Squaring the Slab
An engineering perspective on evaluating foundations for reuse.
Often after an event like a tornado, windstorm, water surge, or fire, the damage sustained to some property structures exceeds the value of it. In these cases, an engineer might be asked to determine if the remaining foundation system is suitable for reconstruction. The motivation for this is fairly straightforward: the property may have been underinsured and saving the foundation represents an effort to reduce the reconstruction cost, or there is a desire to get the structure rebuilt as quickly as possible. In the interest of time and money, an evaluation of the suitability of the foundation for reuse can be performed. There are three main questions for the engineer to answer: Is the slab damaged by the event? Are there any other factors, such as differential foundation movement, that would prevent the slab being reutilized? Are there any code considerations that prevent the slab from being reutilized?
Residences or buildings damaged by a storm or other natural catastrophe event can range from new construction to more than 100 years old, so building codes will vary dramatically. In most parts of the U.S., the governing building code is the International Building Code (IBC) or the International Residential Code (IRC). Each state, city, and county has local requirements in addition to the building code. For example, key items in the IBC 2009 include the following: the building official has the authority to condemn a building if the conditions are deemed dangerous; if the structure has substantial structural damage (50 percent or greater of the market value), then any repairs are considered substantial improvement; and when there is substantial structural damage, a professional engineer shall evaluate the building to determine whether the building will comply with the current provisions of the IBC once repairs are made.
There are exceptions for residential houses and seismic categories A, B, and C. For residential houses, if no dangerous conditions are created, then the house may be redesigned for the loads for which the house was originally designed. Also, if any structural deficiencies are observed during alteration, they must be corrected.
There also may be requirements of the local jurisdiction and the building official. Once the superstructure has been removed, then there is a consideration for reutilizing the existing concrete slab-on-grade. The governing authority may require verification by a licensed professional engineer that the slab is still serviceable and will be capable of supporting current code-specified loads. The term “serviceable” refers to the ability of the foundation to perform within certain code-prescribed parameters beyond its ability to accommodate loads and that the foundation has limitations on its allowable deflection, overall tilt, and slope. If these criteria have been violated, whether event related or not, then it cannot be said that the foundation has adequately performed and its reuse may not be possible, or at least not without modifications. The serviceability issues would be most prominent in areas of differential foundation movement due to expansive soils, or where post-construction settlement has occurred.
With respect to the foundation’s ability to accommodate loads, the evaluation can take at least two paths. The first path might include direct measurement of the actual capacity of the system and its components through sampling and laboratory testing (or even in situ load testing). For concrete, that could mean cutting cores and conducting laboratory compression tests. For reinforcing bars or post-tension tendons, the steel is carefully exposed for observation of its condition and samples might be taken for laboratory analysis.
The second path of the evaluation involves the investigator looking for indicators of conditions that might affect material properties. When evaluating the condition of a slab-on-grade foundation for reuse after a structure fire, for instance, there might be clues that give the investigator an idea of the temperature to which the materials were exposed. These indicators might be in the form of a crazed (small, closed cracks with random pattern) or a spalled concrete surface; discoloration of the concrete; large cracks and/or plastic deformation; charring (or lack of) of wood control joints; and melting or deformation of through-slab plumbing. These indicators are known to occur at specific temperatures that can be compared to the known temperatures at which the strength characteristics of the materials change.
Case Study #1: Conventionally Reinforced Concrete Slab-on-Grade
In May 2013, a devastating tornado hit Moore, Okla., destroying just over 1,000 homes. As the assessments progressed, the one reoccurring question from homeowners was whether the remaining foundations could be reused. Moore’s building department indicated that remaining foundations could be reused, provided that an evaluation was conducted by an engineer licensed by the state of Oklahoma. The engineer had to approve the foundation’s ability to accommodate current design loads and that serviceability requirements had been met.
The city municipality had adopted the IRC 2009 (with local addenda) and construction including this slab was required to be in accordance with this code. The key requirements were as follows: the site around the slab must drain down and away from the foundation at a minimum rate of six inches per 10 feet (five percent), and that the top of the slab must be at least six inches above the top of the surrounding grade. In this case, the slab was six inches above grade, but did not slope away from the foundation at the proper rate. This issue may have been remedied by altering the grading to comply with the requirement.
The city of Moore requires that foundations and turned-down portions of the slab be at least 18 inches below grade to protect the foundation from frost heave. In this case, the stem wall was less than 18 inches (Photo 1).
The slab must be inspected for evidence of structural distress, including large cracks (greater than 1/8 of an inch wide); cracks that have differential surface elevations (faulting); or evidence of previous repairs. In this example, some cracks were observed to be greater than 1/8 of an inch. One crack was observed to be greater than 1/8 of an inch and had differential surface elevations. A separation was also observed between the stem wall and slab-on-grade along two perimeter walls of the house and at the front steps. The separation is the result of the rotation of the stem wall relative to vertical. This means that the foundation it not performing properly (Photo 2).
The slab was inspected for performance-related issues that may have occurred during its service life. In addition to the structural distress mentioned above, a slab that tilts greater than two percent or has a deflection greater than l/360 (a building code-mandated maximum deflection where “l” is the length of the member) may not be performing its serviceability function. The slab surface’s relative elevation was measured using a laser construction level and the slope, deflection, and tilts were calculated. The measured tilt exceeded the allowable two percent and localized deflection exceeded the allowable l/360.
Therefore, the existing slab was found to be unsuitable for reconstruction due to the depth to the bottom of the existing stem wall, differential movement between the stem wall and slab-on-grade foundation, and the amount of deflection and tilt of the slab-on-grade. It was recommended that the slab-on-grade and associated under-slab utilities be removed from the site prior to construction. The observed conditions of the foundation, which ultimately precluded its reuse, were caused by long-term differential foundation movement and were unrelated to the tornado. If a structural component is not performing adequately prior to an event, then its reuse is not recommended.
Case Study #2: Post Tension Slab-on-Grade Foundation
After a fire destroyed the community center of a manufactured home park, the contractor retained an engineer to evaluate the condition of the post-tension slab-on-grade foundation. This decision was made to save on construction costs and get the building rebuilt as quickly as possible. The prevailing building code at the time was the IBC 2009, which, due to the extent of the damage, required an engineering evaluation for wind and seismic loads. This was done and the foundation was found to be adequately designed for the required loads at the location.
A visual assessment found that the surface of the slab did not show cracks greater than 1/8 of an inch wide, cracks with a differential surface, obvious displacement, excessive slope, tilt, or deflection. The foundation was found to be performing adequately from a serviceability standpoint and its ability to accommodate the loads from the previous structure.
What effect did the structure fire have in the foundation? According to the fire investigator’s report, the fire originated in the attic and was extinguished quickly, even though the structure itself was completely destroyed by the fire and suppression activities. Observations of wood expansion joints found them to be void of charring, through floor plumbing was not melted or deformed, the surface of the concrete was not spalled, crazed, or discolored (Photo 3). These observations indicated that the concrete probably did not reach temperatures above 300 degrees Fahrenheit, which is much less than the 1,000 degrees necessary to affect the compressive strength of the concrete and still less than the 600 degrees necessary to affect the post-tension tendons.
The foundation was found to be suitable for reuse with a similar structure due to its wind and seismic load capacity, serviceability requirements, evidence that it performed adequately with the previous structure, and lack of evidence that it had been exposed to temperatures that would have affected the strength characteristics of the concrete or steel.
The ability to reuse a foundation for construction has tremendous cost and schedule implications for the insured, contractor, and insurance company. The reuse of a foundation is allowed by building departments, provided that it can be shown that the foundation would be compliant with current codes for items such as frost-depth protections, drainage, and evidence of compliance with serviceability requirements. In many cases, due to the amount of damage to the structure, the code also may require that an engineer provides a structural analysis to assess the foundation’s capacity of current wind and seismic loads.