A home contains interior and exterior walls. One should always assume that an exterior wall is a load bearing, a wall that supports a load from above, until it is otherwise proven that it not. Load bearing walls must be remodeled removed and maintained much differently than non-load bearing, partition walls; one must always provide for the load to be maintained when working with a load bearing wall.
If you have a pier, beam or post in the basement or crawl space because your span was too great for the lumber sizes available, chances are good that you have one of more interior load bearing walls. This may not be true if your roof is supported with manufactured trusses. You floor may be framed traditionally and you may have roof trusses that at rated for the full span. Other trusses sometimes require a center bearing point. Only a structural engineer or the truss manufacturer can tell you which you have.
Until a few years ago 16" OC, on center, was the standard for both interior and exterior walls in residential construction. A somewhat disturbing trend to me is allowing non-load bearing interior walls to be constructed 24" OC provided bracing in used between the studs. I can see where this might save a builder a few pennies on each house if the construction crew used scraps for the bracing; unfortunately I never see them doing that. By the time you saw up new 2x4's for the bracing you can't be saving enough to make it worth the time and labor cost.
Where there is an opening for a window or door a header, a doubled set of 2X lumber usually with a plywood or OSB core added, is required to support the load above. When dealing with exterior walls most building inspectors keep life simple and require that all headers be of a certain size even if the wall in question is not supporting a roof load as would be the case on the gable ends of a house of gable construction. 2x8 with core is about the minimum exterior header.
The core we mentioned above serves two purposes. 2X dimensional lumber is really 1-1/2" thick and a stud is 3.5". When one adds a 1/2" core to the two 2X's it allows them to fit flush to both sides of the stud wall. Supposedly it also adds some strength to the header. The core is required in my county but not required in most of the surrounding ones. Your local codes may or may not require the core.
A stud wall is constructed with a top and bottom plate to which the studs are nailed. The bottom plate is nailed to the sub floor and into the joists and rim joists where it is possible to do so. The better nailed exterior wall is far more likely to withstand the strong winds it might encounter in its lifetime. A second top plate which overlaps adjacent walls allows the walls to be tied together into a reasonably sound structure is nailed into place once the wall is standing in place. This second top plate also adds rigidity to the top plate so that it is not necessary for every single roof rafter to rest directly over a stud. That might be hard to do since roof rafters or trusses are often 24" OC while the walls are 16" OC.
We will go into more detail in future articles when we show you how to frame a wall you may want to build.