Any Math Wizards - Figuring Pier Diameter to Encompass a 20x20 Square

[danieldd]

Your CEO should be able to sign off on the smaller circle. All they need is to have x amount of concrete in the hole to support the beams. I would ask them again. Do the math for the amount of concrete. The circle you think you need is a lot more concrete and a waste of time, effort, and money.

The 20" by 20" by 12" deep square is 4800 cubic inches of concrete and the 28.24" diameter circle will be 7536 cubic inches. Basically double the amount of concrete. A short load from a concrete company will cost you an arm and a leg. You will probably have to pay for 4 yards at $100 a yard.

After looking at home many bags you will need, you may be better off paying for a short load. If you can get them to agree to the smaller circle with the same area as the 20 x20 square, it wouldn't be that bad and you could save yourself a few hundred bucks for an afternoon's work.

All good points and I will talk to them again - tomorrow. They don't give a rip whether I have a round hole or a square hole. The round hole was my idea simply to avoid the saw "overcut" marks in the concrete.

Agreed on too many 60 or 80 lbs of Quickcrete to accomplish the job on my own. Don't want to break my back doing this.

Just had the concrete contractor by to look at my patio. Since I've referred him to several customers, he was willing to do it for $150 + the cost of the concrete - which as you well know is expensive for anything smaller than 4 cubic feet. Concrete guy said I was looking at $450 total. Then I have to add $100 for the Titan post anchors I was going to buy. This adds up to more than I have in the cost of the wood that I've already cut and was ready to assemble.

If the city doesn't give me some leeway on this, I'm half inclined to put the damn thing up for sale on Craigslist..

 

[danieldd]

So OP . . . gotta tell us what City, and what Country is forcing that massive sub-structure for a pergola??

Any pics of this "massive" pergola structure that will need 4 piers??

How deep do these core holes through concrete have to go??

Huntsville, Alabama..
Madison County


Representation of the same Pergola here: http://www.amazon.com/dp/B000V1DHKS/?tag=atomicindus08-20

 

[danieldd]

Good ideas from those of you suggesting cutting a diagonal to allievate the overcut marks. Thanks!

 

[jcadwell]

A pergola that isn't bolted down could be called an arbor... Will it free stand?

 

[CNGsaves]

Wow, that is nice pergola . . . . but hardly has the "roof" area to warrant those giant concrete cores. Sounds like AL is going overboard after the last few years of tornados through area. Those pergola corner posts might be ONLY thing to survive F4 or F5 tornado !!

Good luck in your decision, and dealing with bureaucracy.

 

[B-Well]

I don't see why you want to cut a huge circle and use double the concrete compared to the required square.

I have a 10' fence with 8x8 posts, finished on each side. Only used 2 bags per hole and its still straight and strong after 4 years and 6+ hurricanes