Shallow Foundation Concrete Piers

1. Introduction

Concrete piers generally serve one or both of two purposes: 1) to support the downward weight (gravity load) of something, like a deck or balcony, and 2) to keep something from tipping over, like a sign or a pole. Piers that support only gravity loads will act as `compression' members. In the case of a pier for a deck, some post typically comes down and is connected to the top of the pier, and then the pier takes the load on down to the bearing soil below. Building codes generally require some minimum penetration into the ground; and where the ground is expected to freeze, this minimum depth is the prevailing `frost depth'. In the case of a pier being used to keep something from tipping over, say a sign, the pier must be big enough and penetrated deep enough so that the reactive pressures from the sides of the pier against the soil don't fail the soil (on the sides of the pier) and allow excessive tip. Let's look at both of these conditions more closely.

2. Piers for Gravity Loads

Piers supporting gravity loads generally support a post or column that is delivering a downward vertical gravity load to the pier. The post or column should be attached with code-approved hardware, which will also generally provide some `anchoring', so that the post isn't knocked sideways off the top of the pier. For those with an engineering or mechanics background, this connection is basically a `pinned' connection. If the post or column is wood, then the connection hardware must provide protection against trapped moisture or moisture wicked through the concrete, and the top of the pier some minimum height above soil or finish grade, and/or the wood must be naturally resistant to decay. Obviously the cross section dimension of the pier (diameter if it is round) must be sufficient to accommodate the post / column and hardware (as well as concrete `cover' and / or edge distance requirements). In many applications, such as residential or light commercial construction, the hardware is pre-manufactured and available (or order-able) at the local building supply store.

The pier itself may be either plain or reinforced concrete. Plain concrete is allowed if the pier is `stout' (my word). A more precise term is `pedestal'. A pedestal is a concrete compression member in which the length (height) is not more than three times the least cross section dimension. So, if it is a round pier, it is (also) considered a pedestal if ... in equation form ...

(Pedestal if) ... L ≤ 3 D,

where,

... L is the height of the pier, from physical bottom to physical top (regardless of ground line), and

... D is the pier diameter (or least cross section dimension, say, if rectangular).

Example Pedestal Height

Local code requires that untreated wood be at least 8 in. above grade. If the frost depth is 30 inches, and a footing of 8 in. thickness is required, let's find the required pier length (height) and the minimum dimension if it is to be designed as a pedestal.

... 8 in. (above grade clearance) + 30 in. (local frost depth) = 8 in. (footing thickness) + L (pier height) ...

... gives L = 30 in. tall pier required.

To be a `pedestal', L ≤ 3 D; or D ≥ h / 3 = 30 in. / 3 = 10 in.

So, to be considered a `pedestal' the pier must be at least 10 in. diameter (or 10 in. across at the narrowest point).

So, around here, where the frost depth is 30 in., I generally specify pier diameters of not less than 10 inches.

Since it is a pedestal it need not be reinforced. However, I generally specify at least one vertical reinforcing bar, either # 4 or # 5, to `hold things together'. Also, the building official might like to see at least some reinforcement.

Pier Footings

Piers that carry significant gravity loads also generally require footings. Stated differently, unless the pier is founded on something really strong, like bedrock, the downward load will punch the pier through the soil. We thus need to spread these loads out with `spread footings'. (Words have meaning.)

Steps in footing design are:

1. Design the footing wide enough to not fail the soil ... in equation form,

   fp ≤ Fp,

   where,

   fp = the applied soil pressure = P / A

   where

   P = the total downward load, including the weight of the pier and footing, and

   A = the (`footprint') area of the footing, and

   Fp = the allowable soil pressure, prescribed by per soil type in the Code, or provided by a Soils Engineer / Report.

2. Make the footing thick enough so it doesn't shear off ... in equation form,

   x ≤ h

   where,

   x = the projection of the footing out past the face of the pier, and

   h = footing thickness.

3. Provide reinforcement, if necessary, to keep it from breaking in bending. For modest footings generally if the footing is thicker than the projection past the face of the pier, it won't break in bending, either.
4. Provide any other reinforcement needed. Generally even though we could demonstrate by calculation that a plain footing is sufficient, local building officials like seeing at least some rebar in the footing.
5. Make sure the pier doesn't punch through the footing. Generally this is not an issue unless we somehow have a really thin footing.

For larger loads it may be impractical to size a footing thick enough so that x ≤ h. In that case we can do some engineering calculations to determine the minimum thickness to keep it from shearing, and then require enough reinforcement so that it also doesn't break (at the face of the pier).

Footing Example

Consider a 10 in. diameter pier 30 in. long carrying a gravity load of 5750 lb (including pier but excluding footing) to be supported by Silty Clay soil. Determine an appropriate size footing.

Let's try 30 in. x 30 in. x 10 in. footing ...

1) Make sure the footing is wide enough to not fail the soil ...

The footing itself will weigh ... 30/12 ft x 30/12 ft x 10/12 ft x 150 pcf = 781 lb.

Add this to the 5750 gives ... 6531 lb.

The footprint area of the footing is 30/12 x 30/12 = 6.25 sq ft.

The applied soil pressure is fp = P/A = 6531 lb / 6.25 sq. ft. = 1045 psf.

The prescriptive allowable soil pressure for Silty-Clay is 1500 psf (2006 International Residential Code, Table R401.4.1).

Is fp = 1045 psf ≤ Fp = 1500 psf? ... Yes, way yes so let's try a smaller footing.

Try 24 x 24 x 8 ...

Weight = 24/12 x 24/12 x 8/12 x 150 = 400 lb.

Total P = 5750 + 400 = 6150 lb.

A = 2 x 2 = 4 sq ft

... fp = 6150 / 4 = 1538 psf. Ughhh ... not quite.

Try 26 x 26 x 8 ...

Weight = 26/12 x 26/12 x 8/12 x 150 = 469 lb.

P = 5750 + 469 = 6219 lb

... fp = 6219 / (26/12 x 26/12) = 1325 psf ...

Is fp = 1325 ≤ 1500 = Fp? ... Yes, Good!

2. Now let's make sure it doesn't shear ...

... x = ( 26 - 10 ) / 2 = 8 in.

Is x 8 in. ≤ h = 8 in.? ... Yes, perfect.

3. Since it is thicker than it projects past the face of the pier the footing is not likely to break in bending, either, but let's still add some steel ... say 2 - # 4 each way, evenly spaced, placed 3 in. clear from the bottom of the footing. And because it is thicker than it projects, the pier can't punch through the footing either.

Concluding Comments

If we have a pier that does not satisfy the `pedestal' requirement it must then be designed as a reinforced concrete compression member, which gets `complicated'. Let's not go there for now. Note that I have still required a vertical bar in the pier (pedestal). This bar will often be embedded (or perhaps spliced) into the footing (and tied to the footing steel), and may go high enough in the pedestal to assist casting the connection hardware at the top. In some cases a big conical footing is cast (using a conical form) that spreads the load out via a concrete cone that can be cast integrally with the pier / pedestal. It may or may not have reinforcement.

Next we will look at the laterally loaded pier (the pier that needs to be designed so it doesn't `tip over') ... (here).

References

Building Code Requirements for Structural Concrete, ACI 318, American Concrete Institute, P.O. Box 9094, Farmington hills, Michigan, 48333.

International Residential Code, International Code Council, 4051 West Flossmoor Road, Country Club Hills, IL 60478.

Residential Construction Guide, Community Development, City of Moscow, 221 East 2^nd Street, Moscow, Idaho 83843.