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Unit 42. Floor Framing. Floor Unit Resting on Sill Plates • Floor Unit Resting on Cripple Walls • Posts and Beams • Floor Joists • Subfloor • Floor Underlayment • Floor Trusses • Wood I-joists.
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Unit 42 Floor Framing Floor Unit Resting on Sill Plates • Floor Unit Resting on Cripple Walls • Posts and Beams • Floor Joists • Subfloor • Floor Underlayment • Floor Trusses • Wood I-joists
Basic parts of a floor unit include posts, beams, joists, bridging or blocking, and a subfloor. Floor framing begins after foundation work is completed.
When constructing a floor unit resting on sill plates with a crawl-space foundation underneath, an 18″ minimum clearance should be maintained between the bottoms of floor joists and the soil, and 12″ between the bottom of the beam and the soil.
Cripple walls, including sill plates and studs, are used to extend the height of low foundation walls.
No bolts or other types of fasteners are necessary to secure the fixed post bases to concrete; the lower section of the base is set into a concrete pier when concrete is placed. An adjustable post base should have 1″ to 2 1/2″ standoff height above the concrete. An adjustable base is secured in position by drilling a hole into hardened concrete and fixing a threaded rod into place with epoxy, or by setting a J-bolt into fresh concrete.
Joints on a built-up beam are staggered. Plank joints occur directly over a post.
Allowable beam sizes are based on the span of supported flooring, clear opening, lumber grade, and live and dead loads. This table is for a floor beam supporting a 40 psf live load and 10 psf dead load.
Steel pipe columns are frequently used to support steel beams or wood beams.
A hollow steel section column may be used to support the end of an LVL beam.
Wood plates may be attached to the top of wide-flange steel beams with a powder-actuated tool.
When placing wood posts and beams, the top of each beam should be aligned with the upper surface of the sill plate.
Allowable floor joist spans are based on the lumber species and grade, and joist size and spacing.
Header joists prevent regular joists from rolling or tipping, support the wall above, and fill spaces between regular joists.
Joists should be lapped the full width of the beam or wall supporting them.
Joists butting over a beam should have the ends scabbed together with a metal or wood tie.
Joist hangers are available in a variety of standard configurations. Face-mount and top-flange joist hangers are the most common types.
Double-shear joist hangers provide greater strength than standard joist hangers.
Always refer to manufacturer installation information regarding the proper nail size and required number of nails in the header and the joist.
A sill plate can be bolted to the top of a steel beam. Joists are toenailed to the plate. The sill plate may also be attached to the steel beam with a powder-actuated tool.
Joists butted against a steel beam may be supported with joist hangers or a steel plate welded to the bottom of the beam.
Blocks separate doubled joists to allow drain pipes, water supply pipes, and heating ducts to extend into a wall cavity above.
Cantilevered joists provide support for a floor or balcony that projects past the wall below it.
When regular floor joists run parallel to the intended overhang of the floor, the inside ends of the cantilevered joists are fastened to doubled joists.
Wood cross bridging is installed by toenailing each end with 6d or 8d nails. Allow 1/2″ space between adjacent bridging members to prevent them from rubbing together and producing squeaks.
Metal cross bridging does not require nails for installation.
When framing a typical floor opening, headers are nailed to trimmers and tail joists are nailed to headers and header joists.
When laying out joists, an “X” mark indicates the side where the joists are to be nailed.
In this application, floor joists are placed 16″ OC and layout has been started from the left side. The first joist is marked at 15 1/4″ to ensure that the 4' or 8' edge of a panel will fall on the center of a joist. The layout for the following joists is then 16″ OC. A doubled joist is laid out at the right.
When laying out a floor unit, joist placement is marked on the sill plates.
A floor unit is framed by setting the header and end joists, placing the floor joists, and installing blocking and bridging. The floor opening is framed as described in Figure 42-30. Most framing members should be precut before construction begins.
Joist hangers can be installed prior to installing the joists. Joist hangers may be installed using a hammer, palm nailer, or pneumatic nailer specifically designed for metal connectors.
Subfloor panels are placed over joists so the long sides run at a right angle to the joists. Note that the panel joints are staggered.
Subfloor panels are attached to joists using a heavy-duty pneumatic nailer or self-feeding screwgun.
Aligning the first row of panels with a chalk line 4′-0″ from the edge of the building ensures that the first row of panels will be perfectly straight even if the edge of the building is not straight. When placing the remaining panels, drive a few fasteners to secure the panels in place. Then, snap chalk lines across the panels in line with the joist centers below and complete nailing or screwing down the panels.
In the glue-nailed panel method, a construction adhesive is applied to the top of the joists before subfloor panels are placed.
In a post-and-beam subfloor, the floor unit receives its main support from beams rather than from floor joists.
Underlayment is placed over the subfloor to provide a smooth and even surface for finish floor materials.
The top and bottom chords of floor trusses are tied together by webs. The chords and webs are typically 2 × 4s and are joined together with metal connector plates.
Floor trusses may be constructed with 2 × 4 wood top and bottom chords and tubular steel webs. The 2 × 4 shown along the bottom chord toward the left side stabilizes and ties the trusses together.
Trademarks provide information regarding recommended clear spans and spacing of wood I-joists.
Span tables specify on-center spacing of various sizes of wood I-joists.
Spacing and layout of wood I-joists is the same as for solid dimension lumber. Engineered wood rim boards are used to support the ends of I-joists.