What does the code say about
foundation performance?
The Texas State
Legislature recently adopted the International Residential Code for One and
Two Family Dwellings as a state building code.
The following is a summary of some key issues from this code:
 | There is
no stated requirement that foundation deflection not be allowed to exceed some
specified amount such as L/360.
|
| 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.
|
| 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. |
|
What do the code-approved
design protocols say about foundation performance?
For engineered slab-on-ground
foundations on expansive soils, there are two code approved design protocols
that may be used, one published by the Post-Tensioning Institute (PTI) and the
other published by the Wire Reinforcement Institute (WRI).
The 1996 edition of the PTI book Design
and Construction of Post- Tensioned Slabs-On-Ground includes the
following wording:
|
“Application of these recommendations results in slab
designs similar to those that have exhibited satisfactory performance.”
|
The WRI publication, Design of Slab-on-Ground Foundations – An Update,
includes similar wording. Neither
code approved design protocol promises that the actual deflection of engineered
slab-on-ground foundations will be less than some stated amount. They do promise
that actual foundation performance in terms of superstructure distress will be
“satisfactory” and the distress “minimal.”
I think it is fair to say that slab-on-ground foundations designed and
constructed in accordance with either design protocol will not deflect enough to
cause structural (load-bearing) damage to the superstructure.
There is likely to be some degree of cosmetic distress and some minor
door problems. The levelness of the
slab surface is not addressed by either design protocol.
