Barn - concrete opinions?

[drm]

Rich,

Use of the #4 vertical rebar and #3 hoops is reasonable since technically no reinforcing is required per the barn engineer. My recommendation is what I would put in; but, I would not be bending the rebar but buying it that way. If you are concerned about the amount of steel; the area of a #4 rebar is 0.20 in**2 and a #5 rebar is 0.31 in**2. Therefore, the #5 would provide 50% more steel. So you could use 9 -#4 bars in place of 6 -#5 rebars. But the call is up to you.

I personally would use more steel but I am an engineer and a bit ****. I get questioned about some things like this situation where the rebar technically is not required and say to the contractor or junior engineer to just do it. It just makes me feel better (sleep at night) to get enough steel and concrete to make it "look good" to me. I have done trouble shooting for steel in power plants, precast concrete manufacturers, and general construction including steel, concrete, and wood. It is very difficult to go back and fix concrete reinforcing problems. I have done many and have developed some unique methods for precast but never like fixing them. It just isn't as good as being done right the first time.

Now that I am off my soap box, what you proposed should work if you and the building engineer are comfortable. Heck, I don't even know the size of the building so my “recommendation” is just based on a 24” diameter pier size.

Sorry I didn't reply sooner got pulled away.

 

[drm]

caylor,

Correct, the minimum required area for a column design is 1% of the gross cross sectional area of concrete. The cross sectional area of a #4 rebar is 0.20 in**2. The area of a #6 rebar is 0.44 in**2. That is why I stated 10 -#6 bars.

For foundation piers it is common to use 1/2 of this amount of steel or 2.26 in**2. So 8 - #5 bars would be appropriate if you take the liberty to reduce the amount of steel. I am not familiar with construction practices in Calif. Since I have never worked there; but, I know seismic requirements are more stringent period. I have done work on east coast is some low to moderate seismic areas. And yes, the local seismic design requirements do need to be considered. I am assuming the building engineer has already done this.

 

[Brad_Blazer]

100 tons of tensile strength in a short slug of concrete attached to a building with a few 5/8" anchors? What am I missing? When I think of a "column" I think of a tall element subject to seismic bending/buckling loads, not a buried pier with an aspect ratio of less than 1:2.

 

[keeney]

I have to agree with Brad - the piers are sounding way over-built. I'd put the steel (money) into the floor slab instead.

If you are still worried about failure in tension, either use a longer anchor bolt, or hook the end of a #4 over the ear of the anchor bolt and put another short bend on the other end down near the bottom.

The 5/8" anchor bolt is plenty. Even at the conservatively estimated 6000 lb capacity, such a bolt could pull the entire pier out of the ground! 2'x2'x5' x 150 lbs/cu-ft is only 3000 lbs.

(neglecting friction, wedging, suction, or otherwise lifting the surrounding soil).

- Rick

 

[RichT]

Since I started this thread, to all those who have contributed, my sincere thanks. The exchanges here have substantially improved my ability to understand and evaluate foundation design tradeoffs for my tractor shelter/barn project.

I recognize that some folks tend to offer "commercial grade" solutions. Such input is valuable for understanding the boundary conditions on a design. In my business (electrical engr) I tend toward the same thing. The customer/advisee can chose to take a more conservative route within the design constraints as cost, schedule, or other factors may dictate.

The quality of commentary in this forum continues to impress me. We should all be thankful for such a resource.
rich

 

[drm]

Brad,

To be blunt two things:
1). Concrete Design
2). Building Code MINIMUM design requirements.

Minimum amounts of steel are dictated in the building codes to ensure that the concrete acts as reinforced concrete if it is designed with reinforcing. In an earlier post I touched on plain concrete design (designing the concrete without any reinforcing). I also stated that even though the minimum amount of reinforcing dictated by the building code is 1% it is common in areas that I have worked to use ½% steel in foundations. The reinforcing steel not only acts to take tensile forces (in any reinforced concrete element) but to ensure the integrity of the element. In this particular case the later is its primary purpose.

Heck I have seen buildings built on posts (wood poles) set in soil with no foundation, concrete, etc. I have also been the one who is called in after a building, roof, wall, etc. collapses to determine what happened and why. Most of the time is because someone did not think meeting the Building Code minimum requirements was necessary. The majority of these are structures that were not engineered or were engineered and built wrong.

My reply may seem hostile but I do not think you have understood the exchange I am trying to make. I am not dictating anything. Only giving and opinion and some code requirements. BTW, I do not take any of this criticism personally and I hope you do not either. I have worked on and around construction sites for over 20 years and have heard it all.

In closing, I have repeatedly stated that the design is totally up to the owner and engineer of record. I am only trying to answer the questions raised with enough information so that the poster of this message can make an intelligent decision.

 

[keeney]

Part of the misunderstanding here is that the pier is being made 24" in diameter all the way up not due to the load it will be bearing, but for convenience. The base or footing might be specified at 24" because its easier to just say make it 24" to handle the worst case instead of figuring out what kind of soil its actually sitting on.

Another way to think of it - its a 24" footing with an 8" dia column. The extra concrete is just being dumped in the hole as a convenience so that the position doesn't have to be so accurate.

The only other reason to make it so big would be as wind-load ballast - just raw weight to keep the thing in the ground.

I do agree that if you really did need a 24" diameter column to support the load, then you would really want all the steel that goes with it.

In this case, however, I suspect that the other poster is right. Very few pole-barns will have loads in the 100's of tons range that would actually necessitate a 24" diameter column. He's not building Trump tower

- Rick

 

[drm]

Rick,

I do understand. Look at Rich's last post and he does also.