The use of Post-Tensioned cables seems to be the preferred choice of reinforcement for slab-on-grade foundations, due to cost savings and design advantages.
Conventional Rebar reinforcing is used primarily as a builder’s preference. It is also used when building an addition to an existing foundation, or when there are deep or multiple finished floor elevation changes throughout the slab.
No, you will need to contact the company who provided your Post-Tension cables.
We do NOT warranty the slab, there are several warranty companies that you or your builder can contact that can provide a warranty for you. While we do stand by our design, there are many other factors that affect the design and we have little to no control over the materials and workmanship used to construct our design.
We recommend having and Original Construction Elevation (OCE) survey done on your foundation after your foundation has been poured and before carpet and flooring has been installed. If you have problems in the future this helps determine if and where a problem has occurred.
We do not do forensic engineering, if you need a recommendation we can send you information on Engineers in the area who do that.
We do not provide the type of home inspection you need for selling your home.
Professional Engineers usually make judgments concerning whether cracks and other forms of damage or distress are due to foundation movement using their knowledge of structural mechanics, the engineering characteristics of building materials and the experience gained from inspecting hundreds and sometimes thousands of homes.
There are two points that need to be recognized at the onset. If the damage is minor, such as a few hairline cracks, it is difficult to say definitively what caused the damage. On the other hand, if the damage is severe, such as ¼ inch wide cracks in the drywall, the damage is almost certainly due to structural distortion. The homes that are difficult to evaluate are those where the damage lays somewhere in between minor and severe. The following are some rules of thumb published by the Institution of Civil Engineers and the Building Research Department in the United Kingdom.
- Foundation movement tends to produce a few large cracks, usually wider than 1/16th inch, rather than a lot of small cracks.
- Cracks in brick veneer due to foundation movement will normally extend from the top of the wall to the bottom of the wall and be tapered. By tapered, I mean that the crack will be wider either at the top or at the bottom. A crack that is the same width at the top and bottom is more likely to be due to thermal stresses than to foundation movement.
- Considered as a whole, the pattern (meaning the location and taper) of the cracking should be consistent with a possible known mode of foundation distortion. For instance, if a brick veneer wall shows cracks that were close to each other and one was wide at the top while the other was narrow at the top, it would usually be unreasonable to consider both cracks to be due to foundation movement since they are not both consistent with a known mode of foundation distortion.
- Foundation movement usually results in cracks in drywall and brick veneer at weak points such as the corners of windows and doors.
- Cracks that show up after long periods of dry weather and tend to close when the weather turns wetter are usually due to foundation movement from expansive soils.
- Foundation movement can distort door openings causing doors and windows to stick and bind. Wallpaper can exhibit rucking at the inside corners of walls and at the intersection of walls and ceilings.
- In some situations, finish floors can become perceptively out-of-level. Unfortunately, floors are frequently constructed out-of-level and foundations that undergo a normal range of movement can also become more or less out-of-level over time. Relating floor levelness to foundation movement is always based to a great degree on the engineering judgment of the inspecting engineer; that judgment is always subjective and interpretive.
- Brick courses, countertops and sills can become noticeably out-of-level due to foundation distortion. These items are normally constructed to a tighter level of tolerance than are floors.
A foundation inspection consists of observing the interior and exterior of the house for signs of structural distortion that might be related to foundation movement. An engineering evaluation of the performance of the foundation consists of taking the data from the inspection and using it, in conjunction with the engineer’s knowledge of structural mechanics, the structural behavior of houses (including the structural behavior of brick veneer walls, stucco walls, drywall walls and door frames) and the engineering properties of building materials to make engineering judgments about the performance of the foundation.
Finish floor elevation profiles are arguably a somewhat more sophisticated way to use finish floor elevation measurements. In this approach, a series of elevation measurements is made across the length or width of a foundation. Each series of measurements are then used to create a profile of the finish floor surface.
- Elevation profiles, like elevation surveys, are always approximations due to the fact that slab-on-ground foundations are not constructed flat.
- Elevation profiles can provide an engineer insight into the distorted shape of a section of the foundation at the time of the inspection if the assumption of an as-constructed foundation surface being flat can be verified.
- If the assumption of an as-constructed foundation surface being flat cannot be verified, then an elevation profile can be misleading.
- Elevation profiles are very easy to interpret in terms of the structural engineering concept of “deflection ratio.” This is a very well accepted concept in the structural engineering profession.
- Normally, if a finish floor elevation profile can be interpreted as not indicating deflection in excess of the span of the profile (which would be the length or the width of the foundation) divided by 360, then the foundation repair would not be recommended unless the foundation damage is severe.
- The surface geometry of the finish floor is not structurally significant in itself, but can be considered to be an approximation of what is called the deflection curve of the foundation. Structural engineers consider the deflection curve to be a very useful piece of information since it is a graphical representation of the distortion of the foundation. Thus, a finish floor elevation profile can be very useful so long as it can be considered to be a reliable approximation to the deflection curve.
Both the finish floor elevation survey and the finish floor profile approach can be misleading if the slab surface was not constructed level and/or flat. Published construction tolerances for slab-on-ground foundations allow for two points on the surface of the slab 10 feet apart to have an elevation difference of as much as 1.25 inches. Some engineers who use finish floor elevation surveys will recommend foundation repair if the survey shows any portion of the floor surface with a slope in excess of 1 inch in 10 feet. This can result in a foundation repair recommendation that is clearly unnecessary. Finish floor elevation profiles can also result in unreliable repair recommendations if the foundation surface was not constructed as flat as normal.
It has been pointed out that the finish floor elevation survey approach can be misleading if the slab surface was not originally constructed level and flat. One way to verify if the foundation was constructed level is to use a four-foot level to check the levelness of countertops, window stools, and other trim that is normally constructed to a much tighter levelness tolerance than slab-on-ground foundations. By using a four-foot level in this way, an experienced engineer can usually make a reliable judgment as to how level the foundation was originally constructed.
For instance, if the floor shows on obvious slope but the window stool above the sloped floor is level, it is reasonable to conclude that the floor un-levelness was probably due to the original construction, not foundation movement.
Structural safety problems that result from foundation movement are clearly intolerable. But beyond that, the correct answer is that there is no one answer that is correct for everyone. Different people have a different tolerance for cracks in wall coverings and sticking doors. It is important when deciding your own tolerance levels to be realistic. Demanding that a house show no sign of foundation movement and never will show damage that could be attributed to foundation movement is just not realistic. Many problems can be resolved without costly repair methods such as underpinning, but the ultimate decision as to how much damage you are willing to tolerate is one only you can make.
There is no answer to this question that is universally accepted. The Post-Tensioning Institute has published a peer-reviewed paper in which it is stated that a diagnosis of excessive expansive soil movement cannot be made unless the slab surface is out of level substantially in excess of published American Concrete Institute (ACI) standardized construction levelness tolerances for Slab-on-ground foundation construction. The ACI publishes several different construction tolerances but recommends the use of what are called F-numbers. The F-number system allows the elevation of two points 10 feet apart to be different by as much as 1.25 inches. If a foundation were to deflect L/360 in both directions (which most engineers would consider acceptable), the resulting slope (adding an as-constructed slope to the slope caused by foundation distortion) could result in a foundation surface slope of 1.65 inches or more over 10 feet. A slope greater than 1% (1.2 inches over 10 feet) is noticeable by most people. Thus, a noticeable floor slope may or may not indicate excessive foundation movement. You should also understand that the as-constructed slope and the slope due to foundation movement may not add together; the foundation may distort in a way that makes the slab surface more level than it was constructed.
Some engineers prefer to judge the levelness of the foundation due to distortion by looking not at the levelness of the slab surface, but at the levelness of first floor countertops and sills since these elements are normally constructed to much tighter levelness tolerances than slab-on-ground foundation surface tolerances. If the countertops and sills are reasonably level within normal construction tolerances, then it is reasonable that any floor out-of-levelness is probably due to original construction.
The Texas Section of the American Society of Civil Engineers (TSASCE) has recognized different levels of residential foundation evaluations including a Level A and a Level B evaluation. The Level A evaluation is usually referred to as a visual evaluation or a report of “First Impressions.” We prefer to describe it as a visible damage evaluation. The Level B evaluation is built on a Level A evaluation but also includes finish floor elevation measurements. Some engineers claim to be able to use elevation measurements to confirm or deny whether the observed damage from a Level A evaluation is due to foundation movement; but, recently published studies have concluded that this belief is founded more on faith than any verifiable empirical data.
One way engineers use finish floor elevation measurements is to create a sketch of the foundation showing contour lines. Every point on a contour line has the same elevation. The contour lines show the approximate shape of the foundation surface. The foundation surface can be considered an approximation of what engineers call the deflected shape of the foundation. Here are several important points about elevation surveys.
- Elevation surveys are always approximations for a number of reasons, such as the fact that slab-on-ground foundations are not constructed flat and level.
- Elevation surveys can provide an engineer a valuable insight into the mode and degree of distortion a foundation exhibits at the time of the inspection, if the assumption that the original as-constructed foundation surface being flat and level can be verified.
- If the assumption of an as-constructed foundation surface being flat and level cannot be verified, then an elevation survey can be very misleading.
- Elevation surveys are often used to calculate maximum floor slopes. Unfortunately, foundation deflection is not related mathematically to slope. It is not mathematically possible to determine if a foundation has or has not distorted excessively on the basis of floor slope
Yes, we can, in fact we have designed many basements. The most common form of a basements structure in Texas is often referred to as a “Walk-out” basement, due to the fact that they are constructed on a sloped hillside and the wall (generally the back wall) is “at grade” with doors and windows that open to the outside.
Yes, and it can be designed for either above ground or below ground, and to withstand an F5 tornado.