Floor systems provide a base for the remainder of the structure to rest on. They transfer the weight of people, furniture, materials, etc., from the subfloor framing to the foundation wall, the footing, and finally to the earth.
Floor systems are constructed over basements or crawl spaces. Single-storey structures built on slabs do not have floor systems; however, multi-level structures may have both a slab and a floor system.
Understanding Floor Systems
A typical platform floor system and its various parts
Sills, also called sill plates, are the lowest member of a structure’s frame. They rest horizontal on the foundation (the supporting portion of structure below the first floor construction including the footing).
Sills fulfill the following purposes:
They serve as …
the attachment point to the concrete or the block foundation for all the other wood framing members
They provide …
a means of leveling the top of the foundation wall
They prevent …
other wood framing member such as lumber, from making contact with the concrete or masonry
2) Beams/Girders and Supports
The distance between two outside walls is frequently too great to be spanned by a single joist.
When two or more joists are needed to cover the span, support for the inboard joist end must be provided by one or more beam, commonly called girders.
A couple of facts about girders:
• Girders carry a very large proportion of the weight of a building.
• Girders may be made of solid timber, built-up lumber, engineered lumber, or steel beam. Each type has its own advantages and disadvantages.
Types of Beams/Girders and Supports
Solid Lumber Girder
Solid timber girder stock used for beams is available in a various sizes with 4 × 6, 4 × 8 and 6 × 6 being typical size. If straight, large timbers are available their use can save time by not having to make built up girders.
Built Up Lumber Girder
Built-up girders have the advantage of not warping as easily as solid wooden girders as they are less likely to have decayed wood in the center. A disadvantage of built-up girder is that it is not capable of carrying the same load as solid timber girder of an equivalent size.
Engineered Lumber Girder
Laminated veneer lumber (LVL) and glue laminated lumber (glulam) are engineered lumber products that are used for girder and other framing members.
Their advantage is that they are stronger than structural lumber of the same size.
Steel I-Beam Girder
Metal beams can span the greatest distance and are often used when there are few or no piers or interior support in a basement. Two types of steel beams are available: standard flange (S-beam) and wide flange (W-beam).
Beam Girders Support
Girders and beams must be properly supported at the foundation wall, and at proper intervals in between either by supporting posts, columns, or piers.
3) Floor Joists
Floor joist are a series of parallel, horizontal framing members that make up the body of the floor frame.
They rest on and transfer the building load to the sills and girder. The flooring or subflooring is attached to them, and its span determines the length of joist that must be used.
Joists must be:
• Strong enough to carry the load that rests on them
• Stiff enough to prevent undue bending (deflection) or vibration
• Doubled where extra load needs to be supported
• Joist hangers are used where the bottom of the girder must be flush with the bottom of the flush.
Called trimmer joists, these additional joists support the headers that carry short joists (tail joists). Double joists should spread where necessary to accommodate plumbing.
Trusses are manufactured joist assemblies made of wood or a combination of steel and wood.
Solid light-gauge steel and open-web steel trusses are also made but these are used mainly in commercial construction.
Floor trusses consist of three components: chords, webs, and connector plates.
Bridging is used to stiffen the floor frame to prevent unequal deflection of the joist. It is also used to enable an overload joist to receive some support from the joist on either side.
Subflooring consists of panels or boards laid directly on and fastened to the floor joist in order to provide a base for underlayment and/or the finished floor material.
Underlayment is a material such as particleboard or plywood, laid on top of the subfloor to provide a smoother surface for some finished flooring.
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