Bell Pier Concrete Calculator

Now you begin digging out the hole for a deck post and something feel wrong with the dirt. No longer is it solid sand or firm clay. Now its loose gravel or even more frightening, soft wet soil that slithers through your hands.

Thats when most DIYers freak. They believe they must call in structural engineer to confirm if that post will sink this coming winter. Usually, it’s not as dramatic, but we still has some math involved, beyond simply calculating the volume of a cylinder. Enter the bell pier, which changes everything. It also lets you avoid any guesswork thanks to the calculator above.

How to Stop Your Deck Posts from Sinking

A belled footing simply expands the footprint of the pier so it spread out the weight. Picture being in deep snow and having to stand on one foot. You’ll sink. Stand on a wide snowshoe instead. You’ll float. That’s what the bell does. It gives you greater bearing area while not needing to excavate a huge trench all over your yard.

Here’s the catch. It’s tricky geometry. You’re working with a straight shaft transitioning down into a cone-shaped base that flares out wider at the bottom. Figuring the volume of that flare section by hand is tedious and prone to errors. Plug in your shaft and bell dimension, and the calculator above do the math for you. It saves you the trouble of fumbling around with cubic conversions and pi.

For example, most folks spend all their time calculating how many yards of concrete they will purchase. They don’t realize the ground they plan to set the post in may not be capable of supporting the weight. The tool makes them consider how much weight the soil can hold. Your dense gravel or sand may be able to give you a couple thousand pounds per square foot of support. You soft clay doesn’t give you nearly as much.

The tool then compares your vertical service load against the allowable soil bearing capacity. If the demand is higher than the allowance it flags it. That’s what most homeowners miss. They pour all the concrete and foundation still settles because the underlying soil wasn’t strong enough to start with.

The other unseen force? Uplift. Your structure isn’t being pushed over by wind only. It wants to pull upward. A heavy concrete pier resists that. However, you also have to consider weight of the surrounding soil cone. To simulate that resistance, the estimator adds in a reserve factor. It’s no substitute for a geotechnical report, but it provides a sanity check. If your uplift reserve is razor thin, you may want to deepen the shaft, or perhaps widen the bell. It’s a little change on screen but it save a big repair bill down the road.

Another silent variable is rebar displacement. Concrete has space for steel bars. Four #4 rebar may displace a noticeable amount of concrete in the narrow shaft. The calculator deducts that volume so you do not need to order extra material. Sounds trivial, yet it can make a difference if you’re mixing your bags by hand or ordering a tiny ready-mix load. You don’t want an overage of wet slurry to carry away and want a clean form instead.

Don’t ignore waste. Holes can close up. Soil moves around. Concrete forms can leak. To be safe, add an estimated 10 percent factor for waste. Better to err on the side of having some excess than to find yourself short when you go to pour.

For bigger projects like this porch and deck, look at the table above (a sample of many included on the page). They’re average figures; tweak them to your site conditions.

Your footings stays in place. Then you realize that concrete isn’t merely gray stuff in a hole. It’s the counterweight to weather that keeps everything level long past a hard rain. Once you get the formula, uplift, volume, bearing, you’ll never think of concrete the same way again. You should of used the tool earlier.