FLOOR FRAMING AND BRIDGING

Floor framing (Figure 5.19) is laid out in such a way that the ends of un-cut 4-foot by 8-foot (1.2-m by 2.4-m) subflooring panels will fall directly over joists; otherwise, many panels will have to be cut, wasting both materials and time. Standard joist spacings are 16 or 24 inches o.c. (406 or 610 mm o.c.; o.c. stands for on  center,  meaning that the spacing is measured from center to center of the joists). Though less common, a joist spacing of 19.2 inches (486 mm) can also be used. Any of these spacings automatically provides a joist to support each end of every 8-foot (2.4-m) panel (Figure 5.23).


Figure 5.19
Step Three in erecting a typical platform frame building: the ground- floor platform.
Compare this drawing with the framing plan in Figure 5.17. Notice that the direction of
the joists must be changed to construct the cantilevered bay on the end of the building. A
cantilevered bay on a long side of the building could be framed by merely extending the  floor joists over the foundation.
Figure 5.23
Installing floor joists. Blocking will be
inserted between the joists over the two
interior beams to prevent overturning of
the joists.

Wood composite I-joists or manufactured trusses are frequently used in place of solid wood joists. Their advantages include the ability to span greater distances, straightness  and uniformity, and reduced drying  shrinkage of the floor  platform. These joist types are most frequently  spaced at 19.2 or 24 inches o.c. (488  or 610 mm o.c.) I-joists, with their thin webs, often require reinforcing where concentrated loads occur,  such as at joist ends, at cantilevers, or where interior loadbearing partitions bear on the joists (Figures 5.44 and 5.45).

Figure 5.44
Alternative details to those shown in Figure 5.43 for a fl oor framed with I-joists rather than solid lumber. For clarity,
the web of the I-joist is rendered light gray and blocking with a darker shade. The web stiffener shown in A is cut
slightly shorter than the height of the web of the joist, and is installed so that a small space remains between the
top of the blocking and the underside of the top fl ange. This prevents the blocking from prying the fl anges apart if
the I-joist itself shrinks or is compressed slightly (see also Figure 5.45). The I-joist blocking panel in B-1 is exactly
analogous to the solid blocking shown in Figure 5.45 B. A squash block is shown in B-2. This is a short section of
2x (38-mm) framing, installed vertically like a very short stud on either side of the I-joist. Squash blocks are used
under points of concentrated load, such as under loadbearing posts or studs on either side of a large opening in
a wall above. They are cut slightly longer than the full height of the joist to ensure that the loads are transmitted
through the blocks and not the joist

Figure 5.45
Blocking where an I-joist frames into a
glue-laminated wood beam, as illustrated
in Figure 5.44A. This blocking acts as
a web stiffener and also prevents the
I-joist from rotating within the metal
hanger, which is not tall enough to
restrain the upper fl ange of the joist.
The need for blocking depends on the
size of the I-joist, the magnitude of the
loads, and the type of hanger used.

After floor framing is complete, subflooring is fastened in place. To reduce squeaking in the finished floor and to increase floor  stiffness, adhesive may be used in conjunction with mechanical fasteners (Figures 5.26 and 5.27). Deformed shank nails or screws, either of which hold subflooring more securely than common nails, also contribute to the prevention of squeaks. Plywood and OSB panels must be laid with  the grain of their face layers perpendicular to the direction of the joists because these panels are considerably stiffer in this orientation (that is, a standard 4-foot by 8-foot (1.2-m by 2.4-m) panel should be laid with its  longer dimension perpendicular to the joists). Sheathing and subflooring panels are normally manufactured  1 /8  inch (3 mm) short in each face dimension so that they may be spaced slightly apart at all their edges  to prevent floor buckling during construction from the expansion of storm-wetted panels. Bridging, consisting of solid blocking or wood or metal crossbracing inserted between joists at  midspan or at intervals not exceeding  8 feet (2.4 m), is a traditional feature of floor framing (Figures 5.19, 5.28). (The term blocking refers, in general, to short lengths of lumber inserted between framing members for any  of a variety of purposes, such as bracing, reinforcing, or providing solid backing where fasteners are required for attachment of finish materials or equipment.) Its function is to hold the joists straight and to help them  share concentrated loads. The International Residential Code requires bridging only for joists deeper than 2 x 12 feet (38 x 286 mm). In better-quality construction, however, bridging is frequently used in all floors, regardless of depth, to improve stiffness and reduce vibration. Solid blocking is also required wherever floor joists span continuously across supporting beams to resist overturning of the joists (Figure 5.18B, D).

Where the ends of joists butt into supporting headers, as around stair openings and at changes of joist di- rection for projecting bays, end nails and toe nails cannot transfer the full  load between members, and sheet  metal joist hangers must be used. Each hanger provides a secure pocket  for the end of the joist and punched holes into which a number of special short nails are driven to make a safe connection.

Figure 5.26
Applying OSB subfl ooring. Note that
the longer dimension of the panel runs
perpendicular to the joist framing.

Figure 5.27
For a stiffer, quieter fl oor, subfl  ooring
should be glued to the joists. The
adhesive is a thick mastic that is
squeezed from a sealant gun on the tops
of the joists just before the subfl  ooring
is laid in place.

Figure 5.28
Bridging between joists may be
solid blocks of joist lumber, wood
crossbridging, or, as seen here, steel
crossbridging. During manufacture, the
thick steel strip is folded across its width
into a V- shape to increase its stiffness.
Steel crossbridging with one end toothed
requires only one nail per piece and
no cutting to size, so it is the fastest to
install.
Figure 5.18
Ground-fl oor framing details, keyed to the lettered circles in Figure 5.19. A
compressible or resilient sill sealer strip should be installed between the sill and the
top of the foundation to reduce air leakage but is not shown on these diagrams. Using
accepted architectural drafting conventions, continuous pieces of lumber are drawn
with an X inside and intermittent blocking with a single diagonal. The metal termite
shield in detail C is used in areas where the risk of termite infestation is high. It
prevents subterranean termites from traveling undetected from cracks in the concrete
into the wood framing above.

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