Piers Can Support Anything From a Deck to a House
Bird's eye view
For use with unusual soils or steep slopes
Concrete piers foundations save time and money because they don’t require extensive excavation or a lot of concrete. Simple pier foundations resemble those used for residential decks. If the site has unusual soils or is steeply sloped, or the house has unusually high loads, an engineered foundation is required.
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Cardboard and plastic footing forms
For a simple foundation, a concrete footing can be poured by placing concrete directly into the bottom of the hole. When soil conditions require a form, many builders choose round plastic footing forms like the Bigfoot system (www.bigfootsystems.com), designed to accommodate different diameters of cardboard Sonotubes.
The Footing Tube (shown) integrates the footing form and pier form into a one-piece system.
The Footing Tube
Discussing foundation options
Builder Mike Martuscello (MGM Construction) talks with and structural engineer Bruce King and Peter Yost of GreenBuildingAdvisor.com about integrated designBuilding design in which different components of design, such as the building envelope, window placement and glazings, and mechanical systems are considered together. High-performance buildings and renovations can be created cost-effectively using integrated design, since higher costs one place can often be paid for through savings elsewhere, for example by improving the performance of the building envelope, the heating and cooling systems can be downsized, or even eliminated. and weighing foundation choices for a site in a seismic region. Seismic considerations narrowed the choices. But the owners of this house wanted the house to go beyond seismic-resistance. They wanted to house to be capable of providing refuge for many in such an event. It should be Earthquake-proof and provide passive survivability.
One choice was a thick reinforced "mat slab" that would allow the house to "float" on the surface. Another was a pier and grade-beam foundation that would anchor the house to it. Both would work, but the environmental consequences of each type was surprising.
The floating slab seemed like the less intensive route, but when all things were factored in, the pier and grade-beam foundation was greener, and a lot less expensive.
Frost-proofing a pier foundation
Concrete-column construction can cause problems in cold areas due to freeze-thaw cycles. But this system can be great if you do it right. As frozen soil heaves in the winter, it can grab the cardboard column form and cause it to lift or to shift slightly. When the ground settles back, the column can be left tilted. Even if you peel off the cardboard, frost is good at grabbing concrete.
To combat frost, do two things
Builder Lynn Hayward of Northport, Maine recommends a couple of strategies to minimize the effects of frost:
1. Control frost action as much as you can;
2. Give the frost something to grab so that it leaves the column alone.
As shown in the drawing, you can keep frost action low by backfilling around a column with crushed stone and gravel; this process helps to drain water away from the column. You also can buffer the ground temperature with foam insulation. Columns on the south side of a foundation experience more freeze/thaw cycles than those on the north side because the sun beats on them during the day. A layer of insulation will keep the sun’s heat out of the ground.
To keep frost from grabbing the column, use a sleeve. In the old days, Hayward used schedule-20 sewer pipe as a sleeve. The frost would grab the sleeve, but the column itself would not move.
He's also wrapped the column with plastic sheeting up to ground level. The plastic acted as a sleeve so that the frost couldn’t grab.
A new product that incorporates the footing, pier, and frost-proof sleeve all in one does a good job of accomplishing Hayward's two goals. It’s called the Footing Tube. Rather than cardboard, the tube is made of high-density plastic, so frost can’t bond to it. The footing base is built in. All you need to do is set the form in a hole, cut off the top to the desired height, backfill, and pour concrete.
Footings a minimum of 12 inches below grade
According to the IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code., piers — like other foundation types — should have footings placed a minimum of 12 inches below undisturbed grade (403.1.4) and protected from frost (403.1.4.1). The bearing capacity of soils underneath a pier should be based on a soils test or IRC Table 401.4.1 which identifies the load-bearing capacity of five soil types from clay (1500 psf) to crystalline bedrock (12,000 psf).
ABOUT PIER FOUNDATIONS
Disturbing as little soil as possible
Pier foundations are unlike more conventional concrete footings and walls in that they support structural loads at a number of distinct points, not continuously. Pier foundations can be as simple as concrete-filled cardboard tubes dropped into hand-dug holes. But more complex foundations incorporating very deep piers or helical screws can support much greater loads.
To some green builders, one advantage of pier foundations is that they cause minimal disruption to the soil environment. Excavation can be performed with a shovel, and existing roots and soil organisms remain mostly undisturbed. At the end of the building's useful life, the site will be easier to restore to a natural state than a site with a full basement.
Design starts in the soil
The number and size of the piers depends on how much weight the underlying soil can hold. The standard design load in the International Residential Code assumes a 40-pound live load; dead load is commonly calculated at between 10 and 15 pounds per square foot, depending on the materials used for construction.
Hard-packed gravel may be able to handle as much as 3,000 psf while soil with high clay content or lots of organic material will support much less weight. Recommendations of the IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. are summarized in a table (referenced in the code as 401.4.1). In situations where the bearing capacity of the soil is questionable, it may be necessary to call in a soils engineer for help.
MORE ABOUT PIER FOUNDATIONS
The most basic foundation consists of a series of concrete posts bearing on concrete footings installed below the frost line.
Pier foundations, however, can be far more complex. On steep or unstable sites, pier and grade-beam foundations are an option when conventional foundations won’t work. These consist of a series of concrete piers, extending up to 20 ft. into the ground, connected by horizontal concrete beams. These foundations require careful engineering, along with a good deal of reinforcing steel and concrete. But they allow building on sites where conventional techniques would not.
Helical screws, another type of specialized pier foundation, can support foundations in soils with low load-bearing capacity. They consist of a steel shaft with helical plates welded onto it that are turned into the ground with hydraulic motors. They can be used in tension (as in a telephone pole guy wire support) or in compression, as they would in a foundation. They can be used in new construction on lots with poor soil, or to save foundations on existing houses that are sinking into the ground.
Building on Piers, Fine Homebuilding
- Dan Thornton/Fine Homebuilding #180
- Chris Ermides/Fine Homebuilding #180
- Dan Thornton/Fine Homebuilding #169
May 13, 2010 4:54 AM ET
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