F-12583
A-1 ENGINEERING, LLC

  A-1 ENGINEERING, LLC                      

Structural + Civil Engineering Consultants                    Serving San Antonio and Austin

Pier and Beam Foundations - Design and Movement

 

Before the use of the conventional slab on grade or floating slabs, the pier and beam foundation was the typical foundation constructed to support a house. Before 1950, the pier and beam foundation was the most economical and practical foundation for homes. With the advancements in concrete design, contractors and developers started to construct floating slabs as opposed to pier and beam foundations.The most important thing to remember about pier and beam foundations is that if you live in an area with expansive soils (e.g. clays), which is most of Texas, the building code does not allow you to use a pier and beam foundation unless the piers are deep.

 

Table of Contents

Performance of a Pier and Beam Foundation

To understand how a pier and beam foundation performs it is important to discuss how the foundation is constructed and what level of performance to expect from this foundation. A true pier and beam foundation consists of pedestals (piers) embedded into the ground and spaced at an average of 5 to 6’-0” on center with a perimeter concrete wall that acts as a beam. The idea was that the perimeter wall, or beam, enclosed the crawl space (air space) created by the pedestals that supported the wood beams that make up the floor and wall framing. The pier and beam foundation is commonly misunderstood to signify a pier supporting wood beams, hence “pier and beam”. The beam of a true pier and beam foundation is the concrete or masonry wall that is along the perimeter of the house.

The performance of the pier and beam foundation is dependent on two major factors. The first is the structural integrity of the wood pier. Since wood decays with moisture, it is common practice to use cedar posts for the piers. The cedar is a naturally preserved wood that is able to resist the decay due to moisture for a much longer period of time than other wood species. We recently worked on leveling project for a 100 year old home that was supported on cedar posts and just recently had them replaced.

Using a Pier and Beam Foundation

Economics and architecture were the driving forces to constructing a slab on grade foundation as opposed to a pier and beam foundation. Prior to 1950, the majority of homes used wood as an exterior finish and for flooring. Since the framing of wood is considered flexible, the exterior finish and the floors seldom cracked or broke. The performance of the pier and beam foundation was incidentally compatible to the finish.

We are frequently asked to design pier and beam foundations in San Antonio, Austin, Houston, and Dallas. Our clients are surprised to know that pier and beam foundations are not allowed by the building code in expansive soils, such as clays. Most of the aforementioned cities have expansive clays as the type of soil and so, unless you use deep piers (usually about 20ft deep), we are not allowed to engineer a foundation using pier and beam. 

Common Problems of a Pier and Beam Foundation

Most pier and beam foundations that have leveling issues are a result of the piers decaying and settling as a result of less wood mass. As the wood decays, the decayed area of the wood become soft and creates a void between the soils and solid internal wood. As a result, the pier settles and so does any wood framing that it supports.

Moisture (water) is the second and most significant cause to foundation issues with a pier and beam foundation. Besides moisture decaying the wood pedestals, the soils that support the wood pier tend to swell up when the amount of water that they are holding increases. Imagine a sponge has dried out and become stiff. As you add water to the sponge, the sponge begins to swell up and become more flexible. Soils tend to act in the same fashion.

Causes of Problems Affecting Pier and Beam Foundations

This picture exhibits a common issue with pier and beam foundation constructed with timber or cedar. Yes, the building and the foundation are actually listing over. The main cause of the listing is lateral instability (side to side stability) and deterioration and decay of the wood pier. This particular building did not have significant cracks in the walls or ceiling because the foundation was uniformly failing all at once. A structural engineer is needed in this case to design a new pier that can provide adequate lateral stability to the foundation AND provide better durability. Also, the debris below the building helps harbor rodents and allows water to accumulate near and around the foundation. 

Most pier and beam foundations that have leveling issues are a result of the piers decaying and settling as a result of less wood mass. As the wood decays, the decayed area of the wood become soft and creates a void between the soils and solid internal wood. As a result, the pier settles and so does any wood framing that it supports.

Moisture (water) is the second and most significant cause to foundation issues with a pier and beam foundation. Besides moisture decaying the wood pedestals, the soils that support the wood pier tend to swell up when the amount of water that they are holding increases. Imagine a sponge has dried out and become stiff. As you add water to the sponge, the sponge begins to swell up and become more flexible. Soils tend to act in the same fashion.  As water is added to the soils the soils begin to swell up and push everything that it supports up as well; to include a pier and beam foundation. It is very common to find a pier and beam foundation that has one corner of the building much lower (drier soils) in elevation (or height) than the area near a water faucet, plumbing or water drain is broken and is leaking water into the soils. Broken roof gutters also add a substantial amount of water to the soils in concentrated areas along the foundation. With water decaying cedar posts and swelling the soils in concentrated areas, a floor of a pier and beam foundation typically is much more uneven than a conventional concrete slab on grade. For this reason, stucco and tile finishes that are very sensitive to foundation movement should be avoided if the owner cannot live with the cracks that will result from the performance of a pier and beam foundation.

Moisture problems, which causes swelling and shrinking to occur in the soil, is the primary reason the building code does not allow for a traditional pier and beam foundation.

Hiring a Knowledgeable Structural Engineer

Designing and inspecting a pier and beam foundation as well as recognizing issues and problems with existing pier and beams foundations, can only be successfully achieved through the knowledge and experience of a licensed Structural Engineer. If the building code allows us to use a pier and beam foundation for your property, we'll be more than glad to engineer it for you. If the building code tells us not to use a pier and beam foundation, then we'll help you engineer a foundation which meets the building code. Most people are surprised to hear that most times you cannot use a pier and beam foundation because they've talked to other engineers who say they can; the problem in this situation may be that those engineers are not qualified structural engineers.

A good tool to know whether you're about to hire the correct Structural Engineer is to check with the Texas Board of Professional Engineers. You can input the company name or the structural engineer doing the work, to check whether that company is actually licensed to practice structural engineering; you will be surprised to see that many engineers out there claiming to be structural engineers in the residential industry are in fact just civil engineers.

The most common exception to using a pier and beam foundation in expansive clays is if there's an existing foundation and you're doing repairs to it or if you're doing an addition to an existing pier and beam foundation.   

We hope this article was very helpful to you. If you have any more questions or need help on your next project in San Antonio, Austin, and across Texas (including New Braunfels, Boerne, Poteet, San Marcos, etc,) please do not hesitate to contact us

And for more on where do you need a Structural Engineer in San Antonio, just click Structural Engineer San Antonio.

For more info please visit our Structural Engineer Austin page or Structural Engineer San Antonio Homepage.