Deck Pier/ Column Footings (Small Loads)- Confused myself completely (theory/ practical discussion)

[WTT1]

Hello all,

This is the first post I have added here. Maybe it will help someone in the future and myself in the near term. I'm a civil engineer (PE) for the DOT so I have a bit of educational knowledge but 0 real world knowledge in the subject I'm posting about. Essentially, I have read so much at this point, that I've probably confused myself more than anything-- I have taken a theoretical approach also because I would like to learn more about how real world works in situations with much more substantial loading.

Below I will give all of the things I have determined about my situation. I will be the engineer and the construction worker on this process.

Loads:
10 psf dead
40 psf live
It looks like the service load per post is 3000 pounds based on the above.
I am in Knoxville TN; Seismic Design Cat. C
90mph asd wind, 115 vult wind
deck (foundations) 6x6 post on 10' centers; The deck is 12' transverse to the back of the house (30' parallel to the house)-- again post on 10' centers
circular foundation's base will be at 2' below the ground surface, it will be 1' thick and 2'x2'
The stachion/ pier/ column that goes down to the base will be 15" long (3" of which will be above the ground)
The "column" will be made by a 12" circular sonotube.

I have learned through ASCE 7 wind/ uplift calcs that I have no net uplift on the edge post nor center post.
I have also learned that I believe the pier/ stanchion/ short column could be calculated based on "plain concrete"-- the 3:1 ratio-- maybe I'm wrong?
I think I have also learned the footing doesn't need rebar by theoretical calculation but I have heard its a good idea.
I plan on putting 3 #4 rebar in, in each direction and tying together with rebar ties, they will be 3" up from the foundations base (12" total thickness).

If I'm understanding correctly, in my situation, the rebar in both the pier and the footing is somewhat "insurance" and good practice for the potential of seeing tension through ground movement/ crack mitigation etc.

But...

Here are the areas I'm confused:

I feel like I need to tie the 12" column to the to the base with rebar but I'm confused if I need to have the bends on the bottom of the main bars to tie them into the rebar that will be 3" up from the base of the foundation. I feel like I've read in the CRSI handbook (1956- freely available) that Case 4 is using only straight dowels as no moment would be transferred in my instance. However their table calculations don't go down to a 2x2 footing and they don't specify a case 4 dowel bar size as you start getting into the smaller sized footings and I don't really understand why. Can someone with more knowledge than me extrapolate this just from experience?

Also, if I do put the 90 degree bend on the portion that is in the footing, do I turn the bends in towards each other or away from each other. Also, do the bends sit on top of the other rebar that is 3" up from the base of the footing and tie in with the wire or do I hook them under that rebar. With my column size being 12" and my footing being 24"x24", I don't know that the 90 degree bends matter as I don't see how there's much bending here due to the proportions.

Also, I'm confused on the need for column ties. I understand what they do but do I need them in this case? In small load situations like this, how do you know where you can "make liberal modifications" or where you need to stick to the code? (I know experience helps, but I don't have this in this area of engineering) :-/; I guess if I'm working off of the "plain concrete" section of aci 318, I guess they aren't need at all, again only if practical to help with construction/ not necessarily engineering calcs for code compliance)

I know my loads are small and the ties apparently help with resistance to buckling of the main/ longitudinal steel reinforcement of the column and confinement of the concrete core.

I was looking at compression developed length to help understand this dowel concept (I feel like their related but don't know/ understand fully). I feel like this idea relies on the bond strength of the concrete to the surface area of the rebar available with no 90 degree bends at the base of the main column steel (as spoken of 4 paragraphs above).

Could you all give the theoretical explanation surrounding my confusion above and then give me the "you're overthinking things, here's what I would do based on what you already know explanation".

 

[jhnblgr]

I think you are over complicating your project. The rebars need embedment to transfer forces, hooks or bends reduce the length into the footing. Ties in columns for buckling, likely your piers are short and stout enough to not warrant much steel but investigate to understand. typically 12” o.c to hold rebar uprights in place. Anchors or base plates would likely determine you pier dimensions based on edge distance required or confining reinforcement. A sketch would help. Think load path, structural members deck to joist to beam to wood columns to base plate to concrete pier to footing to ground. Also think about stability of the columns depending on height.

 

[RWW0002]

If you have not grabbed a copy yet, take a look at DCA6 from AWC

https://web-media.awc.org/wp-content/uploads/2022/02/17210514/AWC-DCA62015-DeckGuide-1804.pdf

Also, I think Knoxville B&C has a deck guide that is similar.

https://permits.knoxvilletn.gov/files/assets/building/v/1/building-code-faq/residential-prescriptive-deck-guide.pdf

 

[WTT1]

If you have not grabbed a copy yet, take a look at DCA6 from AWC

https://web-media.awc.org/wp-content/uploads/2022/02/17210514/AWC-DCA62015-DeckGuide-1804.pdf

Also, I think Knoxville B&C has a deck guide that is similar.

https://permits.knoxvilletn.gov/files/assets/building/v/1/building-code-faq/residential-prescriptive-deck-guide.pdf

Thank you. I do have both of these. I am using them as a cross reference to make sure I don’t do something stupid in any of my calcs. Hoping to eliminate some of the conservative approaches that are discussed in the commentary of dca6.

Also rebar intricacies are not discussed.

 

[WTT1]

I think you are over complicating your project. The rebars need embedment to transfer forces, hooks or bends reduce the length into the footing. Ties in columns for buckling, likely your piers are short and stout enough to not warrant much steel but investigate to understand. typically 12” o.c to hold rebar uprights in place. Anchors or base plates would likely determine you pier dimensions based on edge distance required or confining reinforcement. A sketch would help. Think load path, structural members deck to joist to beam to wood columns to base plate to concrete pier to footing to ground. Also think about stability of the columns depending on height.

Thank you for your response here. I have done extensive investigation at this point. Most of this is all for educational purposes and better understanding even though I don’t do structural for a living.

I believe I found that 1” min tie spacing and max 6” tie spacing are my code limits if I were wanting to design by column code limits rather than “plain concrete” requirements of aci. 12” like you are saying, would be fine too as long as I stay in the chapter 22 design area for my situation.I plan on a “sprinkling” of rebar based on minimum requirements of other code sections that would be applicable if designed outside of chapter 22 aci.

I plan on doing a single expansion anchor to hold the Simpson post base down to the 12” circular “column” that leads down to the footing. (The 6x6 post is screwed to that)

I pretty well have everything else under control, minus the column/ footing details.

I think with your help I have those more understood at this point. I will post my footing cad here in the next little bit.

I would appreciate your thoughts.

I still don’t know how the bent rebar going into the footing from the “column” is supposed to tie in— does it even matter whether the bent portion sits on top of the rebar that’s in the bottom of the footing or if you slide it up under that rebar.

I’ll post my drawing shortly. It’s rudimentary but at the least, I feel like I’m learning.

 

[gte447f]

Based on the small size of your concrete foundation elements and the small magnitude of your design forces, you don't really need any rebar in the foundation (I am including the pier and the footing as the foundation). However, I would provide at least a rebar dowel to tie the pier to the footing. I say this because I have seen residential deck piers sitting on footings in the wild that you could push over by hand. Ordinarily, these dowels would be hooked in the footing, but in this case you won't have enough distance in the pier above the footing to develop the bars anyway, so it doesn't really matter if you hook the dowels in the footings. You also don't really need any bars in the footings because the footings won't really experience any appreciable bending stresses, but if your are planning to dowel the piers to the footings it might be helpful to have some bottom bars in the footing just to tie the dowels to just to hold them in place during concrete placement. It might be nice to have hooks on the dowels for the same reason.

 

[WTT1]

Based on the small size of your concrete foundation elements and the small magnitude of your design forces, you don't really need any rebar in the foundation (I am including the pier and the footing as the foundation). However, I would provide at least a rebar dowel to tie the pier to the footing. I say this because I have seen residential deck piers sitting on footings in the wild that you could push over by hand. Ordinarily, these dowels would be hooked in the footing, but in this case you won't have enough distance in the pier above the footing to develop the bars anyway, so it doesn't really matter if you hook the dowels in the footings. You also don't really need any bars in the footings because the footings won't really experience any appreciable bending stresses, but if your are planning to dowel the piers to the footings it might be helpful to have some bottom bars in the footing just to tie the dowels to just to hold them in place during concrete placement. It might be nice to have hooks on the dowels for the same reason.

Thank you for your response here. I'm sorry it took me a bit to get back to this thread.

Also I understand what you are saying about the small loads and no "structural" requirement for rebar. I think any rebar I provide would be for longevity/ tying the structure together after cracking.

Thank you for the idea about the rebar dowel. I had seen this in some commercial footings where they pushed 4 pieces of rebar down into the pier portion, the bottom ends, ended near the bottom of the footing to tie the 2 together.

I was wandering about the "hooking" and I know very little about the "development length" concept, it has been too long since I was in school and using these concepts. (I don't do much/ hardly any concrete design.

I used a development length calculator that was showing around 12" for compression. For the longitudinal steel with the "hooks", is the length I compare to the calculated min. development length, the amount of straight/ horizontal steel that is actually in the footing that is part of the longitudinal steel (after the radius). I guess that is what you meant by, "the rebar will not be developed anyway".

I understand what you are saying about the steel in the footing not being necessary for "structural" reasons. Again, only for longevity/ tying of the structure.

Thank you for your time you have spent on you response.

In the end, I am planning on doing 3 number 4 bars in each direction at 3" up from the bottom of the footing. Using hooked longitudinal bars. All will be tied with rebar ties and then set down into the excavated area as a full unit.

Below are CAD screenshots of what I think I ended up with:

Please comment with any other information that you believe would be helpful.-- especially if something seems difficult to construct without other additions.