Structural Failure ...

... occurs in two (2) scenarios within a residence due to water impact.  The first impact is the evaporation of water from the soils causing a high humidity; or, saturated environment; and the second is pier elevation descent (sinking) due to laterally traveling subterranean water that causes the soils beneath the physical footing become soft.

We commonly speak of pier impact in a parallel concept of ... "Just because the elephant's ankles appear to be dry, it does not mean that he is not necessarily standing in mud!"

Typically, when one thinks of water removal, they envision standing or pooling water; and its removal.  In terms of residential environments, we also look at the levels of humidity within the crawlspace of a structure.  The air volume within the crawlspace is the environment that the structural members (girders & floor joists) are subjected to daily.  When framing materials are manufactured, they are normally sawn to a specific dimension and then kiln dried to a specific water saturation of about 19%.  At this low percent of water saturation, the materials are structurally sound and yield optimum performance with regard to span and support.  However, when these materials are subjected to high levels of moisture in a damp or saturated crawlspace, the water within the air (humidity) is absorbed into the wood material and weakens its ability to provide design strength.  This saturation promotes decay and failure; but, more over, it causes the wood to distort from its original character and become warped, or at a minimum, not linear (or straight).  The structural static loads imposed on the framing members greatly determines the rate of failure that will occur and to what degree.

Once the water has been controlled; the soft soils conditions & the high humidity environments alleviated, the home's structural framing can then be surveyed and assessed for remedy and restoration.  The exterior foundation wall of a home's perimeter typically stands strong and rarely moves.  However, in contrast, the interior individual pier footings almost always settle or sink into the soils due to subterranean water migration.  The pier locations are referred to as "control points".  These control points are the locations that the girders are placed atop; which carry the transposed loads created by the assembly of interconnecting floor joists. 

If the interior  piers are lower in elevation that they were at the time of construction, for whatever reason, then the general overall net square footage of the floor will be slightly increased.  For those homes having hardwood coverings, this can easily be seen in the openings of the hardwood flooring boards throughout the home. 

Most foundations are not exactly level; that is to say that they are generally flat in a two-dimensional plane; but, the plane may be slightly off.  To level a home, or to place a home's floor system back into plane first requires that the perimeter be surveyed and a general two dimensional plane established.  It is from this mathematical reference that the interior girders are to be aligned; and ultimately, any dramatically sagging floor joists.

If a home's floor system design and pier locations were generally correct at origin, lifting the home back into plane is normally not a huge undertaking.  However, we often find homes hat were constructed with the load bearing walls not having structural members beneath them.  Or, often times, the culprit of failure is in the way that it was physically assembled by the framing contractor at the time of construction.  There are two types of framing systems; 1) support girder (where the boards actually rest atop the girder & the girder is not within the same plane as the floor joists), 2) the joists & girder are in the same plane and the joists intersect the girder perpendicularly and rest atop end bearing 2X ledger boards.  Our repair history shows that a support girder design proves to be a more successful installation.  However, it also provides for less ground space clearance.

Another key element is the Building Code and its vantage of "minimum standard".  As an FYI, structural members are allowed a deflection not greater than 1/360 of their span.  Simply stated, a board eight (8) feet in length can have a 1/4" sag over the length of its span.  Most floor joists are 12 to 13 feet in length.  This is the reason for us having to install midspan girders and pier systems to remedy concave floor issues that are not within the control point guidelines.