This article pertains to the design, analysis, and code provisions pertaining to shallow foundations only (foundations whose depth does not exceed their width).
1. Why does my structure need a foundation?
The purpose of the foundation is to adequately transfer the loads from the superstructure to the soil or subgrade below.
Without the correctly sized foundation, the structure is susceptible to adverse effects such as settling, overturning, sliding and uplift which may lead to strength and serviceability issues such as member failure, cracks, excessive deflection, etc.
2. How do I decide what kind of foundation I need for my project?
The kind of foundation required for a structure depends on a few different factors such as
i. Loads on the structure.
ii. Location of the structure.
iii. Condition of the soil that will support the structure.
iv. Presence and location of other structures (both in-ground and above-ground) in the vicinity.
v. Grading and drainage properties of the land.
Based on the above information and any associated soil reports, surveys, or construction plans, the engineer can determine the type & size of foundation required for your specific project.
3. How do I determine the size of my foundation?
Any shallow foundation (isolated footers, slabs-on-grade, wall footers, mats & rafts, etc.) must meet the following general requirements.
A. It must be wide enough to adequately distribute the loads from the structure it is supporting to the soil without sinking into it.
B. It must have enough weight to counter any uplift caused by wind or earthquakes*.
C. It must have enough weight and soil coverage to counter the lateral forces caused by wind, earthquake, flood which may cause it to slide or overturn.
*The weight that counter’s the effect of wind or earthquakes must be multiplied by a factor of 0.6, per Section 1605.1.1 of the Building Code.
4. I am building an exact replica of a structure I did before but in a different location. Can I build the same foundation too?
Not without certain checks. While the design and layout of your structure would remain the same, the winds acting on it might be different. The soil under the structure might have different values of bearing capacities and grading and drainage properties. Based on the location, it might be subjected to different earthquake loads, snow loads, etc.
In short, each structure must be specifically analyzed for the loads that it will experience at a specific location.
5. I changed the use of my building from Storage to Residential. Does that affect my previously designed foundation?
Yes, it does affect the size of the foundation.
A structure designed for storage is a Risk Category I (low risk) structure, whereas a typical residential structure is a Risk Category II (moderate risk) structure. Risk Category II structures must be designed to a stricter criterion (higher wind speeds, etc.) than Risk Category I structures. This is due to their comparative risk values, in terms of loss of life and property.
While it is possible the existing foundation might be sufficient to take the new loads, without evaluation by a competent structural engineer, such an assumption cannot be safely made.
Please visit our blog on Risk Categories for more information.
6. I am demolishing an old structure and putting up a similar new one in its place. Can I use the same foundation? Or can I rebuild the same foundation after demolishing the old one?
Yes, it may affect the size of the foundation in certain situations.
In all likelihood, the older structure was designed and permitted under an older edition of the building code, and grandfathered in when the new building cycles came into effect.
Any structure that is replacing the demolished one will need to be designed under the building code in effect. This means the existing foundation cannot be re-used without analyzing if it can adequately sustain the new loads applied to it.
7. Can I use plain concrete, aka concrete without the necessary steel reinforcement, for my foundation?
Per ACI, plain concrete may be used for a foundation, if (and only if) the foundation meets both the following requirements:
A. Structure complies with ACI 318-19 Section 14.1.3 & 14.1.4.
(These sections list the few cases in which plain concrete is permitted* and the minimum size and reinforcement** requirements of the same).
B. The foundation has been structurally analyzed and determined adequate to support all the applicable loads without the necessary steel reinforcement.
- The word “permitted” is operative here. It is up to an engineer’s judgement to deem the necessity of reinforcement in a foundation.
** Plain concrete does not mean “concrete with no reinforcement”. It simply means “concrete with less than minimum mandated reinforcement”.
8. If I have a concrete slab on grade, do I need extra footers for the structure?
Broadly speaking, it depends on the location and distribution of loads on the structure.
Concrete slabs, by virtue of their large area, usually are adequate to distribute the downward loads to the soil. Also, slab normally have enough weight to counter the uplift forces and keep the structure grounded.
However, slabs-on-grade, in general, are relatively thinner than wall footers or isolated footers. Hence, they are more susceptible to cracking. A footer, on the other hand, is directly under a wall (or a post) and concentrates all its weight at the exact point of loading, and hence reduces the risk of cracking.
Detailed analysis of a slab’s cross section and reinforcement would be required to make a judgement.
9. Do minor structures such as fences and tool sheds require a foundation?
Yes, they do*.
While failure of these Risk Category I structures, in itself, might not be a concern to human life, the detritus of these failures might cause secondary effects on higher Risk Category structures.
For example, a fence without a foundation is susceptible to being blown away during a hurricane. This debris might harm people or structures sheltering people.
Thus, it is important that even minor structures be adequately anchored down.
*Certain wood, wire & chain fences might be exempt from this requirement, based on the embedment of the post into the soil as well as the lateral bearing capacity of the soil itself.
At Florida Engineering LLC, our team of highly qualified and experienced engineers specializes in the design and layout of different types of foundations, suitable for a wide range of structures, both host attached and freestanding. We endeavor to provide you with the most structurally sound and materially economical design. Reach out to support@fleng.com for a quote today.
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