Minimum Vertical Reinforcement for Octagonal Pedestal

[AJMF]

Dear all,

Good day!

Regarding the minimum vertical reinforcement for octagonal pedestal (footing type foundation), is there a section in code (ACI) that states that minimum vertical reinforcement (0.005Ag for pedestal or 0.01Ag for column) may not be met for octagonal pedestal due to very large area gross? Because we all know that due to large area gross, the requirement for minimum vertical reinforcement will produced very large rebar size (around the octagonal pedestal perimeter) and maybe very small spacing too.

Thanks in advance!

 

[Agent666]

Can you maybe post a sketch of what you mean, are you talking about a octagonal shaped column that has some height to it relative to the width of the cross section, or quite a squat wide pedestal. Say 5 times wider than it is high or something?

I thought ACI318 had something along the lines of you could reduce the minimum reinforcement, if minimum was based on an arbitrarily smaller member that actually worked for strength purposes. Without knowing the exact arrangement you had in mind its hard to give a definite opinion.

 

[AJMF]

Agent666,

Say I have a octagonal pedestal (squat wide pedestal) with edge to edge distance D = 26'-1 7/8" and depth of 3'-10". For this size of pedestal, we only provide above the minimum rebar ([highlight #FCE94F]120-#6, 16 at each face with spacing of 8.4"[/highlight]) as per Section 4.5.5 of PIP STE03350, see below statement.

Section 4.5.5 Minimum pedestal reinforcement should be as follows:
1. Octagons 6' to 8.5':
16-#4 verticals with #3 ties at 15" maximum spacing
2. Octagons larger that 8.5' to 12'"
24-#5 verticals with $4 ties at 15" maximum spacing
3. Octagons larger than 12':
#5 verticals at 18" maximum spacing with #4 ties at 15" maximum spacing

But if we follow the minimum reinforcement provided by the code (Section 16.3.4.1 of ACI318-14), As min = 0.005*0.8284272D^2 = 408.07 in^2. That will produce a rebar of [highlight #FCE94F]280-#11, 36 at each face with spacing of 3.6"[/highlight].

We follow minimum rebar provided by PIP and design the vertical rebar to resist overturning moments from the vessel, I just want to know if ignoring 0.005Ag or 0.01Ag for large pedestal is supported by the code.

Thank you very much.

 

[Agent666]

The minimum steel requirements are generally intended for flexural members. I'd think something of that size isn't intended to resist global flexural forces (like a column might), so maybe the requirement could be relaxed to essentially deal with only whatever requirements are required for dealing with temperature and shrinkage requirements and any localised loading that might be present. (though I'm not familiar with what PIP STE03350 is)

 

[JAE]

If you read the commentary of ACI 318-14, section R16.3.4, you see that the intent of this 0.005Ag provision is to ensure ductility during construction and during the life of the structure.

For a 26 ft. wide octagon pedestal, I think ductile behavior is highly questionable unless there's a huge vertical seismic force going on.
Your "pedestal", to me, is more like a reinforced mat.

And instead of placing all the reinforcement around the perimeter of the octagon, it seems to me you could scatter the vertical bars across the full area of the pedestal.

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[JoshPlumSE]

Yes, I ran into these exact issues early in my career when I worked on vertical vessel foundations. I even worked on an internal program my company used to design the reinforcement for these pedestal and foundations. We got into a long internal discussion about this. My recollection was the following:

1) The ACI code was not intended to handle this type of pedestal. I would argue that a "pedestal" per ACI isn't just a column with a height to width ratio of less than 3. Rather it has (or should have) a lower bound for a height to width ratio of less than about 0.33.

2) The code wouldn't allow us to view this as a "plain concrete" pedestal because the seismic design category was too high. So, per code we would think of this as a pedestal which needed a minimum vertical reinforcement ration of 0.5%. Ugh! That's the argument that you are making.

3) What is the real load path? Well, the moment comes from the vessel into the Anchor Rods and from the anchor rods into the mat foundation.... Not as flexure in the "pedestal". So, as long as that load path is properly designed, you're in good shape.

4) What are these really, really short pedestals? They're really more like a drop panel or a thickened area of the mat foundation. If we view them that way, we don't need to provide 0.5% vertical reinforcement. Instead, we can provide some reinforcement for detailing purposes. Maybe call it skin reinforcement for crack control on the outside surface. But, it's not taking flexure or providing confinement the way it would in a column or a real pedestal.

 

[BridgeSmith]

AASHTO has modifications for ignoring concrete in excess of what is required for the loading conditions. Doesn't ACI have something similar? If it does, what I would do (and have done per AASHTO) is look at what concrete you could remove and still have an adequate load path, stability, capacity, etc. and subtract it from the area you calculate for the 0.5%. In other words, don't count the 'inert' concrete.

 

[WARose]

What I use to do (as someone who did a lot of those types of pedestals (of all shapes)) is to consider a perimeter "wall" (typically where your anchorage or skid is) as being the effective zone where %Ag is applicable. The rest is fill. (Possibly unreinforced....unless the mass is counted on during a dynamic analysis for unbalanced loading from machinery.)

 

[AJMF]

Agent666,

Yes, this squat type pedestal will not be classified as flexural member.

JAE,

You are right, that is why I do not follow that provision for this type of pedestal.

JoshFlum,

Yes, that is the load path, rebars are only to resist tension from anchor bolt (if required by design) then pass it to the footing. Our practice for this type of pedestal is only to provide rebars around the perimeter and skin reinforcement at top of pedestal.

HotRod10,

Thanks for the info. [highlight #FCE94F]What part/section of AASHTO is that?[/highlight]

WARose,

Same with AASHTO provision, right?

Thank you all, maybe provision from AASHTO (or equivalent ACI provision) will be the best reference when client ask.

 

[BridgeSmith]

There's a provision in Section 5.6.4.2 (AASHTO LRFD 8th Ed.) dealing specifically with the reduction for longitudinal reinforcement in regions of low seismic potential, and more general provision for lightly loaded flexural members in 5.6.3.3. There's also a limit on reinforcement ratio of 0.0025 for wall-type piers in 5.11.4.2.

None of these provisions really address your situation, and you're not designing a bridge. Without something more specific from the governing code provisions that you must use for design, you will still be deviating from those governing code provisions. What I've referenced should only be used for the purpose of satisfying your own comfort level with making that deviation from the letter of the code.

That being said, I would consider this to be a mat foundation or a footing, and reinforce it as such. That would likely mean providing any necessary confinement steel or just shrinkage and temperature reinforcement, if no confinement reinforcement was required.

 

[AJMF]

HotRod10,

Yes, that will be the way forward. Consider the "pedestal" as mat and reinforce it to comply S&T and also the provision in PIP STE03350 (Process Industry Practice for Vertical Vessel Foundation).

Thank you all.