day compressive strength increases to 3000 psi (21 MPa). There are no requirements about the water-to-cement ratio (w/cm) of the concrete.

The PTI DC10 requirements are consistent with the minimum strength requirements contained in ACI 332-14, but not with ACI 332-20. The commentary to ACI 332-14 states that the w/cm ratio has a limit in place by controlling the minimum required compressive strength of the concrete. While ACI 332-20 does now contain limits for w/cm ratio, the PTI DC10.5 document does not.

From PTI DC10.5, there are no references to ACI about durability, nor are there references back to the IBC since the PTI receives scope as a standalone document. In fact, in the commentary of DC10.5 Chapter 1, it states (underlines added by the author),

“Post-tensioned concrete foundations designed by this standard generally meet the requirements for plain concrete specified in Chapter 14 of ACI 318-14. These foundations will typically contain less reinforcement – prestressed and non-prestressed – than the ACI318 requirements for reinforced concrete. This standard is intended to be a standalone document uniquely developed for the design of post-tensioned concrete foundations on expansive and stable soils and is supported by the performance of many thousands of existing conformant foundations. As such, it is intended that this standard be independent of ACI 318 and the conflicting parts of the general building code into which this standard is incorporated.”

The PTI DC10.5 document lets the designer know that it is the end destination. There are no secondary exits, roundabouts, or U-turns leading to other standards or back to the IBC and IRC.
While the PTI DC10.5 is demonstrably independent of the other standards, it is interesting to note that Chapter 19 of the IBC directly references ACI 318. Specifically, under IBC section 1904 “Durability,” the first sub-section, 1904.1, lists ACI 318 durability requirements as the standard for the IBC and ACI 201 by reference. However, before leaving 1904.1 an exception exists about Group R-2 and R-3 occupancies. These building types include apartments, condominiums, townhomes, and one-and two-family dwellings - residential and light-commercial applications. The only durability requirement under the exception is that the minimum concrete compressive strength shall be 3000 psi (21 MPa) with no further action given including the w/cm ratio.
The 2024 IRC indicates that concrete slab-on-ground floors adhere to ACI 332 or the provisions of R506—which includes DC10.5 under R506.2 specifically addressing PT slab-on-ground. Therefore, the final destination from the 2024 IRC is DC10.5 independent of ACI 332. While the reference out of the IRC may seem insignificant to this discussion, it is extremely important. Neither the IBC nor the IRC refer out to ACI about PT slab-on-ground. If they did, then there would be a conflict between ACI 318 and ACI 332. This is further justification that DC10.5 is a standalone document.

All the various, independent, durability destinations within the I Codes can be summarized by the adaptation of the original highway map shown in Figure 2.

In July 2017, the PTI Technical Advisory Board (TAB) published Technical Note 21 (PTI Technical Note 21, Issue 21—July 2017, “TN21”) further clarifying the independence of the PTI DC10.5 document. TN21 gives exit signs for those that arrived in the ACI 318, and by reference ACI 201, for the design of PT slab-on-ground foundations.

While TN21 uses ACI 318-14 for its facts, improved and clarified verbiage exists within ACI318-19. According to TN21, ACI 318-14 Section 1.4.7 states that “This Code (ACI 318-14) does not apply to design and construction of slabs-on-ground, unless the slab transmits vertical loads or lateral forces from other portions of the structure to the soil.” At first glance this infers that ACI 318-14 might apply. However, the commentary R1.4.7, clearly states (bold font added by the author), “Detailed recommendations for design and construction of slabs-on-ground and floors that do not transmit vertical loads or lateral forces from other portions of the structure to the soil, and residential post-tensioned slabs-on-ground, are given in the following publications:

In the most current ACI at the time of this article (ACI318-19), Section 1.4.6 states, “For one- and two-family dwellings, multiple single-family dwellings, townhouses, and accessory structures to these types of dwellings, the design and construction of cast-in-place footings, foundation walls, and slabs-on-ground in accordance with ACI 332 shall be permitted.” However, the commentary for section 1.4.6 then indicates an exit route to DC10.5-12 for expansive soils. What do we do about stable soils? The commentary goes on to refer to section 1.4.8 for stable soils which ultimately lands at the ACI 360R standard. Entering Chapter 10 of ACI 360R-10 for the design of post-tensioned slabs-on-ground, we find a reference to the PTI as an applicable design procedure under section 10.2.4. Once again, all roads and all exits have ended within the PTI DC10.5 document.

As simply and clearly stated by TN21 (underline added by the author):

“This Technical Note is to clarify that the post-tensioned residential slabs-on-ground were never intended to be governed by the ACI 318 Building Code.”

Finally, the PTI published an article titled “Code Requirements for Sulfate Durability in Residential Concrete” in February 2008 of the PTI Journal in Volume 6 Issue 1 authored by Mr. Ken Bondy. It is important to note that a review of this article occurred under the PTI Journal publication policies. The article discusses that in historical practice the w/cm ratio did not apply to residential concrete design and construction. Typical compressive strengths used were between 2000 and 3000 psi, which equated to w/cm ratios of 0.8 and 0.6, respectively.

Therefore, litigation ensued because the w/cm exceeded the requirements for sulfates (0.45 or 0.5 depending on conditions) of the ACI tables.

At the time of the journal article, ACI 332 had not yet included limitations in the form of w/cm ratio. ACI 332 now does include limitations, but recall that ACI 318, 201, and 332 do not have scope on the DC10.5 document. Some of the early engineers in the PT slab-on-ground field did not have the luxury of the information presented in this article. As a result, those engineers reacted to the many lawsuits regarding sulfate durability allegations by changing their plan specifications and, according to the journal article, this “… has resulted in a significant increase in the cost of many new homes in California, Nevada and Arizona, with no related benefit.”

The journal article also addresses the question: Why different durability requirements for residential concrete? The journal article suggests that factors of safety in the range of 30 are present under light residential loads. In addition, it makes the point that reducing the w/cm ratio from 0.6 to 0.45 effectively changes the service life of the concrete from 150 to 200 years, while the materials in the structure on top of the concrete only have a service life of roughly 75 to 100 years.

Finally, after going through the various durability requirements for PT slab-on-ground within the PTI, which do not (then or now) include limitations on w/cm ratio, the journal article summarizes as follows (bold font added by the author):

“…They (the codes) will include a requirement for sulfate-resistant cements, but they will not require direct limitations on w/cm, which are difficult to control in fresh concrete and impossible to evaluate precisely in hardened concrete. This will clarify and refute the erroneous allegation that the mere reference to a sulfate-resistant cement by a licensed design professional somehow also triggers a requirement for a limitation in w/cm. In this model code, w/cm ratios will be indirectly controlled, when necessary, by specifying a minimum concrete compressive strength. These new code criteria are consistent with long-standing successful practices for sulfate durability in residential slabs and foundations. They should help in reducing opportunistic lawsuits which have resulted in increased costs to homeowners with no related benefit and are based simply on a lack of clarity in code wording rather than a real deficiency in performance.”

Understanding the scope and code flow within the building code is a necessity. Without a proper map of the route through the code for a post-tensioned slab-on-ground, an engineer, designer or lawyer might get lost along the way. The result is a potential increase in lawsuits regarding the applicability of standards within the code. Should the engineer, designer, or lawyer get lost in the ACI because of a lack of understanding, it is important to note that all those roads in the ACI regarding post-tensioned slab-on-ground still lead back to the PTI (Fig. 3).

Paths exist to the PTI DC10.5 document as the standard for the design and construction of PT slab-on-ground foundations on expansive and stable soil types. There are areas within the I Codes and ACI with references to the PTI DC10.5 document that still need additional wording reflecting the correct application of the document to stable as well as expansive soils. Efforts are underway by the author and others to make the path as clear as possible to engineers, designers, and lawyers.

Even so, the DC10.5 standard is a standalone from the originating codes as well as the ACI. Very simply stated, DC10.5 is “… independent of ACI 318 and the conflicting parts of the general building code into which this standard is incorporated.” ■

About the Author

David Sparks, PE, SE, is vice president of research and development for Felten Group, which is an architectural and structural engineering consulting firm specializing in residential construction.