Reinforced Pier vs. Plain Pedestal (ACI 318-14)

[Lukas PE]

This is my first post on this forum but I come here a lot to do research, etc. I know this topic has been discussed in several instances, but hoping to get a perspective on some nuances...

I've got a CA issue where the GC did not install steel columns per our design. I am being asked to review the as-built condition to determine if it is sufficient in the interest of construction timeline. Our design called for baseplate to footing connection at (-3'-6") with "pier" poured around column up to (-0'-8") below floor slab. (See typical detail attached) The design intent didn't really scrutinze the "pier" as it isn't really doing much other than providing protection for the column. As such, only 0.3% reinforcing steel was provided. Well, the GC put the columns on top of pier... Not to mention the superstructure is 3-ft taller than designed, but I can handle that as a separate issue... Wanting to get some feedback on my thought process below. (Controlling load is pure compression. Lateral loads are negligible.)

Option 1: Consider the "pier" a short column and design per ACI 318-14 Chapter 10. Therefore absolute minimum longitudinal reinforcement would be 0.001 x Ag/2 = 0.005Ag per 10.6.1.1 and 10.3.1.2. If I consider this option, the piers do not provide the minimum reinforcement ratio and will therefore not meet code. However, running through the calcs, I have way more strength than required under design loads.

Option 2: Consider the "pier" a Plain Concrete Pedestal per ACI 318-14 Chapter 14 and design as unreinforced. (Provided 0.3% would be considered T&S reinforcement only). In this case I have a 32" tall pedestal completely below grade that is 20" square. Running through the calcs, I also have plenty of strength under design loads.

I would be curious to hear some feedback regarding interpretations and previous experience. Is it reasonable to consider these piers as plain pedestals to satisfy the code requirements? I guess the difference between a pier and a pedestal is an important nuance here...

Thank you,

 

[JoshPlumSE]

Pure compression, with no lateral loading....

I think your thought process is pretty good. I'd start checking it per option #1 and make sure it works capacity wise (which you've done). But, since it doesn't meet minimum reinforcement ratios, I would also check it per option #2.

Note: I would not tell my client / the owner that this solution "meets code". I'd give a 3rd option (which would be much, much more expensive) that would meet code. Then I'd let the client know that "leaving it as is" doesn't appear to cause any direct danger to current or future occupants. However, that these piers do not meet current code requirements and should be inspected after any significant structural event (earthquake, major wind event, et cetera). Ultimately, I'd let the client / owner make the decision.

 

[Lukas PE]

Thank you for your advice JoshPlum. The wording you provided is very helpful. (I knew that negligible lateral load comment would stick out to folks...) Lets proceed with thread recommendations regarding compression only. I will dig into the model more to make sure I am comfortable with negligible lateral loading. We have low roof diaphragms framing in from the sides that offer lateral restraint.