Buildings With Tall Exterior Walls
Post frame makes large floor to ceiling heights more cost effective.
Many buildings without basements are supported on cast-in-place crawlspace walls or frost walls that rest on continuous cast-in-place concrete footings. The construction time and concrete cost associated with these continuous concrete foundation walls and footings is significantly greater than that associated with a post-frame building that utilizes embedded posts or a post-on-concrete pier system (figure 4) as its foundation system.
Mechanically- and glue-laminated posts are used in the vast majority of today’s post-frame buildings. These posts enable the construction of buildings with relative large floor-to-ceiling heights at prices much less than they could be fabricated with a comparable wood stud wall.
Laminated posts can be fabricated to any length by splicing shorter pieces of wood together (figure 5). Laminated posts are also straight and inherently more stable because of the laminating process. The only way to get a tall, relatively straight wall with wood studs is to use more expensive, engineered lumber products (e.g., laminated strand lumber, parallel strand lumber).
Figure 6: The ability to construct inexpensive buildings with relatively high eave heights makes post-frame ideal for many machinery storage buildings including this airplane hangar.
As wall height increases, bending moments in the wall’s vertical framing elements increase. This increase is not minor. Bending moments induced by uniformly-applied loads increase with the square of the unsupported length of a member. This means that a 20-foot wall stud is subjected to a bending moment that is four times greater than that for a 10-foot wall stud with the same on-center spacing.
Laminated posts are able to deal with increases in bending moment more effectively than solid-sawn posts. This is because weak areas in one layer of a laminated assembly are supported by adjacent layers – layers which have a low probability of having a weakness at the same location. This support of weak areas in one layer by the adjacent layers gives rise to the phenomena of load sharing. Load sharing is very important in any area of a laminated assembly in which there is a butt joint between two members. Because of load sharing, the design strength of a laminated assembly is greater than the summed total of its individual layers prior to lamination.
Figure 7: This building on the Larson Dairy near Evansville, WI is typical of dairy freestall barns. The greater width of these buildings results in (1) tall gable endwalls requiring substantial framing, and (2) the need for interior support posts.