Concrete Pier Design
DEAR TIM: I've heard all kinds of things from friends about concrete foundation piers. To say the least, I'm confused. Some say you just dig a hole and fill it with concrete. Others say you need to be sure it's the right size and shape. Still others say you need to include steel in it. Can you tell me why one would use concrete piers and how you would install them? Are there any products that make it easy for a homeowner? Krista B., Wake Forest, NC
DEAR KRISTA: I'm sure you realize that you need to be careful about taking advice from well-intentioned friends. This is true even if one of them is in the construction industry. Each day I run into folks who have been building for decades using a particular method. It turns out, unfortunately, that they've been doing it wrong all those years.
Concrete piers are vertical structural elements that support the weight of a building or structure. I'm sure a structural engineer would give you a more accurate description, but for the layman, think of them as table legs. The legs of a table support all the weight of the table and things on the table. If the table legs are on a solid floor, the table doesn't sag or tilt.
The same is true for concrete foundation piers. The concrete piers act as the table legs. If they are designed correctly, and placed on solid ground or to a depth that the friction on the sides of the pier is greater than the load placed on the pier, the piers and the structure will not move. That's a good thing!
Some ground beneath houses or buildings is deep sediment where the bedrock can be hundreds of feet below the surface. In these cases, structural engineers rely on the friction between the poured concrete and the soil or sediment to create stability. For the average homeowner that's using a concrete pier to support a deck, room addition or even a home, this is not a big issue. If in doubt, just hire a structural engineer to assist in the design of the pier.
Most concrete piers used around the average home for a deck or other structure are usually no more than 24 or so inches in diameter at the base of the pier. As the pier rises up and out of the ground, it can reduce in size to as small as 10 inches in diameter. The bottom of the pier should always be below the frost line in the area.
If you're building in the northern hemisphere, piers on the north side of a building that might find themselves in the shade should probably go at least 6 or 8 inches deeper than the frost line. The frost can go deeper into the soil in ground that never receives direct warmth from the sun's rays in the winter months.
It's also very important, in my opinion, that the bottom of the pier be wider than the top. The wider base helps distribute the weight of the structure over more soil. This adds to the stability of the design.
A pier design that's wider at the bottom than the top also helps offset the force of the wind on the structure. The concrete pier acts like an anchor in a wall. It requires enormous upward force to pull a one-piece concrete pier out of the ground that has a wide base. If you use a proper hold-down anchor that's connected to the pier with a long anchor bolt embedded in the concrete, your shed, deck or structure should not blow over.
It used to be hard to create a concrete pier that had this shape. You had to first pour the wide bottom in one concrete pour, carefully place reinforcing steel in the base and wait a day. Then you had to install a heavy-duty cardboard tube that extended up towards the surface. You then filled this with concrete. Believe me, it was not easy.
But now you can purchase a plastic concrete pier form that has the correct shape, it comes with all the needed reinforcing steel pre-cut and pre-bent! The steel fits into slots and holes in the concrete form so the concrete flows around it perfectly. These forms are easy to snap together with no tools, and they're easy to level on compacted soil. You pour all the concrete in minutes saving at least one day in the building process.
You can purchase precast concrete piers for your project, but the trouble is they are heavy - I mean really heavy. You'll need a backhoe or small crane to put them into position. It's possible that you can devise a method to move them by hand or with a group of people, but don't count on it.
Don't underestimate the importance of the reinforcing steel in the concrete piers. It's often overlooked. Steel adds enormous strength to concrete. Be sure the soil under the pier is stable and compacted before you place the concrete pier form and pour the concrete.
You can watch two informative videos that show precast concrete deck piers and the innovative plastic deck pier form at www.AsktheBuilder.com. Just type "building a deck pier video" or "precast concrete deck pier video" into the AsktheBuilder.com search engine.