Codes & foundation performance

What does the code say about foundation performance?

The most widely used building code for home design and construction is the International Residential Code for One and Two-Family Dwellings, numerous editions.

The key issues in the code

The following is a summary of some key issues from this code:

1. There is no stated requirement that foundation deflection not be allowed to exceed some specified amount such as L/360.

2. The Building Official has the discretion to permit the construction of a slab-on-ground foundation without an engineered design, if past experience has shown that the proposed slab-on-ground foundation has performed adequately.

The key performance criteria for slab on ground foundations

For a non-engineered slab-on-ground foundation to be judged to be performing adequately, it must meet three stated criteria.

1. The foundation must be able to resist differential volume changes: while this would eliminate some minimal foundation designs, almost all foundation designs typically used in the metropolitan areas of Texas could be said to provide some degree of resistance to differential soil volume changes.

2. The foundation must be able to prevent structural damage to the supported structure. In this context, it is clear that structural damage means damage to the supported structure that reduces the ability of the supported structure to carry the imposed loads in a safe manner. Most foundations would be able to pass this test.

3. Deflection and racking of the supported structure shall be limited to that which will not interfere with the usability and serviceability of the structure. This would imply that the foundation deflection should not result in functional problems such as doors or windows that bind and stick.  This is clearly a problem with some homes.

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