Wood Frame System

 

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According to the class, a system is defined as "an organized group of interacting, interrelated or interdependent elements forming a harmonious complex whole".  I feel as though this definition is accurate and certainly incorporates the basic theory behind wood frame construction.  The general premise is that no individual piece of wood can function on its own, without the support of multiple other pieces of the framework.  Wood framed structures are a perfect example of how simple elements, such as pieces of lumber, can be formed into complex systems, such as a house.  When creating a complex system from basic materials, one must understand the specific function of each element and then understand how the addition of another element will change the overall properties of the system being created.

There are many different types of wood frame construction, the most common being post and beam and standard balloon framing.  Post and beam framing relies on heavy, oversized lumber to transfer loads into the ground.  Neither the post, nor beam are effective at doing this without the compliment of the other.  For example, a post can transfer loads axially in the vertical direction.  However, it cannot stand on its own and be stable enough to prevent being effected by forces in any horizontal direction.  This is where the role of the beam comes into play.  The beam is used to connect multiple posts together and effectively combat and forces in the direction parallel to its orientation.  Also, the beam can take any point load acting on it and transfer it horizontally onto the top of the post, where it can be transferred to the ground.  Post and beam construction was popular in the past, when trees could be chopped down and roughly milled into usable members.  However, technology has created better, more efficient and effective methods of using wood as a structural member.  Nowadays, post and beam is used more for its architectural impact rather than its convenience and effectiveness. 

The problem with wooden post and beam construction is that the members are very large and bulky.  Having a large framework that carries the loads of a building is effective at supporting the structure, but it lacks the ability to define spaces.  To do that, non-load bearing walls had to be erected between structural members.  This is what eventually led to the use of smaller materials to perform the same job.  Instead of using two large posts to carry the weight of a certain span, you can use multiple smaller posts to carry the same load of that same span.  This technique better utilizes the areas between the posts and in essence defines spaces naturally.  This is the idea behind conventional balloon framing.  These smaller posts are easier to work with, as well as easier to produce, easier to machine, and cheaper for the consumer. 

Nowadays, balloon framing is standard in almost all residential applications as well as smaller commercial applications.  It is regarded as the easiest and most effective option for creating small buildings or structures.  It's only limited by the size of a structure and by the spans that are required.  Once the size and/or span reaches a certain point, other methods become the most convenient and practical.  Since post and beam construction is generally very simple, the primary focus of this assignment will be on traditional balloon framing.

The Foundation

http://imgsrv.homes.com/imgsrv/d8/53/93541538.jpg

Since the objective of a building is to transmit loads around a centralized space and into the ground, it is clear why the foundation is so important.  As with all aspects of construction, their are many different ways to get the job done.  However, the most common types of foundations are concrete block and poured concrete.  Poured concrete is the standard and is used most frequently.  The main job of the foundation is to connect the bottom of the building or structure to some form of stabilized earth.  To do this, a contractor must excavate the topsoil until a suitable soil type is reached.  In larger construction, some foundations tie into bedrock, but since wood frame construction is rarely used in large projects, that almost never applies. 

There are three different common types of foundations; slab foundation, crawl space foundation, and full basement foundation.  A slab foundation consists of a simple shallow footing around the perimeter and a poured concrete slab that rests directly on the ground.  A crawl space foundation consists of a higher footing and allows for the floor to be constructed off the ground but does not allow for any usable space underneath.  A full basement foundation has the deepest footings which also serve as walls for the interior basement space.  This foundation is fairly popular with new home construction due to its usable space.  Any and all of these footings can be created using either poured concrete or concrete block but both the slab foundation and basement floor almost always utilize poured concrete.

Poured concrete foundations use forms to define the foundation and the concrete is simply poured into the forms and allowed to properly set.  While the concrete is still wet, anchors are placed vertically to be used later to attach the wooden frame.  After the concrete has set, the forms are removed and the foundation is left standing.  This technique is very simple and straight forward and has been in use for a long time.

The Wooden Frame

The first piece of wood to come in contact with the foundation is referred to as the sill plate.  The sill plate is generally a piece of pressure treated lumber to combat the moisture that is inevitably present in the concrete foundation.  The sill plate is then bolted down using the anchors that have been cast into the foundation.  This sill plate provides a nice dry surface for the remainder of the framework to be attached too.  In the case of a basement and crawl space foundation, the next step is to span the space below with the floor joists.  The perimeter of the foundation is first framed out by what is referred to as the rim joist.  Then the subsequent floor joists are laid within that framework.  These joists are generally fairly large, depending on the required spans.  Most often they are standard 2x10 or 2x12 lumber spaced out at 16 inches on center. 16 inches on center is chosen because 4'x8' plywood will always be able to start and end in the center of a structural member.  This floor system is tied down to the foundation via the sill plate.  These joists are then sheathed with plywood to create what is called the sub-floor.  After the sub-floor is complete, the first floor walls can be built and erected. 

The construction of a wood framed wall is pretty straight forward.  Studs are laid out on 16 inch centers and connected with a top and bottom plates.  Studs are commonly sized at 2x4 or 2x6 lumber, depending on the specifics of the structure.  These studs are responsible for carrying all vertical loads from above the space onto the foundation below. 

Drawing Courtesy of Mike Schick

However, on their own, these studs are just like any other free standing post in the fact that they have no way to deal with horizontal forces.  If a stud wall were to be pushed on parallel to the direction of the wall, it would deform.  This deformation is what is called racking.  The wall needs to be braced to prevent it from racking.  This bracing comes in the form of exterior plywood sheathing.  A sheathed wall is capable of withstanding forces in two directions and is no longer prone to racking.

Drawing Courtesy of Mike Schick

Support in the third direction comes from another load bearing wall being attached to it perpendicularly.  This system of walls is the only way in which loads can be accounted for in three directions.  All of these aspects work together to create a vertical wall system that is capable of withstanding forces in all directions.

Drawing Courtesy of Mike Schick

Those diagrams accurately depict a wall that is unblemished by windows or doors.  However, that is rarely the case.  When there are windows or doors that are designed to be placed in a wall, certain precautions have to be taken to transfer the loads across them.  The most important piece is the header.  The header is a piece of doubled up lumber that carries the load from the top of either a window or door and sends it down adjacent studs.  The following diagram contains all the information that relates to framing a window or door into a load bearing wall. 

Drawing Courtesy of Mike Schick

After all the first floor load bearing walls have been constructed, the process is repeated until all subsequent floors are built.  The rim joists and floor joists are installed on the second floor and attached to the first floor top plate just as the first floor joists were attached to the sill plate.  Once the second story sub-floor is in place, the second floor load bearing walls can be erected in the same fashion as the first floor walls.  This pattern continues using the same technique until the desired number of stories have been reached.  Once that has occurred, a roof must be constructed overhead.

The Roof

Drawing Courtesy of Mike Schick

The most critical aspect of finishing off a building is providing a roof overhead.  This roof not only keeps the spaces inside dry, it also ties together the entire structure.  The roof is framed by building up gable end walls.  These walls dictate the slope that the roof will be at.  This slope can be any number of different angles, provided it can carry the required loads and run-off water.  Once the gable ends have been constructed, they get tied together by the ridge beam.  Once that is in place, the rafters can be laid in place at 16 inch centers.  The rafters are placed evenly on both sides and they are what transfers the loads from the roof onto the load bearing walls that make up the structure.  Once all the rafters are set in place, the roof can be sheathed and then prepared to be finished.  There are many different ways to finish off a roof from the inside, but most commonly, joists are laid across the top plates off the wall to support the ceiling.  These joists not only support the ceiling, but they also hold together the tops of the walls in which they are connected, this provides added support and rigidness to the entire structure.  Other methods include leaving a vaulted ceiling exposed, or even a cathedral type exposed ceiling.

With the construction of the roof, the structure is now complete in its rough phase.  This includes all of the structural elements that will support the building for years to come.  After the rough building is complete, the surfaces can be finished in a various number of ways to enhance the appearance of these structural elements.  The way in which these surfaces are finished general has little impact on the performance of the structure as a whole.  Almost all forms of finishing are merely aesthetic. 

The Loads

Drawing Courtesy of Mike Schick

All loads that arise in a building are general handled the same way.  That is, the load is initially dissipated by the face it is acting on and is then transferred to the foundation by structural members that are oriented perpendicular to the face in which the load acted.  In a vertical sense, this means that a load acting on the roof will be dissipated by the roof plane and then transferred via the rafters onto the vertical walls, in which the load will be distributed down the studs and onto the foundation.  For loads that act horizontally on the face of a building, the procedure is the same.  The load is dissipated and transferred by the face on which it acts to the walls that are mounted perpendicular to the face and are then tapered off into the foundation. 

The loads that effect a building are generally broken up into 2 categories; live loads and dead loads.  Dead loads are those associated with the actual structure itself.  These loads are constant and generally won't change throughout the life of the structure.  They include all weights of the materials used, both structurally and aesthetically.  Any and all weights of the structure itself must be dealt with and handled accordingly.  Live loads are described as those that are variable.  These include people and furniture as well as elemental loads, such as wind and snow.  These loads get accounted for based on the general designed use for the structure and the geographic area that the structure is located in. 

Generalization

In conclusion, it is clear that there are many ways to successfully build a wood framed structure.  However, in every single one of these variations, the concept that remains the same is having individual elements working together to accomplish a common goal of supporting all necessary loads.  As technology advances, the methods used for wood framed construction keeps getting more and more refined.  Previous span problems have been dealt with by the introduction of engineered beams.  These beams can span much further than an all natural beam could.  These advances keep adding to the possibilities and creating new ways to build a structure.  One thing that all wood framed structures have in common in the ease of use.  Any average Joe can pound in a few nails on a weekend, whereas not anybody could weld up steel as needed in steel construction.  One of the greatest benefits of wood framed construction is its ability to be easily manipulated into creating virtually any shape that is needed.  Also, its easy to change shapes or add onto them.  The benefits of wood framed construction are clear and it is easy to see why it is so widely used in this day and age. 

 

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This site was last updated 04/18/05