The Building Code and the Post-Tensioned Institute (PTI) - Part 1

It has recently become more apparent to FGI that there are not only engineers in training coming out of school that have no idea of how to apply building code to what they are taught in school, but also that there is an abundance of senior engineers, those that even call themselves “experts”, that do not understand the building code.  With the lack of understanding comes a lack of knowledge regarding how to apply the various other standards referenced by the building code.  The PTI is just one of those many referenced standards.
In order to understand the building code, specifically the International Building and Residential Codes (IBC and IRC aka “I Codes”), we should think of the codes like a highway.  The highway itself is marked with “exit” signs that branch off in paths to other standards (referenced in Chapter 35 and 44 for the IBC and IRC respectively)– and even some loops that lead back from one code or standard to another.  Sometimes municipalities will adopt the “I Codes” by reference unconditionally.  Other times, the municipalities will amend the “I Codes” as necessary to achieve the path through the codes that makes 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”.

Amendments to the “I Codes” are much like road closures or detours.  In those cases, exits from the code to other standards may be closed and/or specific paths through the code could be eliminated or changed, like a road closure and detour.  Researching amendments, whether municipal or state, is the important first step before beginning your journey into and through the “I Codes”.  This step provides us with the “closures and detours” so that we don’t get lost along our journey.

For the purposes of furthering this discussion, it is helpful to draw out a flow chart to assist in visualizing the path through the “I Codes”.  While both the IBC and IRC have references out to other standards, this flow chart will only include those pertinent to the discussion for establishing a path to the PTI and those that are more commonly used for residential construction in particular.

It is extremely important to note that the arrows indicate a “one way” direction on the path through the code.  In order to arrive at the IRC, the scope must be delegated to it either directly from the municipal or state codes, or as typically done from 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 prior to arriving in the IRC.  Once you arrive in the IRC, there are some alternate reference standards listed, but in IRC Chapter 3 direction is given that two triggers exist to leave the IRC other than the alternate reference standards:

  1. Engineered rational analysis which requires the use of the IBC is always allowed in lieu of the IRC provisions
  2. Portions of the structure that are non-compliant with the IRC shall use the IBC

As of 2024, both the IBC and the IRC contain exit ramps to the PTI DC10.5 document.  Of note, and a recent point of contention among less knowledgeable engineers, the 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”.  While the IBC only references the PTI DC10.5 and omits the actual title of the document.  The IBC exit ramp to the DC10.5 document occurs in IBC Chapter 18 under 1808.6 (1808.6.2 to be exact) which pertains to “Design for Expansive Soils”.  The remainder of IBC Chapter 18 does not address slab-on-grade foundations on stable soils.  Therefore, even though you can see the destination, especially considering the IRC exit ramp in Chapter 5, you technically can’t get there because there is no exit ramp from Chapter 18.

While a reasonably intelligent person can infer that the DC10.5 document refers to both expansive and stable soils (because it says so in its title), there exists a notable problem within the IBC.  Engineers have used the DC10.5 document for stable soils for years as the standard of practice to design PT Slabs on stable soils, but the IBC technically does not explicitly state it.

Prior to the 2024 IRC, the path to the DC10.5 document for both residential and commercial projects only existed in the IBC.  This is because the DC10.5 document used to fall outside the scope of the IRC, and, therefore, the engineer was required to leave the IRC and return to the IBC Chapter 18 to access it (refer back to the chart above).  Thankfully, the 2024 IRC added its own exit ramp and reference to the DC10.5 document for both expansive and stable soils, but the issue in the IBC remains to this day.

While there exists those in the legal realm that profit because of these inconsistencies, it is important for the industry to speak up as to the principal intent of the DC10.5 reference from the IBC until a correction can be made.  The inappropriate attack on PT Slabs on Stable soils that are under the scope of the IBC does harm to the industry as a whole.

Stay tuned, as we will discuss improper usage of ACI codes pertaining to the PTI document in our next post!