Chart for Estimating Post-Tensioned Slab Thickness

While McClure is nationally known for its expertise in the design of Cold-Formed Steel structures, we also do an extensive amount of work with wood, structural steel, and post-tensioned (PT) concrete. In a review of the many PT projects we’ve done, we developed a reference chart for high-level estimates of flat PT slab thicknesses based on the number of stories of wood or cold-formed steel it is supporting. 

While these numbers will not hold true for every project, they are a great starting point. For example, one of our recent 11-story CFS load-bearing projects was supported by a 22-24” podium. This chart is conservative to cover unique conditions or odd load distribution. 

Furthermore, we’ve found that PT podiums will be approximately 2” thicker if you run your floor framing from exterior to interior corridor instead of demising wall to demising wall. Exterior to interior corridor spanning floor framing is the direction assumed in this chart as it is the most efficient layout for CFS structures, a topic we will explore further in a future post. The additional thickness of the PT podium is due to the corridor walls typically landing in the middle of the drive lanes in the garage below. 

Another consideration is the use of drop panels. Drop panels—also known as column caps, drop caps, and shear caps—are 6”-15” thicker portions of podium slab directly over columns. For podiums, drop panels are typically 6’-10’ wide. The use of drop panels will reduce podium slab thickness by 2”-4”. 

Drop Panel

If you have questions about the design of Cold-Formed Steel load bearing structures, Wood, Post-Tensioned, or Structural Steel Mid-Rise Structures, please reach out via our request a quote page.