Vertical vessel footing - when is vertical flexure reinforcement needed?

[LearnerN]

Newbie structural engineer question. I am considering a 30ft tall vessel (36" OD) that weighs 34,000 pounds. Originally, I was thinking of using a standard square footing with vertical axial/flexure reinforcing in the column. But now I'm thinking of just a square block foundation. What is the typical way to determine if a support needs any vertical reinforcing for flexure (not just axial)? Would that be to check if e/h...the eccentricity M/(P*h) is less than 0.1 for tied columns? (Like ACI 318-14 Section 22.4.2.1)

The footing will definitely have some moment primarily from wind load...I guess I'm trying to conceptualize how a block foundation would behave and how to check if vertical flexure reinforcing is needed, or if the reinforcing will just be for bearing/axial. Thank you.

 

[JStephen]

Depending on dimensions, details- use vertical reinforcing to support a top mat, as general shear reinforcing, to anchor bolts that don't develop adequate breakout resistance on their own.

 

[JoshPlumSE]

Vertical reinforcement in the pedestal or the footing? I always put vertical reinforcement in the pedestal. For the footing, I might hook the bottom bars up to help support the top bars. Or, vice versa.

 

[LearnerN]

Vertical reinforcement in the pedestal. Yes, there'd always need to be some vertical reinforcement in the pedestal for axial for sure, but how to determine when the vertical reinforcement in the pedestal needs sized for both axial AND flexure...and more specifically, how to determine if this is needed in a simple square block foundation.

 

[KootK]

how to determine when the vertical reinforcement in the pedestal needs sized for both axial AND flexure

Perform a structural analysis to get your axial and bending loads then calculate the resulting compression stress distribution on your member. If it turns out that some portion of your member would need to be in tension to satisfy equilibrium, you'll probably need some reinforcing for flexural purposes.

and more specifically, how to determine if this is needed in a simple square block foundation.

I'm still confused as to whether this is a footing and pier assembly or just a giant blob of concrete. I suspect that latter in which case JStephen's comments will be of particular relevance. Consider posting a sketch of what you're proposing to do. Better advice would surely ensue.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[LearnerN]

KootK, not a footing and pier assembly, just like a 5'x5'x5 reinforced concrete pier.

You said: "Perform a structural analysis to get your axial and bending loads then calculate the resulting compression stress distribution on your member. If it turns out that some portion of your member would need to be in tension to satisfy equilibrium, you'll probably need some reinforcing for flexural purposes."

Got it, that makes sense. Basically size the reinforcing to ensure the concrete member is in compression.

 

[Leftwow]

The minimum's we use are min compression steel for the pedestal 0.005, and min flexural steel in the footing 0.0033. When you have a moment due to wind or seismic, you need to calculate the eccentricity, in order to figure out your corner pressures. The eccentricity should be an empty tank with a wind load or a operating volume tank with a seismic load. Use that eccentricity to figure your corner pressures, then calculate the moment from that NON-UNIFORMED distributed load.

 

[KootK]

KootK, not a footing and pier assembly, just like a 5'x5'x5 reinforced concrete pier.

At those proportions, you won't need any flexural reinforcement per se. Your primary design issues will be soil bearing stress, overturning, sliding, and proper anchorage design. Emphasis on the proper anchorage design. I'm willing to bet that, in your head, you're thinking of a failure mode where an upper portion of the block separates from a lower portion of the block as a result of tensile stress. And you should be concerned about that. With these proportions, however, that is very much an anchorage issue rather than a flexural reinforcing issue.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.