Foundations Design
By: Nexstar Staff
Updated: February 3, 2012
It's no secret that a foundation with structural damage can be a homeowner's worst nightmare. That's why it's important to make sure the design of the foundation for your new house meets the conditions specific to your parcel of land and to your section of the country. Our segment featured a look at an excavated home-site that gave us an "x-ray view" of the foundation before contractors poured the concrete.
Foundations serve as the interface between nature and man-made structures. All foundations are designed to minimize the effect nature can have on a man-made structure. Nature's biggest effect comes in the form of shifting soil. The ground will move for one of two reasons:
1) a high clay content. We shot our segment in a section of the country that has some of the worst soil in the US for causing foundation damage, because of an extremely high clay content.
2) a changing water content in the soil. Moisture changes that influence the foundation occur within relatively shallow soil depths. When soil gets wet, it expands. When it dries out, it contracts.
Soil shifts cause the two kinds of Major Foundation Problems - Upheaval & Settlement
For either problem, water is usually the culprit.
Slab Upheaval is more prevalent & more costly by far. Upheaval is caused by Moisture Increase (rain that does not drain away adequately from the foundation, plumbing leaks that pool water under the house, or subsurface water). Even a slow drip can doom a foundation to failure. As little as 6 drops of water an hour, accumulated over 12 months, can heave a slab foundation a full inch, which is usually sufficient to cause structural damage. Frost in winter can lead to foundation upheaval because water expands when it freezes.
The pressure of built-up water under a foundation can be immense. The pressure can easily be two or three times greater than the weight of the entire house. A water leak can bend re-bar, so even if you fix a leak you'll still have bent re-bar in your foundation.
Slab Settlement is caused by Moisture Loss. Soil shrinks as it dries out, so the foundation's support hollows out and disappears, causing the foundation to sink or settle. Fixing a settled foundation is easier than fixing a heaved foundation. The main goal is to raise settled sections of the foundation to meet original grade.
There are Three Basic Foundation Designs -
1) Floating Slabs - A horizontal slab basically comes in direct contact with the ground. Concrete piers are sometimes poured to provide additional support. Our segment featured a "floating slab" that was to be poured over piers that ran 30 feet into the ground. This design is popular in the warmer sections of the country, where a frost line is not a big issue.
"Floating Slab" is a bit of a misnomer for the foundation in our segment. This is more of a "Waffle" design. Piers for this foundation were poured weeks before the foundation concrete was to be poured. The piers provide a load-bearing support capacity in addition to the support of the fill soil that surrounds the foundation. As concrete fills the hole it's poured into, it will form a waffle design because it will create a grid-work of horizontal "beams" attached to the bottom of the horizontal slab. This single piece of concrete forms because of a single pouring of concrete (it's called a "monolithic pour"). The beams are formed by trenches dug in the dirt (about 10 or 12 inches wide and about 24 to 36 inches deep on average). These beams make the slab bite into the surrounding soil, so it resists side-to-side soil movement.
If the soil ever expands underneath the slab, this design allows it to "float" off the piers, then resettle once the cycle of expansion and contraction ends. The slab is "stiff" enough to float off the piers because of post-tension design.
1A) Post-tension Slabs - the foundation we featured will have a slab poured over a latticework of tension cables. About ten days after the concrete is poured, engineers will bring in machines to induce about 4500 pounds of pressure on the cables from all four sides of the slab. The cables resist this active tension, so they try to contract back to a state of rest. This creates an opposite compressive force on the concrete, directed towards the center of the slab. Since concrete is stronger when it's compressed, this design stiffens the slab so it will be stronger if soil expansion ever forces it to float off the piers. A simpler design (a floating slab reinforced with rebar, but without active tension applied) creates a passive situation, where tension within the slab will occur only after the ground moves the concrete and causes tension in the bent rebar.
2) Pier & Beam Foundations- Piers of concrete are poured deep enough into the ground to reach stable soil or bedrock (as deep as 35 feet or more). These piers are capped with a grid-work of beams, which support a hard flooring surface that sits above the ground surface. This floor bears the weight of the house built on top of it. With this design, soil can heave and shift without touching or affecting the floor. A pier & beam foundation design can cost twice as much to build as a floating slab foundation.
3) Cast or Poured Concrete Foundation - these are usually built in regions where a foundation must extend below the frost line to rest on soil that does not freeze and thaw (expand and contract). The depth of the frost line may determine if the foundation goes deep enough to allow enough room for a basement, or only a crawlspace. Walls built into the ground support the weight of a house. The walls rest on some type of concrete footing, which distributes the weight of the house to the surrounding ground.
Inspection Points - A professional engineer will inspect a foundation site and check these items:
- Proper number and location of tension cables. The cables should be at least 4 inches below the top of the form (the framework that will contain and shape the wet concrete when it's poured) and at least 6" from corners. Tension cables cannot have kinks.
- All copper wiring must be wrapped to prevent electrolysis (concrete can generate a chemical reaction if it touches copper)
- Beam trenches must be dug to proper depth and they must be cleaned with no roots showing.
- Pier tops must be cleaned (remember these were poured weeks before the slab) and the piers must be placed and spaced properly.
- All fixtures that will stick out of the foundation (plumbing, electric and gas lines, ventilation systems, etc.) must have sufficient clearance above the concrete, sufficient coverage by the concrete, and sufficient support in the concrete.
Common Misconceptions concerning damaged foundations-
Damaged Foundations don't always crack (They'll crack no more than 25% of the time). Structural elements of the house itself (like interior or exterior walls, ceilings, drywall, etc) will crack to signal structural shifting or damage.
You don't lift a house off its foundation to fix it. You lift the house and foundation off the ground together, then work on securing or filling in the ground underneath the foundation to avoid further damage (it's called "underpinning").
Preventive Foundation Maintenance - 4 Main Points:
Although your house may be designed by an architect or a designer, your foundation should definitely be designed by an engineer, to ensure all structural and safety concerns are addressed. Foundation problems can cost you tens of thousands of dollars, so proper design is the best prevention for future foundation headaches.
Make sure all landscaping drains water away from the foundation, so water won't pool or saturate the ground around or under a foundation. If water pools on one side of the foundation more than the other, the foundation could rise up on one end and cause major structural damage.
In dry periods (both winter & summer), wate
Foundations serve as the interface between nature and man-made structures. All foundations are designed to minimize the effect nature can have on a man-made structure. Nature's biggest effect comes in the form of shifting soil. The ground will move for one of two reasons:
1) a high clay content. We shot our segment in a section of the country that has some of the worst soil in the US for causing foundation damage, because of an extremely high clay content.
2) a changing water content in the soil. Moisture changes that influence the foundation occur within relatively shallow soil depths. When soil gets wet, it expands. When it dries out, it contracts.
Soil shifts cause the two kinds of Major Foundation Problems - Upheaval & Settlement
For either problem, water is usually the culprit.
Slab Upheaval is more prevalent & more costly by far. Upheaval is caused by Moisture Increase (rain that does not drain away adequately from the foundation, plumbing leaks that pool water under the house, or subsurface water). Even a slow drip can doom a foundation to failure. As little as 6 drops of water an hour, accumulated over 12 months, can heave a slab foundation a full inch, which is usually sufficient to cause structural damage. Frost in winter can lead to foundation upheaval because water expands when it freezes.
The pressure of built-up water under a foundation can be immense. The pressure can easily be two or three times greater than the weight of the entire house. A water leak can bend re-bar, so even if you fix a leak you'll still have bent re-bar in your foundation.
Slab Settlement is caused by Moisture Loss. Soil shrinks as it dries out, so the foundation's support hollows out and disappears, causing the foundation to sink or settle. Fixing a settled foundation is easier than fixing a heaved foundation. The main goal is to raise settled sections of the foundation to meet original grade.
There are Three Basic Foundation Designs -
1) Floating Slabs - A horizontal slab basically comes in direct contact with the ground. Concrete piers are sometimes poured to provide additional support. Our segment featured a "floating slab" that was to be poured over piers that ran 30 feet into the ground. This design is popular in the warmer sections of the country, where a frost line is not a big issue.
"Floating Slab" is a bit of a misnomer for the foundation in our segment. This is more of a "Waffle" design. Piers for this foundation were poured weeks before the foundation concrete was to be poured. The piers provide a load-bearing support capacity in addition to the support of the fill soil that surrounds the foundation. As concrete fills the hole it's poured into, it will form a waffle design because it will create a grid-work of horizontal "beams" attached to the bottom of the horizontal slab. This single piece of concrete forms because of a single pouring of concrete (it's called a "monolithic pour"). The beams are formed by trenches dug in the dirt (about 10 or 12 inches wide and about 24 to 36 inches deep on average). These beams make the slab bite into the surrounding soil, so it resists side-to-side soil movement.
If the soil ever expands underneath the slab, this design allows it to "float" off the piers, then resettle once the cycle of expansion and contraction ends. The slab is "stiff" enough to float off the piers because of post-tension design.
1A) Post-tension Slabs - the foundation we featured will have a slab poured over a latticework of tension cables. About ten days after the concrete is poured, engineers will bring in machines to induce about 4500 pounds of pressure on the cables from all four sides of the slab. The cables resist this active tension, so they try to contract back to a state of rest. This creates an opposite compressive force on the concrete, directed towards the center of the slab. Since concrete is stronger when it's compressed, this design stiffens the slab so it will be stronger if soil expansion ever forces it to float off the piers. A simpler design (a floating slab reinforced with rebar, but without active tension applied) creates a passive situation, where tension within the slab will occur only after the ground moves the concrete and causes tension in the bent rebar.
2) Pier & Beam Foundations- Piers of concrete are poured deep enough into the ground to reach stable soil or bedrock (as deep as 35 feet or more). These piers are capped with a grid-work of beams, which support a hard flooring surface that sits above the ground surface. This floor bears the weight of the house built on top of it. With this design, soil can heave and shift without touching or affecting the floor. A pier & beam foundation design can cost twice as much to build as a floating slab foundation.
3) Cast or Poured Concrete Foundation - these are usually built in regions where a foundation must extend below the frost line to rest on soil that does not freeze and thaw (expand and contract). The depth of the frost line may determine if the foundation goes deep enough to allow enough room for a basement, or only a crawlspace. Walls built into the ground support the weight of a house. The walls rest on some type of concrete footing, which distributes the weight of the house to the surrounding ground.
Inspection Points - A professional engineer will inspect a foundation site and check these items:
- Proper number and location of tension cables. The cables should be at least 4 inches below the top of the form (the framework that will contain and shape the wet concrete when it's poured) and at least 6" from corners. Tension cables cannot have kinks.
- All copper wiring must be wrapped to prevent electrolysis (concrete can generate a chemical reaction if it touches copper)
- Beam trenches must be dug to proper depth and they must be cleaned with no roots showing.
- Pier tops must be cleaned (remember these were poured weeks before the slab) and the piers must be placed and spaced properly.
- All fixtures that will stick out of the foundation (plumbing, electric and gas lines, ventilation systems, etc.) must have sufficient clearance above the concrete, sufficient coverage by the concrete, and sufficient support in the concrete.
Common Misconceptions concerning damaged foundations-
Damaged Foundations don't always crack (They'll crack no more than 25% of the time). Structural elements of the house itself (like interior or exterior walls, ceilings, drywall, etc) will crack to signal structural shifting or damage.
You don't lift a house off its foundation to fix it. You lift the house and foundation off the ground together, then work on securing or filling in the ground underneath the foundation to avoid further damage (it's called "underpinning").
Preventive Foundation Maintenance - 4 Main Points:
Although your house may be designed by an architect or a designer, your foundation should definitely be designed by an engineer, to ensure all structural and safety concerns are addressed. Foundation problems can cost you tens of thousands of dollars, so proper design is the best prevention for future foundation headaches.
Make sure all landscaping drains water away from the foundation, so water won't pool or saturate the ground around or under a foundation. If water pools on one side of the foundation more than the other, the foundation could rise up on one end and cause major structural damage.
In dry periods (both winter & summer), wate
